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JOHNSON MATTHEY TECHNOLOGY REVIEW

Johnson Matthey’s international journal of research exploring science and technology in industrial applications

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Volume 59, Issue 3, July 2015 Published by Johnson Matthey

ISSN 2056-5135

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Contents Volume 59, Issue 3, July 2015

JOHNSON MATTHEY TECHNOLOGY REVIEW

Johnson Matthey’s international journal of research exploring science and technology in industrial applications


174 Selected Electrical Resistivity Values for the Platinum Group of Metals Part I: Palladium and PlatinumBy John W. Arblaster

182 12th Greenhouse Gas Control Technologies ConferenceA conference review by Christopher Starkie

188 Platinum Group Metal and Washcoat Chemistry Effects on Coated Gasoline Particulate Filter DesignBy Chris Morgan

193 “Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing”, 2nd EditionA book review by Jonathan Edgar and Saxon Tint

199 Temperature Dependent Heat Transfer Performance of Multi-walled Carbon Nanotube-based Aqueous Nanofl uids at Very Low Particle LoadingsBy Meher Wan, Raja Ram Yadav, Giridhar Mishra, Devraj Singh and Bipin Joshi

207 The Effects of Hot Isostatic Pressing of Platinum Alloy Castings on Mechanical Properties and MicrostructuresBy Teresa Fryé, Joseph Tunick Strauss, Jörg Fischer-Bühner and Ulrich E. Klotz

218 “Exploring Materials through Patent Information”A book review by Julia O’Farrelly

221 “Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts” An essay book review by Martyn V. Twigg

233 Sintering and Additive Manufacturing: The New Paradigm for the Jewellery ManufacturerBy Frank Cooper

243 Introduction to the Additive Manufacturing Powder Metallurgy Supply ChainBy Jason Dawes, Robert Bowerman and Ross Trepleton

257 Atomic-Scale Modelling and its Application to Catalytic Materials ScienceBy Misbah Sarwar, Crispin Cooper, Ludovic Briquet, Aniekan Ukpong, Christopher Perry and Glenn Jones

284 In the Lab: Combining Catalyst and Reagent Design for Electrophilic AlkynylationFeaturing Professor Jérôme Waser

287 Johnson Matthey Highlights

www.technology.matthey.comJOHNSON MATTHEY TECHNOLOGY REVIEW

http://dx.doi.org/10.1595/205651315X688091 Johnson Matthey Technol. Rev., 2015, 59, (3), 174–181

174 © 2015 Johnson Matthey

Selected Electrical Resistivity Values for the Platinum Group of Metals Part I: Palladium and PlatinumImproved values obtained for liquid phases of palladium and platinum

John W. ArblasterWombourne, West Midlands, UK

Email: [email protected]

Electrical resistivity values for both the solid and liquid phases of the platinum group metals (pgms) palladium and platinum are evaluated. In particular improved values are obtained for the liquid phases of these metals. Previous reviews on electrical resistivity which included evaluations for the pgms included those of Meaden (1), Bass (2), Savitskii et al. (3) and Binkele and Brunen (4) as well as individual reviews by Matula (5) on palladium and White (6) on platinum.

1. Introduction

Electrical resistivity (ρ) is defi ned in terms of the International System of Units (SI units) as:

ρ = R A / l (i)

whereR is the electrical resistance of a uniform specimen of material in ohms (Ω)A is the cross-sectional area of the specimen in square metres (m2)l is the length of the specimen in metres (m)

The units of ρ are therefore Ω m although practically the most useful units are μΩ cm.

The measured electrical resistivity (ρ) usually consists of a temperature dependent intrinsic resistivity, ρi, which is due to the pure metal and is caused by the scattering of the charge carriers (electrons or holes) by phonons (quantised vibrations of the lattice) and by their collisions with each other, and a residual resistivity (ρ0) due to impurities which also scatter the carriers and increase the resistivity. The quantity ρ0 is considered to be a summation of the effects of different impurities and is also considered to be temperature independent. The two contributions to the total resistivity are combined according to Matthiessen’s Rule: ρ = ρ0 + ρi and because ρ0 may vary from sample to sample then attempts are made to evaluate values of ρi which should be universal for a specifi c metal.

1.1 Correction for Thermal Expansion Effects

In order to obtain a reference value to which all other measurements are adjusted the electrical resistivity is evaluated at 273.15 K (0ºC).

In the low temperature region below about 30 K the resistivity can be represented by ρ = ρ0 + A T2 + B T5

where the temperature dependent terms represent the intrinsic resistivity, whilst up to room temperature the experimental values are generally given in such a form that interpolation can be achieved by using simple polynomials rather than using the complicated Bloch-Grüneisen formula (7–9). In the defi nition of resistivity as ρ = R A / l then A and l are usually measured at room temperature and therefore at different


http://dx.doi.org/10.1595/205651315X688091 Johnson Matthey Technol. Rev., 2015, 59, (3)

temperatures both A and l have to be corrected for thermal expansion effects. It is found below room temperature that for the level of accuracy given for ρ, thermal expansion corrections are generally negligible but at higher temperature the measurements have to be corrected, especially if they are based entirely on the room temperature values for A and l which are usually measured at 293.15 K, the accepted reference temperature for length change measurements:

ρ (corrected) = ρ (uncorrected) [(AT / A293.15) × (l293.15 / lT)] (ii)

= ρ (uncorrected) [1 + (lT – l293.15) / l293.15] (iii)

where Equation (iii) can be considered to be a close approximation of Equation (ii). However since 273.15 K is the actual reference temperature then corrected values of ρ(T) should be further corrected for thermal expansion from 293.15 K to 273.15 K. Since this correction is usually negligible at the level of accuracy given then it is not applied.

In the case of rapid pulse heating to high temperatures, because of inertia l generally is unaltered and it is A that changes. If D is the diameter of the wire then:

ρ (T) = ρ (measured) (DT2 / D293.15

2) = ρ (measured) (VT / V293.15) (iv)

where VT is the volume of the sample at temperature T and V293.15 is the volume at 293.15 K. These are essentially DT

2 and D293.152 respectively since l is

assumed to be unaltered.

2. Palladium

Palladium has a face-centred cubic structure and the melting point is a secondary fi xed point on the International Temperature Scale of 1990 (ITS-90) at 1828.0 ± 0.1 K (10).

2.1 Solid

Electrical resistivity values for solid palladium at 273.15 K are given in Table I. The selected value is an average of the last three determinations. The ρ0 correction to the measurement of Laubitz and Matsumura (14) was suggested by Matula (5) who also appears to have selected this value as the reference value.

From 71 data sets for solid palladium Matula (5) selected only the measurements of Schriempf (17) (1.6 K–10.6 K), White and Woods (13) (10 K–295 K) and Laubitz and Matsumura (14) (90 K–1300 K). However it is considered that the values of White and Woods

have been superseded by the later high precision measurements of Williams and Weaver (15) (0 K–300 K) and Khellar and Vuillemin (16) (17 K–300 K), with the latter given only in the form of an equation which was evaluated at 17 K and then at 10 K intervals from 20 K to 270 K. The measurements of Williams and Weaver were interpolated above 100 K so as to also obtain a full evaluation at 10 K intervals from 20 K to 270 K. The measurements of Schriempf and of Williams and Weaver agree satisfactorily and were averaged to 10 K with the measurements of Williams and Weaver being extended to 16 K. The measurements of the latter and of Khellar and Vuillemin do not agree below 35 K. However the equation of Khellar and Vuillemin showed peculiar behaviour below this temperature with derived values being 6% higher than those of Williams and Weaver at 17 K but 31% lower at 20 K. Therefore the latter measurements were given preference up to 35 K. At this temperature and above values from the two sets of measurements were averaged. Overall agreement is to within 0.5% between 60 K and 180 K and to within 0.1% above 180 K. The selected values of Matula below 273.15 K are based on a combination of the measurements of White and Woods and of Laubitz and Matsumura and on average the intrinsic values show a bias of 0.02 μΩ cm above the more recently selected values. Other measurements in the low temperature region were discussed by Matula.

In the high temperature region Matula (5) selected only the measurements of Laubitz and Matsumura (14) (90 K–1300 K). After correction for ρ0 = 0.020 μΩ cm the values were calculated at 50 K intervals from 350 to 1300 K. In the present evaluation these measurements were combined with the more recent measurements of Khellaf et al. (18) (295 K–1700 K) which were given in the form of an equation which was also evaluated at 50 K intervals but over the range 350 K to 1750 K. After correction of both sets of measurements for thermal expansion using the values selected by the present author (19) they were fi tted to Equation (v) which has an overall accuracy as a standard deviation of ± 0.13 μΩ cm. The two sets of measurements show a maximum disagreement of 1.0% at 1300 K. The equation was extrapolated to the melting point and selected values are given in Table II.

Measurements of Milošević and Babić (20) (250 K–1800 K) were independently corrected for thermal expansion. Their equation differs from the selected equation sinusoidally by trending from initially 0.3% high to 1.7% high at 400 K to 0.9% low at 1400 K



Table I Electrical Resistivity of Palladium at 273.15 K

Authors Ref. ρi,μΩ cm Temperature of data

Powell et al. 11 9.79 At 273.15 K. Corrected for ρ0 0.144 μΩ m

Powell et al. 12 9.75 Interpolated 200 – 400 K. Corrected for ρ0 0.143 μΩ m

White and Woods 13 9.70 At 273.15 K. Average of three samples

Laubitz and Matsumura 14 9.760 Interpolated 250–300 K. Corrected for ρ0 0.020 μΩ m

Williams and Weaver 15 9.751 At 273.15 K. Corrected for ρ0 0.007 μΩ m

Khellar and Vuillemin 16 9.765 Calculated. Fit 17–300 K

Selected 9.76 ± 0.01 At 273.15 K

Table II Intrinsic Electrical Resistivity of Palladium

Temperature, K

ρi,μΩ cm

Temperature, K

ρi,μΩ cm

Temperature, K

ρi,μΩ cm

Solid

5

10

15

20

25

30

35

40

45

50

60

70

80

90

100

110

120

130

0.0008

0.0038

0.011

0.028

0.061

0.113

0.189

0.294

0.420

0.566

0.908

1.29

1.71

2.14

2.59

3.04

3.48

3.92

140

150

160

170

180

190

200

210

220

230

240

250

260

270

273.15

280

290

300

4.36

4.79

5.21

5.63

6.04

6.45

6.86

7.26

7.66

8.06

8.46

8.85

9.25

9.64

9.76

10.02

10.41

10.79

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1828

14.47

17.92

21.14

24.15

26.96

29.59

32.03

34.30

36.42

38.39

40.23

41.95

43.55

45.05

46.46

46.84

Liquid

1828

1850

1900

2000

2100

81.4

81.5

81.6

81.8

82.0

2200

2300

2400

2500

2600

82.2

82.4

82.6

82.8

83.1

2700

2800

2900

83.3

83.5

83.7



to 0.4% high at 1800 K. Figure 1 shows the deviations of the selected values of Matula (which are considered as incorporating the measurements of Laubitz and Matsumura) and the experimental values of Khellaf et al. and Milošević and Babić from the fi tted curve. Measurements of Binkele and Brunen (4) (273–1423 K) which were also independently corrected for thermal expansion, showed systematic biases of 1.3% high for runs 1 and 2 and 1.7% high for run 3.

Also in the high temperature region there are a number of other measurements which were published after the review of Matula. After correction for thermal expansion (19) the electrical resistivity measurements of Miiller and Cezairliyan (21) (1400 K–1800 K) trend from 4.0% to 6.9% high whilst the measurement of Pottlacher (22) at the melting point is 5.9% high. Resistivity ratio measurements of García and

Löffl er (23) (295 K–1100 K) were corrected from RT/R295 to RT/R273.15 and were also corrected for thermal expansion. On this basis the differences reached a maximum of 4.1% high at 450 K but then showed some scatter varying between 1.0% low at 800 K and 1.6% high at 1100 K. Figure 2 shows the deviations of these three sets of measurements from the fi tted curve where the resistivity ratios of García and Löffl er were converted to electrical resistivity values for comparison purposes.

2.2 Liquid

Electrical resistivity values for palladium at the melting point are given in Table III. In the liquid state neither Dupree et al. (24) (1832 K–1924 K) nor Güntherodt et al. (25) (1864 K–2019 K) obtained evidence for any variation of resistivity with temperature. Although Seydel and Fischer (26) (1825 K–3000 K) did obtain evidence of such a variation, the values of Pottlacher (22) (1828 K–2900 K) were selected and fi tted to

Ref. (5)Ref. (18)Ref. (20)

Dev

iatio

n, %

2

1.5

1

0.5

0

–0.5

–1

–1.5

200 400Temperature, K

Fig. 1. Solid palladium – percentage deviations from selected curve

600 800 1000 1200 1400 1600 1800

Fig. 2. Solid palladium – percentage deviations from selected curve

Ref. (21)Ref. (23)Ref. (22)

Dev

iatio

n, %

7

6

5

4

3

2

1

0300 500 700 900 1100 1300 1500 1700 1900

Temperature, K

Table III Differences Between the Solid and Liquid Electrical Resistivity of Palladium at the Melting Point

Authors Reference ρS,μΩ cm

ρL,μΩ cm ρL /ρS Notes

Dupree et al.

Güntherodt et al.

Seydel and Fischer

Khellaf et al.

Pottlacher

Present assessment

24

25

26

18

22

–

(48.8)

47.3

50.2

(45.2)

49.6

46.84

83.0

78.8

79.1

77.3

81.4

81.4

1.700

1.666

1.576

1.710

1.641

1.738

(a)

(b)

Notes to Table III

(a) Solid value based on (ρL – ρS)/ ρS = 0.70 ± 0.05

(b) Solid value based on ρL /ρS = 1.71



Table IV Intrinsic Electrical Resistivity of Platinum

Temperature, K

ρi,μΩ cm

Temperature, K

ρi,μΩ cm

Temperature, K

ρi,μΩ cm

Solid

10152025303540455060 708090100110120130140

0.00260.01190.03670.08550.1630.2700.4030.5600.7341.121.531.952.382.803.233.654.064.48

150160170180190200210220230240250260270273.15280290300400

4.895.305.706.116.526.927.327.728.128.518.919.309.709.82

10.0910.4810.8714.71

500 600 700 800 900100011001200130014001500160017001800190020002041.3

18.4522.0725.5929.0032.2935.4738.5441.5044.3547.0949.7452.3454.9357.5160.1162.7663.87

Liquid

2041.3205021002200

102.8102.9103.4104.3

2300240025002600

105.3106.2107.2108.2

270028002900

109.1110.1111.1

Equation (vi) with selected values for the electrical resistivity of the liquid and are also given in Table II.

3. Platinum

Platinum has a face-centred cubic structure and the melting point is a secondary fi xed point on ITS-90 at 2041.3 ± 0.4 K (10).

3.1 Solid

The resistance ratio of platinum, WT = RT/R273.15, forms the basis of the International Temperature Scale which White (6) extended to 1300 K and calculated values of intrinsic resistivity using the fi xed reference value of 9.82 ± 0.01 μΩ cm at 273.15 K. Above 1300 K White combined the selected values to this temperature with the electrical resistivity measurements of Righini and

Rosso (27) (1000 K–2000 K), Laubitz and van der Meer (28) (300 K–1500 K), and Flynn and O’Hagan (29) (273 K–1373 K) and the resistance ratios of Roeser (30) (73 K–1773 K) and Kraftmakher (31) (1000 K–2000 K) together with resistivity measurements given by Martin et al. (32) (300 K–1200 K). White fi tted all selected values from 100 K to 2000 K to Equation (vii) which was extrapolated to the melting point. Differences between values derived from this equation and the tabulated values of White as given in Table IV do not exceed 0.01 μΩ cm. An abridged version of the values for the solid phase as given in Table IV was originally given in Platinum Metals Review by Corti (33).

For comparison between these measurements and the selected values as given in Figure 3, the resistivity ratios of Roeser (30) and Kraftmakher (31) were converted to electrical resistivity values and all



measurements except those of Flynn and O’Hagan (29) were corrected for thermal expansion using values selected by the present author (34). In addition the measurements of Martin et al. (32) were corrected to correspond to the selected electrical resistivity value at 273.15 K. Because of their larger deviations values of Righini and Rosso (27) are compared with the selected values in Figure 4.

In the case of additional electrical resistivity measurements of Birkele and Brunen (4) (273–1497 K), combined runs 1 and 5 trend from initially 0.8% high to 0.1% high at 1200 K to 0.4% high at 1373 K whilst combined runs 2, 3 and 4 trend to an average of 0.5% low above 1000 K. These trends are also shown in Figure 3.

Electrical resistivity measurements of Pottlacher (22) (473 K–1573 K and 1740 K–2042 K in the solid range) are initially 1% higher then trend to an average of 3% higher between 900 and 1573 K before trending to 1.2% higher and then to 0.5% higher between 1740 K and the melting point. These differences are also shown in Figure 5.

3.2 Liquid

Electrical resistivity values of platinum at the melting point are given in Table V. In the liquid state electrical resistivity measurements of Pottlacher (22) (2042 K–2900 K) were selected as Equation (viii) since in the overlap region they are closely confi rmed by measurements of Gathers et al. (36) (2100 K–7300 K) obtained at a pressure of 0.3 GPa which trend from 0.5% low at 2100 K to 1.0% high at 2900 K. Measurements of Hixson and Winkler (37) (2042 K–5100 K) are initially 7% low at the melting point and trend 1% low to 1% high between 2100 K and 2900 K but above 3000 K, in direct comparison with the measurements of Gathers et al., the trend is to an average of 2% low. Selected values for the electrical resistivity of liquid platinum from the melting point to 2900 K are also given in Table IV.

Fig. 3. Solid platinum – percentage deviations from selected curve

Dev

iatio

n, %

1

0.8

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

–0.8

Temperature, K300 500 700 900 1100 1300 1500

Ref. (4), runs 1 and 5Ref. (4), runs 2, 3 and 4Ref. (28)Ref. (29)


Ref. (30)

Ref. (31)

Ref. (32)

Dev

iatio

n, %

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

Temperature, K200 400 600 800 1000 1200 1400 1600 1800 2000


Ref. (27)Ref. (22), run 1Ref. (22), run 2

Dev

iatio

n, %

3.5

3

2.5

2

1.5

1

0.5

0

Temperature, K300 500 700 900 1100 1300 1500 1700 1900 2100



High Temperature Intrinsic Resistivity of Solid Palladium (273.15 to 1828 K)ρi (μΩ cm) = 4.58639 × 10–2 T – 1.39098 × 10–5 T 2 + 1.84118 × 10–9 T 3 – 1.76742 (v)

Intrinsic Resistivity of Liquid Palladium (1828 to 2900 K)ρi (μΩ cm) = 2.058 × 10–3 T + 77.7 (vi)

Intrinsic Resistivity of Solid Platinum (100 to 2041.3 K)ρi (μΩ cm) = 4.681197 × 10–2 T – 3.258075 × 10–5 T 2 + 8.554023 × 10–8 T 3

– 1.594242 × 10–10 T 4 + 1.837342 × 10–13 T 5 – 1.316886 × 10–16 T 6

+ 5.678222 × 10–20 T 7 – 1.340980 × 10–23 T 8 + 1.329896 × 10–27 T 9 – 1.621733 (vii)

Intrinsic Resistivity of Liquid Platinum (2041.3 to 2900 K)ρi (μΩ cm) = 9.604 × 10–3 T + 83.2 (viii)

Table V Differences Between the Solid and Liquid Electrical Resistivity of Platinum at the Melting Point

Authors Reference ρS,μΩ cm

ρL,μΩ cm ρL /ρS

Martynyuk and Tsapkov 35 62.1 92.6 1.491

Pottlacher 22 64.2 102.8 1.601

Present assessment – 63.87 102.8 1.610

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Plenum Press, New York, USA, 1965

2. J. Bass, ‘Electrical Resistivity of Pure Metals and Dilute Alloys’, in “Electrical Resistivity, Kondo and Spin Fluctuation Systems, Spin Glasses and Thermopower”, eds. K.-H. Hellwege and J. L. Olsen, Landolt-Börnstein Numerical Data and Functional Relationships in Science and Technology, New Series, Group III: Crystal and Solid State Physics, Vol. 15a, Springer-Verlag, Berlin, Heidelberg, New York, 1982, p. 1

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The Author

John W. Arblaster is interested in the history of science and the evaluation of the thermodynamic and crystallographic properties of the elements. Now retired, he previously worked as a metallurgical chemist in a number of commercial laboratories and was involved in the analysis of a wide range of ferrous and non-ferrous alloys.




12th Greenhouse Gas Control Technologies ConferenceAdvances in carbon capture and storage research

Reviewed by Christopher StarkieJohnson Matthey Technology Centre,Sonning Common, Reading RG4 9NH, UK


Introduction

The International Conference on Greenhouse Gas Control Technology is a biennial meeting now in its twelfth incarnation and is a highlight for carbon dioxide sequestration researchers around the globe. The conference was held between 4th–9th October 2014 at the Austin Convention Center, Texas, USA. Over four days the conference encompassed all aspects of the carbon capture value chain. Approximately 30% of the sessions focused on CO2 capture technology and 30% on CO2 storage, with the remaining sessions covering case studies, CO2 utilisation, commercial issues, CO2 transport, policy and social science. The conference was attended by 1166 delegates comprising of an almost even distribution of students, academics, industrial representatives, research institutes and government agencies. There was a high level of participation with 874 contributions presented throughout the seven parallel sessions and well-planned poster sessions. Furthermore a small selection of exhibitors complimented the technical programme providing details of commercial ventures and institutional programmes.

This selective review highlights interesting advances presented at the conference. For a more comprehensive overview of the area the reader is directed to a number of encompassing reviews on the topic (1–3).

Plenary SpeakersJulio Friedmann (US Government, USA)

From Vision to Inheritance – The Technical Foundation for the Next Decade of CCS Projects

David G. Victor (University of California, San Diego, USA)

Global Climate Policy and the Future of CCS

Juho Lipponen (International Energy Agency, France)

Gas-Fired Power Generation With CCS – A Competitive Option

Michael J. Monea (SaskPower, Canada)

Boundary Dam – The Future is Here

Suk Yee Lam (Department for Energy and Climate Change, UK)

The UK’s CCS Programme: Policy and Delivery

Gary T. Rochelle (The University of Texas at Austin, USA)

From Lubbock TX to Thompsons, TX: Amine Scrubbing for Commercial CO2 Capture From Power Plants

Xu Shishen (Huaneng Clean Energy Research Institute, China)

Greengen and CO2 Capture Projects in China

Emma ter Mors (Leiden University, Netherlands)

The Value of Social Science Research for CCS Deployment

Greg Schnacke (Denbury Resources Inc, USA)

CO2 EOR: U.S. Opportunities and Challenges



Status of Carbon Capture and StorageCarbon capture and storage is at a pivotal stage. SaskPower’s Boundary Dam power station has started to operate the fi rst large scale carbon capture unit on a commercial power plant. The next two years will see the commissioning of two larger power generation projects: Kemper County in 2016 and Petra Nova, Texas in 2016. There are over twenty large scale projects in operation or under construction across the globe (4). These developments are built upon the lessons of hundreds of pilot plants around the globe and thousands of research hours. In the past ten years the energy consumption of amine scrubbing has been reduced from 300–350 kWh per tonne CO2 to 200–250 kWh per tonne CO2. Although promising, this approach to capture is limited by the large energies required to regenerate the solvent, and by long term solvent durability. Cost reduction is the motivation of much of the ongoing research with advances that increase the rate of adsorption or reduce the regeneration energy highly sought after. It was a core theme of the conference that carbon capture and storage has the potential to mitigate emissions from power generation and industrial processes for which there are no substitutes.

New Capture Technologies

Owing to differences in their adsorption mechanism, tertiary amines are promising candidates for CO2 adsorption with the potential to offer higher amine effi ciency and lower desorption requirements compared to simple primary and secondary amines. The main limitation of tertiary amines in liquid scrubber systems is the sluggish formation of carbonic acid. Cameron Lippert (University of Kentucky, USA) took inspiration from the enzyme carbonic anhydrase which readily catalyses the hydration of CO2 in the majority of plants and animals. The mechanism of carbonic anhydrase is well understood, beginning with the deprotonation of the ligated water coordinated to the zinc centre (Figure 1) (5). This is followed by the nucleophilic attack of the enzyme on CO2. Finally ligand exchange occurs with the Zn bound HCO3

– replaced by water regenerating the starting species. Previous studies have shown that this enzyme denatures under conditions akin to industrial capture units (6, 7). Much prior research has focused on mimics, however these are retarded by the coordination of anions blocking the active site. To circumvent this issue Lippert investigated ligands with

strongly electron donating groups thereby facilitating bicarbonate dissociation and retarding inhibition. The work used Zn salen (N,N’-bis(salicylidene)ethylenediamine) hexafl uorophosphate complexes as catalysts for liquid amine systems (Figure 2). The incorporation of such catalysts led to a signifi cant increase in the rates of CO2 hydration. The catalytic cycle is thought to mimic that of the enzyme. If scalable the addition of these catalysts could vastly improve the CO2 adsorption kinetics leading to more effi cient adsorption systems.

Carbonic anhydrase also provided inspiration for Richard Blom et al. (SINTEF, Norway) who built upon the work by Murthy et al. presenting the Zn complex [Zn{N[CH2(2-py)]3}(μ-OH)]2(NO3)2 (8). As well as catalysing the hydration of CO2 such complexes can directly react with CO2 to form a metastable complex. The complex was found to absorb CO2 in the presence of water forming a trimeric Zn species bridged by a carbonate species (Figure 3) (9). The absorption could be completed at 40ºC with purely thermal regeneration possible at 80ºC; a temperature at which liquid amines show very little desorption. Such low desorption

H+

HHH O O

N NN

NNN

ZnZn I

HCO3

H2O

–

–

IV

III

II

H

O

NZn

HOOOO

O

NNN

NN

Zn

C

CO2

–

Fig. 1. Carbonic anhydrase cycle

N N

Zn

PF6–

PF6–

+

O

N N+

O

N N

Fig. 2. Carbonic anhydrase mimic



temperatures are promising, offering signifi cant reductions in regeneration requirements. Furthermore this complex was found to readily remove CO2 from air at low temperatures opening up a series of niche applications for this technology. The behaviour of the complex is also being investigated under different solvents to avoid the additional heating requirements brought about by the high heat capacity of water and to overcome issues relating to solubility.

There were a number of talks highlighting new CO2 capture technologies for post combustion capture. Joshuah Stolaroff (Lawrence Livermore National Laboratory, USA) gave a talk on microencapsulated sorbents. These double emulsion materials feature an active liquid phase constrained within a porous polymer shell. They are produced using a microfl uidic device in a linear process at a rate of 50 Hz (Figure 4). In such a process mixtures of monoethanolamine (MEA) and water are passed through the inner capillary with a silicone polymer pumped through the middle tube

to yield a coaxial fl ow. A poly(vinyl alcohol) (PVA) stabiliser is pumped in a counter current manner to achieve hydrodynamic focusing with the three streams passing through a single exit where they form spheres. By using a silicone polymer and additives the outer PVA and silicone shells can be cured under ultraviolet (UV) light to yield aqueous monoethanolamine solutions encapsulated in a porous polymer shell.

Microfl uidic devices enable monodisperse spheres to be easily produced with particle sizes controlled by the dimensions of the device. The team began encapsulating simple amines such as aqueous solutions of MEA and piperazine which yielded rapid absorption of CO2. Their recent work in this area highlighted that the rate determining step was not mass transit through the polymer shell with encapsulated absorbents having a rate twelve times that of their liquid counterparts (11). They found that the working fl uid was stable over numerous absorption-desorption cycles. Furthermore the capsules have soft gelatinous physical characteristics negating the issue of attrition prevalent in solid adsorbents and catalysts. Encapsulated sodium carbonate slurries mitigate the issue of precipitates and offer signifi cantly better rates of absorption. Currently such double emulsions are made in a serial fashion and Stolaroff and his team are working to improve the production capacity of such systems by combining parallel arrays of the microfl uidic devices.

Amine Degradation

Liquid amine based CO2 absorption is the most developed capture technology and the most likely candidate for initial full scale deployment. A typical liquid amine absorber includes a water wash at the

Fig. 3. Trimeric zinc complex, adapted from (9)

Zn Zn

Zn

Zn

Zn

N

HO

N

N

N N

N N

O O

O N

NN

N

N

(NO3)3(OH) + H2O(NO3)2 + CO2

1

3/2

N

N

N

NO H

NN

NN

Outer fl uid Middle fl uid

Inner fl uid

Collection tube Injection tube

Fig. 4. Microfl uidic device (Reproduced with permission from (10))



end of the absorption process to remove the amine and other water soluble products from the exhaust gases. With liquid amine columns appearing to be the first generation of carbon capture technology the issue of amine slip and degradation products needs to be thoroughly investigated. There is much discussion surrounding the potential release of nitrosamines and nitroamines formed by the reaction of nitrogen oxides and amines (12–14). Various nitrosamine structures are believed to be carcinogenic and could present a significantly increased hazard compared to the parent amine. Nitrosamines have been detected in laboratory experiments, photochemical experiments, numerous pilot plant studies and are formed by the reaction between an amine and a suitable nitrosating agent (15, 16). In the flue gas, nitrogen oxide can react with nitrogen dioxide forming dinitrogen trioxide (Equation (i)). Nitrogen dioxide can also dimerise to form dinitrogen tetraoxide (N2O4) (Equation (ii)). These species can then react with various amines to yield nitrosamines (Equations (iii) and (iv)) or nitroamines (Equation (v)) depending on the reacting isomer. Furthermore nitrate can be generated in solution by the decomposition of either N2O3 or N2O4 which can directly react with amines.

HO

HO

HO

HO

NH + N2O3 N NO + HNO2 (iii)

HO HONH + ONONO2 N NO + HNO3 (iv)

HO

HO HO

HO

NH + O2NH2NO2 N NO2 + HNO2 (v)

These species and their tautomers can react with amines to form nitrosamines or nitroamines either in the absorber column or desorber and recirculating wash water. There is potential for these pollutants to volatilise and contaminate local air or water resources. Studies suggest that NOx concentrations down to around 25 ppm lead to the formation of nitrosamines (17). For reference a typical fl ue gas from a coal fi red power station contains 50–100 ppm NOx. After fl ue gas desulfurisation and deNOx this drops to 5 ppm. Traditionally the performance of deNOx systems is

driven by changing legislation and as such systems to reduce these emissions further are still under development.

The extent of solvent loss, nitrosamine formation and potential release is highly dependent on the fl ue gas and exact nature of the process used. A number of solutions were presented at the conference. Nathan Fine (The University of Texas at Austin) investigated the rates of thermal decomposition of MEA and piperazine nitrosamines. For fresh solutions of MEA they found minimal conversion to nitrosamine. In cases where 1% of the MEA had degraded to N-(2-hydroxyethyl)glycine a build-up of nitroso-2-hydroxyglycine was apparent. MEA is regenerated at 120ºC to preserve the structure of the amine and at this temperature the nitrosamines were not suffi ciently thermally degraded. It was found that 15% of the NO2 absorbed by piperazine was converted to nitropiperazine. Piperazine systems could be regenerated at 150ºC leading to appreciable thermal decomposition of the nitropiperazine.

A forward thinking presentation by Jesse Thompson (University of Kentucky) explored the possibility of nitrosamine destruction as an end of pipe treatment. The benefi t of nitrosamine reduction is twofold; mitigating the release of nitrosamine and regenerating degraded solvent. Using nitrosopyrrolidine as a probe molecule and a circulating up-fl ow reactor Thompson et al. demonstrated that a commercial palladium catalyst cleanly regenerated pyrrolidine at typical desorption temperatures. Given the low fraction of nitrosamine in the solution, selectivity between the nitrosamine and the parent amine is crucial. Silica based Pd, Ni and Fe catalysts were investigated but found to have limited stability over successive cycling. This limited stability was attributed to the highly alkaline environment slowly degrading the silica surface leading to metal leaching. In a move to combat this, 2×2 manganese octahedral molecular sieves (OMS-2) (Figure 5) co-impregnated with Fe, Pd and Ni were prepared.

In such structures the guest metal ion resides in the centre of the cage with catalytic activity believed to be maintained by electron transfer from the vacant states of the OMS-2 structure. The Pd OMS-2 showed enhanced activity compared to Ni and Fe–OMS-2 however nitrosamine destruction was limited to 60%. Although possessing a lower activity than Pd, iron(II) oxide (FeO) was also investigated as a catalyst owing to its ready availability in fl y ash. Fly ash containing 5.37 wt% FeO yielded fair nitrosamine destruction

NO2 + NO2 N2O4 (ii)

NO + NO2 N2O3 (i)



behaviour under hydrogenation conditions. Successive cycling of the fl y ash catalyst exhibited a decrease in performance attributed to active phase leaching into the amine solution. Given the conditions, fl ow rates and competitor species this is a challenging hydrogenation however this work shows that hydrogenation has potential to limit nitrosamine formation and regenerate the solvent. Thompson and his team demonstrated that supported Pd, Ni and Fe catalysts show promise for the hydrogenation of nitrosamines back into their parent amines in the solvent native to the process (Figure 6).

Conclusion

This review presents only a fragment of the presentations and posters disclosed. The popularity of the conference, combined with the large number of key stakeholders present highlights the importance of carbon capture and storage. A signifi cant number of the talks and posters discussed pilot plant experiences of both capture and storage technologies. Carbon capture and storage is at a crucial stage in development with fi rst generation systems becoming a commercial reality. The large variety of novel concepts and technologies indicate the potential for further advances in second and third generation systems. These technologies look to address the core issues of regeneration requirements, material stability and the possible behaviour of

nitrosamines. The closing session presented the clear message that as a technology carbon capture and storage is ready, with the greatest challenge being policy support to incentivise deployment.

References1 M. E. Boot-Handford, J. C. Abanades, E. J. Anthony, M.

J. Blunt, S. Brandani, N. M. Dowell, J. R. Fernández, M.-C. Ferrari, R. Gross, J. P. Hallett, R. S. Haszeldine, P. Heptonstall, A. Lyngfelt, Z. Makuch, E. Mangano, R. T. J. Porter, M. Pourkashanian, G. T. Rochelle, N. Shah, J. G. Yao and P. S. Fennell, Energy Environ. Sci., 2014, 7, (1), 130

2 T. C. Drage, C. E. Snape, L. A. Stevens, J. Wood, J. Wang, A. I. Cooper, R. Dawson, X. Guo, C. Satterley and R. Irons, J. Mater. Chem., 2012, 22, (7), 2815

3 D. M. D’Alessandro, B. Smit and J. R. Long, Angew. Chem. Int. Ed., 2010, 49, (35), 6058

4 The Global Status of CCS 2014, Global CCS Institute, 2014, Melbourne, Australia

5 M. Bräuer, J. L. Pérez-Lustres, J. Weston and E. Anders, Inorg. Chem., 2002, 41, (6), 1454

6 R. Lavecchia and M. Zugaro, FEBS Lett,. 1991, 292, (1–2), 162

7 G. M. Bond, J. Stringer, D. K. Brandvold, F. A. Simsek, M. G. Medina and G. Egeland, Energy Fuels, 2001, 15, (2), 309

8 N. N. Murthy and K. D. Karlin, J. Chem. Soc., Chem. Commun., 1993, (15), 1236

9 R. H. Heyn, U. E. Aronu, S. J. Vevelstad, K. A. Hoff, T. Didriksen, B. Arstad and R. Blom, Energy Procedia, 2014, 63, 1805

10 A. S. Utada, E. Lorenceau, D. R. Link, P. D. Kaplan, H. A. Stone and D. A. Weitz, Science, 2005, 308, (5721), 537

11 J. J. Vericella, S. E. Baker, J. K. Stolaroff, E. B. Duoss, J. O. Hardin IV, J. Lewicki, E. Glogowski, W. C. Floyd, C. A. Valdez, W. L. Smith, J. H. Satcher Jr, W. L. Bourcier, C. M. Spadaccini, J. A. Lewis and R. D. Aines, Nature Commun, 2015, 6, 6124

12 C. J. Nielsen, H. Herrmann and C. Weller, Chem. Soc. Rev., 2012, 41, (19), 6684

13 A. D. Shah, N. Dai, and W. A. Mitch, Environ. Sci. Technol., 2013, 47, (6), 2799

14 E. D. Wagner, J. Osiol, W. A. Mitch and M. J. Plewa, Environ. Sci. Technol., 2014, 48, (14), 8203

15 B. R. Strazisar, R. R. Anderson and C. M. White, Energy Fuels, 2003, 17, (4), 1034

16 B. Fostås, A. Gangstad, B. Nenseter, S. Pedersen, M Sjøvoll and A. L. Sørensen, Energy Procedia, 2011, 4, 1566

Fig. 6. The nitrosamine reduction concept put forward by Thompson et al.

N N

NO H

H2, catalyst

4.6 Å

Fig. 5. Structure of OMS-2 (Reprinted with permission from (18). Copyright (2012) American Chemical Society)



17 N. Dai, A. D. Shah, L. Hu, M. J. Plewa, B. McKague and W. A. Mitch, Environ. Sci. Technol., 2012, 46, (17), 9793

18 G. D. Yadav, P. A. Chandan and D. P. Tekale, Ind. Eng. Chem Res., 2012, 51, (4), 1549

The Reviewer

Christopher Starkie graduated with a MSci in Chemistry from the University of Nottingham, UK. He is currently undertaking postgraduate study at the Engineering Doctorate Centre in Effi cient Fossil Energy Technologies under the supervision of Professor Ed Lester, Professor Sean Rigby and Professor Trevor Drage. Working in collaboration with Johnson Matthey he is investigating novel functionalised materials for CO2 separation. His research interests include adsorbents, surface functionalisation of porous materials and novel material synthesis.




Platinum Group Metal and Washcoat Chemistry Effects on Coated Gasoline Particulate Filter DesignDevelopment of gasoline particulate fi lters to meet Euro 6c

By Chris MorganJohnson Matthey Emission Control Technologies,Orchard Road, Royston, Hertfordshire, SG8 5HE, UK


Gasoline particulate fi lters (GPFs) are being developed to enable compliance with future particulate number (PN) limits for passenger cars equipped with gasoline direct injection (GDI) engines. A PN emissions limit of 6 × 1011 km–1 over the New European Drive Cycle (NEDC) will apply for new GDI vehicles from September 2017. (A three year derogation allowing a higher PN limit of 6 × 1012 km–1 is currently in force.) Real-world Driving Emissions (RDE) legislation is being fi nalised by the European Commission, which is expected to impose additional restrictions on particle number emissions in typical driving conditions. Legislation proposed for Beijing, China, (commonly known as Beijing 6) is also expected to set a limit on PN emissions from GDI vehicles.

This paper, based on a presentation given to the Society of Automotive Engineers (SAE) Light Duty Emissions Symposium in Detroit, USA, in December 2014, discusses the results from Johnson Matthey test programmes to understand the effects of different driving conditions on engine out PN emissions, the benefi ts obtained from applying a platinum group metal (pgm)-containing coating onto a GPF and the impact of such a coating on soot combustion properties.

Effect of Driving Conditions on Particle Number Emissions

Almost all GDI vehicles can meet the current 6 × 1012 km–1 PN target over the NEDC. It has been reported that a limit of 6 × 1011 km–1 can be achieved through engine design measures such as the use of high pressure fuel injectors, injection and combustion timing and careful design of spray patterns and piston heads to improve mixture formation and to avoid wall wetting (1–3). However, the NEDC is characterised as having moderate acceleration and deceleration rates and extended cruises at constant speed, which are generally unrepresentative of typical real-world driving. Harder acceleration rates and more transient driving are observed to give higher PN emissions. For example a Euro 5 GDI vehicle equipped with a fl ow through three-way catalyst (TWC) system emitted 2.8 × 1012 particles km–1 over the NEDC. PN emissions from the same vehicle increased almost fourfold to 9.6 × 1012 km–1 when a transient drive cycle with harsh acceleration rates was used. Similarly, experiments on multiple GDI vehicles indicated 30–85% higher PN emissions from testing using the more transient US Federal Test Procedure (FTP)-75 drive cycle compared to NEDC data. Driver to driver variability has also been observed. Therefore, it is likely that real-world PN emissions from GDI vehicles are signifi cantly higher than NEDC test data indicate.

Furthermore, ambient temperature has a signifi cant effect on PN emissions. NEDC testing is conducted at a temperature of 23ºC, signifi cantly above the average



temperature in the UK and many other European countries. A Euro 6 GDI vehicle equipped with a fl ow through TWC system was measured to emit <4 × 1011 particles km–1 over the NEDC when tested at 23ºC (Figure 1). When the test car was cooled to 10ºC before the start of the test, PN emissions increased threefold to 1.1 × 1012 km–1. Cooling the car to 0ºC before the start of the test doubled PN emissions again to 2.3 × 1012 km–1. Therefore, even vehicles designed to give low PN emissions during the certifi ed test at 23ºC are likely to emit signifi cantly more in the ambient conditions many drivers experience daily.

Benefi ts of a Coated Gasoline Particulate Filter

GPFs have been shown to be very effective at attenuating PN emissions (4, 5). Cordierite wall fl ow fi lters are most commonly used in development programmes and at least one series application employing uncoated cordierite GPFs is on sale today. Adding an uncoated fi lter downstream of the existing aftertreatment system allows PN control without requiring signifi cant changes to engine calibration or on-board diagnostic (OBD) strategies. However, the addition of an extra unit adds canning cost and the space required can be problematic on smaller vehicles. Applying a suitable TWC coating onto the GPF allows it to be substituted for fl ow through TWC volume in the existing aftertreatment system, resulting in a more compact architecture. Johnson Matthey and other coaters (6–8) have demonstrated such technologies previously.

However, expected RDE limits on nitrogen oxides (NOx) emissions will add further demands on the

gasoline aftertreatment system. Many European exhaust systems have their catalyst volume and precious metal content optimised to meet current NEDC emissions targets. It is well known that conversion of NOx requires more catalyst volume than conversion of CO or hydrocarbons. NEDC-optimised catalyst systems may not provide suffi cient NOx conversion activity under more demanding conditions with higher space velocities. Adding effective catalyst volume by applying a pgm-containing coating onto the GPF can increase the conversion of gaseous pollutants under such conditions. For example, a Euro 5 1.0 l GDI application was tested over the NEDC and World-Harmonised Light-Duty Test Cycle (WLTC) with uncoated or coated GPFs fi tted downstream of the series TWC. With the uncoated fi lter system NOx emissions over the NEDC were 56 mg km–1, increasing to 82 mg km–1 over the more transient WLTC for which the TWC was undersized. With a coated GPF NEDC emissions were ~10% lower than with the uncoated fi lter, at 50 mg km–1. Furthermore, the coated GPF gave good control over the WLTC with emissions of 47 mg km–1. The coated GPF is particularly effective in avoiding NOx slip during the higher speed, higher space velocity conditions experienced in the fi nal 600 s of the WLTC (Figure 2).

The benefi ts of a coated GPF can be further enhanced by optimising the precious metal content of the coating. For example, Figure 3 shows NOx emissions from testing of a TWC plus coated GPF system on a Euro 5 2.0 l GDI vehicle over the transient Artemis test cycle. Thrifting precious metal from the close-coupled TWC resulted in a ca. 20% increase in NOx emissions. However, increasing the rhodium loading on the

0 200 400 600 800 1000 1200Time, s

4.0E + 123.5E + 123.0E + 122.5E + 122.0E + 121.5E + 121.0E + 12

5.0E + 11

0.0E + 00

Cum

ulat

ive

parti

cula

te

NO

per

km

PN dependent on soak temperature

23ºC 14ºC 10ºC 0ºC –7ºC –9ºC Scheduled speed, kph160140120100

80

6040200

Speed, kph

Fig. 1. Impact of vehicle soak temperature on PN emissions over the NEDC



downstream coated GPF from 2 g ft–3 to 5 g ft–3 more than compensated for this effect, with signifi cantly improved conversion at higher speeds.

Therefore, as well as controlling PN emissions, coated GPFs offer advantages over uncoated GPFs in conversion of gaseous pollutants for RDE and more transient drive cycles. Additional pgm and oxygen storage capacity (OSC) on the coated fi lter help to control emissions breakthroughs during harsh accelerations and high speed driving.

Soot Combustion

A concern about the use of uncoated GPFs, particularly when located in remote underfl oor locations, is whether they will regularly reach suffi cient temperatures to regenerate collected soot without the use of extreme engine operating conditions to artifi cially increase the fi lter temperature. It was hypothesised that the presence of an active coating on the GPF would enhance soot combustion. To investigate this soot-loaded coated

0 200 400 600 800 1000 1200 1400 1600 1800

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Time, s

Acc

umul

ated

NO

x em

issi

ons,

g

300

250

200

150

100

50

0

Vehicle speed, kph

Engine out NOx Post TWC NOx Tailpipe NOx Scheduled speed

NOx conversion by the coated GPF

Fig. 2. Comparison of TWC plus coated or uncoated GPF over the WLTC

0 500 1000 1500 2000 2500 3000Time, s

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

140

120

100

80

60

40

20

0

Increased Rh loading on fi lter helps with NOx breakthrough

TWC 100 // cGPF Rh@2TWC 80 // cGPF Rh@5

TWC 80 // cGPF Rh@2Scheduled speed, kph

Cum

ulat

ive

NO

x em

issi

ons,

g

Scheduled speed, kph

Fig. 3. Effect of GPF Rh loading in compensating for emissions breakthroughs from pgm thrifting of the upstream TWC



and uncoated GPFs were fi tted to a Euro 5 1.4 l GDI vehicle and tested over a cycle comprising the NEDC with the fi nal, higher speed, Extra-Urban Drive Cycle (EUDC) repeated ten times. The fi nal EUDC deceleration from 120 kph triggers a fuel cut-off and the resulting oxygen-rich environment can lead to soot combustion at suitable fi lter temperatures. Peak fi lter inlet temperatures during the NEDC were controlled to values between 520ºC and 650ºC by varying the dynamometer gradient from 0 to 3%. Soot combustion was monitored through measurement of pressure drop over the aftertreatment system and by weighing the fi lter before and after each test to measure the mass of soot removed.

In the uncoated fi lter only 50% of the soot was removed at a peak temperature of 600ºC, increasing to 90% at 650ºC (Figure 4). In contrast, soot combustion in the coated fi lter increased rapidly at temperatures above 550ºC, with complete removal of the soot at ca. 570ºC. Therefore, the presence of a pgm-containing TWC coating reduced soot combustion temperatures by approximately 100ºC, delivering signifi cant benefi ts for vehicle strategies to control soot build up in a GPF. Detailed analysis of the differential pressure data showed that at 570ºC the backpressure reduced to a stable level after the fi rst EUDC, indicating rapid promotion of soot combustion.

The chemistry of the TWC coating can also be optimised to enhance soot combustion properties. Powder reactor studies confi rmed that the presence of pgm and ceria-containing OSC materials signifi cantly reduce soot combustion temperatures and that the choice of OSC material can infl uence peak soot combustion temperatures by more than 50ºC.

Conclusions

Control of PN emissions from GDI engines is of increasing importance to meet Euro 6c and proposed RDE and Beijing 6 emissions standards and GPFs are effective in reducing tailpipe PN emissions. However, emissions of particulates and gaseous pollutants can be signifi cantly higher in real-world conditions, with more transient driving, higher maximum speeds and lower ambient temperatures all shown to have a detrimental effect. Use of a pgm-containing coated GPF offers benefi ts over an uncoated GPF, including the abilities to control emissions breakthroughs from upstream TWC components, to enable regeneration of collected soot at lower temperatures and at a faster rate and to allow substitution for TWC volume for system compactness. These benefi ts can be enhanced through careful design of the washcoat chemistry and precious metal content.

Acknowledgements

The author would like to thank the Gasoline Product Development and Catalyst Test Laboratory teams at the Johnson Matthey Emission Control Technologies Centre in Royston for the development work and test data described in this paper.

References1 G. Fraidl, P. Hollerer, P. Kapus, M. Ogris and K.

Vidmar, ‘Particulate Number for EU6+: Challenges and Solutions’, IQPC Advanced Emission Control Concepts for Gasoline Engines, Stuttgart, Germany, 21st–23rd May, 2012

2 M. Winkler, ‘Particle Number Emissions of Direct Injected Gasoline Engines’, IQPC Advanced Emission Control Concepts for Gasoline Engines, Stuttgart, Germany, 21st–23rd May, 2012

3 O. Berkemeier, K. Grieser, K. Hohenboeken, E. Kervounis and K. M. Springer, ‘Strategies to Control Particulate Emissions of Gasoline Direct Injection Engines’, FISITA 2012 World Automotive Congress, Beijing, China, 27th–30th November, 2012

4 T. Shimoda, Y. Ito, C. Saito, T. Nakatani, Y. Shibagaki, K. Yuuki, H. Sakamoto, C. Vogt, T. Matsumoto, Y. Furuta, W. Heuss, P. Kattouah and M. Makino, ‘Potential of a Low Pressure Drop Filter Concept for Direct Injection Gasoline Engines to Reduce Particulate Number Emission’, SAE Technical Paper 2012-01-1241

450 500 550 600 650 700Filter inlet temperature, ºC

1009080706050403020100%

Soo

t bur

nt a

fter 1

0 fu

el c

uts

Bare GPF Coated GPF

Fig. 4. Effect of a pgm-containing GPF coating on soot combustion under vehicle deceleration fuel cut-off conditions



5 K. Ogyu, ‘Requirement of Exhaust Emission Control on Direct Injection Gasoline Engines – a New Approach on Gasoline Particulate Filters’, IQPC Advanced Emission Control Concepts for Gasoline Engines, Stuttgart, Germany, 21st–23rd May, 2012

6 C. Morgan, ‘Three Way Filters for Particulate Number Control’, IQPC Advanced Emission Control Concepts for Gasoline Engines, Stuttgart, Germany, 21st–23rd May, 2012

7 J. M. Richter, R. Klingmann, S. Spiess and K.-F. Wong, ‘Application of Catalyzed Gasoline Particulate Filters to GDI Vehicles’, SAE Technical Paper 2012-01-1244

8 K. Harth, K. Wassermann, M. Arnold, S. Siemund, A. Siani, T. Schmitz and T. Neubauer, “Catalyzed Gasoline Particulate Filters: Integrated Solutions for Stringent Emission Control”, 34th International Vienna Motor Symposium, Austria, 25th–26th April, 2013

The Author

Chris Morgan is Technology Director for Johnson Matthey’s European Emission Control Technologies business, responsible for the development and scale-up of autocatalyst coatings for light duty gasoline and diesel applications. Chris previously managed the Gasoline Product Development team, developing new families of three-way catalysts and leading Johnson Matthey’s early work on coatings for gasoline particulate fi lters. He joined Johnson Matthey in 1997, after completing a DPhil at the University of Oxford, UK, on high temperature ceramic superconductors.




“Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing”, 2nd EditionBy Ian Gibson (Deakin University, Australia), David Rosen (Georgia Institute of Technology, USA) and Brent Stucker (University of Louisville, USA), Springer Science+Business Media, New York, USA, 2015, 498 pages, ISBN: 978-1-4939-2112-6, £81.00, €96.29, US$119.00

Reviewed by Jonathan Edgar*Johnson Matthey Technology Centre,Blount’s Court, Sonning Common, Reading RG4 9NH, UK

Saxon TintJohnson Matthey Noble Metals,Orchard Road, Royston, Hertfordshire SG8 5HE, UK

*Email: [email protected]

Introduction

“Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing” is authored by Ian Gibson, David Rosen and Brent Stucker, who collectively possess 60 years’ experience in the fi eld of additive manufacturing (AM). This is the second edition of the book which aims to include current developments and innovations in a rapidly changing fi eld. Its primary aim is to serve as a teaching aid for developing and established curricula, therefore becoming an all-encompassing introductory text for this purpose. It is also noted that researchers may fi nd the text useful as a guide to the ‘state-of-the-art’ and to identify research opportunities.

The book is structured to provide justifi cation and information for the use and development of AM by using standardised terminology to conform to standards (American Society for Testing and Materials (ASTM) F42) introduced since the fi rst edition. The basic principles and historical developments for AM are introduced in summary in the fi rst three chapters of the book and this serves as an excellent introduction for the uninitiated. Chapters 4–11 focus on the core technologies of AM individually and, in most cases, in comprehensive detail which gives those interested in the technical application and development of the technologies a solid footing. The remaining chapters provide guidelines and examples for various stages of the process including machine and/or materials selection, design considerations and software limitations, applications and post-processing considerations.

Principles and Processes

The fi rst three chapters provide a basic understanding of why someone might want to utilise AM in the context of traditional methods as well as developments in AM since its inception. In the initial chapters the reason for introducing standardised terminology is justifi ed in contrast to the historical terms. For example, the use



of ‘rapid prototyping’ to describe the fi eld is no longer relevant as the technology is now used for functional components as well.

Brief summaries of the process are also provided in these initial chapters in increasing detail as the reader approaches the technical chapters. This provides a realistic view of actions required throughout the process and gives context to references made throughout the book. In its simplest form the process can be universally summarised as:• conceptualisation and computer aided design

(CAD)• conversion to STereoLithography (STL)/additive

manufacturing fi le (AMF)• transfer to AM machine and fi le manipulation• machine setup• build• removal and cleanup• post-processing• application.

Additive Layer Manufacturing Technologies in Detail

Chapters 4–11 describe seven different printing processes in differing degrees of detail. Topics such as ‘vat photopolymerisation’ (VP) and ‘powder bed fusion’ (PBF) are covered in such comprehensive detail that their chapters are double the length compared with ‘binder jetting’ (BJ), despite the latter being a very well used industrial technique. Printable materials and delivery mechanisms are also listed here and the chapters include a ‘process benefi ts and drawbacks’ section, useful for the beginner. It should be noted that some processes (extrusion, sheet lamination and directed-energy deposition) are omitted from this review.

Vat Photopolymerisation

VP processes make use of liquid, light-curable resins as their primary materials. Upon irradiation, these materials undergo photopolymerisation, becoming solid. Ultraviolet (UV) light is projected onto the build plate, curing a layer before recoating. Methods for illuminating the photopolymers are presented, including masked projection, layer-wise processing and vector scan point-wise processing. Expressions relating laser power, scan speed, spot size and cure depth are derived, forming the basis of a process model which can

be applied generally. Curable materials are discussed, including an overview of photopolymer chemistry and their interactions with radiation. Another method, called the ‘two-photon’ approach, describes how a dual light source can increase part resolution – feature sizes of 0.2 μm have been achieved. Advantages of VP include part accuracy, surface fi nish and process fl exibility. The main drawback is their usage of photopolymers, which generally do not have the impact strength or durability of good quality injection moulded thermoplastics.

Powder Bed Fusion

PBF methods deposit layers of powder which are sequentially fused together by an energy source resulting in solid parts residing in a powder bed. The prominent methods are selective laser sintering (SLS) and electron beam melting (EBM), and these two are well compared: EBM processes benefi t from the fl exibility and high energy of their heat source, which can move nearly instantaneously and with split beams; EBM builds typically maintain the bed at high temperature and can create parts with a cast, low porosity, low residual stress microstructure. However their requirement for a vacuum chamber and conductive target material restricts material capabilities whereas SLS machines can process materials such as polymers and ceramics in gaseous atmospheres. This chapter builds on the process model from the previous chapter, applying the concepts to the fusion process of solids and methods for reducing internal stresses in solidifi ed parts. PBF is a well commercialised technology, but due to the lapsing of major patents there are some open-source machines available which have led inventors to create non-engineering applications of the technology too.

Materials Jetting

The fi rst generation of materials jetting (MJ) machines was commercialised in the 1980s. They relied on heated waxy thermoplastics deposited by inkjet printheads, lending themselves to modelling and investment casting manufacture; however the more recent focus has been on deposition of acrylate photopolymers, wherein droplets of liquid monomer are formed and then exposed to UV light to initiate polymerisation. Machines with build spaces as big as 1000 × 800 × 800 mm are available, as well as multi-material capability which can print 1000+ materials by varying the composition of several photopolymers. Research groups around



the world are working on other material groups such as non-photopolymers, ceramic suspensions and low melting point metals (including aluminium). The challenge of forming molten droplets, then controlling deposition and solidifi cation characteristics has kept these material systems in the research arena. A section on the fl uid mechanics of droplet formation and jetting is included in a process model for the MJ process (a section which is also relevant to Chapter 8).

Binder Jetting

BJ methods were primarily developed at Massachusetts Institute of Technology (MIT), USA, during the early 1990s and feature a powder bed similar to that of the PBF process (Chapter 5). However, instead of using an energy source to fuse the material together, an inkjet head deposits a binder onto each layer to form part cross-sections. The binder forms agglomerates with the powder particles and provides bonding with the layer below. The chapter describes some commercial materials available, and notes that many of them require post-processing to increase strength, for example by infi ltration with another material. Materials include polymers (for example, poly(methyl methacrylate) (PMMA)), ceramics, foundry sands and metals such as 420 stainless steel or Inconel alloy 625 infi ltrated with bronze. This technology can be scaled up quite easily, as demonstrated by machines with an incredible 4 × 2 × 1 m build volume. Although the subject of ink drop formation is covered in the previous chapter, a discussion surrounding ink interaction with the powder bed is missing here. Disappointingly for a process described as one of the most readily scalable, the chapter is one of the shortest – although some aspects, such as powder handling, are discussed in other chapters.

Guidelines for Process Selection and Ancillary Process Tasks

The remainder of the book provides outlines and information on software applications and physical processes necessary for gaining maximum advantage from AM.

Process Selection

There are software applications where the performance attributes of each AM technology option are weighted

and ranked based on the relevance of the input parameters (for example geometric considerations and mechanical properties). A particular example, RM Select, is provided (installation fi le and manual are available (1, 2)). This information is particularly pertinent to those interested in production applications of AM.

Post-processing

AM is often viewed as a complete process and therefore, to the uninitiated, it can be a surprise to learn of the requirement for post-processing. The book introduces the various types of support structures required to achieve and maintain all levels of geometric complexity and the subsequent post-processing methods and design considerations to aid their removal. Depending on the application of the part, surface fi nishing may also be required. In this case it may be necessary to design extra material in the areas to be post-processed to ensure the desired dimensions are maintained.

Powder bed methods possess ‘natural’ supports where the built object is encased in excess material. They are named as such as they are an intrinsic part of the process. The main disadvantage of natural supports is that the part must be designed so that the excess material can be removed, for example for powder bed methods parts that are designed to be hollow must have an escape hole for loose powder. In this case the excess material is recycled as much as possible with some sieving usually required.

‘Synthetic’ supports are required for methods that do not have ‘natural’ supports or for methods that have stresses as an intrinsic part of the process, for example powder bed fusion parts need to be tethered to the substrate to prevent warping from stresses created by the presence of signifi cant temperature gradients. It is noted that these stresses can be reduced, and therefore the requirement for synthetic supports is decreased, by raising the temperature of the build environment. For MJ and materials extrusion methods the support structures can be constructed from the build material or a secondary material. In the case of secondary materials a second ‘printhead’ or a purge cycle is required to prevent contamination. If the ‘synthetic’ supports are constructed of the build material then they must be designed such that they can be easily removed in post-processing, it is common for printer software to include automatic support generation. Alternatively, the ‘synthetic’ supports can



be constructed of a secondary material with differential solubility (with respect to the build material) and can simply be dissolved away post-process.

Post-processing is an integral procedure of AM and understanding the many factors to streamline and safeguard the integrity of the printed part is paramount.

Software Issues and Considerations

The fi le standard for AM technologies is the STL fi le format. The origin of this fi le extension hails to the fi rst stereolithography machines commercialised by 3D Systems Inc, USA. This format is expressed as a list, either in binary or American Standard Code for Information Interchange (ASCII) (text), of the vertices of triangular facets used to approximate the surface of the digital part. This approximation is most relevant, with respect to deviation from the true geometry, when there are a signifi cant number of curves in the part. The orientation of the facets is defi ned by the unit normally expressed in vector coordinates.

Due to the nature of the process, i.e. layer-by-layer, there is also the requirement to ‘slice’ the fi le prior to initiating the printing process. The generation of the support structures may be done before or during the slicing operation.

One problem with STL, unlike other CAD formats, is that there are no units associated with the fi le itself. Therefore the scale/dimensions must be checked to ensure the correct dimensions are applied. This can be particularly important for international communications, for example USA to EU or vice versa. Furthermore, it should be noted that not all graphics software packages can create STL fi les with adequate accuracy. One common issue when converting to STL is the creation of parts with holes (i.e. incomplete surfaces), which are not compatible with AM machines. More modern dedicated AM software packages are emerging, making this issue less prevalent.

Colour and materials properties are another factor to include into the defi nition of a part to print. For coloured parts this can be done by colouring of individual facets with solid colour (colour STL) or an image (virtual reality modelling language (VRML)). The advantage of applying an image is that the applied colour is not limited by the resolution of the facet. As the STL fi le format is surface data only there is the assumption that the underlying material is homogeneous. For AM technologies capable of multi-material printing the fi le typically has to be defi ned as distinct objects

with materials properties defi ned for each. As yet, there is not an agreed upon standard fi le format for multi-material objects.

Design Considerations

Particular focus is made on the use of designed cellular structures and void fi lling. These structures can be symmetrical or conformal (with variable cell size to appropriately fi ll the void space). The primary advantage of using cellular structures is the reduction of the required build material without compromising overall strength. Cellular structures are a particular strength of AM processes given the fact that the entire structure can be designed, and with the help of software applications, optimised to achieve the most effi cient strength-to-weight properties. The output cannot always be reproduced but a near approximation can be achieved with little compromise in performance. Examples of topology optimisation software given are Abaqus from Dassault Systèmes and OptiStruct from Altair.

Applications

This section offers a fl avour of the multitude of possible applications of AM technologies. ‘Rapid tooling’ refers to the use of AM to create production tools. Typically these are reusable moulds, impressions or patterns from which the tools can be created. This method may be applied, in particular, if the material required for part production is not currently available in an AM technology or the mould can be improved upon by utilising the design freedoms of AM, for example conformal cooling channels for injection moulds. Medical and aerospace/automotive applications have received signifi cant attention from the implementation of AM technologies. Medical procedures can be streamlined by developing surgical guides and tools. Prostheses and implants are of particular interest for the application of AM as they can be customised on a case-by-case basis. Signifi cant research efforts are currently being employed to print with living cells and biomaterials for direct transplant. Another method is to print a biocompatible scaffold for post-process treatment with living cells to encourage natural material growth, for example Osteopore produces ‘bioresorbable’ implants to encourage natural bone growth over trephination holes from neurosurgery.

Aerospace applications of AM have been quite diverse with examples of engine system and



non-structural components. The primary benefi t for the aerospace industry is improved fuel effi ciency by better performance and lighter weight components.

Business Opportunities and Future Directions

The book is rounded out by highlighting the fact that AM offers genuinely new avenues for production and product development and thus adjusted business models and practices could be required to accommodate the rapidly changing landscape of manufacturing.

Conclusion

Readers desiring a comprehensive introduction to the many technologies of AM should be satisfi ed. Although it is aimed primarily at students and educators, the authors do very well to appeal to those in research and manufacturing positions too. Excellent explanations of basic concepts through to the state-of-the-art make this a great starting point for in-depth research, whilst the process selection tools and business opportunities chapters will be very useful for manufacturers looking to explore this technology.

References1 D. Rosen, ‘RM Selection, Software User Manual’,

Version 1, Georgia Institute of Technology, Atlanta, USA, 12th August, 2005

2 The Georgia Institute of Technology: The Systems Realization Laboratory: http://www.srl.gatech.edu/Members/drosen (Accessed on 28th May 2015)

“Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing”

The Reviewers

Jonathan Edgar received a BSc and PhD in nanotechnology from the University of Technology, Sydney, Australia. Currently he is working for Johnson Matthey on a core science and materials development project using additive manufacturing technologies.

Saxon Tint has an MEng in Materials Science and Engineering from Imperial College London, UK. He joined Johnson Matthey Noble Metals in 2011 as a Materials Scientist and has worked on a range of projects including alloy development for spark plug tips, powder metallurgy and AM.



As a FTSE100 company

that develops advanced materials and specialises in

precious metals, Johnson Matthey has a strong interest in precious metal and speciality metal powders. If you have an interest in precious metal powders or speciality metal powders for additive layer manufacturing, 3D

printing or other applications we’d be keen to hear from

you. Please contact Alexandra French, Sales and Marketing Director, Noble Metals on: a l e x a n d r a . f r e n c h @ m a t t h e y . c o m o r

+44 (0) 1763 253856 / +44 (0) 7968 568532

For more information about Johnson Matthey’s expertise as well as its investment in research and development, please visit

the corporate website: http://www.matthey.com/




Temperature Dependent Heat Transfer Performance of Multi-walled Carbon Nanotube-based Aqueous Nanofluids at Very Low Particle LoadingsInvestigating the mechanism of thermal conductivity enhancement

By Meher WanDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721302, India

Raja Ram YadavDepartment of Physics, University of Allahabad, Allahabad-211002, India

Giridhar Mishra and Devraj Singh*Department of Applied Physics, Amity School of Engineering and Technology, An affiliated institute of Guru Gobind Singh Indraprastha University, Bijwasan, New Delhi-110061, India


Bipin JoshiDepartment of Science and Technology, Technology Bhavan, New Mehrauli Road, New Delhi-110016, India

Aqueous suspensions of multi-walled carbon nanotubes (MWCNTs + deionised water) have been synthesised. Carbon nanotubes (CNTs) were derived by chemical vapour deposition (CVD). Transmission electron microscopy (TEM) measurements show the formation of MWCNTs. Three samples of CNT-based aqueous nanofluids having MWCNT concentrations of 0.01 vol%, 0.03 vol% and 0.05 vol% were prepared with the help of ultrasonic irradiation. A very small amount of

sodium dodecyl sulfate (SDS) was used as a surfactant to minimise the agglomeration of the MWCNTs. An effective enhancement in thermal conductivity was observed at different temperatures. The obtained results are explained with percolation theory.

1. Introduction

Heat transfer management is becoming increasingly critical in the infrastructure, industry, transportation, defence and aerospace sectors. Several cooling methods have been investigated recently to meet the heat transfer requirements of 21st century high-technology, high density heat producing industrial equipment. Water, ethylene glycol and mineral oil have been used as conventional coolants for different industries. Conventional heat transfer systems used in a wide range of applications, including petrochemicals, refining, power generation and microelectronic devices, are rather large and involve a significant amount of heat transfer fluids in comparison to biomedical engineering based applications (1). Managing high density heat generation in microelectronic industries is a good example (2, 3).

Modern, small scale cooling applications require very effective coolants. Existing heat management systems can be improved by enhancing the performance of heat transfer fluids through the use of nanofluids, resulting in lower heat exchange surface area, lower capital costs and higher energy efficiencies (4). Several techniques have been investigated to enhance the thermal exchange performance of the fluids. Out of many tried methods,



one is to add a very small percentage of nanoparticles having high thermal conductivity into heat transfer fluids to improve their overall thermal conductivity (5–8). Such additives may include novel metals, metal oxides and carbon nanostructures. Nanoparticles may possess either spherical, cylindrical, fibril or sheet-like structures. Cylindrical carbon nanostructures (4, 9, 10) or CNTs have very high thermal conductivity of the order of 3000 W m–1 K–1 (11). MWCNTs have very exotic physical properties and are relatively easy to synthesise. They have been shown to be promising additives in conventional heat transfer fluids for diverse heat transfer applications (9, 12). Several researchers have tested different combinations and permutations of CNTs, the effects of their aspect ratios (diameter and length) and base fluids on thermal conductivity in CNT-based nanofluids (13). For a more uniform dispersion of CNTs, the effects of different quantities of surfactant materials on thermal conductivity have also been studied (14). However the addition of a large weight percentage of filler in a fluid affects its viscous properties. Higher viscosity does not support better heat conduction due to the need for higher pumping power as well as changes in other fluidic properties.

The present work is focused on the preparation of nanofluids by dispersing MWCNTs in deionised water and measuring the temperature dependent thermal conductivity of the nanofluids at very small particle loadings. The experimental observations regarding thermal conductivity enhancements can be explained with the help of appropriate theoretical models. It has now been established that when CNTs are suspended in conventional heat transfer fluids, anomalous enhancements in thermal conductivity are observed (15, 16). The motivation behind the present study is to find the mechanism of thermal conductivity enhancement at very small concentrations of MWCNTs in suspension and the effects of MWCNT aqueous nanofluids on thermal performance at a range of temperatures. It is assumed that the viscous properties of the base fluid do not change with the inclusion of very low CNT loadings.

2. Materials and Methods

Deionised water and MWCNTs were used to produce nanofluids. The MWCNTs were synthesised by CVD using nickel salen [N,N’-ethylene-bis(salicylideneiminato)]-nickel(II); Ni(C16H14N2O2) as a catalyst. The details of the CNT growth are described elsewhere (17). Ni

salen was found to produce CNTs via a tip-growth mechanism. Small Ni particles were observed at the tips of the CNTs which were otherwise free of Ni. Since Ni is used as a catalyst in relatively small amounts and is embedded inside the CNT tips, its contribution to the thermal conductivity enhancement can be ignored. The MWCNTs have an average outside diameter of 30 nm, and a length of several micrometers, as observed by electron microscopy (field emission scanning electron microscopy (FE-SEM) and TEM) (Figures 1(a) and 1(b)).

Nanofluids were prepared using a two step method. First the required amounts of MWCNTs and deionised water needed for the sample preparation were determined. Then a very small amount of surfactant (SDS) was dissolved in the liquid matrix, followed by appropriate amounts of MWCNTs for synthesis of different concentrations of nanofluids. SDS was used to minimise the agglomeration of nanotubes in the base fluid. The samples having MWCNTs were stirred using a magnetic stirrer and were ultrasonicated for 20 minutes with 100 W intensity at 20 kHz ultrasonic frequency (Sonics VC 505). As the probe sonicates within a limited conic volume, to facilitate uniform dispersion, sonication was followed by 10 minutes magnetic stirring. According to the density data provided by Jana et al. (18), 0.026 g, 0.078 g and 0.130 g of CNTs were dispersed in 100 ml of deionised water to prepare nanofluids with 0.01 vol%, 0.03 vol% and 0.05 vol% CNTs. 100 ml suspensions of each composition were prepared.

Thermal conductivity of the synthesised nanofluids was measured with a Hot Disk TPS 500 Thermal Constant Analyser which works on the temperature coefficient of resistance principle. The transient plane source (TPS) method is an updated version of the transient hot wire (THW) technique (19). The uncertainty in the measurements in this method is ±4%. TPS overcomes many of the drawbacks of THW due to its sensor structure and shape. TPS has a computer controlled temperature controller for accurate temperature settings and precise results at typical temperatures. The TPS based instrument can be used to measure thermal constants such as conductivity and diffusivity via a Ni sensor, using the temperature coefficient of resistance. Inside the instrument, the TPS element behaves both like a temperature sensor and a heat source. TPS uses the Fourier law of heat conduction as its fundamental principle for measurements.



3. Results and Discussion

Thermal conductivity measurements were taken for all the nanofluid samples at temperatures from 10ºC to 80ºC. The experimental data for thermal conductivity of the nanofluid samples are presented in Figure 2. For comparison, the temperature dependent thermal conductivity data of the base fluid (water) are also given. It can be seen that there is no significant increase in the thermal conductivity with the suspension of 0.01 vol% MWCNTs. On the other hand, there is a significant anomalous increase in the thermal conductivity of water with suspension of 0.03 vol% CNTs. Thus, it can

be claimed that the percolation threshold exists below 0.03 vol%, leading to the conclusion that the CNTs do not have continuous chains from one end to the other below a certain particle loading, as predicted by Sastry et al. (20).

Although a number of models based on different mechanisms are available currently in the literature, no theoretical model is able to explain the anomalous thermal conductivity enhancement observed experimentally due to controlled and uncontrolled parameters in the experimental setup (21). Some models, for example the Sastry model, are in reasonable agreement with the experimental data under different assumptions and conditions. A critical analysis of the theoretical models has been carried out by Lamas et al. (21) who concluded that the available models show a negative effect of temperature on thermal conductivity enhancement in these systems. However, this is contradictory to the available experimental results. The agreement of theory and experiment was achieved by adjusting parameters such as the CNT geometry (22) and the value of interfacial conductance (23). These theoretical models had various degrees of empiricism and provided a limited physical insight into the experimental observations. In past studies, experimental data have showed that the dispersion technique and interaction between the CNTs and the base liquid also play a strong role, causing the enhancement to vary by as much as 7%–40% for water-MWCNT nanofluid (24).

For the present study, very low concentrations were chosen for two reasons. Firstly, to maintain the fluidic

(a)

200 nm

(b)

0.2 µm

Fig. 1. (a) FE-SEM images of the as-grown MWCNTs; (b) TEM of the MWCNTs

0 10 20 30 40 50 60 70 80 90Temperature, ºC

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.70.60.5

Ther

mal

con

duct

ivity

, W m

–1K

–1

DI-water + SDS0.01 vol%0.03 vol%0.05 vol%

Fig. 2. Thermal conductivity of nanofluids containing CNTs at different temperatures



or viscous properties of the base fluid; and secondly to investigate the existence of a percolation threshold to explain anomalous thermal conductivity enhancements as predicted by Sastry and his coworkers. A number of physical mechanisms have been proposed to explain the thermal conductivity enhancement of a nanofluid. An early concept was interfacial layering: the formation of a solid-like, liquid molecular layer close to the CNT interface, which has much higher thermal conductivity than the bulk liquid itself (25). Alternatively, due to the high aspect ratio of CNTs, interaction between them is thought to be highly probable resulting in the formation of a network. The phenomenon is often termed percolation. The formation of a highly conductive heat flow path in the liquid by the percolation network can potentially explain the enhancement due to concentration of CNTs in relatively low temperature zones where Brownian motion is not dominant. A critical volume fraction of MWCNT loading, called the percolation threshold, was thought to exist above which the electrical conductivity of the MWCNT nanofluid would abruptly increase multifold. However, initial experiments did not show any such sudden rise in the effective thermal conductivity of the nanofluid. Thus it was concluded that there is no percolation threshold in thermal transport in those experiments (25, 26), a view that was contradicted recently (20). Sastry et al. considered percolation without specifically approaching it from a threshold perspective (20). Their approach relies on the basic premise of three-dimensional MWCNT chain formation in the liquid (percolation) and consideration of a thermal resistance network with higher resistance contact points of CNTs and very low resistance CNT channels. Random CNT orientation and CNT-CNT contacts are introduced by probability density function. They considered a cubical volume of nanofluid formed by dispersion of CNTs. The fundamental basis of the model is an interaction between the CNTs touching each other to form percolating chains in the suspension. In this mechanism, resistive points are formed at the contact centres of the CNTs. Thus, a series of resistive contact points offer resistance in the thermally conductive medium of a CNT chain. The net resistance has been calculated by Sastry et al. (20) as given in Equation (i):

R

R RM

R

R RM

Rnet

CNTi P CFi p

CNTi P CFi p

=

+

×

+

+

,,

,,

2

2

=∑i

N

1

(i)

where R

LK ACNTi P

i P

CNT CS,

,= ; RL

K AFi Pi P i i

liq liq,

, sin cos=

φ θ

and

RGdC

N

= 12

Finally effective thermal conductivity can be written as

kL

R Aeffcell

net Cell

= ; where Lcell is the length of the cell and

Acell is the effective area of the cell in which the percolation exists. The thermal conductivity enhancement will not be observed below a certain particle loading, where the formation of chains of CNTs does not occur. The enhancement in thermal conductivity for 0.01 vol% at higher temperatures is due to Brownian motion, which dominates the percolation chain mechanism at high temperatures. The percolation mechanism can be applied for mechanically stable suspensions only. At high temperatures, suspensions become less stable due to Brownian motion.

Figure 3 shows the variation of thermal conductivity enhancement ratio (Keff/Kliq) with temperature at different MWCNT loadings. Thermal conductivity ratio is considered as a much more suitable parameter to understand the enhancement in thermal conductivity, irrespective of thermal conductivity of the base fluid or liquid in which MWCNTs are dispersed to form nanofluids.

The present experimental study was carried out in the temperature range 10ºC to 80ºC. The thermal conductivity was observed to increase with temperature, contradictory to predictions made by

0 10 20 30 40 50 60 70 80 90

1.55

1.501.451.401.351.301.251.201.151.101.051.000.95

Temperature, ºC

Ther

mal

con

duct

ivity

enh

ance

men

t, K

eff/K

liq

0.01 vol%0.03 vol%0.05 vol%Theoretical curve (Prasher et al.)

Fig. 3. K/Kf of nanofluids containing CNTs at different temperatures



available theoretical models (21). The temperature effect on thermal conductivity of nanofluids can be explained with the help of Brownian motion. The thermal conductivity of the CNT-water nanofluids was therefore calculated to examine the Brownian motion effect. Prasher et al. (27) developed a model for water based spherical nanoparticle nanofluids. Based on Prasher’s approach, the present experimental observations were modelled for CNT containing water-based nanofluids at high temperatures. Since the CNTs have very small diameters when suspended in the liquid, Brownian movement of the CNTs is quite possible. The root-mean-square velocity (vN) of Brownian particles can be defined (18) as (Equation (ii)):

vk Tm d

k TdN

B

N N

B

N N

= =3 1 18

πρ (ii)

where kB is the Boltzmann constant, T the temperature, mN the particle mass, ρN the density, and dN the average diameter of the CNTs.

Now we consider the effect of the convection of the liquid near the CNTs due to their Brownian movement. The Reynolds number (Re) based on νN given by Equation (ii) can be written as Equation (iii):

Rk TdeB

N N

= 1 18ν πρ

(iii)

where n is the kinematic viscosity of the liquid.The Re for 30 nm CNTs in water, Re = 0.0132, is very

small and therefore for convection the flow falls in Stokes regime. If a particle is embedded in a semi-infinite medium of thermal conductivity Km, then the Nusselt number (Nu) based on the radius of the CNTs can be shown to be 1; i.e., h = (Km/a). In Stokes regime ‘h’ is given (28) as h = (Kf/a)[1+ (1/4)Re + Pr] where Pr is the Prandtl number, ‘a’ is the radius of the particle and ‘h’ is the heat transfer coefficient. Note these relations are derived analytically from first principles. This means that the effective K of the fluid due to the convection caused by the movement of a single sphere is:

Km = Kf[1+ (1/4)Re ×Pr] (iv)

Note that this is based on a single isolated CNT. In a real system there will be interaction in the convection currents from different CNTs. The value of Km is substituted from Equation (iv) in Equation (v):

KKm

= + + −+ − −

( ) ( )( ) ( )1 2 2 11 2 1

α φ αα φ α

(v)

which is based on the traditional Maxwell-Garnett (MG) model where f is the particle volume fraction, α = 2R K db m N/ , and Rb is the interfacial resistance.The effective K of the nanofluid can be written as

Equation (vi):

KK

R

f

e= +× + + −

+ − −

(

Pr) ( ) ( )

( ) ( )1

41 2 2 11 2 1

α φ αα φ α

(vi)

Equation (vi), together with the definition of Re given in Equation (iii) has all the necessary ingredients for predicting K, because it includes:(a) conduction contribution of the CNTs (b) Rb between the nanotubes and the water (c) convection contribution (effective for smaller dN).

So far no empiricism has been included in the model. Taking the clue from correlations for particle-to-fluid heat transfer a general correlation for h of the form: h = (Kf/a) [1+ A Re

mPr0.333f] is proposed for the Brownian

motion induced convection from multiple nanotubes, where A and m are constants. Convective heat transfer relations are regime dependent, and so depending on Re, these relations can change. Therefore it is most likely A should be independent of the fluid type whereas m depends on the fluid type. This modification leads to Equation (vii):

KK

AR Pf

em

r= + + + −+ − −

( ) ( ) ( )

( ) ( ).1 1 2 2 1

1 2 10 333φ

α φ αα φ α

(vii)

If Equation (vii) is valid, A and m should be the same for different experimental data for a particular fluid.

Figure 3 shows the semi-empirical model for K of various water based nanofluids (different f), taking Rb = 2.5 × 10–8 Km2 W–1, m = 2.4 and A = 40,000 (as in similar materials) for the best fit. Here in the model a thermal boundary layer δT = 3 δF/Pr is arbitrarily defined, where Pr is the Prandtl number and δF is the diameter of the base fluid molecule. Here Pr = 10 has been taken. Nusselt number (Nu) is given as Nu ~ Re

2Pr2; α = 2R K db m N/ = 0.967 has been

computed; Km = thermal conductivity of water. Re has been computed as 0.0132.

It is important to note that in the present case an increase in the viscosity at higher temperature increases Brownian motion. Figure 3 reveals



significant enhancement in the thermal conductivity of water containing a suspension of 0.05 vol% of CNTs at different temperatures, in good agreement with the theoretically modelled curve based on order of magnitude calculations.

The temperature variation of the enhancement of the thermal conductivity (K/Kf) on experimental observation is also in good agreement with the theoretical curve within the measurement accuracy of the apparatus. Thus, it may be concluded that convection induced due to Brownian movement of the CNTs is the main cause for the observed enhancement in K of the nanofluid at higher temperatures in the present investigation. The interfacial resistance Rb has also been incorporated as interfacial resistance greatly depends on bonding between the solid and the liquid (27).

The shelf life of the nanofluids was also investigated. In Figure 4 the temporal stability of nanofluid containing 0.03 vol% CNTs is shown at room temperature. The nanofluid is stable for a long time.

4. Conclusions

It can be concluded that the observed thermal conductivity enhancement in prepared nanofluids is due to the suspension of CNTs in deionised water. Sudden enhancement is observed at a concentration of 0.03 vol% CNTs in water at lower temperatures, which is attributed to formation of CNT chains with very high conductivity in the medium. The abrupt enhancement of thermal conductivity at 0.03 vol% can be explained with the help of percolation phenomena. From

observations, it can be claimed that the percolation threshold is between 0.01 and 0.03 vol% CNT loadings in base fluid. Thermal conductivity enhancement increases with temperature and concentration of CNTs in the base fluid. The temperature dependence of the thermal conductivity enhancement is explained well with Brownian motion theory as chains of CNTs will not form at higher temperatures due to instability imposed by the temperature increase. These MWCNT + deionised water nanofluids are very stable with time. They therefore show potential to be used as coolants in different fields of industry.

Acknowledgements

Meher Wan is thankful to the Council of Scientific and Industrial Research (CSIR) of the Government of India for a senior research fellowship to carry out the work.

References1 D. G. Cahill, P. V. Braun, G. Chen, D. R. Clarke,

S. Fan, K. E. Goodson, P. Keblinski, W. P. King, G. D. Mahan, A. Majumdar, H. J. Maris, S. R. Phillpot, E. Pop and L. Shi, Appl. Phys. Rev., 2014, 1, (1), 011305

2 Z. Ling, Z. Zhang, G. Shi, X. Fang, L. Wang, X. Gao, Y. Fang, T. Xu, S. Wang and X. Liu, Renew. Sustain. Energy Rev., 2014, 31, 427

3 T. Dixit and I. Ghosh, Renew. Sustain. Energy Rev., 2015, 41, 1298

4 R. Taylor, S. Coulombe, T. Otanicar, P. Phelan, A. Gunawan, W. Lv, G. Rosengarten, R. Prasher and H. Tyagi, J. Appl. Phys., 2013, 113, (1), 011301

5 R. Saidur, K.Y. Leong and H. A. Mohammad, Renew. Sustain. Energy Rev., 2011, 15, (3), 1646

6 S. Özerinç, S. Kakaç and A. G. Yazıcıoğlu, Microfluid.Nanofluid., 2010, 8, (2), 145

7 S.-H. Lee and S. P. Jang, Int. J. Heat Mass Transfer, 2013, 67, 930

8 O. Mahian, A. Kianifar, S. A. Kalogirou, I. Pop and S. Wongwises, Int. J. Heat Mass Transfer, 2013, 57, (2), 582

9 Z. Haddad, C. Abid, H. F. Oztop and A. Mataoui, Int. J. Therm. Sci., 2014, 76, 168

10 C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Millán, V. S. Amaral, F. Palacio and L. D. Carlos, Nanoscale, 2013, 5, (16), 7572

11 H. Maruyama, R. Kariya and F. Arai, Appl. Phys. Lett., 2013, 103, (16),161905

0 5 10 15 20 25 30Time, days

Ther

mal

con

duct

ivity

, W m

–1K

–1

0.80

0.75

0.70

0.65

0.60

CNT nanofluid (0.03 vol%)

Fig. 4. Thermal conductivity of nanofluid containing 0.03 vol% CNTs at different times



12 S. Halelfadl, T. Maré and P. Estellé, Exp. Therm. Fluid Sci., 2014, 53, 104

13 S. M. S. Murshed and C. A. Nieto de Castro, Renew. Sustain. Energy Rev., 2014, 37, 155

14 P. Estellé, S. Halelfadl and T. Maré, Int. Commun. Heat Mass, 2014, 57, 8

15 Y. Ding, H. Alias, D. Wen and R. A. Williams, Int. J. Heat Mass Transfer, 2006, 49, (1–2), 240

16 S. U. S. Choi, Z. G. Zhang, W. Yu, F. E. Lockwood and E. A. Grulke, Appl. Phys. Lett., 2001, 79, (14), 2252

17 J. Sengupta, A. Jana, N. D. P. Singh, C. Mitra and C. Jacob, Nanotechnology, 2010, 21, (41), 415605

18 S. Jana, A. Salehi-Khojin and W.-H. Zhong, Thermochim. Acta, 2007, 462, (1–2), 45

19 S. E. Gustafsson, Rev. Sci. Instrum., 1991, 62, (3), 797

20 N. N. V. Sastry, A. Bhunia, T. Sundararajan and S. K. Das, Nanotechnology, 2008, 19, (5), 055704

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22 W. Yu and S. U. S. Choi, J. Nanopart. Res., 2004, 6, (4), 355

23 L. Xue, P. Keblinski, S. R. Phillpot, S. U.-S. Choi and J. A. Eastman, Int. J. Heat Mass Transfer, 2004, 47, (19–20), 4277

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The Authors

Dr Meher Wan obtained his DPhil from the Physics Department, University of Allahabad, India. He is currently working as research fellow in the Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Kharagpur, India. His research interests are thermal conductivity, phonon transport in nanostructured composite materials and non-destructive techniques for characterisation of materials including ultrasonics. He has published several important studies on heat transfer phenomena of nanofluids in internationally reputed journals.

Professor Dr Raja Ram Yadav is presently Professor of Physics at the Department of Physics, University of Allahabad. His research interests are in the non-destructive ultrasonic and thermal characterisation of nanomaterials, lyotropic liquid crystalline materials, intermetallics and semiconductors; the development of nanomaterials for biomedical applications; and theoretical calculations of nonlinear elastic and ultrasonic properties of crystalline materials. He was awarded the prestigious Indian National Science Academy (INSA) Teachers’ Award for the year 2012.

Dr Giridhar Mishra is Assistant Professor at the Department of Applied Physics, Amity School of Engineering and Technology, New Delhi, India. He obtained his DPhil in Physics from the University of Allahabad. He has worked as a Research Fellow in a project sponsored by the Department of Science and Technology, New Delhi, in the field of materials science. His current research interests are focused on the study of ultrasonic and thermal properties of nanofluids, nanomaterials and other materials. He is a life member of the Indian Association of Physics Teachers (IAPT).

Dr Devraj Singh is Assistant Professor and Head of the Department of Applied Physics at Amity School of Engineering and Technology. His research interests are in the ultrasonic non-destructive characterisation of condensed materials. Presently, he is working on ultrasonic studies of rare earth materials for engineering applications.



Dr Bipin Joshi is a Scientist at the Department of Science and Technology, Government of India, New Delhi. His research area is optical characterisation of nanostructured fabric semiconductors. He is a life member of various scientific societies.




The Effects of Hot Isostatic Pressing of Platinum Alloy Castings on Mechanical Properties and MicrostructuresPost processing of parts for jewellery and other applications

Teresa Fryé*TechForm Advanced Casting Technology, 5558-D SE International Way, Portland, Oregon 97222, USA


Joseph Tunick StraussHJE Company, 820 Quaker Rd, Queensbury, New York 12804, USA

Jörg Fischer-BühnerIndutherm Erwärmungsanlagen GmbH, 75045 Walzbachtal-Wössingen, Germany; and Legor Group SpA, Via del Lavoro, 1 36050 Bressanvido (VI), Italy

Ulrich E. Klotzfem Research Institute for Precious Metals & Metals Chemistry, Katharinenstraße 17, 73525 Schwäbisch Gmünd, Germany

The effects of hot isostatic pressing (HIP) on castings produced in a variety of platinum alloys was investigated. A number of benefi ts were observed, including a reduction in porosity and improvements to the microstructure and mechanical properties. Differences in the response to HIP of individual alloys is evaluated as well as some inherent limitations of the HIP process.

Introduction

In earlier research a better understanding of the solidifi cation characteristics for a number of platinum-based alloys was established (1). The fi ndings demonstrated a strong tendency toward the formation of shrinkage and gas porosity upon solidifi cation, and HIP, a high-pressure thermal treatment developed as a densifi cation process, was proven to be an effective method to minimise or eliminate this porosity.

While the previous results made it clear that porosity had been signifi cantly reduced following the HIP process, the authors had not yet explored the full range of HIP’s effects in terms of post-processed microstructure and mechanical properties. The goal of this new phase of research is to further our understanding by characterising the post-HIP effects on platinum based castings with respect to grain size and shape, chemical distribution and mechanical strength.

Overview of Hot Isostatic Pressing

Companies that build HIP equipment or perform the HIP process describe an isostatic press as something that forms and densifi es powdered and cast materials using liquid or gas under extremely high pressure. Unlike mechanical force which compresses a workpiece from one or two sides, the isostatic pressure is applied



uniformly on all sides of an object, eliminating internal porosity without changing net shape. Typical product improvements cited by the HIP industry are the elimination of internal voids, improvements in product consistency, and improvements in the soundness and mechanical properties of materials. The fundamental material change underlying these improvements is the attainment of a higher density material in comparison with its pre-HIP condition.

The isostatic nature of pressure in the HIP process as described above is key to maintaining the dimensional integrity of a casting during HIP; the pressure being equal on all sides lends to a uniform compression of the material with product dimensions typically remaining intact. Although, as seen in Figure 1, local deformations in the form of ‘dimpling’ can occur when the sizes of internal pores are extremely large and diffusion bonding collapses exterior surfaces inward. This diffi cult to feed thick-to-thin geometry for the channel band represents an extreme example of subsurface shrinkage porosity

and is a powerful representation of the pore collapsing that can occur with HIP.

To better demonstrate how the HIP process works, Figure 2 shows the schematic for a typical HIP unit. The unit contains a high temperature furnace enclosed in a pressure vessel. Parts are typically placed within the chamber in vertical layers with the use of graphite, steel or ceramic shelving to maximise load capacity. During operation the HIP chamber is fi rst placed under vacuum, followed by fl ooding with an inert gas, usually argon, which is used to apply the isostatic pressure. The temperature and pressure is then ramped up and left to dwell for a specifi ed period of time depending upon the material’s properties. Parts become densifi ed when the material’s yield strength is surpassed, creating a plastic fl ow that forces internal voids to collapse under differential pressure. The internal surfaces of the voids diffusion bond together, increasing density and thereby improving the material properties. HIP unit sizes span from small laboratory size up to large-scale industrial. The unit shown in Figure 3 is an example of a medium scale unit.

Not all metals will HIP effectively and the extent to which an alloy will respond to HIP is a function of its creep resistance. Creep is a solid material’s tendency to move slowly and deform permanently under stress. In metals, creep increases with temperature and starts at approximately 30% to 50% of an alloy’s melt temperature (2).The rate of creep is a function of temperature, the material’s properties, and the amount of pressure that is applied. In order to achieve optimum material properties, the parameters used in a HIP cycle

2 cm

Fig. 1. HIP dimples in a 95Pt5Ru channel band

(a)(b)

Inert gas

Compressor

Cooling pump

Cover

Cooling jacket

Pressure vessel

Pressurised gas

Heater

Vacuum pump

Vacuum line

Exhaust valve

Pressureline

Ceramic pieceViscous coating (optional)ThermocoupleElectricline

Temperature controller

Power controller

Fig. 2. Schematic of a typical HIP unit



must be precisely dialled in according to the needs of the alloy. Typical parameters including both metals and ceramics will generally fall into the ranges shown in Table I.

Table I HIP Parameter Rangesa

Parameter Typical lower limit

Typical upper limit

Temperature 500ºC 1400ºC

Pressure 7000 PSI 45,000 PSI

Dwell Time 2 hours 4 hours

Cooling Rate 1ºC per minute 100ºC per minuteaCourtesy Avure Technologies

Comparative Study of the Effects of HIP on Platinum Alloy Microstructure and Mechanical Properties

The goal of this research was to characterise post-HIP effects on castings with respect to grain size and

shape, chemical distribution, and mechanical strength. The following sections report the methods used, results and conclusions.

Test Geometry

A tapered test specimen was chosen to assess microscopic porosity levels, density and hardness before and after HIP. As shown in Figure 4, the test specimen was designed to promote directional solidifi cation with a single heavy sprue attached to the thickest end.

Casting Parameters

The casting parameters and conditions for these trials are shown in Table II. Standard pour temperatures, fl ask temperatures and fi ring curves were used. The fl asks were cast using a centrifugal casting machine with induction heating and each tree contained two test geometries for each alloy. One casting was retained for as-cast sampling and the second casting was HIPed.

Table II Casting ParametersAlloy Pour temp., ºC Flask temp., ºC Casting condition

95Pt5Ru 1870 850 1 As-cast; 1 HIPed

90Pt10Ir 1870 850 1 As-cast; 1 HIPed

90Pt10Rh 1960 850 1 As-cast; 1 HIPed

95Pt5Co 1850 850 1 As-cast; 1 HIPed

Notes to Table II(a) All patterns 3D printed for dimensional precision(b) All trees were identically assembled with two samples per tree(c) All fl asks air cooled identically(d) All alloys were HIPed in the same load

Fig. 3. HIP unit, courtesy Avure Technologies, USA

Fig. 4. Tapered test specimen (units in mm)

1.686

23.586

4.941

1.917 5.682

(a)

(b)



Effects of HIP on Microstructure

Given the high levels of porosity seen in 95Pt5Ru and the relatively low levels seen in 95Pt5Co, these two alloys were chosen for the present report on microstructural changes brought about by the HIP process. Figure 5 demonstrates signifi cant porosity levels in the as-cast state of 95Pt5Ru. The pores in this alloy are interdendritic microshrinkage pores; such pores form during the spontaneous solidifi cation of the alloy that occurs so rapidly that continued feeding is not possible. The HIP process has successfully closed these microshrinkage pores, such that the microstructure is completely dense after HIPing.

Another important fi nding is that grain size is not negatively affected by the HIP process (Figure 6). A simple heat treatment to the same thermal parameters

as the HIP processing (without the use of pressure) is neither capable of fully closing the pores from the as-cast condition, nor maintaining grain size (Figure 7). While the amount and the size of pores are clearly reduced, grains are growing substantially during heat treatment. Thus, any benefi cial effect of porosity reduction is compromised by grain growth. Based on this result, it would appear that the pressure used in the HIP process has the added benefi t of retarding grain growth. Figure 8 demonstrates the comparative grain sizes of 95Pt5Ru in the as-cast, HIPed and heat-treated conditions.

The casting sample of 95Pt5Co shows no visible macroscopic or microscopic porosity in the as-cast condition (Figure 9). However, compared to 95Pt5Ru the grains are extremely large. Their size and shape indicates a relatively slow solidifi cation process

50 m

Fig. 5. 95Pt5Ru as-cast showing numerous interdentric microshrinkage pores present in the as-cast condition and uneven grain size distribution with coarse columnar grains at the surface

50 m

Fig. 6. 95Pt5Ru HIP which has dense and pore-free microstructure and even grain size distribution. HIP pressure appears to retard grain growth



where a few grains were nucleated at the surface of the part, which then grew into the centre. As a consequence, there was suffi cient time for feeding and the microstructure is free of pores. Therefore, during HIPing few changes of the microstructure occurred in the 95Pt5Co (Figure 10). This is not to

say that HIP does not provide any benefi t to 95Pt5Co castings. The previous research on larger samples demonstrated a tendency of this alloy to form large gas pores and centreline shrinkage porosity that were either eliminated or reduced in size by the HIP process.

50 m

Fig. 7. 95Pt5Ru heat treatment using same thermal curve as HIP, showing reduction of microshrinkage porosity and heavy grain coarsening during thermal processing

Fig. 8. Comparative microstructures of 95Pt5Ru: (a) as-cast; (b) heat treated; (c) HIPed

(a) (b) (c)

50 m 50 m50 m

50 m

Fig. 9. 95Pt5Co as-cast showing very large columnar grains growing from the surface during solidifi cation and no macroscopic or microscopic porosity in as-cast condition



Fig. 10. 95Pt5Co HIP showing no signifi cant change of microstructure during HIPing

50 m

Alloy Density

Density of the test samples was measured using the Archimedes’ Principle (3) to determine levels of densifi cation achieved through the HIP process. Although the density of the as-cast samples was already very high, HIP increased the levels to near 100%. This result is impressive and effectively puts castings on a par with wrought material. As can be seen in Table III, HIP most effectively increased density in 95Pt5Ru and 90Pt10 Rh and was slightly less effective for 90Pt10Ir, which we can see already starts out with higher as-cast density. This result correlates well with the generally lower levels of visible porosity seen in 90Pt10Ir cross-sections from the 2011 study (1). Although 95Pt5Co was not tested for density, one would expect similar fi ndings as in the case of 90Pt10Ir due to the lower levels of porosity in the as-cast state.

Certain defect types either do not respond to HIP or have a lower densifi cation response. A key limitation of the process is that only porosity that is fully subsurface will collapse; if pores are open to the surface of the casting in any way, they will not respond to HIP. This effect can be seen in the shrinkage porosity (Figure 11(a)). Another limitation of HIP is seen in gas pores. Pores created by gas are less responsive to HIP than shrinkage pores due to the pressure they contain. Rather than being eliminated, the pores are typically reduced in size by HIP as can be seen in the cross-section (Figure 11(b)).

Alloy Homogeneity and its Effects on Segregation

Another aspect of the present study was determining whether there had been any change in segregation

Table III Alloy Density Results

Alloy Condition Density, g cm–3 Relative density

95Pt5Ru As-cast 20.32 98.4%

95Pt5Ru HIPed 20.62 99.9%

90Pt10Ir As-cast 21.39 99.5%

90Pt10Ir HIPed 21.48 99.9%

90Pt10Rh As-cast 19.58 98.2%

90Pt10Rh HIPed 19.89 99.7%



of the alloying elements during high temperature heat treatment or HIP. By contrasting EDX mapping of 95Pt5Ru in the as-cast, heat treated, and HIPed conditions, we found that Ru segregated to the primary dendrites during solidifi cation in a similar manner for all three conditions. Thus, neither heat treatment nor HIP changed the segregation of Ru. As can be seen in the comparative images in Figure 12, after HIP the dendrites have coarsened, but the microstructure is not negatively affected because the dendrites have arms that can coarsen without changing the overall size of the dendrite (Figure 12(c)).

Tensile Testing

A literature search for mechanical properties data on cast platinum alloys showed that there are few publications in this area, with the exception of microhardness values that are frequently cited by jewellery industry sources. A publication from 1978 by Ainsley and Rushforth (4) was likely one of the earliest to look at tensile properties from actual castings versus the more commonly cited mill product values. These authors published values on nearly a dozen different

platinum casting alloys including two that are also covered in the present testing. It is notable that the values they published for the same alloys tested here were appreciably lower in the as-cast state. Although it is not known why this was the case, a plausible explanation might be the diffi culty of obtaining high quality cast test bars with the technologies available in 1978.

This relative scarcity of hard data is not so surprising given that sophisticated platinum casting is a relatively new development. It was not until the mid-1990s that induction machines capable of handling platinum’s high temperature requirements became mainstream. Prior to that, small-scale oxyhydrogen torch melting was the only method available and inconsistent quality coupled with low pour weight capacity prevented investment casting from becoming a mainstream industrial process for platinum. All of that has of course changed and platinum based alloys are now routinely investment cast with induction melting methods on a global basis.

While as-cast tensile properties of platinum alloys are of keen interest in their own right, an additional motivation to perform this testing was as a means to compare the strength characteristics of as-cast versus HIPed platinum alloys. In theory, the higher density

200 m

Fig. 11. (a) Surface connected shrinkage porosity in 95Pt5Ru; (b) sectioned Pt90Ir10 gas pore after HIP

(a) (b)

Fig. 12. (a) Pt95Ru5 as-cast; (b) Pt95Ru5 heat treated; (c) Pt95Ru5 HIPed

90 m 90 m 90 m

(a) (b) (c)

1000 m



of HIPed product would show increased values for a number of tensile properties. Table IV outlines the testing plan that was followed to produce the data, followed by Figure 13 depicting the test bar geometry used for the tensile tests. Tensile testing was carried out in accordance with international standards (5).

The results in Table V report values for yield strength (YS), ultimate tensile strength (UTS), elongation (ε) and reduction of area (ROA). Yield strength describes stress levels above which plastic deformation occurs and will generally increase with decreasing grain size. Following yielding, the material work hardens by the generation of dislocations. As a consequence, the required stress for further deformation increases until the ultimate tensile strength is reached. In metals, the UTS values will generally correlate with Vickers hardness values.

At strains above UTS, which marks the maximum of the stress-strain curve (Figure 14), the cross-section is locally reduced through necking of the sample. Further deformation is localised in the necking region and as a consequence the required stress for further deformation is continually decreasing until failure occurs. The total elongation (ε) indicates how much plastic deformation the material can withstand. Pores in the material will signifi cantly reduce the elongation because they act as stress concentration sites. The effect of pores is even more pronounced on the reduction of area, which indicates how much necking occurs until the sample fi nally fails.

While UTS or hardness are clearly important properties to measure, they are not necessarily the most critical properties to predict failure in a broad number of applications. When it comes to fatigue life, elongation and reduction of area are generally viewed as being more important. Specifi cally, in cases where

Table IV Casting Specifi cations for Tensile Bars

Alloy Pour temp., ºC Flask temp., ºC Number of test bars Casting condition

95Pt5Ru 1870 850 12 6 As-cast; 6 HIPed

90Pt10Ir 1870 850 8 4 As-cast; 4 HIPed

90Pt10Rh 1960 850 8 4 As-cast; 4 HIPed

95Pt5Co 1850 850 8 4 As-cast; 4 HIPed

Notes to Table IV

(a) Total bars: 36; minimum tests required: 18

(b) Test bar locations in ‘upper’ and ‘lower’ centrifuge orientation

(c) All waxes turned on lathe for dimensional precision

(d) All bars identically wax assembled with double end gates

(e) All bars cooled identically

(f) All bars HIPed to the same parameters

Fig. 13. Test bar geometry: simple design, heavy molten feed to gauge area with double end gates, directional solidifi cation from interior of bar towards outer heavy sections optimizes gauge area (units in mm)

4.000

19.000

59.720

19.000

3.000



subsequent cold working of the material is involved, an increased ability to bend before cracking is of paramount importance.

The HIP treatment affects the mechanical properties of the four alloys in a different way. For all alloys the effect on YS and UTS is rather low. For 90Pt10Ir and 95Pt5Co there is little effect on elongation and ROA. However, for 95Pt5Ru and 90Pt10Rh a signifi cant increase in elongation and ROA is found through

HIPing. These results correlate well with the porosity levels of the different alloys, and are a clear indication that reduction of porosity increases ductility in the alloys.

Another interesting observation came from an analysis of UTS scatter in the sample population. The graphs in Figure 14 demonstrate the difference in spread between the as-cast and HIPed groups. The HIPed bars exhibit a very tight distribution, whereas the

Table V Tensile Properties

Alloy composition

Condition Yield strength, MPa

Ultimate tensile strength, MPa

Elongation, % Reduction of area,%

Change in reduction of area,%

95Pt5Ru As-cast 225 412 30 55 –

95Pt5Ru HIPed 236 420 39 87 +32

90Pt10Ir As-cast 219 353 33 90 –

90Pt10Ir HIPed 226 358 36 87 –3

90Pt10Rh As-cast 140 330 37 64 –

90Pt10Rh HIPed 144 333 43 89 +25

95Pt5Co As-cast 220 452 36 76 –

95Pt5Co HIPed 189 449 38 82 +6

Fig. 14. UTS scatter of 95Pt5Ru: (a) as-cast; (b) HIPed

Stre

ngth

, MP

aS

treng

th, M

Pa

500

400

300

200

100

0

500

400

300

200

100

0

0 10 20 30 40Strain, %

0 10 20 30 40Strain, %

UTS

YSROA

(a)

(b)



as-cast bars are more scattered. This result correlates well with observations of lower porosity levels together with a more homogeneous grain size and structure in the HIPed samples.

As stated above, reduction of area values posted the most impressive gains in the HIPed product. This property is of particular interest in the jewellery industry given the substantial amount of bending and forming that is inherent in stone setting, engraving, sizing and myriad bench operations. Reduction of area indicates a material’s ductility and is crucial to successful performance in many of these operations. Figures 15(a) and 15(b) demonstrate a profound visual difference in ductility between the test bar fractures in the as-cast and HIPed 95Pt5Ru.

Hardness Testing

Table VI reports Vickers hardness values (6) for three of the tested alloys. With the possible exception of

95Pt5Ru, all alloys report values that are so close in the before and after HIP conditions that any difference is seen as essentially inconsequential. Even the 95Pt5Ru that shows a 12-point spread is not considered enough to be characterised as an appreciably harder material by performance. Thus we can conclude that hardness is not signifi cantly impacted by HIP on the platinum-based alloys we tested.

Conclusions

The most signifi cant impact of HIP on platinum-based alloys is a reduction in porosity. Reduced levels of porosity have several associated benefi ts, including a marked increase in ductility in the majority of the alloys tested without sacrifi cing strength. Of the tensile properties tested, the most impressive response was found in the values for ROA, a key indicator of an alloy’s ductility.

Another meaningful result demonstrated here was HIP’s effect on grain structure and size. Although further testing is needed to fully characterise this aspect, our initial research confi rms a more uniform grain size and structure in the HIPed samples without any increases in grain size, at least for the alloy 95Pt5Ru.

Findings also confi rm that the response to HIP is strongly impacted by the alloy’s composition. 95Pt5Ru benefi ts the most due to its higher levels of porosity present in the as-cast condition, and 95Pt5Co having lower porosity benefi ts the least.

Further work is recommended to more closely assess the impact of qualitative changes in the HIPed product on manufacturing operations. Empirical evidence strongly suggests greater ease in post-cast operations due to the elimination of sub-surface micro-porosity and a generally more consistent metallurgical condition, including uniformity of grains. In addition, the increased ductility of HIPed platinum alloys should, in theory, result in a lower number of failures during metal bending and forming operations.

Acknowledgements

The authors would like to acknowledge the original publishers, Santa Fe Symposium on Jewelry Manufacturing Technology 2014, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2014) for permission to re-publish this work.

Fig. 15. (a) 95Pt5Ru as-cast 55% ROA; (b) 95Pt5Ru HIPed 87% ROA

(a)

(b)

Table VI Vickers Microhardness Results

Vickers Hardness HV1a

Alloy As-cast HIPed Heat treated

90Pt10Ir 113 111 123

90Pt10Rh 89 89 n/a

95Pt10Ru 113 125 128

95Pt5Co 126 122 n/aa Error: ± 3 HV1



This research was supported in part by the German Federal Ministry for Economic Affairs and Energy (BMWi) under the IGF program (Project No. AiF-IGF 16413N).

References1. T. Fryé and J. Fischer-Bühner, ‘Platinum Alloys in the

21st Century: A Comparative Study’, in "The Santa Fe Symposium on Jewelry Manufacturing Technology 2011", ed. E. Bell, Proceedings of the 25th Symposium in Albuquerque, New Mexico, 15th–18th May, 2011, Met-Chem Research Inc, Albuquerque, New Mexico, USA, 2011, pp. 201–230

2. “Elements of Metallurgy and Engineering Alloys”, ed. F. C. Campbell, ASM International, Ohio, USA, 2008, p. 265

3. “Standard Test Method for Density, Oil Content, and Interconnected Porosity of Sintered Metal Structural Parts and Oil-Impregnated Bearings”, (Withdrawn 2009), ASTM Standard B328-96(2003)e1, ASTM International, West Conshohocken, Pennsylvania, USA, 2003

4. G. Ainsley, A. A. Bourne and R. W. E. Rushforth, Platinum Metals Rev.,1978, 22, (3), 78

5. “Metallic Materials – Tensile Testing – Part 1: Method of Test at Room Temperature”, DIN EN ISO 6892-1: 2009-12, Deutsches Institut für Normung e.V., Berlin, Germany, 2009

6. “Metallic Materials – Vickers Hardness Test – Part 1: Test Method”, DIN EN ISO 6507-1:2006-03, Deutsches Institut für Normung e.V., Berlin, Germany, 2006

The Authors

Ms Teresa Fryé has over 25 years’ experience working in the investment casting industry. She started her career at Precision Castparts Corp, USA, one of the world’s largest investment casters, serving the international customer base for high-temperature aerospace castings. In 1994 she co-founded TechForm Advanced Casting Technology, a company that specialises in shell casting of platinum group metals. She holds a BA in International Affairs and graduate studies in Psychology from Lewis & Clark College in Portland, Oregon, USA.

Joe Strauss has a PhD in Materials Engineering from Rensselaer Polytechnic Institute, USA. He has over 30 years of experience in developing atomisation and powder metallurgy technologies for non-traditional materials including precious metals for jewellery, dental, electronic and medical applications. His work includes design and fabrication of atomisation systems, research and development in powder metallurgy and engineering troubleshooting for manufacturing processes.

Dr-Ing Jörg Fischer-Bühner holds a PhD in Physical Metallurgy and Materials Technology from the technical university RWTH Aachen, Germany. He is currently active in Research & Development for Legor Group SpA (Bressanvido, Italy) as well as INDUTHERM Erwärmungsanlagen GmbH (Walzbachtal, Germany). His work has included manufacturing support, failure analysis, training and consultancy to manufacturing companies, and his research has focused on alloy properties and manufacturing technologies, especially precious metal alloys for jewellery, dental and electrical engineering applications.

Ulrich E. Klotz graduated from the University of Stuttgart, Germany, as a Diploma Engineer in Physical Metallurgy and has a PhD in Materials Science from ETH Zürich, Switzerland. He is Head of the Department of Physical Metallurgy at the fem.




“Exploring Materials through Patent Information”By David Segal (Abingdon, Oxfordshire, UK), Royal Society of Chemistry, Cambridge, UK, 2015, 244 pages, ISBN:978-1-78262-112-6, £24.99, €31.24, US$40.00

Reviewed by Julia O’FarrellyJohnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK


The majority of books and reviews on any area of technology development tend to focus on information published in the journal literature; reviews of the patent literature are more often confi ned to the prior art sections of patent documents. However, patents remain one of the best sources of detailed technical information, particularly where the invention may have commercial signifi cance, and it is good to see this reviewed in a book.

The author, David Segal, is a graduate of Trinity College Cambridge, UK. In his long career he has worked widely in the fi elds of chemicals and materials and has published a number of patents and papers.

Introduction: Exploring the World of Materials through Patent Information

In Chapter 1 Segal provides a general introduction to the topic of materials for the non-specialist. He makes the point that materials underpin many of the technological advances that we know in everyday life; for example, the use of lanthanide elements as phosphors to generate colour in display screens, or the application of neodymium iron boron (NdFeB) magnets in computer

hard drives and magnetic resonance imaging (MRI) scanners. The success of additive layer manufacturing (ALM), also known as additive manufacturing (AM) or ‘three-dimensional (3D) printing’ technology, depends on the availability of plastics that can be converted into molten droplets and of ceramic and metal powders that can be formulated for use in a range of different ALM techniques.

Chapter 1 also provides a basic introduction to patents, covering ‘what is a patent?’ and patent structure (front page data, background to the invention, summary of the invention, embodiments, drawings and claims). Sections are included on patent fi ling, patent infringement and patent searching. The book stresses the importance of seeking specialist advice from qualifi ed practitioners. As with much of the book, however, too many topics are included, making coverage superfi cial.

Chapters 2–13 review a wide range of emerging technologies in the context of patent information: light emitting diodes, quantum dots, organic light emitting diodes (OLEDs), liquid crystals and liquid crystal displays, ALM or ‘3D printing’, healthcare, block copolymers, aerogels, ionic liquids, fl ame retardants, graphene, hydrogels and super-hydrophobic materials. I have chosen three of these chapters to review below.

Additive Layer Manufacturing

Chapter 6 is titled ‘3D Printing’. Segal describes the evolution of this process starting from Swanson and



Kremer’s photopolymerisation by computer controlled laser beams. In fused deposition modelling, 3D parts are built up layer-by-layer from a thermoplastic material using a digital representation of the part. In selective layer sintering a layer of metal, ceramic or polymer powder is deposited under computer control and sintered by scanning a laser across the surface. In each case the process is repeated to ultimately form a 3D part. The use of the term ‘3D printing’ was coined later by Sachs et al. and used, for example, in their 1993 patent (1).

The chapter goes on to give a number of examples of patents on applications of ALM (Figure 1). Aerospace components are discussed, as are dental prostheses and biomedical implants. Among the aerospace examples, Segal describes a high temperature AM process to make titanium near-net-shape metal leading edge protective strips for aerofoils (2). A key feature is heat transfer away from the support mandrel. In the dental fi eld he describes the use of selective laser powder processing to build up a jaw structure from titanium (3). Although Segal reproduces technical details of the laser required to melt the metal particles, he does not discuss the signifi cance of such a process, which in this case is the ability to fabricate complex shapes without lengthy pre- or post-processing and the ability to customise parts to individual patient requirements in the same production run.

Some examples are also given of components needed for ALM machines such as shutter mechanisms and wiper blades.

Organic Light Emitting Diodes

Chapter 4 discusses the development of OLEDs. It starts with a review of the early development of electroluminescence, for example the work by Gurnee and Fernandez which showed that a doped host material with a conjugated structure, such as anthracene, gave off green light when placed between two electrodes (4). Pioneering work by Friend et al. on conjugated polymers such as poly(p-phenylvinylene) (PPV) formed the basis of polymer light emitting diodes (PLEDs) (5). Later it was discovered that organic phosphorescent materials, which can emit light in the triplet state, are potentially more effi cient than fl uorescent materials which can emit only in the singlet state. The example is given of the green phosphorescent emitter tris(2-phenylpyridine)iridium, Ir(ppy)3. Of course the phosphorescent emitter layer is only one of a complex series of material layers and the development of complex circuitry is an essential part of the realisation of a working OLED device.

Graphene

Chapter 12 discusses graphene, a material which has received much interest since its discovery in 2004. The development of methods for making it, including wet chemical processes, vapour phase processes, the use of ionic liquids and electrochemical methods, have been key; details of these processes are best found in the patent literature. The chapter goes on to

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discuss potential methods for large scale production of graphene, essential for successful exploitation; chemical vapour deposition is one such method. Incorporation of graphene fl akes into the polymer matrix of a fi bre reinforced composite can increase its compressive strength, potentially useful in racquets and other sports equipment. In lithium-ion batteries, multilayer graphene fi lms have been evaluated for use as an anode and graphene platelets have been used to reinforce the matrix of a cathode material.

Conclusion

Overall, the book does effectively make the point that patents contain a wealth of technical information and should be considered alongside journals and other sources when researching a topic. It successfully gives a broad overview of the historical development of each topic, with references in the form of patent numbers which serve as a starting point for further reading. The focus is very much geared to demonstrating the technical detail that can be found, with examples of material compositions and experimental data as claimed in individual patents.

However, this reviewer believes that it tries to cover too much material in too small a volume and as a result does not really do any individual topic full justice. Too many patents are referenced and too much non-essential experimental detail from the patents is included. At the same time, the signifi cance of each invention is often not clear without going back to the original patent source and this detracts from the overall readability.

Segal’s book does not touch on how patents have altered the course of development of a technology by impacting different companies’ freedom to exploit it; nor does it make any reference to how the existence of patents has affected the commercial development of a technology. In my opinion, such a discussion would

have helped to round out a promising but somewhat disappointing volume.

References1 E. M. Sachs, J. S. Haggerty, M. J. Cima and P. A.

Williams, Massachusetts Institute of Technology, ‘Three-dimensional Printing Techniques’, US Patent 5,204,055; 1993

2 M. W. Peretti and T. Trapp, General Electric Company, ‘Methods for Making Near Net Shape Airfoil Leading Edge Protection’, US Patent Appl. 2011/0,143,042

3 J.-P. Kruth, I. Naert and B. Vandenbroucke, ‘Procedure for Design and Production of Implant-based Frameworks for Complex Dental Prostheses’, US Patent Appl. 2008/0,206,710

4 E. F. Gurnee and R. T. Fernandez, The Dow Chemical Company, ‘Organic Electroluminescent Phosphors’, US Patent 3,172,862; 1965

5 R. H. Friend, J. H. Burroughes and D. D. Bradley, Cambridge Research and Innovation Ltd, Cambridge Capital Management Ltd and Lynxvale Ltd, ‘Electroluminescent Devices’, US Patent 5,247,190; 1993

“Exploring Materials through Patent Information”

The Reviewer

Julia O’Farrelly is a Principal Information Analyst in the Technology Forecasting and Information group at Johnson Matthey Technology Centre, Sonning Common, UK. She is interested in emerging technologies and their development and commercialisation and the use of patents as a tool in understanding technology landscapes.




“Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts”Edited by Isabella Nova and Enrico Tronconi (Politecnico di Milano, Italy), Fundamental and Applied Catalysis, Springer Science+Business Media, New York, USA, 2014, 716 pages, ISBN: 978-1-4899-8071-7, £171.00, €239.99, US$249.00

An essay book review by Martyn V. TwiggTST Ltd, Caxton, Cambridge CB23 3PQ, UK

*Correspondence may be sent via Johnson Matthey Technology Review: [email protected]

The introduction and development of catalytic control for exhaust gas emissions from vehicles has been one of the major technical achievements over the last four decades. A huge number of cars were manufactured during this time that provided society with a high degree of personal mobility and without the continuous development of emissions control technologies the atmospheric pollution derived from them would have been overwhelming. Three-way catalysts (TWC) were introduced on traditional gasoline powered cars in the early 1980s to control the emissions of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) and have since been developed so that today tailpipe emissions of these pollutants can be reduced by more than 99.5% and tailpipe emission levels can be less than in the surrounding ambient air. During more recent years, and especially in Europe, the proportion of diesel powered cars has increased rapidly so now about half of new European cars have a diesel engine. Control of their tailpipe emissions has been particularly challenging because of their low exhaust gas temperature and the presence of excess oxygen.

Under these conditions TWCs cannot be used and alternative technologies were developed for the control of HCs and CO by oxidation catalysts. An undesirable characteristic of older diesel engines was the black soot they produced. This was considerably reduced by fuelling and combustion engineering improvements and was effectively eliminated by the use of diesel particulate filters (DPFs) which were introduced a decade ago. The remaining difficult challenge has been the control of NOx emissions from both light and heavy duty diesel vehicles. Two technologies have been recently introduced to do this, though only one, ammonia selective catalytic reduction (SCR), appears to be able to provide the necessary performance for future demands under a wide range of driving conditions. The present book is about diesel engine NOx emissions control by ammonia (derived from urea) SCR, and before detailing the book’s contents some background information is given which provides a suitable context. Because of higher exhaust gas temperatures control of emissions from heavy duty diesel vehicles is less demanding than with light duty ones, so the emphasis here is on diesel cars.

1. Background1.1 Exhaust Gas Temperature

The control of tailpipe emissions from vehicles powered by traditional stoichiometric gasoline engines with



TWC is now highly advanced and can achieve almost complete removal of the three gaseous pollutants CO, HCs and NOx under normal driving conditions. In practice reductions of more than 99.5% are possible, and a contributing factor for such a good performance is the relatively high temperature of the exhaust gas. In contrast the control of diesel engine exhaust gas emissions under lean conditions has been more problematic for two main reasons. The first problem results from the efficiency of diesel engines that under part load conditions can result in particularly low exhaust gas temperatures. For example, the exhaust gas temperature of a small European car with a gasoline engine may typically be in the region of 350ºC to 475ºC in the urban part of the European test cycle, whereas the same car with a diesel engine may be around 150ºC as shown in Figure 1, and designing catalysts to operate efficiently at such low temperatures has been a major challenge, particularly when fuel sulfur levels were higher than they are today!

Heavy duty diesel engines in trucks generally operate under higher engine percentage loads and over appropriate duty cycles they can have much higher exhaust gas temperatures than their passenger car counterparts, typically up to around 400ºC. So here there is considerably more scope for catalytic emissions control.

Once diesel fuel sulfur levels were reduced from the very high levels of two decades ago in Europe control of CO and HC emissions from all diesel engines by oxidation catalysts became feasible although special catalysts had to be developed for the low operating temperature cars.

1.2 Particulate Control with Filters

This was followed by control of particulate matter (PM or soot) by the introduction of filter technologies that enabled engine measures to further reduce engine-out NOx levels without being overly concerned about increased PM that was handled by the filter system. Traditionally the main approach for controlling NOx from small diesel engines has been via engine measures, including the use of exhaust gas recycle (EGR) and improving injection fuelling to produce ever finer sprays via multiple smaller injector nozzles and via increasing fuel pressures. EGR works by decreasing the amount of oxygen in the combustion charge that reduces the fuel burn rate and the peak temperatures as well as somewhat increasing heat capacity of the combusting charge.

There is a trade-off between NOx and PM. Reducing engine-out NOx normally results in an increase in PM. This is because a higher combustion temperature reduces PM by increased burning of residual

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carbonaceous PM but thermodynamically high temperatures favour formation of endothermic NOx (see Figure 2). This trade-off was broken by fitment of DPF that recently enabled achievement of car diesel engines with engine-out NOx levels significantly below 0.1 g km–1 in the combined European test cycle.

1.3 NOx Control Technologies

Notwithstanding the improvements just mentioned, more recently NOx control has become increasingly important, driven by ever more stringent NOx emissions legislation. This legislation requires additional catalytic aftertreatment to meet the NOx standards for diesel vehicles, especially cars, and two approaches have become established.

In the first of these catalytic approaches, NOx trapping, under normal driving lean conditions NO is oxidised to NO2 as in Equation (i). This undergoes further oxidation as it is stored as a metal nitrate, Equation (ii), followed at intervals by a reductive regeneration that converts the stored NOx to nitrogen. In this process NOx is liberated usually as NO, Equation (iii), that is reduced over a rhodium component in much the same way as a TWC functions on a traditional gasoline car, Equation (iv).

NO + ½O2 à NO2 (i)

MCO3 + NO2 à MNO3 + CO2 (ii)

2M(NO3)2 → 2MO + 4NO + 3O2 (iii)

2NO + 2CO → N2 + 2CO2 (iv)

This technology works well on smaller diesel cars, although it has temperature limitations reflecting the thermodynamic stability of the metal nitrates concerned.

The second NOx control technology, and the subject of the present book, is ammonia SCR that involves reaction of NOx with ammonia to form nitrogen and water. Ammonia SCR technology was introduced on power plant applications in Japan in the early 1970s, and some twenty years later it was adopted for use in heavy duty diesel vehicles that have exhaust gas temperatures appropriately high to use the traditional vanadium-based catalysts. Ammonia was derived from urea solution that was injected into the exhaust gas where it hydrolyses forming ammonia and carbon dioxide (see Equation (v)).

However, the temperatures on light duty diesel vehicles are too low for efficient operation of the older SCR vanadium-based catalyst formulations and so after much effort base metal zeolite catalysts were introduced that can operate effectively at remarkably low temperatures and already increasingly large numbers of cars on European roads are equipped with this SCR technology.

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More than thirty years ago a very important discovery was made about the effect of the ratio of NO2 to NO on the rate of the ammonia SCR reaction over vanadium-based catalysts. The reaction is much faster when both are present. The reactions involved when urea ((NH2)2CO) is the source of ammonia are: urea hydrolysis to give ammonia, shown overall in Equation (v); a rapid reaction when just NO is present, Equation (vi); a particularly slow reaction when NO2

is present alone, Equation (vii); and an amazingly fast reaction when the ratio of NO2 to NO is 1:1, that is known as the fast SCR reaction, Equation (viii). Depending on the actual SCR catalyst used it can therefore be important that an appropriate upstream oxidation catalyst provides the SCR catalyst with a suitable mixture of NO and NO2, although some modern copper zeolite catalysts are less sensitive to the NO/NO2 ratio than other catalysts.

Urea hydrolysis (NH2)2CO + H2O à 2NH3 + CO2 (v)

Standard reaction 4NH3 + 4NO + O2 à 4N2 + 6H2O (vi)

Slow reaction 4NH3 + 4NO2 à 4N2 + 6H2O + O2 (vii)

Fast SCR reaction 4NH3 + 2NO + 2NO2 à 4N2 + 6H2O

There have been extensive studies on the mechanism of ammonia SCR reactions, and by analogy with known reactions of discrete compounds it may be suggested that rapid decomposition of ammonium nitrite (Equation (ix)) is important in the SCR surface catalysed process forming nitrogen. At higher temperatures one route to undesirable nitrous oxide (N2O) emissions might be from decomposition of ammonium nitrate-like surface species, Equation (x). The former reaction has been used to prepare chemically pure nitrogen (free of atmospheric argon) and the latter to manufacture N2O.

NH4NO2 à N2 + 2H2O (ix)

NH4NO3 à N2O + 2H2O (x)

However, the surface SCR reactions are complex and recently nitrate species have been shown to have important roles in the fast SCR reaction. Chapters in this book go a long way to help the reader to unravel some of the mechanistic details.

2. Topics Covered

This book has 22 chapters by eminent contributors and is appropriately edited by Professors Isabella Nova and Enrico Tronconi from the Politecnico di Milano, Italy, whose Laboratory of Catalysis and Catalytic Processes (LCCP) has a worldwide reputation for research on the control of NOx emissions especially by ammonia SCR reactions. This very well produced book includes some colour illustrations and it is divided into eight parts that are detailed in the following sections.

2.1 Part 1. Selective Catalytic Reduction Technology

The first part of the book has two chapters with the first entitled ‘Review of Selective Catalytic Reduction (SCR) and Related Technologies for Mobile Applications’ by Timothy Johnson (Corning, USA). It provides an overview of relevant legislation and progress in engine developments to reduce engine-out NOx levels, before detailing mobile SCR systems using urea in solution as the source of ammonia. This chapter relies heavily on illustrations reproduced from a variety of original publications that appear not to have been redrawn so there is, unfortunately, a lack of style consistency. Notwithstanding this the chapter collects together much valuable and practical information.

The second chapter called ‘SCR Technology for Off-highway (Large Diesel Engine) Applications’ is by Daniel Chatterjee and Klaus Rusch (MTU Friedrichshafen GmbH, Germany) and is concerned with large diesel engines used in marine applications, mining trucks and trains as well as in electrical power generation units. These engines usually operate under high load conditions so have high temperature exhaust gas, enabling good SCR performance with conventional vanadium-based catalysts, but their fuel invariably contains high sulfur levels and this can cause a variety of problems. For example, newer zeolite-based SCR catalysts are poisoned and do not work well, and at these quite high operating temperatures some sulfur dioxide (SO2) can be oxidised to sulfur trioxide (SO3), Equation (xi). If any ammonia slip is present this can form particulate ammonium sulfate and/or ammonium bisulfate according to Equations (xii) and (xiii), as well as sulfuric acid mist that can itself cause difficulties, Equation (xiv).

SO2 + ½O2 à SO3 (xi)

(viii)



2NH3 + SO3 + H2O à (NH4)2SO4 (xii)

NH3 + SO3 + H2O à NH4HSO4 (xiii)

SO3 + H2O à H2SO4 (xiv)

This well-illustrated chapter goes on to discuss combined SCR systems such as SCR/filter combinations, and large scale SCR units, as well the automated control strategies that are usually involved.

2.2 Part 2. Catalysts

The second part of the book has four chapters that focus on SCR catalysts, and the first of these by Jonas Jansson (Volvo, Sweden) discusses vanadium-based catalysts used in heavy duty mobile SCR applications and highlights the legislative requirements before considering catalyst properties. Because the vanadium-based catalyst operates in the temperature range of optimum activity (say 300ºC–500ºC) they have been widely used. Typical catalyst compositions are given as 1%–3% V2O5 plus about 10% tungsten trioxide (WO3) impregnated onto a high surface area titania (normally anatase) that is coated onto flow-through substrates. Related extruded catalyst compositions have also been widely used. Practical aspects such as selecting appropriate catalyst size (dimensioning), the effects of space velocity and ageing effects (thermal and poisoning) are considered, and it is clear vanadium catalysts have had and will continue to have a major role in this area. However, with reduced sulfur fuel levels the newer, higher activity zeolite-based catalysts discussed in the following chapter will probably become increasingly important.

Appropriately the next chapter, by Todd J. Toop, John A. Pihl and William P. Partridge (Oak Ridge National Laboratory, USA), is about iron-zeolite SCR catalysts. These were amongst the first metal zeolite catalysts used in SCR applications, and because they can have good high-temperature performance coupled with reasonable stability, they were introduced into gas turbine applications at an early stage. In contrast copper zeolite catalysts usually have better low-temperature activity that falls off at higher temperatures as ammonia is oxidised to NOx. Making sweeping generalisations about the relative performance of SCR catalysts can be problematic because several factors are involved such as: the type of zeolite involved, its silica to alumina ratio, the metal loading and importantly the preparation method. However, in general copper-based catalysts

absorb more ammonia than do iron ones and under dynamic transient conditions this can provide a significant performance advantage.

Because of their superior low-temperature performance copper zeolite catalysts have been adopted for use in car applications and these are discussed in the third chapter in this section by Hai-Ying Chen (Johnson Matthey, USA). The emphasis is on the impact of the nature and physical properties of the zeolite type on catalytic performance, and in particular the size of the zeolite pores classified as small, medium or large. Small pore zeolites such as chabazites and other small pore molecular sieve materials such as the silicon substituted aluminium phosphate SAPO-34 have outstanding hydrothermal stability, excellent SCR activity and importantly they form very low amounts of the undesirable byproduct N2O. The introduction of these copper molecular sieve SCR catalysts into the series production of diesel cars in Europe was an outstanding technical achievement that will provide a high degree of NOx emissions control into the future. Indeed one might expect that new materials will be discovered that provide the necessary acidity and environment around the copper atoms to provide good SCR activity and durability. However, these features are not unique in providing excellent ammonia SCR performance. Some simple metal oxide catalysts have been shown to perform well and these are the subject of the last chapter in the part on SCR catalysts.

The last chapter in this part on catalysts is the result of a collaboration by Gongshin Qi (General Motors, USA) and Lifeng Wang and Ralf T. Yang (University of Michigan, USA) that deals with low-temperature SCR involving both zeolite and metal oxide ammonia SCR catalysts as well as touching on developments with hydrogen SCR. They highlight the importance of the method of making iron-ZSM5 catalysts. Aqueous impregnation with iron(III) salts does not lead to full metal incorporation into the pores because, it is suggested, the heavily hydrated metal cations are too large for easy penetration, whereas impregnation with iron(II) species makes highly active catalysts. The interpretation of the origin of this effect may be more complex because reduction of catalysts derived from iron(III) salts gives improved activity. Again the importance of small pore acidic molecular sieves is noted, as is the wide range of activities that can be obtained with different copper zeolite catalysts and their dual role in providing acid sites for formation of ammonium cations and metal-based oxidation of NO



to NO2, leading to highly reactive ammonium nitrite-like species that decompose to nitrogen and water. Manganese oxides can have excellent low-temperature ammonia SCR activity, and clearly their oxidation capability is important. A wide variety of promoted oxides have been investigated, but it appears their adoption has been restricted by a lack of tolerance to water and in particular sulfur poisoning. Moreover, increasing activity by using higher manganese loadings appears to result in the formation of more N2O. It is noted perhaps the most successful development in this area was made by Shell who in the early 1990s developed a relatively low-temperature ammonia SCR process using a vanadium on titania catalyst promoted by transition metal species.

Hydrogen can be a reductant in NOx SCR reactions and over platinum group metal (pgm) catalysts. The reactions that can take place are shown in Equations (xv)–(xvii).

2NO + 4H2 + O2 à N2 + 4H2O (xv)

½O2 + H2 à H2O (xvi)

2NO + 3H2 + O2 à N2O + 3H2O (xvii)

High conversions of NO in the presence of oxygen at low temperatures are possible, although as might be expected, at higher temperatures the direct reaction of hydrogen with oxygen, Equation (xvi), increasingly takes place, and NO conversion decreases because less hydrogen is available for the SCR reaction. As a result an operational temperature window is formed in which NO conversion is optimised. A serious detraction from these hydrogen SCR pgm catalyst systems is the high proportion of N2O that can be formed. A better catalyst in this respect appears to be a palladium promoted vanadium on titania/alumina that retains good low-temperature SCR performance and has reduced N2O formation although this is probably still too high for practical applications.

2.3 Part 3. Mechanistic Aspects

This part of the book is concerned with the mechanistic aspects of SCR reactions and has three chapters, the first of which is by Wolfgang Grunert (Ruhr University Bochum, Germany) on the nature of SCR active sites. The range of available characterisation techniques are first outlined before the surface science techniques that have been used are highlighted. Vanadium-based catalysts are considered first, and there is general

agreement that binary V—O—V moieties including a Brønsted site are the most active structures and a well-accepted mechanism is available for this site. Isolated VO2+ species exchanged into zeolite structures are also active, apparently via a different mechanism. Tungsten promoted vanadium appears to be effective by encouraging formation of isolated V—O—V species. The active sites in iron and copper exchanged zeolites are then considered; here a huge amount of research has been done over several decades trying to identify the intimate mechanistic details and the nature of the active SCR sites. Much of the earlier work involved exchanged ZSM-5, and then more recently beta-zeolite and small pore molecular sieves were studied. As previously noted a key feature is the low temperature performance of the copper catalysts and the higher temperature durability of the iron catalysts. The metal centres may be associated with NO oxidation. An added advantage of the iron catalysts, like the earlier vanadium ones, is tolerance towards sulfur that is in marked contrast to the poison sensitive copper catalysts. Although there has been considerable speculation about the roles of monomeric, dimeric and oligomeric metal active centres their general relative importance is unclear. Brønsted acidic zeolite sites have been thought to be a means of concentrating ammonium ions close to the metal centres, but the importance of this is questioned by more recent work on non-zeolite conventional oxide catalysts some of which can have good performance.

The next chapter by Masaoki Iwasaki (Toyota, Japan), is about mechanistic aspects of the ammonia/NO reaction in excess oxygen, Equation (vi), that is the traditional standard or rapid ammonia SCR reaction. Results are given for reactions involving copper and iron exchanged ZSM-5, a tungsten-promoted vanadium on titania catalyst as well as the acid form of ZSM-5. The expected reaction order of copper was greater than iron and vanadium catalysts. Kinetic parameters such as apparent activation energies and apparent reaction orders were reported for the separate oxidation of ammonia and NO as well as the ammonia-NO-oxygen reaction. Generalising for the ammonia NO SCR reaction the order in NO is positive and close to one, the order in oxygen is fractional and that for ammonia is negative, reflecting its strong adsorption that can result in reaction inhibition. There is a strong correlation between SCR activity and NO oxidation activity. A considerable amount of carefully determined transient



response data is reported and several catalytic cycles are presented. The mechanistic conclusions are similar to those previously noted.

There then follows an important chapter on the role of NO2 in ammonia SCR reactions by the editors Isabella Nova and Enrico Tronconi (Politecnico di Milano, Italy). The most obvious role of NO2 is in combination with NO in the fast SCR reaction. Ammonia and NO2 are strongly adsorbed and interact on the catalyst surface. The reaction of NO2 with surface oxide ions affords nitrate and nitrite ions, according to Equation (xviii), with the latter being further oxidised by NO2 to nitrate and NO via Equation (xix) so the overall stoichiometry is as shown in Equation (xx). These reactions are in equilibrium and depend on concentration, temperature and catalyst oxidation state.

2NO2 + O2– ⇌ NO3– + NO2

– (xviii)

NO2– + NO2

⇌ NO + NO3– (xix)

3NO2 + O2– ⇌ 2NO3– + NO (xx)

The intimate mechanism of the SCR process is based on nitrogen redox chemistry. In the standard slow SCR reaction oxygen is the oxidant taking NO to nitrite, and in the fast SCR reaction the more powerful oxidiser NO2 is available and so the mixed reaction involving NO

and NO2 is much faster. The overall fast SCR reaction may be considered to go via the disproportionation of NO2 to nitrite, the nitrate oxidation of NO to nitrite and the formation of surface ammonium nitrite that spontaneously decomposes to water and nitrogen. All the steps involved in the fast SCR reaction are summarised in Table I. The required oxidant provided by NO2 in the fast SCR reaction can also be supplied by addition of ammonium nitrate in what is called enhanced SCR.

2.4 Part 4. Reaction Kinetics

There are three chapters in the part of the book on the reaction kinetics of ammonia SCR reactions, and fittingly the first is by Isabella Nova and Enrico Tronconi. This is on SCR reactions over vanadium(V) oxide (V2O5)/WO3 supported on titania catalyst, and they explain how measured unsteady state kinetic parameters for all of the reactions concerned can be incorporated into a computer model for the control of heavy duty diesel NOx control systems. At an intimate mechanism level surface sites are indicated that include a surface redox site at which oxygen is adsorbed, a reaction site at which NO is adsorbed and an acidic site to bond to ammonia. Reduced vanadium centres are reoxidised by nitrate. It is concluded the fast SCR reaction proceeds via dimerisation of NO2 followed

Table I Summary of the Individual Steps Involved in the Fast SCR Reaction over Vanadium-based and Zeolite Metal Promoted Catalysts

Involving NO2 only

2NO2 ⇌ N2O4 NO2 dimerisation

N2O4 + O2– ⇌ NO2– + NO3

– Disproportionation

NO2 + NO2– ⇌ NO + NO3

– Nitrite oxidation by NO2

In the presence of NH3

2NH3 + H2O ⇌ 2NH4+ + O2– NH3 adsorption

NH4+ + NO2

– ⇌ [NH4NO2] → N2 + 2H2O Nitrite reduction by NH3

NH4+ + NO3

– ⇌ NH4NO3 Formation/dissociation of NH4NO3

NH4NO3 → N2O + 2H2O Formation of N2O

In the presence of NO

NO + NO3– ⇌ NO2 + NO2

– Reduction of nitrate by NO

Fast SCR

2NH3 + NO + NO2 → 2N2 + 3H2O Overall reaction



by its disproportionation to surface nitrite and nitrate. Ammonium nitrite decomposes to nitrogen while nitrate is reduced by NO to reform NO2.

The next chapter by Michael P. Harold (University of Houston, USA) and Parnit Metkar (DuPont, USA) provides a very good overview of the published mechanistic work on ammonia SCR of NOx. They consider not only kinetics and mechanisms but also the role of transport effects, especially in reactions over iron exchanged zeolites and layered catalysts comprising separate copper and iron zeolite layers. A number of particularly important points are highlighted. Diffusion limitations can become significant for the fast SCR reaction at temperatures just above 200ºC, first diffusion within the catalyst pores; and increasing the amount of ammonia rather than increasing the rate of the standard SCR reaction with NO does not enhance the reaction rate but rather slows it due to strong ammonia adsorption causing site blocking. The reaction orders are one in NO, half in oxygen and –0.3 in ammonia and the corresponding activation energy of around 10 kcal mol–1 could reflect a relatively low barrier for the rate limiting step since this was estimated under conditions where diffusion effects were thought to be absent. Curiously on iron zeolite NO oxidation is inhibited by water, but the standard SCR reaction is not. However, the results of isotopic labelling experiments are consistent with the decomposition of ammonium nitrite being involved, Equation (ix), and a potentially important route to ammonium nitrite is from NO reduction of the nitrate. It is clear the mechanistic situation for the fast SCR reaction can be significantly complex, and the chapter concludes with an examination of two layer copper zeolite/iron zeolite catalyst arrangement, and aspects of reactor modelling.

The last chapter in this part, by Louise Olsson (Charmers University, Sweden), complements the previous one because it concerns the kinetic modelling of ammonia SCR reactions over copper zeolite catalysts. An often unappreciated fact, that is highlighted, is under operating conditions the zeolite will adsorb a large amount of water in addition to ammonia with an enthalpy of adsorption of around 100 kJ mol–1 in the absence of competing adsorbates.

2.5 Part 5. Modelling and Control

The first chapter in this part, about reactor models for flow-through and wall-flow converters, is by Dimitrios Karamitros and Grigorios Koltsakis (Aristotle University Thessaloniki, Greece). The arrangement of the different

aftertreatment components are discussed because of the consequences it has on their rate of heating after the engine starts. A number of other interesting aspects are discussed including the complex behaviour of SCR catalysts incorporated into a particulate filter. In the absence of soot in the filter the pressure driven flow of gas through the filter walls containing catalyst provides better performance than the same amount of catalyst on a flow-through substrate because of the absence of diffusion resistances. However, the presence of a substantial layer of soot can modify the situation: there is the potential reaction of NO2 with soot that reduces the NO2/NO ratio which, with some SCR catalysts, can reduce its performance. To compensate for this effect more catalyst will be required. This might not be physically possible and if it were possible more catalyst would increase backpressure across the filter. It is therefore important SCR catalyst incorporated into filters lack NO2/NO ratio sensitivity.

The other chapter in this part includes discussion about the understanding and measurements needed for SCR control systems by Ming-Feng Hsieh (Cummins, USA) and Junmin Wang (Ohio State University, USA). SCR control systems have to take into account varying engine NOx emissions during real world driving and adopt the urea solution injections accordingly. Forward control strategies have been used which make major assumptions about catalyst ageing and degradation of ammonia capacity, but alone they are not adequate and some degree of feedback control using sensors is necessary. However, the present NOx sensors suffer interference from ammonia, and this has to be taken into consideration via sophisticated algorithms. In fact NOx sensors also have a sensitivity to the NO2/NO ratio resulting from the extra oxygen present in NO2. Ammonia sensors are being experimented with to overcome some of the practical difficulties, but there remain significant challenges so SCR control system development is an area of much activity.

2.6 Part 6. Ammonia Supply

The three chapters in this part are about the production of a spray of urea solution in the exhaust gas flow, its conversion into ammonia gas, storage of ammonia in SCR catalysts and the modelling of these processes. The first chapter by Ryan Floyd (Tenneco, USA) and Levin Michael and Zafar Shaikh (Ford, USA) is about system architecture and includes the design of injectors and mixing devices. The computer-based design of these systems has resulted in reliable production of gaseous



ammonia with minimal deposition of troublesome solids. The second chapter is about ammonia storage and release by Daniel Peitz, Andreas Bernhard (Paul Scherrer Institute, Switzerland) and Oliver Kröcher (EPFL, Switzerland) that focuses attention on the chemical reactions involved in converting urea to ammonia, Equations (xxi) and (xxii), before going on to discuss alternative ammonia sources. While some of these alternatives have some attraction, the use and distribution of urea solution is now so well established it seems unlikely it will be displaced. The third and final chapter in this part is about gas flow modelling by Gianluca Montenegro and Angelo Onorati (Politecnico di Milano). Computational fluid dynamics (CFD) have been used for many years to optimise distributed flow through monolithic honeycomb catalysts and the exhaust system as a whole, and these techniques have been successfully applied to systems involving SCR NOx reduction (Figure 3). A high degree of mixing ammonia with the exhaust gas is essential for high overall performance.

CO(NH2)2 à NH3 + HNCO (xxi)

HNCO + H2O à NH3 + CO2 (xxii)

2.7 Part 7. Integrated SystemsFor performance, space constraints and cost considerations it is desirable to integrate emissions control functionality as much as possible, and the three chapters in this part of the book are about this topic. The first chapter details an experimental and modelling study of dual-layer ammonia slip catalysts (ASCs) by Isabella Nova, Massimo Colombo and Enrico Tronconi (Politecnico di Milano) and Volker Schmeiβer, Brigitte Bandl-Konrad and Lisa Zimmermann (Daimler, Germany). The amount of ammonia fed to a SCR catalyst must be sufficient to reduce the varying amounts of NOx produced by the engine while maintaining the quantity of ammonia stored in the catalyst to ensure optimum NOx reduction performance. As highlighted elsewhere in this review, the control systems designed to maintain this situation under dynamic transient

Fig. 3. Schematic diagram of a car exhaust gas emissions control system comprising an oxidation catalyst, wall-flow particulate filter, and flow-through SCR catalyst. Key components include a urea solution tank (heated in cold weather), dosing spray module and static mixer, temperature and NOx sensors. (Source: Robert Bosch GmbH)

Tank

Actuators

SCR-on-filterMixer

Oxi-cat

Coolant

Exhaust

Lambda sensor

Sensors

Electronic control unit (EDC 17) or dosing control unit (DCU 17) incl. SCR functions

Heater control unit (HPU-PC) (only with EDC 17)

Glow control unit (GCU) with integrated heater control (only with EDC 17)

Dosing module DM 3.4

2 temp. sensors

Differential pressure sensor

Particle sensor

NOx sensor

Supply module SM 5.1 (PC) or SM 5.2 (LD) (Defined welding interface to the tank. Heater, lifetime filter, level and temperature sensor on module for tank integration. Pump module consisting of supply and emptying pump as replacement part)

Engine CAN



conditions can be effective but occasionally in some circumstances excess ammonia may slip from the SCR catalyst, and oxidation catalysts have been developed to control the amount of escaping ammonia by converting it to nitrogen. They need to have high selectivity towards the production of nitrogen, and they can have SCR activity should any NOx be present. The situations examined were the traditional arrangement of a separate special oxidation catalyst after a SCR catalyst, a layer of oxidation catalyst above which was a layer of SCR catalyst as well as a physical mixture of the two catalysts. An iron zeolite catalyst was used and when this was present as an upper layer on the oxidation catalyst there was enhanced selectivity to nitrogen and a small amount of additional NOx reduction.

The second chapter in this part is about combining NOx-trapping catalysts with downstream SCR catalysts on diesel cars and is by Fabien Can, Xavier Courtois and Daniel Duprez (University of Pointiers, France). When a NOx-trap is regenerated by periodic enrichment of the exhaust gas ammonia can be formed, and the reactions involved in this process are detailed before giving the fascinating history of the use of this ammonia with a SCR catalyst. The ability of the SCR catalyst to store significant amounts of ammonia enables it to reduce NOx that is not retained in the NOx-trap during normal lean operation. Although optimised systems have been used on series production cars it seems likely further advances will be made in the future in this important area because it has the practical advantage of not requiring to store and inject urea solution into the exhaust system.

The final chapter in this part is by Thorsten Boger (Corning, Germany) about the integration of SCR catalysts into DPFs. The DPF materials in series production are cordierite, various forms of aluminium titanate, and silicon carbide. To reduce component count, cost and possibly improve performance there has been a move to incorporate catalytic functionality into particulate filters especially those in light duty diesel vehicles. This was first done with oxidation catalyst that removes CO and HCs during normal driving and periodically provides high temperature to initiate filter regeneration. This is done by oxidising partial combustion products from late injection of fuel into the engine. Recently SCR catalyst has been incorporated into filters, and a large amount of catalyst is required so that exceptionally high porosity filters are needed. Having sufficient strength and filtration efficiency with the necessary high porosity material

has been a major challenge that has been overcome and such filters are now in series production on some European diesel cars.

2.8 Part 8. Case Histories

There are two chapters in the last part of the book about practical applications of urea SCR systems to series of production vehicles. The first, entitled ‘Development of the 2010 Ford Diesel Truck Catalyst System’ by Christine Lambert and Giovanni Cavataio (Ford, USA), is a well written contribution with well sized clear illustrations. It provides a nice overview of the SCR work done in Ford since the early 1990s. By 1995 they had demonstrated a SCR NOx-control system on a light duty diesel vehicle, and development continued culminating in the USA with the introduction of the 2010 truck system. The evolution of copper zeolite catalysts is detailed and practical aspects such as the importance of durability of the upstream oxidation catalyst to maintain high NOx conversion through the then necessary appropriate NO2 to NO ratio. Also covered is the influence of packaging constraints, backpressure problems, and the temperature requirements for the NOx conversions required. It was clear a rapid heating cold start strategy was needed to enhance the exhaust gas temperature so the emissions control system would work efficiently at a sufficiently early stage. The 2011 model year system comprised two oxidation catalysts, urea solution injection, two SCR catalysts, and a silicon carbide particulate filter. The optimisation work included substituting a proportion of the platinum for palladium in the oxidation catalyst as a cost save, although this resulted in poor (if any with an aged catalyst) NO oxidation to NO2. This was acceptable because a NO2 insensitive copper/zeolite catalyst had been selected. Platinum/palladium formulations also had the advantages of reducing potential volatilisation of traces of platinum via its oxide that could influence SCR catalyst selectivity, reduced low level emissions of N2O and not oxidising traces of SO2 to the more potent catalyst poison SO3. Both the oxidation catalyst and the SCR catalyst had to have high thermal stability because they experience high temperatures during active filter regeneration. The palladium-containing oxidation catalyst had durability, but early copper/zeolite SCR catalysts and even those based on beta zeolite did not have sufficient thermal stability. The availability of SCR catalysts based on small pore zeolites in 2007 provided the required higher thermal stability. The ammonia storage capacity of SCR catalyst with a



suitable urea solution dosing strategy can significantly enhance low temperature NOx conversion, although this has to be balanced with the possibility of ammonia slip during high temperature filter regeneration. On the vehicle this requirement was obtained by the control system. Exotherm problems on the SCR catalyst during filter regeneration caused by HC adsorption and carbon formation were all but eliminated with the small pore zeolite SCR catalyst. This chapter illustrates the huge amount of fundamental and development work that goes into the introduction of a successful vehicle emissions control system incorporating urea-based SCR that, of course, continues to be improved upon.

The final chapter in this part, and the last in the book, is by Michel Weibel, Volker Schmeiβer and Frank Hofmann (Daimler, Germany) and is a short contribution about computer models for simulation and development of exhaust gas systems incorporating urea-based SCR NOx control. Factors such as maintaining the level of ammonia stored in the catalyst are particularly important with copper-based catalysts that operate best with a significant amount of stored ammonia. The urea solution dosing strategy has to satisfy this requirement under most engine operating conditions without there being excess ammonia that would be wasteful and potentially be an emission problem. Independently determined kinetics for each of the catalytic reactions and catalysts involved are parameterised for ease of use in computer modules, and in some instances compiled in data maps. The resulting simulation models are important during the development and optimisation of the individual components and in identifying practical ways of obtaining optimum overall operating performance. This is complex and made more so by a need to take into account the engine operation that determines engine-out NOx emissions.

3. Conclusions

Ammonia SCR has become the technology of choice for control of NOx emissions from all but the smallest diesel vehicles, and its importance is reflected in the

huge number of publications cited in this multi-author book. A wide variety of materials are catalytically active in ammonia SCR reactions, and several high performance catalysts have become established commercially. These have the attributes of high activity, the necessary good selectivity with minimal undesirable formation of N2O as well as very good longevity associated with high thermal durability. The book provides an important up-to-date survey of the state of SCR science and technology that over recent years has undergone tremendous advances. Exceptionally high conversions of NOx to nitrogen with amazingly high selectivity are now possible at temperatures so low they were thought impossible a decade ago. These improvements resulted from development work targeting low-temperature NOx control of emissions from diesel engine powered cars. Development work continues in this area and further exciting developments are likely in the not too distant future that could take the form of substituting urea as a source of ammonia for some other reductant derived from on board sources such as water or diesel fuel.

The lack of consistent illustration style, equation numbering that could have been unified during copyediting are easily criticised, as could the all too brief index that does not for instance have important terms such as chabazite and SAPO. However, these failings do not detract from this book being a mine of information that will be of value to researchers working in the SCR area as well as a reference for students in chemistry, catalysis and chemical engineering. The editors are to be congratulated for bringing together so many eminent contributors and completing such a major endeavour. This book should therefore be made available in academic and industrial research libraries alike.

Reference

1 W. Nernst, Z. Anorg. Chem., 1906, 49, (1), 213



The Reviewer

Following Fellowships at the Universities of Toronto and Cambridge Martyn Twigg joined a polymer group at ICI’s Corporate Laboratory in the North West of England, and after involvement in projects at Agricultural Division at Billingham moved there in 1977. Martyn worked on catalysts and catalytic processes including synthesis gas production via naphtha and natural gas steam reforming, methanol and ammonia synthesis, and proprietary catalysts and processes for herbicide manufacture and environmental protection applications. He studied catalyst activation and built a much used off-site catalyst reduction unit. After managing an international polymerisation project he was head-hunted to work at Johnson Matthey as Technology Director in the autocatalyst area that he successfully led until being appointed Chief Scientist. This provided an opportunity for research diversity that included carbon nanotube manufacture and catalysts for medical applications. He was associated with four Queen’s Awards, and was awarded the Royal Society of Chemistry Applied Catalysis Prize. He has more than 200 papers, co-authored books on transition metal mediated organic syntheses and catalytic carbonylation. He produced the “Catalyst Handbook”, co-edits the Fundamental and Applied Catalysis series with Michael Spencer, and has 150 published patent families on catalysts and catalytic processes. Martyn has on-going collaborations with universities and holds honorary academic positions, and runs an active consultancy and catalyst development business.

It has come to our attention that there was a mistake in the published article: M. Ahmadinejad, J. E. Etheridge, T. C. Watling, Å. Johansson and G. John, Johnson Matthey Technol. Rev., 2015, 59, (2), 152

(Equation (xix)):

kk

x=

+Max A

1 A (xix)

The equation should read:

(xix)

Erratum

Computer Simulation of Automotive Emission Control Systems

k = kMax A x

1 + A x




Sintering and Additive Manufacturing: The New Paradigm for the Jewellery ManufacturerEuropean jewellery industry poised to develop potential of direct metal laser melting in precious metals

By Frank Cooper Jewellery Industry Innovation Centre, School of Jewellery, Birmingham City University, Birmingham, UK


The use of various sintering technologies, allied to suitable powder metallurgy, has long been the subject of discussion within the global jewellery manufacturing community. This exciting, once theoretical and experimental technology is now undoubtedly a practical application suitable for the jewellery industry. All parts of the jewellery industry supply and value chains, and especially design and manufacturing, now need to become aware very quickly of just how unsettling and disruptive this technology introduction has the potential to become. This paper will offer various viewpoints that consider not only the technology and its application to jewellery manufacture but will also consider the new design potentials of the technology to the jewellery industry. It will also briefl y consider how that design potential is being taught to future generations of jewellery designers at the Birmingham School of Jewellery. We shall also discuss in some detail the economics of and potential for new and different business models that this technological paradigm might offer the jewellery industry.

Rationale

This paper intends to explore and open up for debate by the jewellery industry what actions and understanding might be required in order to facilitate the transfer and acceptance of precious metal direct metal laser melting (DMLM) technologies and processes into a manufacturing process specifi cally tailored for the jewellery manufacturing industries and their related value and supply chains. The goal of the Jewellery Industry Innovation Centre (JIIC) and its parent institution, the Birmingham School of Jewellery, UK, is to encourage its students to develop, design and produce computer aided design (CAD) examples of jewellery products to challenge, prove, and democratise the processes and materials required for the application of precious metal DMLM technology into the production facilities of small and medium-sized enterprises (SMEs) within the jewellery manufacturing sector. The paper also assesses and attempts to quantify the current perceived industry needs for an adaptable, low-volume and innovative new technology that will facilitate rapid responses by SMEs to the consumer’s demands for more custom-made, individually designed and personalised jewellery products. Typical jewellery manufacturing processes like lost-wax investment casting or stamping do not have either the necessary quick response times or, more importantly, the design and production fl exibility required to address these



issues. This paper is intended to help increase industry awareness, knowledge, and especially it is hoped to speed up jewellery industry uptake of the new design and production capabilities offered by the DMLM processes for working with precious metals.

The Economic Argument

This section will focus primarily on the European jewellery market sector and its fi nancial models. The European Union (EU) has traditionally been an important supplier of high-quality jewellery to the world’s markets, and is also considered to be the second or third largest market for jewellery consumption, after the USA (China and India vie for the other places depending on the statistical analysis method used). Sales of jewellery in most EU countries are thought to have risen steadily in the decade from 2005 to the present; however, this volume market is predominantly supplied with jewellery items manufactured outside the EU. Socio-economic factors mean that it is very diffi cult for the European jewellery manufacturing industry to be competitive on price alone. Recent global, and particularly EU, economic and fi nancial crises have further impacted the EU jewellery manufacturing industry and recent massive price rises in all precious metals have somewhat weakened jewellery sales. Consumers seem to have reduced their expenditure on jewellery and sought personalised pieces with greater associated personal value (1).

High-quality jewellery manufacturing has long been an important sector within the EU economy. Detailed information on employment statistics for the EU jewellery industry is diffi cult to source and defi ne accurately but there are thought to be some 30,000 smaller companies, with less than 250 employees each, and around 200 larger companies with an estimated total of 180,000 employees. These are companies that specialise either in a style of jewellery design or in a stage of production (2). The World Gold Council has estimated that in 2014 the global demand for gold jewellery was US $100 billion (3).

Within the EU and other developed economies, consumers have been educated by the fast-moving, digital, online revolution to expect a continuous and regularly updated choice of new and innovative products. This has impacted consumer buying patterns, resulting in a surfeit of choice and an ever increasing competition for their disposable incomes. It could safely be predicted that the future high-value jewellery market

could well be increasingly driven by a growing demand for custom-made, personalised, individually designed and innovative designs of high-quality and high-value jewellery as high precious metal prices have resulted in many consumers now considering the design and innovation within a jewellery item as equal to if not more important than its base intrinsic value (4).

Increased affl uence in the newly emerging economies, coupled with a rising number of marriages, working women, increased shopping opportunities and an online interest in fashion, are thought to be the main driving forces behind the latest growth in precious metal jewellery sales in these areas. However, consumers are also more careful with their spending. There is a growing fatigue towards ‘fast turnover fashions’ and many consumers now have increasing opportunity to favour good and uniquely personalised ‘statement’ designs and regard these as more important than the intrinsic value of a jewellery piece.

Global economic uncertainty makes it diffi cult to predict future jewellery industry trends, but the market is predicted in some quarters to begin to expand. In the near future the EU jewellery market is expected to grow, especially in the Eastern EU and accession countries due in part to their newly emerging middle classes. It is the contention of the present article that the EU market in particular will increasingly demand higher quality products, coupled with original designs and statement jewellery with added perceived value, personalisation, or new production technologies (5–8).

A key question is therefore: “Is there a viable economic argument for considering adoption of DMLM technology?”. The present author believes that the jewellery manufacturing sector in the EU has the potential to grow signifi cantly further if new high-technology approaches such as DMLM are adopted and exploited effectively. While all materials used in the production of precious metal jewellery are intrinsically expensive, DMLM offers a technology shift that is able to potentially reduce material usage while offering new market opportunities.

Technology and Design

DMLM was developed during the 1990s in Germany (9). Beginning with CAD data, several layers of metallic powder are successively deposited one on top of the other. Each layer of powder is heated using a focused laser beam corresponding to a selected cross-section of the part to be produced. The powder bed is then dropped



incrementally and another layer of powder is applied and smoothed by a blade prior to application of the next pass of the laser beam, simultaneously fusing each new layer of powder to the layer below it. The method differs from the related technique of direct metal laser sintering (DMLS) in that the layer of metallic powder is fully molten throughout. The method does not require any binders or fl uxing agents. Each run of the laser beam partly overlaps the preceding run, and a protective gas atmosphere is maintained above the interaction zone of the laser beam and the metallic powder. Once fi nished the powder bed is removed from the machine and excess powder is then removed and can be fully recycled, although some sieving may be required. Figure 1 shows a schematic of the process (10).

A key opportunity is presented to the EU jewellery manufacturing sector through the harnessing of the emergent and rapidly maturing DMLM technologies and processes, which will facilitate the manufacture of uniquely designed, high-value-added, often custom-made, personalised, jewellery products that will be inherently resistant to being copied (11).

The initial concept and design phases defi ne the innovative nature of a jewellery product during the early stages of its development. The process of design enables the defi nition and development of concepts and ideas and individual personalisation or customisation of an item, facilitating commercially viable new product development. The transfer of these

concepts and ideas into jewellery products is achieved through a variety of technical processes including CAD (12), prototyping and light engineering based processes and technical feasibility is traditionally a vital consideration at each stage of the design process. As previously discussed, consumer demand for increasingly novel products has resulted in the need for extreme fl exibility in the design and production of jewellery, the ability to respond rapidly to ever changing demands, and the implementation of a streamlined product development process by manufacturers of personalised and custom-made products, not just jewellery. The UK jewellery industry has a signifi cant global reputation for producing well-designed, well-made, high quality jewellery products, manufactured in controlled and regulated environments that meet the high expectations of their end consumers. A radically new manufacturing approach could be considered as being useful to help re-energise the precious metal jewellery manufacturing base and to help facilitate new opportunities to boost production, increase profi tability and regain market share. DMLM, which is nowadays routinely considered to be part of the rapid or additive manufacturing (AM) stable of technologies, has now become an accepted production solution within a range of industrial manufacturing sectors including aerospace, automotive, dental and medical, where AM is used to manufacture parts in a range of ceramic, polymeric and base metallic materials.

Fig. 1. Schematic of the DMLM process (10) (Reproduced with the permission of the Verein Deutscher Ingenieure eV, Germany)

Laser

Tilted mirror with focus

Object

Powder

Platform and retractable table

Build spacePowder delivery system

Blade for spreading powder

Burning point (partially sintered granules)



Early research undertaken in Sweden in 2005 (13) and much more recently in the UK (14) and Europe has demonstrated that DMLM technologies could be extended to the manufacture of products in precious metals, including 18 carat golds in various colours, silver alloys and even platinum group metals. This discovery led to some early global interest in the use of DMLM for precious metal parts manufacture. However, the principal experience of precious metal DMLM to date has been largely limited to the cosmetic dental industry, which has adopted some digital production solutions in precious metals (15). Additionally, this use of the technology in dentistry was restricted to a small number of specialist gold alloys used for restorative dental crowns in non-jewellery specifi c alloys. Many potential alternative uses for precious metal DMLM have also been identifi ed including electronics, fuel cell, medical, catalytic and satellite applications and in the manufacture of low-volume, high-value components in the prestige automotive, biomedical and marine sectors (16). There has also been much discussion concerning the potential for precious metal DMLM and its intrinsic design benefi ts within the jewellery and high-value goods sectors, but the research, capital investment, and metallurgical knowledge base required to set up a precious metals DMLM sector have until now been considered largely prohibitive (17).

Currently there are a small number of different DMLM technologies at various stages of development and use in and around the European jewellery sector. In 2011 the JIIC introduced and continues to deliver a teaching module specifi cally about DMLM technologies and their adaptation for jewellery design and manufacture to the cohort of the Design for Industry (DFI) students

at the Birmingham School of Jewellery. Figure 2 shows just a few examples of their work. Each of them explores and takes advantage various aspects of the geometric design freedoms that the DMLM process offers. These items were produced for the students in a number of different metals by a UK based supplier of a DMLM technology and were built on a Concept Laser Mlab LaserCUSING® by ES Technology Ltd.

The JIIC is also currently actively involved with a UK government funded DMLM of precious metals research scheme called the Precious project, whose mission statement reads:

“To demonstrate the viability of precious metal additive manufacturing within the UK Jewellery industry from design and manufacturing through to fi nishing, polishing and retail” (18).

This is a two-year project aimed at elevating the current state of the art of DMLM AM within the UK precious metal jewellery industry.

After a piece of jewellery has been designed and before it can be manufactured using DMLM a small number of core activities need to take place: • Pre-Processing (Preparing pieces for

manufacture). This essentially refers to all front end software-related activities including the design process

• Processing (Manufacturing jewellery items using DMLM). This refers to the actual manufacturing process using DMLM technologies

• Post-Processing (Manual and automated fi nishing and polishing processes). This refers to the post-DMLM manufacture processing stages up to the point where an article is ready for sale.

Fig. 2. Examples of students’ work from the Birmingham School of Jewellery DFI DMLM module. From left to right: Natalia Antunovity, Suyang Li, Tesni Odonnell, Tomas Binkevic



Each of these steps is interdependent on the others and they must take place in a logical sequence. When combined together effectively this can result in the production of novel and unique quality jewellery items.

Understanding these various activity interrelationships has a profound effect on the eventual quality of any DMLM printed jewellery. If a jewellery designer understands, even on a fairly superfi cial level, what is involved in each of these core activities then they will be able to better design jewellery that not only takes advantage of the geometric freedoms that DMLM offers but also can be suitably post-processed to an acceptable quality of fi nish.

Case Study: The Ojo Project

To illustrate this a pendant piece from the Precious project, called ‘the Ojo’, will be used to show the various stages of designing and manufacturing jewellery for the DMLM process. The Ojo is intended to be an iterative design series of 100 pendants where each pendant produced is signifi cantly different from the pendant before and the pendant after. This is achieved by the use of a CAD design algorithm that continually morphs the basic pendant design. This pendant was conceived and designed by Lionel T. Dean of Future Factories who is a member of the Precious consortium of companies.

The CAD fi le is created, saved as a stereolithography (STL) fi le and is shown in Figure 3. The next step is to use a suitable software to generate and place the support structures (Figure 4) that are a necessary part of the DMLM process which requires the jewellery design to be ‘sliced’ in the software. The parts are built up of multiples of these slices printed by the DMLM

technology one on top of the other. Parts being built using DMLM require a support structure (19), this is a scaffold-like construction, and supports all overhanging parts enabling undercuts, voids and holes to be produced. A jewellery designer will not need to be able to create these supports, as they are added by the DMLM machine setter and the machine’s software package, but they will need an appreciation of the use and application of supports as they leave a witness mark or scarring when they have been removed, which will require extra cleaning up and fi nishing (similar to that required when removing a casting sprue). In theory it is possible to build any shape, however if supported areas are visible but inaccessible, then the result will be perceivably poor surface quality. A small change to a design can eliminate the need for a lot of support structures.

Support structures are required in most, if not all, laser-powered, metal-based DMLM processes, and there are a number of divergent reasons for their presence. To build complex geometries with overhanging and undercut surfaces, support structures are required to assist in controlling any potential defects in or on the part being built. These defects may be caused by the typical thermal stresses of the DMLM process, occasionally by overheating, or most commonly by being dragged over and disturbed by the re-coater blade applying the next layer of powder. Supports are most principally required because the powder bed surrounding the very small melt pool created by the laser is not suffi cient to support the liquid metal phase in place. Other functions of supports are the bonding of the part to the build plate and providing a thermally conductive connection between the part and the build

Fig. 3. CAD images of the Ojo

Fig. 4. An example of the CAD created support structures on an Ojo



plate to rapidly and effectively dissipate heat from the melt zone.

The next step is to fi ll the machine with powder, load up the STL fi le and set it into motion. A re-coater blade, or brush, pushes fi ne, powdered, gold build material from a carefully measured powder supply hopper to create a uniform layer over the build platform. The laser scanning system literally draws the two-dimensional (2D) cross-section of the CAD fi le slice on the surface of the build material, melting it into a solid deposit layer or slice (Figure 5). After the fi rst layer is produced, the piston beneath the build platform is lowered fractionally and another powder layer is pushed into place using the blade. The laser beam melts the second layer and at the same time fuses or bonds it to the layer below. This process is repeated layer by layer until the part is completed. It is this layer adding process that leads to this technology being described as ‘additive manufacturing’ in many quarters.

Once the build process has been completed the build plate piston is slowly raised, the surplus unused powder is carefully swept away and the ‘additively manufactured’ jewellery item is exposed (Figure 6).

The part is then removed from the build platform and the support structures are also removed (Figure 7). The witness marks or scarring left by the supports also have to be removed in much the same way as a casting sprue has to be removed and cleaned up. Because the support structures are much smaller than a typical sprue, a small, fi ne, burr on a pendant or Dremmel drill will often suffi ce.

Once the supports have been removed the parts are then ready for fi nishing and polishing (Figure 8). Mechanical, or mass fi nishing, techniques are often found to be the best for this stage because the DMLM of jewellery will probably prove to be most commercially effective when used to produce geometrically complex designs. These designs by their very nature will

Fig. 5. A laser beam scanning over the surface of the powder and melting the gold

Fig. 6. The Ojo emerges from the powder bed

Fig. 7. Support removal from the Ojo

Fig. 8. Three variants of the Ojo before fi nishing



present many unique and product specifi c challenges when ready to be polished and fi nished. Mass fi nishing technologies are based on the correct application of media fl ow pressure and speed to the jewellery item to be polished. Generally, the higher the pressure exerted by the media on the jewellery, and the faster the media fl ows across the jewellery parts, the faster the desired fi nishing results can be achieved. But this fl ow has to be either uniform or directed, depending where the polishing is required.

Centrifugal disc fi nishing is an industrial mass fi nishing process adapted for the surface treatment of jewellery. The process is carried out in a cylindrical container which is open at the top, while the bottom consists of a turntable-like disc separated from the container wall by a microscopically small gap. During operation, the work pieces and the grinding or polishing media in which they are immersed rotate at a high speed, creating a toroidal abrasive fl ow; the relative difference in speed of the components and media produces the polishing effect. The contact between the jewellery pieces and the medium generates a very intense fi nishing effect which is up to 20 times more effi cient than can be achieved with conventional systems like vibratory fi nishers.

A process refi ned by Precious consortium partner Finishing Techniques Ltd is the ‘stream fi nishing’ process (sometimes referred to as immersion polishing). This is a fairly new concept to jewellery polishing and features short processing times because the medium is compressed against the wall of a large spinning bowl (centrifugal disc fi nishing) and the parts are held and rotated in this fl ow by use of a rotating

fi xture similar to an electric drill chuck on an extended shaft (Figure 9). Because the rotating head is fi xed but with an adjustable angle of attack when immersed into the bowl, it can be easily automated and has shown excellent reliability and repeatability. The use of small, light media can produce an excellent fi nish, the fi nishing energy coming from the relative speed of both the jewellery part and the medium. A fi nal polish by hand completes the process (Figure 10).

Comparison with Casting

The most widely used technology in jewellery manufacturing worldwide continues to be the lost-wax investment casting process, accounting for an estimated 80% of all jewellery production. Current investment casting processes have been highly developed over time principally to facilitate traditional historical demands for high-volume, batch- and mass-produced jewellery products. In contrast, there is an emerging need for a low-volume, rapid response to consumer demands for custom-made, individually designed products, in simple terms producing in volumes of one. DMLM could meet this need. Additionally, yet more pragmatically, it also offers the potential for creating items that appear solid yet, if sectioned through, would prove to be entirely hollow or contain a simple honeycomb or scaffolding structure added for strength, a process sometimes described as ‘volume without mass’. Such forms and designs are not currently achievable using the traditional jewellery manufacturing processes. Caution must be applied, of course, in selecting appropriate

Fig. 9. Stream fi nishing of an 18 carat gold Ojo: (a) the part held in a rotating fi xture; (b) immersed in the polishing medium; (c) removed from the medium

(a) (b) (c)



items for this type of manufacture. For instance making a typical wedding band hollow would result in an item that feels valueless and cheap; whereas a bulky, heavy, watch bezel could be produced using a weight reducing hollow profi le and the resultant watch would still feel right, with weight being added by the watch movement and wrist band (Figure 11). The potential

savings in the intrinsic cost of expensive raw materials is undoubtedly now approaching the point at which this novel manufacturing process can become more commercially viable and attractive to the jewellery manufacturer and the consumer.

Furthermore, it is entirely possible and correct to consider at this point that the widespread business model used in jewellery manufacturing throughout the UK, especially in relation to lost-wax investment casting – namely the use of sub-contract bureau service providers – is equally applicable to smaller jewellery designers and manufacturers accessing the DMLM technological advance.

In conventional jewellery manufacturing there is a measurable correlation between part complexity and its manufactured cost. Using DMLM means that not only is complexity independent of tooling costs, but also that virtually any geometry conceived by the designer is theoretically possible to produce. Conventional design methods are based on the ‘design for manufacture’ principle, in which manufacturing constraints are included at the earliest stages of the design process. This often results in modular designs with standardised components, meaning designers inevitably modify their design intent to enable the item to be manufactured using a specifi c manufacturing process. Using DMLM would allow the removal of many of these constraints, although (as in any other manufacturing process) DMLM has its own limitations. It will be necessary to develop specifi c ‘design rules’ expertise to manage and optimise these new and exciting possibilities. Current research at the JIIC in conjunction with the School of

Fig. 10. 18 carat gold Ojo: (a) after 1 hour of stream polishing; (b) after approximately 3 hours of stream polishing; (c) after fi nal hand/mop polishing; (d) the fi nished Ojo (Ojo is Spanish for eye!)

(a)

(b) (d)

(c)

Fig. 11. Volume without mass (right and wrong): (a) watch bezel; (b) CAD image of a hollow wedding band

(a)

(b)



Jewellery and its students is based around discovering these rules, which will minimise the limitations imposed by machine modifi cations and CAD design adjustments, to attain the objective of being able to produce the widest possible range of geometries.

The Jewellery Industry Innovation Centre has recently purchased a Cooksongold Precious M 080 direct metal ‘3D printing’ DMLM machine for teaching and research purposes, and Figure 12 shows an example of one of the Centre’s early explorations of the machine’s capabilities. The ship is a little over 1.5 centimetres high.

The Future

Manufacturers are generally limited in their methods of fabrication by the cost of tooling, which must be amortised over the number of parts produced during the life cycle of a tooling product. In existing conventional jewellery manufacturing there is a direct link between the complexity of the part and its manufactured cost; this can be signifi cantly reduced with use of the DMLM processes. In gaining knowledge and understanding of the potential design and manufacturing advantages of DMLM, jewellery manufacturers should be provided with the economic impetus to consider adopting DMLM processes, as appropriate to their company’s needs. Many high-value products are made in small volumes or require individual, personalised adaptations for each customer or application. The ability to provide such

modifi cations, as well as the availability of tool-less fabrication, will infl uence what is designed, how it is designed, and the quantity of products offered.

Innovative design could be considered vital for the survival of the high-value-added industries, including jewellery manufacturing, though it should be remembered that the manufacture of well-designed unique products remains an intensive, expensive, and consumer-centred process. The commercial pressures to reduce costs to remain competitive, while retaining design quality, challenge jewellery manufacturers to fi nd ever more innovative manufacturing techniques as well as consider alternative routes to their markets and consumers. The jewellery industry and other design-led creative industries are ideally suited for developing the new interfaces between the customer and designers, and they will also need to consider new production technology approaches that maintain and exploit this competitive edge. Jewellery is therefore in a unique position to capitalise and further develop the potential of DMLM, while using generic fabrication criteria which are relevant to many other high-value-added industries where custom-made products command correspondingly higher consumer prices.

References 1. M. Krijger, ‘CBI Trade Statistics for Jewellery’, Global

Intelligence Alliance/Centre for the Promotion of Imports from developing countries (CBI), Ministry of Foreign Affairs, The Hague, The Netherlands, 2014

Fig. 12. (a) An 18 carat gold galleon ‘printed’ on (b) a Cooksongold Precious M 080 machine

(a) (b)



2. ‘Growth, Dedicated Call 10/00’, Topic IV 31, The European Virtual Institute For Jewellery Technology, EC Funded Project, Reference G7RT-CT-2001-05065, Milan, Italy, 2002

3. L. Street, K. Gopaul, M. Kumar, C. Lu and A. Hewitt, ‘Gold Demand Trends: First Quarter 2015’, World Gold Council, London, UK, 2015

4. M. Krijger, ‘CBI Trends: Jewellery’, Global Intelligence Alliance/Centre for the Promotion of Imports from Developing Countries (CBI), Ministry of Foreign Affairs, The Hague, The Netherlands, 2015

5. D. G. Penfold, ‘New Product Development in the West Midlands Region of the UK Jewelry Manufacturing Industry – Evaluating the Impact of Design Support’, in “The Santa Fe Symposium on Jewelry Manufacturing Technology 2007”, ed. E. Bell, Met-Chem Research, Albuquerque, New Mexico, USA, 2007, pp. 445–456

6. ‘Jewellery & Watches’, Key Note Market Report, Key Note Ltd, Richmond on Thames, UK, 2014

7. D. G. Penfold, Design J., 2007, 10, (1), 3

8. M. Karydes, ‘Bold gold jewelry is back in style’, Fortune, 9th May, 2015

9. W. Meiners, K. Wissenbach and A. Gasser, Fraunhofer Ges Forschung, Germany, ‘Shaped Body Especially Prototype or Replacement Part Production’, German Patent 19,649,865; 1998

10. ‘VDI-Richtlinie: VDI 3404 Generative Fertigungsverfahren – Rapid-Technologien (Rapid Prototyping) – Grundlagen, Begriffe, Qualitätskenngrößen, Liefervereinbarungen’, (Engl. Transl. ‘VDI Guideline: VDI 3404 Additive fabrication – Rapid Technologies (Rapid Prototyping) – Fundamentals, Terms, Quality Parameters, Supply

Agreements’), December 2009 (Withdrawn December 2014), Verein Deutscher Ingenieure eV, Düsseldorf, Germany, 2009

11. A. M. Carey, ‘The Changing Demands on the Creative Process as a Consequence of New Technologies’, in “The Santa Fe Symposium on Jewelry Manufacturing Technology 2010”, ed. E. Bell, Met-Chem Research, Albuquerque, New Mexico, USA, 2010, pp. 101–118

12. S. Adler and T. Fryé, ‘The Revolution of CAD/CAM in the Casting of Fine Jewelry’, in “The Santa Fe Symposium on Jewelry Manufacturing Technology 2005”, ed. E. Bell Met-Chem Research, Albuquerque, New Mexico, USA, 2005, pp. 1–24

13. N. Towe, ‘Laser Sintering Process for Making Hollow Jewellery’, Jewellery Technology Forum Proceedings, June, 2006, Vicenza, Italy, Legor Group, Italy, 2006

14. M. Khan and P. Dickens, Gold Bull., 2010, 43, (2), 114

15. A. L. Hancox and J. A. McDaniel, Int. J. Powder Metall., 2009, 45, (5), 43

16. L. S. Bertol, W. K. Júnior, F. P. da Silva and C. Aumund-Kopp, Mater. Design, 2010, 31, (8), 3982

17. A. Simchi, Mater. Sci. Eng.: A, 2006, 428, (1–2), 148

18. Precious, Innovate UK, The Technology Strategy Board, Swindon, UK: http://www.precious-project.co.uk/ (Accessed on 29th June 2015)

19. F. Cooper, ‘DMLM Supports: are they the Jewellery Industry’s New Sprue, Riser and Gate Feed?’, in “The Santa Fe Symposium on Jewelry Manufacturing Technology 2014”, ed. E. Bell, Met-Chem Research, Albuquerque, New Mexico, USA, 2014, pp. 89–109

The Author

Frank Cooper is a lifelong jewellery industry professional and is now a Senior Lecturer in Jewellery Manufacturing Technologies, and Technical Manager of the Jewellery Industry Innovation Centre, at the Birmingham School of Jewellery, UK. He sits on the Goldsmiths’ Craft and Design Council and is a globally recognised expert in the application of various additive manufacturing, prototyping, CAD and ‘3D printing’ technologies used in the jewellery industry. He is an active participant in a number of jewellery industry-related research initiatives and has published and presented many technical papers and articles in the UK and Europe, as well as at the Santa Fe Symposium in the USA.




Introduction to the Additive Manufacturing Powder Metallurgy Supply ChainExploring the production and supply of metal powders for AM processes

By Jason Dawes*, Robert Bowerman and Ross TrepletonComponent Technologies, Manufacturing Technology Centre, Pilot Way, Ansty Business Park, Coventry, CV7 9JU, UK


The supply chain for metal powders used in additive manufacturing (AM) is currently experiencing exponential growth and with this growth come new powder suppliers, new powder manufacturing methods and increased competition. The high number of potential supply chain options provides AM service providers with a significant challenge when making decisions on powder procurement. This paper provides an overview of the metal powder supply chain for the AM market and aims to give AM service providers the information necessary to make informed decisions when procuring metal powders. The procurement options are categorised into three main groups, namely: procuring powders from AM equipment suppliers, procuring powders from third party suppliers and procuring powders directly from powder atomisers. Each of the procurement options has its own unique advantages and disadvantages. The relative importance of these will depend on what the AM equipment is being used for, for example research, rapid prototyping or productionisation. The future of the metal AM powder market is also discussed.

1. Introduction

A component fabricated using powder bed may consist of many thousands of finely spread powder layers. The uniformity of these layers can affect the properties of the final component. The way in which a powder ‘spreads’ during AM depends on the properties of the powder used. As will be discussed in this paper, even when chemically equivalent, the properties of metal powders vary widely depending on both the atomisation method used and the manufacturing process conditions. To obtain a greater degree of control over AM processes service providers must be able to control the quality of the raw powder feedstock.

The overall AM market has seen exponential growth over the last five years and during this time the sale of powder bed metal AM equipment, services and products has also followed an exponential trend (1) due to increased adoption from the aerospace, oil and gas, marine, automobile and medical sectors. As the benefits of using AM to manufacture functional metallic components start to outweigh the blockers, more component manufacturers are looking towards metal powder bed technologies to allow them to realise their next generation of innovatively and functionally designed products.

Research has shown that metal powder costs will be the biggest continuous expense through the life of an AM machine (1). The quality and consistency of the AM components depends, in part, on the characteristics of the starting powder feedstock. Hence, controlling and understanding the quality of the powder both in



its as-supplied and reused condition is essential in order to achieve the desired mechanical properties of the laser melted components. Given the significance of the metal powder feedstock it is important that AM users make informed decisions when procuring the raw metal powder. The current state of the AM metal powder supply chain is that there are multiple possible methods for the manufacture of metal powders and many times as many potential suppliers. Furthermore not all metal powders are equal in terms of their fundamental properties even when manufactured via the same technique (when procured from different vendors). This presents quite a challenge to beginners in AM technology when deciding on a powder supplier. However, some AM equipment suppliers, such as EOS, sell ‘validated powder’. ‘Validated powder’ is a powder which has been identified as suitable for use in AM. Whilst validated powder can de-risk procuring powders for AM it does limit users to a single source supplier and inhibits the development of in-house expertise.

Given the complexity of the AM metal powder supply chain this review article aims to resolve some of the confusion involved and address some of the frequently asked questions by users. Additionally, the article will highlight key issues that the market needs to address, and make potential users aware of some of the key factors to consider when selecting the most appropriate powder supplier.

2. Overview of the Powder Bed AM Market

Over the last 20 year period the AM market has grown rapidly from an industry worth <US$100 million in 1993 to around US$3000 million in 2013, see Figure 1.

Current predictions forecast that this rapid growth will continue and that there will be a five-fold market value increase by 2021 (1). This growth trend in AM technology is also reflected in global raw materials (powder) sales. Powder sales in the AM sector over the past decade are shown in Figure 2. After a decline in sales in 2009, due to market reaction at the beginning of the fiscal crisis, both the AM sector value and AM material sales have seen rapid growth since 2010. Specific to metal powder AM, the sale of powder for metal processes is also shown in Figure 2. It can be seen that metal sales have followed the market trend since 2010, with sales more than doubling in a three year period. However, the value of the metal powder AM market is a relatively small proportion of the whole market. This highlights the opportunity for growth for powder suppliers offering products into the metals AM market.

Based on data from 2013 there are 855 powder manufacturers worldwide (425 located in North America, 205 in Europe and 225 in the Asia-Pacific region) capable of producing an estimated 1.12 million metric tonnes, to a value of approximately US$6.9 billion (2). The top six powder manufacturers have a combined market share of 44% and generally serve the press and sinter market. The remaining market share is made up of small businesses, likely producing powder for a specific purpose or application. When the metal powder market is evaluated, only US$32.6 million was sold for AM usage (0.0047%). This shows that despite the enormous anticipation of the impact of AM, traditional powder processes such

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Fig. 1. AM market growth 1991–2013 (1)

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Fig. 2. History of materials sales for AM systems worldwide: (–) all AM materials, (–) metallic AM materials only (metallic powder sale data not available prior to 2009) (1)



as press and sinter and metal injection moulding (MIM) still dominate the marketplace. However, as AM processes become more established as component manufacturing routes, rather than rapid prototyping technologies, the potential for growth in the metal AM powder supply is considerable.

3. Selection of AM Powder

3.1 The Importance of Powder

The AM process uses powder as its raw material feedstock, as such the consistency of the powders used to build AM components will have a critical influence on the final component properties. During the build sequence of an AM component, the raw powder feedstock is stored in a hopper, the design of the hopper and the method by which powder is introduced into the build chamber depends entirely on the equipment manufacturer. A discrete amount of powder from the hopper is spread (either using a rake or roller system) across the build chamber to form a thin (no more than one to two particle diameters) continuous layer of powder. After spreading it is critical that the layer is homogenous over the entire area of the build chamber, any degree of inhomogeneity may result in porosity (in the absence of powder) or incomplete through-thickness melting (too much powder pooled up in one area). The spread layer is selectively fused using either a laser source or an electron beam based on an input sliced three-dimensional (3D) computer aided design (CAD) model. Following selective sintering another layer of powder is spread over the first. This iterative process of powder spreading followed by selective melting is continued until the build is complete. The total number of powder layers spread will of course depend on the size of component being built but the number could be in the region of 7000 layers. Furthermore it is common to build multiple components during one build event.

The layer spreading, hopper dosing and bulk packing performance of the AM powder will depend entirely on the properties of the powder being used. Further complicating the use of AM powder is that the volume of the actual component built can be significantly less than the total volume of powder that has been spread. As a consequence there is a large amount of unused powder left over in the build chamber, given the high cost of metal powders it is essential that the unused powder is effectively recovered and reused in future builds. However, the effect of continued recycling of

the unused powder on the actual powder properties and hence subsequent component properties has not been the subject of intensive scientific study. In the few scientific journals published in the field of metal powder recycling in AM, it has been observed that recycling powders in powder bed AM processes results in an increase in powder particle size distribution (PSD) (2, 3). The thermal effects that result from the process, such as chamber temperature and the radiation energy in selective laser melting (SLM) of metal powders, may cause physical as well as chemical changes to the recycled powder. Furthermore contamination, either through impurities, foreign bodies or interstitial elements may be introduced to the powder as a result of handling during pre-processing or post-processing stages.

The first step in understanding powder requirements for AM processes is to assess the types of metallic powder that are available. The following sections provide an overview of the methods of metal powder production routes.

3.2 Routes of Powder Production

The production of AM metal powder generally consists of three major stages as outlined in the flow diagram shown in Figure 3. Briefly, the first stage involves the mining and extracting of ore to form a pure or alloyed metal product (ingot, billet and wire) appropriate for powder production; the second stage is powder production and the final stage is classification and validation.

The supply chain of taking ore and extracting a metal is well established and supplies a vast range of pure metals and specific alloys to global markets. Once an ingot of the metal or alloy has been formed a number of additional processing steps may be required to make the feedstock suitable for the chosen atomisation process. For example, plasma atomisation requires the feedstock material to be either in wire form or powder form, thus adding additional rolling and drawing work or a first step powder production route.

Once the first processing form has been obtained there are a number of methods available to produce metal powders including, but not limited to: solid-state reduction, electrolysis, various chemical processes, atomisation and milling. Historically, for reasons that will be discussed, atomisation has been identified as the best way to form metal powders for AM due to the geometrical properties of the powder it yields.



None of the powder production routes actually produce a 100% powder yield in the required size fractions. Some post processing is therefore necessary. As a minimum, the as-produced metal powder must be classified into a well-defined particle size distribution suitable for the required process: typically 15–45 µm for SLM and 45–106 µm for electron beam melting (EBM).

3.2.1 Water Atomisation

All atomisation processes begin with melting the feedstock alloy. The melting process has a number of variations, but generally when atomising using water, the feedstock is first melted in a furnace before being transferred to a tundish (a crucible that regulates the flow rate of the melt into the atomiser). The liquid alloy enters the atomisation chamber from above; here it is free to fall through the chamber. Water jets are symmetrically positioned around the stream of liquid metal, atomising and solidifying the particles. The final

powder exits at the bottom of the chamber, where it is collected. Additional processing steps are then required to dry the powder. Metal powder produced in this way is typically highly irregular in morphology which reduces both packing properties and flow properties. Water atomisation is the main method of producing iron and steel powders and typically feeds into the press-and-sintered industries rather than the specialised AM industry.

3.2.2 Gas Atomisation

The gas atomisation process mimics water atomisation, with the differentiator being the use of gas instead of water during processing. Air can be used as the atomising media, but it’s more likely that an inert gas (nitrogen or argon) will be used to reduce the risk of oxidation and contamination of the metal. The process of melting the metal ingots can be the same as described for water atomisation, however for powders produced

Fig. 3. From ore to validated AM powder – powder production steps flow chart

Extraction

Ore

Forming

Hydrogenation and dehydrogenation

Forming (billet, wire)

Plasma, PREP, REP or EIGA atomisation

AtomisationGas or centrifugal atomisation

Water atomisation

Atomisation

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for high end applications such as aerospace, the need to control interstitial elements has led to increased use of vacuum induction melting (VIM) furnaces. A VIM furnace is typically installed directly above the atomisation chamber such that the molten stream of liquid metal enters the atomisation chamber directly from the furnace rather than through a tundish, similar to the set-up shown in Figure 4. The stream of liquid metal is atomised by high pressure jets of gas. Due to the lower heat capacity of the gas (compared to water) the metal droplets have an increased solidification time which results in comparatively more spherical powder particles (i.e. droplet spheroidisation time is shorter than the solidification time). Whilst it is not possible to have complete control over the particle size of as-atomised powder, the distribution can be influenced by varying the ratio of the gas to melt flow rate. Research in the field of gas atomisation has shown that even finer particle size distributions can be achieved through the use of hot gas atomisation (4).

Although interstitial elements can be well controlled in gas atomised powders, there are still potential contamination risks. Contamination most pertinent to non-static critical components, such as aero-engine parts, include refractory materials which can originate from the ceramic crucibles and atomising nozzles used. One solution to this is to use electrode induction melting gas atomisation (EIGA). EIGA is a variation of gas atomisation where the metal is fed into the atomiser in the form of a rod that is melted by an induction coil

just before entering the atomisation chamber, as shown in Figure 5. This application is used when processing reactive alloys, such as Ti-6Al-4V, minimising the risk of contamination from exposure of the molten titanium to the crucible and the atmosphere (5).

3.2.3 Plasma Atomisation

Plasma atomisation is a method of producing highly spherical particles. The feedstock used in the process can either be in wire form such as the method used by AP&C Advanced Powders and Coatings Inc, Canada, or in powder form such as the method used by Tekna Plasma Systems Inc, Canada. The spool of wire or powder feedstock is fed into the atomisation chamber, where it is simultaneously melted and atomised by co-axial plasma torches and gas jets, such as that shown in Figure 6.

Plasma rotating electrode process (PREP) is a variation of plasma atomisation whereby a bar of rotating feedstock is used instead of a wire feed. As the rotating bar enters the atomisation chamber plasma torches melt the end of the bar, ejecting material from its surface. The melt solidifies before hitting the walls of the chamber.

3.2.4 Hydride-Dehydride Process

The hydride-dehydride (HDH) method (6) of powder production differs from the atomisation processes described above due to the fact that it does not involve melting of the metal feedstock. Instead, it involves crushing, milling and screening to resize larger lumps of metal feedstock into finer powder particles. The HDH process relies on the brittle nature of certain metals, such as titanium, when exposed to hydrogen. In the case of titanium, titanium hydrides are formed

Fig. 4. Schematic of the gas atomisation process for production of AM powders (Courtesy of LPW Technology, UK)

Collection chamber

Gas source and pump

Fine powder

Nozzle

Melt

Fig. 5. Schematic of the EIGA process for production of AM powders (Courtesy of LPW Technology, UK)



in a hydride unit by introducing hydrogen and heat. The brittle lumps can then be crushed and screened into the required particle size distribution (PSD). The powder is then returned to the hydride unit to remove the excess hydrogen from the metal powder particles. Powder particles produced using HDH are typically highly irregular. HDH powders are typically used either in their as-made condition or used as the powder feedstock for plasma atomisation.

3.2.5 TiROTM Process

The TiROTM process (7, 8) is a relatively new method for the production of pure titanium powder developed by CSIRO, Australia. The TiROTM process is a two stage continuous production method in which titanium tetrachloride (TiCl4) is first thermally reduced to an MgCl2/Ti composite under the presence of magnesium in a fluidised bed reactor. The MgCl2/Ti composite is then separated using vacuum distillation to produce high purity Ti powder. The as-processed Ti powder is unsuitable for AM processes due to the particle size range primarily being 150–600 µm. As such it is necessary that the powder is modified using a high shear milling process in a controlled environment to resize the powder to a range suitable for AM.

3.2.6 Summary of Powder Production RoutesEach of these processes yields powder with varying characteristics, a summary of which can be seen in Table I. A series of micrographs highlighting the various particle morphologies obtained from each manufacturing route is shown in Figure 7.

3.3 Powder Key Process Variables

The quality of a component built in an SLM process is assessed based on part density, dimensional accuracy, surface finish, build rate and mechanical properties. In order to achieve predictable and consistent component qualities it is desirable that the characteristics of the powder bed and the parameters of the machine are maintained at a constant level since the powder bed and machine parameters are closely correlated. In order to maintain a constant powder bed during each SLM build process it is important to understand and control characteristics such as powder bed temperature and density. These characteristics are governed by the KPVs of the starting powder. Due to the complex nature of powders, characterising their performance is not a trivial task. A list of variables (and analysis techniques) that may be considered to have an impact on performance is provided in Table II.

3.3.1 Particle Morphology

Particle morphology will have a significant impact on the bulk packing and flow properties of a powder batch. Spherical or regular, equiaxed particles are likely to arrange and pack more efficiently than irregular particles (9). Research into the effect of particle morphology on the AM process has shown that morphology can have a significant influence on the powder bed packing density and consequently on the final component density (10–12), where the more irregular the particle morphology the lower the final density. As a consequence of this highly spherical particles tend to be favoured in the AM process. This limits the use of potentially cheaper powder production routes such as water atomisation and HDH. Furthermore as can be observed in Figure 7(b) gas atomised powders are only nominally spherical. In the case of the titanium alloy Ti-6Al-4V, this has led to widespread adoption of plasma atomised powder. Plasma atomised powder is typically highly spherical, but is currently produced by a single source – AP&C, Canada.

Fig. 6. Schematic of the plasma atomisation process for production of AM powders (Courtesy of LPW Technology, UK)

Plasma torches

Titanium spool

Collection chamber

Vacuum pump



Table I Summary of Powder Characteristics by Manufacturing Process

Manufacturing Process Particle size, µm Advantages Disadvantages Common uses

Water atomisation

0–500 High throughputRange of particle sizesOnly requires feedstock in ingot form

Post processing required to remove waterIrregular particle morphologySatellites presentWide PSDLow yield of powder between 20–150 μm

Non-reactive

Gas atomisation (inc. EIGA)

0–500 Wide range of alloys availableSuitable for reactive alloysOnly requires feedstock in ingot formHigh throughputRange of particle sizesUse of EIGA allows for reactive powders to be processedSpherical particles

Satellites presentWide PSDLow yield of powder between 20–150 μm

Ni, Co, Fe, Ti (EIGA), Al

Plasma atomisation

0–200 Extremely spherical particles

Requires feedstock to either be in wire form or powder formHigh cost

Ti (Ti64 most common)

Plasma rotating electrode process

0–100 High purity powdersHighly spherical powder

Low productivityHigh cost

TiExotics

Centrifugal atomisation

0–600 Wide range of particle sizes with very narrow PSD

Difficult to make extremely fine powder unless very high speed can be achieved

Solder pastes, Zinc of alkaline batteries, Ti and steel shot

Hydride–dehydride process

45–500 Low cost option Irregular particle morphologyHigh interstitial content (H, O)

Ti6/4Limited to metals which form a brittle hydride

3.3.2 Particle Size Distribution

Characterisation of PSD in a batch of powder ensures that the optimum range of particles, by size, are used in each process. In general, EBM uses a nominal PSD between 45–106 µm, whilst SLM uses a finer PSD between 15–45 µm. PSD will have an obvious impact on both the minimum layer thickness and the resolution of the finest detail in the component. An inappropriate combination of PSD and layer thickness can potentially lead to in situ segregation due to the mechanical re-coater pushing coarser particles away from the bed (13), segregation in this sense could lead to variation in build quality in the vertical direction. It is generally well reported that using powders with a wide PSD and a high fine

content produce components with a higher fractional density (13, 14). However, the use of fine materials increases the risk of health and safety issues. This is particularly true when processing reactive materials such as titanium where finer particulates are likely to be more flammable and explosive.

3.3.3 Bulk Packing and Flow Properties

Powder flowability is one of the most important technological requirements for powders used in AM. The density homogeneity of the final part depends on the layer-by-layer melting being performed on thin and uniform layers that are accurately deposited by the feeding device. Cohesive powders which exhibit poor



Fig. 7. Example SEM micrographs of typical particle morphologies obtained using: (a) HDH process; (b) gas atomisation; (c) plasma atomisation; and (d) plasma rotating electrode process. In all micrographs the powder shown is Ti-6Al-4V and images were taken using a Hitachi TM3000 SEM

(a) (b)

(c) (d)

Table II Powder KPVs and Techniques that Could be Used for their Measurement

Particulate properties Bulk properties

Powder property Assessment technique Powder property Assessment technique

Particle shape (morphology) SEMOptical microscopy

Apparent density Hall flowFreeman FT4

Tap density Tapped density tester

Particle size and particle size distribution

SieveLaser diffractionOptical microscopy

Flowability Hall flowDynamic flow testing (e.g. revolution, Freeman FT4)Shear cellAngle of repose

Cohesiveness Freeman FT4

Particle Porosity Particle polishing and optical microscope

Surface Area BET surface area analysisChemical composition ICP-OES

XRDInert gas fusionCombustion infrared detection



flow properties are likely to be more problematic in terms of obtaining homogenous density layers throughout the build than powders which are comparatively more free flowing. Powder flow is difficult to relate to any one given parameter of a powder but there are some general rules which can typically be applied (15, 16):(a) Spherical particles are generally more free flowing

than irregular or angular particles(b) Particle size has a significant influence on flow

– larger particles are generally more free flowing than smaller particles

(c) Moisture content in powders can reduce flow due to capillary forces acting between particles

(d) Flow properties often show a dependency on the packing density at the time of measurement – powders with a higher packing density are less free flowing than powders with a lower packing density

(e) Short range attractive forces such as van der Waals forces and electrostatic forces can adversely affect powder flow and may cause particle agglomeration (short range forces have a bigger impact on finer particles).

3.3.4 Chemical Composition

The laser sintering behaviour of a metal powder will not only depend on the physical properties, it will of course also depend on the chemical properties. Powder chemical composition for AM should ideally be optimised for the machine or application. Validating chemical composition helps to ensure that the manufactured component has homogenous material properties.

As well as the bulk alloying chemistry, it is important to understand the effect of interstitial elements, such as O and N, since component properties will depend on the amount of interstitial elements present. For example, it is well known that the tensile strength and ductility properties of Ti-6Al-4V are influenced by O content whereby an increase in O results in an increase in tensile strength and a subsequent decrease in elongation (17). Research has also shown that interstitial elements can influence the melting kinetics of the powder by interfering with the surface tension of the melt pool resulting in Marangoni flow (18). Marangoni flow can have a negative influence on the porosity of the final component (11, 19).

3.4 Powder Recycling

It has been mentioned previously that for a powder bed AM process to be economically viable it is necessary

to recycle the large amount of unused powder. The effect of continuous powder reuse on the KPVs is an area that has up until recently received relatively little scientific attention. A handful of researchers have investigated the effect of continuous reuse of powders (2, 3) for example, however further study is required to fully understand the impact of recycling on process performance.

4. Procurement Options

Once a suitable atomisation route has been selected, the AM user then faces more decisions around powder supply. The market opportunity for metal AM powder supply has not gone unnoticed, and three main options for powder supply have emerged. Firstly, users can choose to procure powder directly from the AM machine provider. Secondly, users could choose to procure powder from third party companies, who offer AM machine ‘validated’ powders. Finally powder can be sourced direct from an atomisation company. Indeed several powder manufacturers are now offering AM specific powders as part of their product portfolio (the largest of these, and the alloys they provide are listed in Table III). A summary of the advantages and disadvantages of each procurement option is presented in Table IV.

At present the majority of powder sales are through AM machine manufacturers or third party suppliers. The powder provided by these suppliers has been optimised for each additive process, also known as being ‘validated’.

By validating a powder, the supplier is ensuring that the powder given to the customer is of suitable quality so that, when being processed, it will behave as intended, leading to a successfully built part that will adhere to the chemical composition and the mechanical properties of the given metal or alloy. Simply put, the machine will successfully build with that material, thus de-risking powder supply for the end user.

Machine suppliers can validate powder as they will develop processing parameters on their machines for each specific material. Once the machine repeatedly builds reliable, mechanically suitable parts then the parameters will be stored and sent out with batches of that material to users. The powder being used will be characterised using some of the techniques discussed in Section 3.3 and all subsequent batches will undertake the same testing to ensure that they adhere to the same specification. This ensures that the



end user is receiving consistent powder across batches that has been validated for their specific process. Third party suppliers offer a similar service, refining powder size and morphology to ensure they work in process.

From this it can be seen that machine manufacturers and third party suppliers undertake a lot of work to ensure the powder they provide is suitable for their AM process. There are a number of obvious advantages from procuring powder through these routes. However, this increased level of material supply security does also draw some limitations.

Alternatively, it is possible to procure powder directly from the powder manufacturer. There are a range of benefits from purchasing powder directly from the manufacturer; however, there are some underlying risks that a customer must also be aware of. The most pertinent of these risks was alluded to in Section 2. The current state of the metallic AM powder market is that

it contributes an almost insignificant proportion of the income generated by powder producers (i.e. 0.0047% of the income generated from metal powders was due to sales into the AM market). Since the AM powder market is not currently a major source of income for powder producers it is likely that powders will not be produced to the strict requirements of AM. A further risk with this procurement method is losing the support of the AM machine manufacturer. This can be slightly de-risked by purchasing material from one of the small number of powder manufacturers who are starting validate their own powder for AM processes.

The major advantage of procuring powder from this route is the increased choice of materials and cheaper procurement costs. To ensure that the specification of the powder is met, a customer may need to undertake their own characterisation analysis. Further to this a manufacturer may only supply the powder in a wider

Table III Supplier List of Powders Specified for AMa

Company

Supplier type Material Building process Manufacturing processes

Loca

tion

Man

ufac

ture

r

Third

par

ty

Fe-B

ased

Al-B

ased

Ti-B

ased

Ni-B

ased

Co-

Bas

edC

u-B

ased

Prec

ious

m

etal

s

EBM

SLM

Wat

er

Gas

Plas

ma

Cen

trifu

gal

Advanced Powders and Coatings – AP&C

Canada P P P P P

Carpenter Technology Corp

US P P P P P P

GKN Hoeganaes Corp

US P P P P

H.C. Starck GmbH

EU P P

Höganäs Sweden AB

EU P P

Sandvik Materials Technology

EU

TLS Technik GmbH & Co. Spezialpulver KG

EU P P

LPW Technology Ltd

UK

P

PP P P P P

PP P P P P

P P PPP P P P

PPPP P

P PPPPP P P P P P P

aInformation obtained from the supplier websites (Accessed April 2015)



PSD than the customer wants, therefore additional sieving may be required. Both of these add complexity to the powder supply.

5. Customer Considerations

When deciding where to purchase powder from, a customer has a number of considerations to make. Most importantly:• Can they supply me with the material I require?• Is the price of the powder competitive?• Can they provide the batch size I require?• Is there traceability of the material source? Do I

require traceability?• Do I require knowledge over the powder

specification? Do I require control over the powder specification?

Additionally, the customer should always consider the use of the machine and powder. For instance: is the machine being used in production or research? What is the use of the end component? A machine used purely for production purposes will be required to make parts of the highest quality, therefore powder from the AM machine supplier or third party supplier is likely to be the best procurement route. The added benefit of this is that the machine manufacturer will support the customer if there are any build issues. However, if complete control and traceability of the powder used for the build is required, then there may be a lack of transparency from the AM machine supplier as to the complete history of their powder.

Research-based machines have a different range of considerations to make. If the primary use of the machine is to prototype or develop the technology,

Table IV Advantages and Disadvantages of Procuring Powder from Machine Manufacturers, Third Party Suppliers and Direct from Powder Manufacturers

Supplier type Advantages Disadvantages

Machine manufacturers Standard machine parameters are provided and are ready to use

Potentially higher cost of materials

Powder that has been ‘tried and tested’ Material options are limited

Support from the supplier should a build have issues

Experimenting with powder of a different specification is limited

Ease of sourcing Lack of traceability of material source and manufacturing process

Already established procurement routesValidated third party suppliers Able to select powder from the entire

powder metallurgy industryLack of traceability of material source and manufacturing process

A wide range of batch sizes are offered Lack of support from the machine manufacturer should a build fail

Powder that has been ‘tried and tested’ Potentially higher cost of materialsEase of sourcing

Direct from powder manufacturers Wide range of material choices Lack of support from the machine manufacturer should a build fail

Potentially lower cost of powder (highly dependent of material/process)

No guarantee that the material will produce a successful build

Choice of manufacturing process, allowing a degree of control over powder characteristics

Minimum batch orders may apply, due to minimum powder yields from a manufacturing process

Can use local manufacturers Will powders be produced to the exacting standards required for AM?

An increased level of material traceability Lack of powder specification with each order



then the customer will likely want the additional control over the powder that they are using. The customer will also likely want to attempt building with new materials or experiment with machine parameters. Therefore sourcing material directly from the powder manufacturer may be best suited here.

There is no right answer for the procurement route that customer takes. However, careful consideration needs to be given to the application and desired end results of the AM system.

6. Future of Powder Supply Chain

There are a number of theories of how this growing material market will develop over the next five to ten years. Here, some ideas are explored.(a) Increased production and industrialisation of

AM drive the price of powder down: all market predictions show the continued growth of metal AM over the coming years. Naturally, as the technology becomes widely used, the supply chain of materials will also grow. Factors such as increasing competition and larger production runs should see the cost of powder per kilogram be driven down. As the powder metallurgy supply chain infrastructure is already well established, the increase in suppliers of specific AM powders is likely to happen rapidly. This theory also applies to processes that are currently expensive to run, for example plasma atomisation. If, as the market develops, this is seen as the best atomisation route then the amount of powder produced by it will significantly increase. Feedstock for the process will become cheaper and it is likely that the range of materials available will increase.

(b) Game changing powder production techniques emerge: throughout this article only atomisation as a way of producing powders for AM has been discussed. However, in the near future, there is the likelihood that an altogether new manufacturing method will eclipse atomisation, providing suitable powder for a fraction of the production cost. Companies such as Metalysis, UK, are developing new ways in which powder can be made at a significantly reduced price.

(c) Introduction of third party suppliers increases competition: the emergence of third party suppliers could see the price of powder driven down further. Purely through competition, suppliers will be able

to procure large batches of powder and pass the savings onto the end user.

(d) Will machine and powder supply lock down or open up? This will be an extremely important moment for metal AM. Other industries, namely polymer-based AM, have seen companies use bar-coded systems on their machines, such that a system will not physically build unless a bar code of a material that they supply is scanned. If this is the case with metal AM machines then it could cause a severe tightening on the research capabilities of these machines. However, this is only likely to happen on the highest value, production use systems. Again, as seen with polymer-based machines, competitors with machines of more open architecture are likely to enter the market. It is highly likely that a lot of machines of this nature will emerge as patents from the major machine manufacturers start to expire.

7. Conclusions

The three main options available for the procurement of AM powders include AM equipment providers, third party suppliers and directly from powder atomisers.

There is some degree of security in purchasing powder directly from AM equipment suppliers. This is because the powder batch they supply will be at least nominally the same grade as the powder batch used to develop melt theme parameters. However, the AM equipment supplier has ownership over the powder source thus limiting the powder supply chain competiveness. This results in powder costs which remain the highest of all procurement options.

Procuring powder directly from the atomisers may be a cheaper alternative. However, despite the recent exponential growth, the AM market is currently a relatively small source of revenue for most powder atomisers. Furthermore, because of the specific particle size fractions used in AM, powder atomisers may not produce powder specifically for AM. Instead atomisers may obtain the required size fractions from an atomised powder batch intended for use in other industries such as powder hot isostatic pressing (PHIP) or press and sinter. The originally intended process for these powder batches may not require the same high quality as powders used in AM and as such their performance in an AM process may not be adequate or as expected.

This risk of going direct to atomisers can be limited by using a third party powder supplier. In this case



the third party supplier takes on the associated risks of procuring powder batches in much higher quantities than would be needed by any one AM service provider. This then allows the AM service provider to procure only the amount they need. The higher powder quantities procured by a third party supplier potentially provides them with the necessary influence to demand higher quality powder batches that are atomised specifically with AM as the intended end use. The level of pre-sale powder qualification is also much more detailed from third party suppliers than from the atomisers themselves. Even with this option the procurement costs can be higher than going directly to the atomisers and less support will be made available from equipment suppliers should the procured powder be considered a potential factor for build failures.

What increases the complexity, and indeed uncertainty, of procuring powders for AM is the lack of AM specific powder specifications. It is commonplace to make decisions to accept or reject powder batches based on specifications used for press and sinter applications. These specifications would at best include chemical composition, sizing by sieve analysis and flow assessment by Hall flow. Such specifications can be inadequate to use as a benchmark for AM powder quality. As discussed throughout this article predicting powder performance is highly complex and can be difficult to characterise using simple techniques. Future work needs to be aimed at systematically identifying the properties of metal powders that have the biggest influence on the powder performance in terms of hopper discharge and powder spreading and also how the powder responds to the AM melting process. Work in this field will allow the development of specifications which adequately define and control the key process variables of powders used in AM.

References1. ‘Wohlers Report 2014: 3D Printing and Additive

Manufacturing State of the Industry Annual Worldwide Progress Report’, Wohlers Associates, Inc, Fort Collins, Colorado, USA, 2014

2. P. A. Carroll, P. Brown, G. Ng, R. Scudamore, A. J. Pinkerton, W. U. H. Syed, H. K. Sezer, L. Li and J. Allen, ‘The Effect of Powder Recycling in Direct Metal Laser Deposition on Powder and Manufactured Part Characteristics’, in “Proceedings of AVT-139 Specialists Meeting on Cost Effective Manufacture via Net Shape Processing”, 15th–19th May, 2006, Amsterdam, The Netherlands, NATO Science and

Technology Organisation, Brussels, Belgium, 2006

3. V. Seyda, N. Kaufmann and C. Emmelmann, Phys. Proc., 2012, 39, 425

4. J. J. Dunkley, ‘Hot Gas Atomisation – Economic and Engineering Aspects’, in Proceedings of the World Congress and Exhibition on Powder Metallurgy, PM2004, 17th–22nd October, 2004, Vienna, Austria, The European Powder Metallurgy Association, Shrewsbury, UK, 2005

5. R. Gerling, H. Clemens and F. P. Schimansky, Adv. Eng. Mater., 2004, 6, (1–2), 23

6. C. McCracken, Powder Injection Moulding Int., 2008, 2, (2), 55

7. C. Doblin, D. Freeman and M. Richards, Key Eng. Mater., 2013, 551, 37

8. C. Doblin, A. Chryss and A. Monch, Key Eng. Mater., 2013, 520, 95

9. N. P. Karapatis, G. Egger, P.-E. Gygax and R. Glardon, ‘Optimization of Powder Layer Density in Selective Laser Sintering’, in Proceedings of the 10th International Solid Freeform Fabrication Symposium, The University of Texas at Austin, USA, 9th–11th August, 1999, pp. 255–264

10. I. Gibson, D. W. Rosen and B. Stucker, “Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing”, Springer, New York, USA, 2010

11. H. J. Niu and I. T. H. Chang, Scripta Mater., 1999, 41, (1), 25

12. D. Manfredi, F. Calignano, M. Krishnan, R. Canali, E. P. Ambrosio and E. Atzeni, Materials, 2013, 6, (3), 856

13. A. Simchi, Metall. Mater. Trans. B, 2004, 35, (5), 937

14. A. B. Spierings and G. Levy, ‘Comparison of Density of Stainless Steel 316L Parts Produced with Selective Laser Melting using Different Powder Grades’, in Proceedings of the 20th Annual International Solid Freeform Fabrication Symposium, The University of Texas at Austin, USA, 3rd–5th August, 2009, pp. 342–353

15. P. C. Angelo and R. Subramanian, “Powder Metallurgy: Science, Technology and Applications”, PHI Learning Pvt Ltd, New Delhi, India, 2008, pp. 76–80

16. R. M. German, “Powder Metallurgy Science”, Metal Powder Industries Federation, Princeton, New Jersey, USA, 1994, pp. 9–58

17. J.-M. Oh, B.-G. Lee, S.-W. Cho, S.-W. Lee, G.-S. Choi and J.-W. Lim, Metal. Mater. Int., 2011, 17, (5), 733

18. M. Rombouts, J. P. Kruth, L. Froyen and P. Mercelis, CIRP Annals, 2006, 55, (1), 187

19. D. Boisselier and S. Sankaré, Phys. Procedia, 2012, 39, 455



The Authors Jason Dawes is a Senior Research Engineer at the Manufacturing Technology Centre (MTC) where he leads the Particulate Engineering research group. His role is in the technical management of highly innovative research projects involving powder based manufacturing technologies such as additive manufacturing, laser cladding and hot isostatic pressing. He was awarded EngD from University of Birmingham, UK, in 2014 in the field of Chemical Engineering.

Robert Bowerman is a Research Engineer at MTC where he leads projects within the field of additive manufacturing. He has experience in the following technologies: electron beam melting (EBM), selective laser melting (SLM), stereolithography (SLA), digital light processing (DLP) and fused deposition melting (FDM). His projects focus on innovative research and development work primarily on the Manufacturing Capability Readiness Levels (MCRLs) 4 to 6. Work carried out covers all areas of additive manufacturing, including development and productionisation of technology and designing for AM. He is presently working towards Chartered Engineer status.

Ross Trepleton is the Component Technologies Group Technology Manager at the MTC. He is responsible for the coordination and management of the MTC Research Programme, in the area of Component Technology to meet the needs of clients and stakeholders. The aim of this area of research is to develop new technologies that enable improved component manufacturing. He was awarded a PhD from University of Birmingham in the field of Metallurgy and Materials.




Atomic-Scale Modelling and its Application to Catalytic Materials ScienceDeveloping an interdisciplinary approach to modelling

By Misbah Sarwar, Crispin Cooper and Ludovic BriquetJohnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH, UK

Aniekan Ukpong, Christopher Perry and Glenn Jones*Johnson Matthey Research Centre, CSIR, Meiring Naude Road, Brummeria, Pretoria, 0184, South Africa


Computational methods are a burgeoning science within industry. In particular, recent advances have seen first-principles atomic-scale modelling leave the realm of the academic theory lab and enter mainstream industrial research. Herein we present an overview, focusing on catalytic applications in fuel cells, emission control and process catalysis and looking at some real industrial examples being undertaken within the Johnson Matthey Technology Centre. We proceed to discuss some underpinning research projects and give a perspective on where developments will come in the short to mid-term.

1. Introduction

The use of atomic-scale modelling, whether force-field methods or electronic structure theory calculations (for example density functional theory (DFT) (1, 2)) in chemical and material research within industry is

becoming increasingly commonplace. This is a field that entails linking fundamental chemical properties, for instance electronic and geometric structure, to activity, ultimately providing the basis from which enhanced materials can be predicted (3). A particularly attractive feature of computational approaches is the flexibility and applicability to all divisions of Johnson Matthey’s business. For instance within New Business Development methods to simulate optical properties of materials (for smart glass applications) are being applied; within platinum group metals (pgm) refining we are exploring metal extraction and applying engineering process modelling; and within glass and colour technologies we are using thermodynamic tools to help us understand the formation and properties of glass compositions. Of particular importance to Johnson Matthey is catalyst technology (Chemical Catalysis, Emission Control, Process Technologies and Chiral Catalysis Technologies) where we are working towards a multiscale modelling capability that links the macroscopic engineering world to the atomic-scale chemistry and physical properties of materials.

A key philosophy within the research group and what makes the field so important for industrially relevant applications, is the importance of linking theory with experiment whether through measurement and validation of the computational methods or by actively seeking collaboration with experimentalists who can synthesise, characterise and test interesting materials. This overview article will firstly discuss some of the background to the methods employed, then move on to discuss some recent projects that have been carried



out with a particular focus on catalytic applications. The aim is to give a feel for the types of application we have been tackling, and the underpinning research we are conducting, before moving on to conclude by highlighting areas where continued research effort will be required in the future.

Computational catalysis is a field that has been flourishing over the past two decades. This is a result of two primary drivers: the development of efficient generic codes or algorithms and the improvement in available computer power on which to run these codes (Figure 1) (4, 5). Computational catalysis as a sub-discipline has arguably emerged from surface science (6, 7). This is unsurprising given that most of heterogeneous catalytic chemistry occurs at the surface of materials. However, it is also inextricably linked to solid-state materials chemistry, for example the bulk properties of complex oxides determine the surface facets that are present and the complex reaction atmospheres present during catalysis lead to various oxidation states of the material, which all play a crucial role in the activity and stability of a given catalyst (8–11).

A key interest of computational catalysis to the industrial arena is that of material prediction. Here

state-of-the-art approaches, with the implicit limitations thereof, can be applied to a specific catalytic reaction or application of interest (12–15). Critical in this approach is the development of a suitable atomic-scale model, from which we can extract information about the electronic properties or certain chemical quantities, for example reaction energies and activation barriers. The idea is to understand the reaction sufficiently, such that we can develop predictive models that can influence the materials development process. This can be achieved at a number of levels starting from potential energies and electronic properties (occupancies, d-band centres and highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gaps), before moving on to thermodynamic models and ultimately full kinetic models (Figure 2) (16). One key concept is the identification of a key property or trend that can then be used as a descriptor to ‘screen’ through other potential candidates (3, 12). This approach has met with some success, and has led to a number of initiatives working on generating databases and computational tools to accelerate or aid the discovery of novel materials using ab initio approaches (17–22). Close collaboration with experimentalists then completes the material discovery

Perdew Wang

DFT for chemists

Nobel PrizeWalter Kohn and John Pople

DFT and computational chemistry

JMTC Computational Chemistry

Group

JMTC – Pretoria; Computational Catalysis and Materials

Science

Modelling methods across the length scales

integral in Johnson Matthey’s development

Development of DFT for chemisorption

problems

Application of DFT to reactions; predictive modelling

of catalysts and other materials

Multi-scale modelling; large scale DFT

calculations; holistic simulation of systems

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

2012

2014

2016

2018

2020

Year

20

19

18

17

16

15

14

13

12

11

10

log 1

0 (c

ompu

ting

pow

er/F

LOP

PS

)

Fig. 1. Log plot showing the most powerful supercomputer, as measured in floating point operations per second (FLOPPS) (5) vs. year. In grey are key developments in algorithms and then in blue, on the development of atomic-scale modelling within Johnson Matthey, as an example of an industrial user



process, allowing iterative refinements of models and ultimately the synthesis of candidate materials.

Beyond the immediate task of predicting materials for current needs, the second important focus of computational catalysis is to look to the future: asking questions such as how to go beyond the current state of the art to describe systems more accurately or how to place the atomic scale picture in the context of the complete catalytic solution. Often this involves trying out new methods and establishing collaborations with academic researchers.

In order to illustrate some of the above we now proceed to discuss some laboratory projects from the areas of homogeneous catalysis, heterogeneous catalysis and electrocatalysis that are, or have been, conducted within the Johnson Matthey Technology Centre in more detail. After which we shall proceed to discuss some of the future directions of our research and show how by working at the cusp of the industrial and academic

interface of fundamental research we are working towards the future development of Johnson Matthey technology.

2. Homogeneous Catalysis

Homogeneous catalysts operate in the same phase as the reactants, usually but not exclusively dissolved in a solvent. A wide variety of homogeneous catalysts exist: these include Lewis and Brønsted acids, metal ions, metal complexes, organometallic complexes and biocatalysts. However, more recently, the term homogeneous catalyst is often applied to organometallic or coordination complexes.

Owing to the fact that upon completion of a reaction, the catalyst must be recovered from the products (often a costly process) and that in general heterogeneous catalysts are more stable, homogeneous catalysts enjoy only limited application in industry. There are

NiAl_1 NiAl_2

NiAl_3 NiAl_4

Fig. 2. The process of developing a surface model, studying the adsorption of intermediates and transition states, culminating in the kinetic model that is used to screen for new catalyst candidates. (Republished with permission of Maney Publishing, from (16); permission conveyed through Copyright Clearance Center Inc)

(a) (b)

(c)Ni:Sb

Ni:Sn

Ni:CNi:GaNi:Zn

Ni

Ni:Al

Ni:MoNi:Cr

5

2

–1

–4

–7

–10

–13

–16

–19–1.0 –0.5 0.0 0.5 1.0 1.5 2.0

C binding, eV

1.5

1.0

0.5

0.0

–0.5

–1.0

–1.5

O b

indi

ng, e

V

Ni:ZrNi:Ti



however, a number of important industrial reactions catalysed by homogeneous catalysts, including hydroformylation, methathesis, carbonylation and polymerisation using Ziegler-Natta catalysts. Perhaps it is in the pharmaceuticals and fine chemicals industries where homogeneous catalysts enjoy their greatest success. Coupling reactions such as the Suzuki-Miyaura reaction, Sonogashira coupling, the Heck reaction, Stille cross-coupling and the Buchwald-Hartwig reaction have long been employed to bring about various organic transformations.

Computational studies with DFT methods on the transmetallation step of the various cross-coupling reactions first appeared just over a decade ago (23–28) and provided useful insight into the energetics and mechanism of the reaction. Within a couple of years, DFT results for the full catalytic cycles were reported (29–34). DFT theoretical studies have also recently been used to investigate cross-coupling reactions involving metals other than palladium; examples include: rhodium-catalysed coupling via C–H bond activation (35, 36), nickel-catalysed cross-coupling reactions (37–42), delineating the mechanism of iron-catalysed cross-coupling reaction (43–47), dialkylzinc-mediated cross-coupling (48), copper-catalysed C–N cyclisation (49) and enantioselective nucleophilic borylation (50).

2.1 Palladium-Catalysed C–H Bond Activation of Heterocycles

Over the past decade, Pd-catalysed direct C–H arylation has emerged as an important alternative to traditional cross-coupling reactions for the synthesis of a wide range of arylated heterocycles (51–56). Compared to traditional Suzuki-type cross-coupling methods, direct arylation has the advantage that it does not require the preparation of stoichiometric quantities of organometallic reagents (typically alkyl or aryl boronic acids), thereby eliminating steps in the synthesis and reducing the amount of toxic metallic waste.

The most widely accepted mechanism for these reactions is the concerted metallation-deprotonation (CMD) pathway, involving simultaneous Pd–C bond formation and aromatic C–H bond cleavage to yield a diaryl Pd(II) species (57–59). A second mechanism, involving a cyclometallated complex, has also recently been proposed to account for the much lower than expected reactivity observed when certain tri-tert-butylphosphine-containing Pd catalysts are reacted with heteroarenes in isolation to the catalytic cycle (60, 61).

In collaboration with experimentalists from Johnson Matthey Catalysis and Chiral Technologies, a modelling study was undertaken to explore some anomalies pertaining to the direct arylation of oxazole and 4-bromotoluene. Oxazole, 1, with three reactive C–H sites, usually displays the reactivity order: C5>C2>C4. However, in certain instances, the reaction at the C5 site appears to be disfavoured, leading to a predominance of C2 product. Table I shows how the product distribution is affected by altering the amount of added phosphine ligand.

Table I Product Distribution for the Reaction of Oxazole with 4-Bromotoluene Using the Pre-Catalyst Palladium Acetate (Pd(OAc)2)

Catalyst Ligand % Mono 5

% Mono 2

% Di 2,5

% Di 4,5

Pd(OAc)2 – 66 0 28 3

Pd(OAc)2 1 × PtBu3 31 4 59 1

Pd(OAc)2 2 × PtBu3 6 35 50 0

In the absence of added phosphine ligand, dimethylacetamide (DMA) solvent is believed to coordinate to Pd. Figure 3 shows the reaction coordinate diagram for the C–H activation step (believed to be rate-limiting) of oxazole at positions C5, C4 and C2, by the well-established CMD mechanism. The results correlate well with the experimental observation that for Pd(OAc)2 in the absence of added phosphine, mono-5- product is formed in preference to both mono-2- and mono-4-substituted product.

Figure 4 shows the reaction coordinate for oxazole arylation, via the CMD mechanism, in the presence of PtBu3 ligand. Formation of the initial intermediate appears to be disfavoured for this mechanism, for both coordination through N3, as well as through the C4=C5 pi bond. The barrier heights for all three intermediates are also fairly large. Our calculations thus suggest that C–H activation at any of the reactive sites via the

Oxazole, 1

O

N

1

234

5



O H

O PdDMA

NO

Ar

O H

O PdDMA

ON

Ar

O H

O PdDMA

NO

Ar

DMAO Pd Ar

OH ON

ON+

H–OO Pd Ar

DMA

ON+

H–OO Pd Ar

DMA

OO

O

Pd ArDMA

N

OO

O

Pd ArDMA

N

OO

Pd ArDMA

O

N

NO

+

OO Pd

DMAAr

14.0

30.00

25.00

20.00

15.00

10.00

5.00

0.00

–5.00

–10.00

–15.00

ΔG

, Kca

l mol

–1

Fig. 3. Free energy diagram for direct arylation of oxazole with [(DMA)Pd(Ar)(OAc)] via the CMD mechanism

O H

O PdPtBu3

NO

Ar

O H

O PdPtBu3

ON

Ar

PtBu3

O Pd Ar

OH ON

ON+

H–OO Pd Ar

PtBu3

ON+

H–OO Pd Ar

PtBu3

OO

O

Pd ArPtBu3

N

OO

O

Pd ArPtBu3

N

OO

Pd ArPtBu3

O

N

NO

+

OO Pd

PtBu3

Ar

11.0

30.00

25.00

20.00

15.00

10.00

5.00

0.00

–5.00

–10.00

–15.00

ΔG

, Kca

l mol

–1

O H

O PdPtBu3

NO

Ar

Fig. 4. Free energy diagram for direct arylation of oxazole with [(PtBu3)Pd(Ar)(OAc)] via the CMD mechanism



CMD mechanism in the presence of PtBu3 ligand is unfavourable.

Hartwig et al. (60, 61) have shown that direct arylation of pyridine N-oxide with (PtBu3)Pd(Ar)(OAc) involves cooperative catalysis between two distinct Pd centres. Figure 5 shows the modelling results for oxazole C–H activation via this recently proposed ‘cooperative’ mechanism and is in agreement with our experimental findings. The initial complex that forms has oxazole coordinated to Pd either through N3 or through the C4=C5 pi bond. In this case, however, formation of the latter is unfavourable by around 2 kcal mol–1. C5 activation still has a marginally lower barrier height, but initial intermediate formation is unfavourable. The energy barrier for C2–H activation is still quite large (around 21 kcal mol–1), but not as large as for the DMA ligand. These findings suggest that C5–H activation is not significantly favoured over C2–H activation as is observed in the absence of a phosphine ligand.

This work has shown how modelling can be instrumental in explaining experimental observations.

A better understanding of the factors that control the reactivity and regioselectivity of various heteroaromatic substrates is important in designing reaction conditions that will favour activating a specific C–H bond in a particular substrate. Additionally, it would allow Catalysis and Chiral Technologies to better market their range of Pd-catalysed cross-coupling catalysts towards specific applications. Future work includes investigating the regioselectivities of the di-substituted products, as well as the effect that the amount of PtBu3 has on the product distribution.

3. Heterogeneous Catalysis

Heterogeneous catalytic materials come in several forms. Generically one could say that these are either unsupported (for example zeolites or PdO) or supported (for example nickel/alumina). The majority of our research has focused on the latter type, where either pure, doped or alloy nanoparticles are deposited on a support that may be either inert, acid/base or redox active. Historically, extended infinite surfaces have been used as models corresponding to the limit

ON+

H–OO

tBu

PdPtBu

12.8

30.00

25.00

20.00

15.00

10.00

5.00

0.00

–5.00

–10.00

–15.00

ΔG

, Kca

l mol

–1

tBu

O H

O

ON

PdPtBu

tBu

O

OH ON

PdPtBu

ON+

H–OO

tBu

PdPtBu

O H

O

NO

PdPtBu

tBu

O H

O

NO

PdPtBu

tBu

OO

ON

PdPtBu

tBu

OO

ON

PdPtBu

tBu

OO

tBu

O

N

PdPtBuN

O

+

OO

tBu

PdPtBu

Fig. 5. Free energy diagram for direct arylation of oxazole with [(PtBu3)Pd(Ar)(OAc)] via the cooperative mechanism



of large nanoparticles (>3 nm – this number is slightly arbitrary due to the computational effort required to explicitly define this value). There is a plethora of real examples where materials design has been influenced by the result of calculations on such systems (62, 63) and indeed this is often the first approach taken with our in-house modelling. However, the complexity of a catalyst is such that detailed questions remain, surrounding for example the influence of particle size

on specific reactions, which may be a combination of electronic and geometric influences (Figures 6 and 7), and also the influence of the so-called ‘metal support interaction’ in which electronic, geometric and ‘spill-over effects’ play a role (64). In fact, these influences can be reaction specific and sometimes even condition specific. It still remains a challenge to theoretically address these questions fully and this leads to much vital underpinning research.

Fig. 6. (a) Graph showing the fraction of atoms of a given type (terrace, step or corner group by coordination number (CN)) as a function of particle size. The plot has been generated for Pt particles using a Wulff construction from surface energies; for clarity only CN 6, 7, 8, 9 and 12 are shown. (b) From top to bottom: three representative surface models are shown: {111} for a terrace (CN: 9), {211} for a step (CN: 8), {532} for a corner atom (CN: 6)

0 1 2 3 4 5 6d{[100]–[–100]}, nm

0.8

0.6

0.4

0.2

0

Frac

tion

of a

tom

s

Terraces

Steps

Corners

CN: 6CN: 7CN: 8 or 9CN: 12

(a) (b)

Fig. 7. Density of states for: (a) small gold clusters as a function of size (increasing from top to bottom); (b) Pt clusters as a function of size (decreasing from top bottom). When viewed in conjunction with Figure 3, it can clearly be seen that overlaid on the geometric effect of coordination is an electronic effect, partly as a result of low coordination, but also as a result of localised electronic states intermediate between atomic and bulk-like

–10 –5 0 5 10

Single atom

6 cluster

13 cluster

79 cluster

Bulk

0.80.40.0To

tal D

OS

Tota

l DO

S 0.80.40.0

Tota

l DO

S 0.80.40.0

Tota

l DO

S 0.80.40.0

Tota

l DO

S 0.80.40.0–10 –5 0 5 10

–10 –5 0 5 10

–10 –5 0 5 10

–10 –5 0 5 10

(a)

–10 –8 –6 –4 –2 0 2 4E–Ef, eV

10

8

6

4

2

0

DO

S

Bulk

201

165

135

79

55

43

19

13

1

(b)



3.1 Methane Activation

The activation of methane is central to many technologies, from hydrocarbon combustion and methane oxidation in catalytic emission control to the synthesis of hydrogen and syngas as a feedstock for Fischer-Tropsch and ammonia syntheses (65).

In the area of steam reforming catalysis, models have been developed to go beyond understanding the intrinsic activity of steam reforming catalysts and begin to consider the influence of poisons and unwanted side reactions. For instance, building on published models (66), extensions have been made to account for: (a) site blocking and (b) the electronic deactivation caused by the presence of sulfur (67). Using the modified model the influence of sulfur can be explicitly simulated (68), enabling strategies to be developed to mitigate its influence. Figure 8 shows an example of a sulfur overlayer, along with a calculated thermodynamic isostere for sulfur coverage on Ni {111}, as a function of temperature and partial pressure of hydrogen sulfide (H2S). This figure allows direct comparison to published experimental work (69), against which calculated enthalpies (ΔH) and entropies (ΔS) can be validated. This exercise showed excellent agreement (ΔH (800 K): –145 (Exp. –155.2) kJ mol–1, ΔS (800 K) = +38 (Exp. +35.9) J K–1 mol–1), indicating that these more complex reactivity studies result in good trends. Figure 9 (70) illustrates the influence

of sulfur adsorption on the steam reforming reaction over a Ni{211} and the resulting activity trends for a series of alternative alloys.

During the steam reforming reaction the dissociation of methane on a nickel catalyst can lead to the formation of polymeric carbon on the catalyst surface (Figure 10). The carbon filaments so formed are very problematic, ultimately leading to deactivation of the catalyst and shutting down of the reactor. Using DFT modelling, the origin of the deactivation has previously been investigated along with possible mitigation strategies (71–73). Two fundamental approaches have been studied in-house: the first is to examine the role of dopants and alloys on the initial formation and nucleation of the problematic carbon, the second to look at burn-off of any formed carbon using oxygen based species and how this can be promoted.

The work has shown that in general if one inhibits the formation of carbon there is a negative effect on the activity of the catalyst; there is the unfortunate problem that the very surface sites responsible for catalytic activity are also the primary nucleation sites for carbon formation. This leads one to consider the promotion of burn-off as the pragmatic route to deal with it (Figure 10). Through modification of the support or surface composition of the catalyst it is hoped that promotion of this mechanistic route can be achieved and this is the subject of ongoing research.

Ordinarily, the presence of excess carbon is unwanted. However, there are also reactions where the presence

–10

–12

–14

–16

–18

–20

–221.0 1.2 1.4 1.6 1.8 2.0

1000 k/T

(a) (b)

Fig. 8. (a) Optimised structure of 0.25 ml S on a Ni surface; (b) isostere of S calculated from first-principles. Good agreement for saturation coverage (just below 0.25 ml) and entropies is found with the work of McCarty and Wise (69)



of carbon growth has been found to promote catalytic activity (74). This has inspired a fundamental research project looking at how defective graphene sheets can potentially act as catalysts for converting propane to propene (75). Furthermore, carbon is also used as a support in applications such as fuel cells. These observations highlight that it is not straightforward to classify whether a given element is a poison; as exemplified by the case of carbon in catalysis, it can be both a friend and a foe.

Partial oxidation of methane over yttrium stabilised zirconia (YSZ) has been experimentally shown to be catalytically active for the partial oxidation of methane (76, 77). First-principles electronic structure calculations have been applied to explore the possible mechanism of methane C–H bond activation over the oxygenated YSZ [111] surface (Figure 11) (78). Previous theoretical

studies (79) and experimental evidence (80) suggest that YSZ will favourably adsorb molecular oxygen over intrinsic vacancy sites and partially reduce it creating stabilised superoxide-like species. DFT calculations of molecular oxygen adsorbed over a vacancy site on the YSZ [111] surface showed that the O2 species was partially reduced. Charge distribution analysis showing it to be in an approximately –0.5 oxidation state, with the O–O bond lengthened slightly and large regions of unpaired electron density on each oxygen atom, all demonstrating transfer of electron density from the surface in to the oxygen π* anti-bonding orbital. The adsorption was found to be energetically favourable, the DFT model giving an adsorption energy of –0.47eV.

The oxygenated surface model described above was used to study the methane activation process. The pure zirconia (ZrO2) and unoxygenated YSZ surfaces were

Fig. 10. Illustration of: (a) nucleation of C at step edge (or defect); (b) growth of graphitic-like carbon; (c) reaction of graphitic carbon with surface OH (burn-off mechanism)

(a) (b) (c)

0

–1

–2

–3

–4

–5

log

(θ)

0

–1

–2

–3

–4

–5–9 –8 –7 –6 –5 –4 –3 –2

log (PH2S/Pstan.)

50% drop in activity

TOFθHθCθSθCH2θOθOHθ*

log (TOF/s

–1)

0

–1–9 –8 –7 –6 –5

(a)

1500

1000

500

0

Pric

e, $

km

ol–1

0 0.2 0.4 0.6Activity

Rh

Rh3NiRh3Ru

Ni3Rh

Ni3NiRu3NiNi3Ru

Ru3RhPt3Ru

Pd3RuRu3PdPd3Co

Ru3RuCo3Pd

(b)

Fig. 9. (a) Calculated turn over frequency and coverage of surface species for methane steam reforming in the presence of S over a Ni {211} surface. Kinetic models like this can be used as the basis for theoretical screening studies; (b) Pareto optimal, intrinsic metal cost (2009), vs. activity for methanol steam reforming (MSR) catalyst, oxidised catalysts have been filtered out following the method outlined in (70)



found to be relatively inert towards methane. However, on the oxygenated surface, methane was found to transfer a hydrogen to a surface lattice oxygen ion in a site neighbouring an adsorbed O2 species to yield a surface OH group and a triangular planar CH3 molecular species in the gas phase. Analysis of the charge density distribution confirms that the CH3 entity is a charge neutral methyl radical and that the H3C–H bond has undergone homolytic cleavage. The overall process is predicted to be energetically favourable (–0.23 eV). Transition state calculations using a constrained algorithm in combination with the nudged elastic band method (81), revealed a relatively low activation barrier of approximately 0.4 eV, suggesting that the reaction should occur readily. Analysis of the changing geometry and electronic structure allowed the prediction of the simplified mechanism given in Figure 11, with the adsorbed oxygen acting as an electron acceptor. It is likely that similar mechanisms may operate over other related metal oxide materials. Further information and analysis of this work is available (78).

4. Electrocatalysis

Proton exchange membrane (PEM) fuel cells offer a promising clean energy source for a range of both stationary and automotive applications. However, there are a number of issues which must be overcome in order for these to be commercialised. These include the high cost of the platinum electrodes, slow kinetics of the oxygen reduction reaction (ORR) and stability of the electrodes under operating conditions (82, 83).

Alloying Pt with another transition metal is one possible way of overcoming some of these issues and this is an area that has been subject to intensive research in the last few years (84–89).

The ORR at the cathode consists of several reaction pathways, the detailed understanding of which is still the subject of ongoing research (90). Generally, the reaction can follow two pathways: the four electron route to water (Equation (i)) and the two electron route to H2O2 (Equation (ii)):

O2 + 4H+ + 4e–à 2H2O (i)

O2 + 2H+ + 2e–à H2O2 (ii)

On Pt and Pt alloy surfaces it is generally agreed that the ORR follows the four electron pathway to water. This reaction can be broken down into the following elementary steps (Equations (iii)–(v)):

½O2à O* (iii)

O* + H+ + e-à OH* (iv)

OH* + H+ + e-àH2O + * (v)

(where * denotes a surface atom or empty surface site).In order for a surface to be an effective catalyst it

should follow Sabatier’s principle (91, 92). This states that a good working catalyst should have the ability to break bonds and generate intermediates. However, it should also have a low enough interaction energy with these intermediates not to stabilise them on the surface, so that they can react further and free up adsorption sites. It has been suggested however that the reason for the slow ORR may be due to the O intermediate binding too strongly to the Pt surface, therefore accumulating on the surface and blocking active sites. The surface d-band centre and oxygen binding energies have been widely used as descriptors for the ORR. Both these show a volcano type relationship with catalytic activity (85).

4.1 Transition Metal Alloy Nanocatalysts

High throughput screening of materials using computational approaches such as DFT has become a powerful and valuable tool in catalyst design. The search for the optimal material, based on predicted activity and stability, among a great number of alloy combinations is both a materials and a combinatorial problem. A high throughput approach was developed and adopted for

Fig. 11. Schematic diagram of the proposed methane activation mechanism over oxygenated YSZ surface. The excess charge balance on the product is delocalised throughout the ZrO2 system (78)

HH

H

O O

HHH

H–O

Zr4+Zr4+O2–

H~0.5–

O O–

DelocalisedYSZ surface YSZ surface



investigating the impact of varying the ratios of metals in the alloy surface layer to determine which compositions are most stable and electrochemically active. In this work, through collaboration with project partners Accelrys and CMR Fuel Cells, a combined theoretical and experimental approach was taken to investigate trends in the stability of Pt-M-Pt and M-Pt-Pt core-shell type catalysts (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Au, Ta, Hf, Cr, Nb, V, Y, Sc, Ti and Zr).

The surfaces were modelled using a 2×2 unit cell of the face-centred cubic (fcc) crystal of Pt (lattice constant bulk Pt system: a = 4.010 Å). The Pt-M-Pt configuration was modelled by substitution of the second atomic layer of Pt with M atoms, whereas the M-Pt-Pt configuration consisted of the Pt atoms in the first atomic layer being substituted with M atoms (Figure 12).

The stability of the electrode under fuel cell operating conditions is a significant challenge to understand, as the highly corrosive environment can promote surface segregation of the alloying component, which can then be leached away removing any benefit obtained from alloying (93–95). The adsorption of O and OH on the surfaces of Pt-M-Pt and M-Pt-Pt alloys was investigated, sampling all possible adsorption sites to identify the most stable. The stability of the surface was assessed by calculating the segregation energy, Eseg, firstly in vacuum (Equation (vi)) and then with the O or OH atom adsorbed (Equation (vii)). The segregation energy is defined as the difference in total energy

between the Pt-M-Pt and M-Pt-Pt structures with and without O/OH adsorbed. If the segregation energy is negative the Pt skin structure is favoured, if it is positive a surface of M atoms is favoured (Equation (vi)):

Eseg = E (Pt-M-Pt) – E (M-Pt-Pt) (vi)

and in the presence of adsorbate (O or OH) (Equation (vii)):

Eseg = E (Pt-M-Pt)(O/OH) – E (M-Pt-Pt)(O/OH) (vii)

Figure 13 shows the segregation energies for the structures with and without O and OH adsorbed. The value of the segregation energy indicates how strongly a particular configuration is favoured. A more negative segregation energy indicates that a Pt skin is

Fig. 12. Schematic depicting the under/overlayer structure of core-shell type catalysts: (a) Pt-M-Pt structure; (b) M-Pt-Pt structure

2.00

1.00

0.00

–1.00

–2.00

–3.00

–4.00

–5.00

–6.00

–7.00

PtA

gP

tAu

PtC

oP

tCu

PtF

eP

tIrP

tNi

PtO

sP

tPd

PtR

hP

tRu

PtA

lP

tBe

PtC

rP

tHf

PtM

nP

tNb

PtS

iP

tTa

PtT

iP

tV

Alloy

CleanOads

OHads

Ese

g, e

V

Fig. 13. Calculated segregation energies for clean {111} surfaces of various Pt alloys in vacuum and with ¼ ml of O or OH adsorbed



strongly favoured. In vacuum it can be seen that the Pt-M-Pt structure is favoured in all cases except Au and Ag (purple bars). However when O is adsorbed the segregation energy is weakened in comparison to the clean surface (blue bars). For the ORR reaction, the materials that retain a Pt skin are most promising. For Pt-Ni-Pt, Pt-Pd-Pt, Pt-Be-Pt and Pt-Y-Pt surface segregation of M is predicted in the presence of adsorbed O. For the remaining alloys a Pt skin is predicted though this is not favoured as strongly as for the clean surface. A reversal in segregation energies is observed for Pt-Ag-Pt and Pt-Au-Pt, which are more stable with a Pt skin in the presence of O.

Adsorption of OH on the surface destabilises the system resulting in a weaker segregation energy compared to the clean surface. This destabilisation is less pronounced than previously seen for O at the surface, with more negative segregation energies obtained for OH. This indicates that the tendency to retain a Pt skin in the presence of OH is stronger than in the presence of O.

The most promising ORR catalysts are ones in which a Pt skin is retained, acting as a protective barrier to prevent the leaching of M from the subsurface. Analysis of the segregation energies in the presence of both O and OH indicate that in terms of stability all compositions except Pt-Be-Pt, Pt-Ag-Pt, Pt-Au-Pt, Pt-Ni-Pt and Pt-Pd-Pt are promising, as a Pt skin is retained in the presence of O and OH when adsorption takes place.

Figure 14 shows the adsorption energy of ¼ ml of oxygen on the surface of each Pt-M-Pt alloy. Alloys to the left of Pt have a lower d-band centre and are

predicted to bind O more weakly than Pt. It is these alloys which bind O slightly more weakly than Pt that are predicted to have enhanced ORR activity. However alloys such as Pt-Ti-Pt and Pt-Al-Pt are predicted to have poor ORR activity as they bind O too weakly.

Using this approach over 2000 alloy combinations were screened and several potential candidates identified to put forward for further experimental testing. This theoretical pre-screening approach allowed the list of combinations for testing to be significantly narrowed down, saving valuable experimental time and resource.

4.2 Carbide Core Nanocatalysts

An alternative strategy to using a transition metal alloy as a core particle is to explore the possibility of using transition metal nanoparticles (96). The use of carbides as non-transition metal cores supporting thin films of Pt has been investigated. Through the study of various carbides and their interaction with Pt overlayers the project has been able to elucidate (a) the geometric and electronic influences on the stability of Pt on these carbides; and (b) how the presence of the different carbide cores influences predicted catalytic activity (Figure 15). Working closely with the synthetic chemists at Johnson Matthey Technology Centre, Sonning Common, the research has created inspiration and directions for research which are being followed in-house.

It is understood that synthesising a material where Pt wets a carbide surface or other non-metal cores may be challenging from a thermodynamic perspective.

–2.9 –2.4 –1.9 –1.4

PtAl PtTiPtSc

PtHf/PtZrPtNb

PtV PtTa

PtCrPtMn

PtCoPtRu

PtOs PtFe

PtIr

PtCu

PtRhPt

PtAg

PtAu

PtPd

Ed–Ef, eV

–2

–2.2

–2.4

–2.6

–2.8

–3

–3.2

–3.4

–3.6

–3.8

–4

E O

ads,

eV

–3.4

Fig. 14. Correlation of d-band centre and O adsorption energy for Pt-M-Pt alloys



Therefore the work, in its second phase, investigated strategies to promote the stability of Pt, for instance through the use of so-called tie-layers. Figure 15 illustrates the stability of a test set of tie-layer/Pt combinations, showing the propensity for the Pt to remain as a skin or to be sandwiched between the tie-layer and carbide support.

5. Emission Control Catalysis

The use of catalysts in emission control is wide and varied. Along with the supported nanoparticle catalysts discussed to this point, another significant class of catalyst is the zeolite. Zeolites are crystalline, microporous materials in which the atoms are arranged to form a network of molecular sized pores and channels. This unique porous structure combined with their huge internal surface area gives rise to a vast number of applications. By tailoring the pores and channels, molecules can be excluded on the basis of size and shape and catalytic processes can be driven to yield only preferred reaction products. Within Johnson Matthey’s business areas alone, zeolites are key components of selective catalytic reduction (SCR) catalysts, as additives for fluidised catalytic cracking (FCC) catalysts, and as catalysts in the petrochemicals refining process.

Zeolites have been extensively studied over the last decade using a wide range of modelling techniques. The structure of the framework, effects of doping, location of ions and adsorbates within the pores, diffusion of reactants and products within the structure and the reaction pathways leading to the catalytic breakdown of molecules can be readily computed. Atomistic modelling has been used to aid the characterisation of

zeolites used for SCR of nitrogen oxides (NOx) species for emission control – in particular to help elucidate the location of ions within the pores and to understand how these interact with other species located within the voids, such as water, nitrogen oxide (NO) and ammonia (NH3).

Small pore zeolites such as CHA have recently gained a lot of interest due to their very good low-temperature activity and enhanced stability compared to medium and large pore zeolites such as MFI and beta (97–103). An understanding of the nature and location of transition metal ions such as Cu and Fe within the pores and how these interact with key NH3-SCR adsorbates such as NO and H2O is crucial in understanding the mechanism of NOx reduction in these zeolites and this in turn can help in devising strategies to improve the performance of these catalysts. A combined simulation and experimental approach was used to investigate Cu location in CHA using techniques such as energy minimisation based on interatomic potentials and quantum mechanics/molecular mechanics (QM/MM) methods to identify stable cation exchange sites within the pores and study the influence of adsorbate interactions (Figure 16). These results were compared to high resolution X-ray diffraction (HR-XRD) and infrared (IR) measurements of probe molecules to elucidate the ion location and how this is modified by interaction with adsorbates.

Diffusion of reactants and products to and from the active sites within the zeolite pores is a key part of the catalytic cycle and can have a significant impact on a catalyst’s performance. Therefore an understanding of this process is crucial to optimising the use of zeolites as catalysts. An ongoing project is using modelling to understand diffusion of various molecules within these

1.5

0.5

–0.5

–1.5

–2.5

Ir Pd Ga Sn

Ese

g, e

V a

tom

–1

Eseg (100)Eseg (111)Eseg (001)ΔEads (100)ΔEads (111)Eads (001)

(a) (b)

Fig. 15. (a) Stability of Pt forming an outer shell with several tie-layer candidates. Overlaid is the oxygen adsorption energy relative to pure Pt; (b) schematic illustrating the carbide core material with the tie-layer/Pt overlayers



materials, including NO, NH3, H2O, propane, xylene and CO2. The atoms in a zeolite can be arranged in a variety of ways to form rings, channels and pores with different sizes and dimensionalities, leading to a large number of possible framework structures. Molecular dynamics (MD) simulations are being used to try and understand the influence of ring size, pore volume, dimensionality and chemical composition on diffusion through the framework and to predict which structures and compositions would allow optimal diffusion.

The self-diffusion coefficients calculated using MD simulations can also be measured using experimental techniques such as pulse field gradient-nuclear magnetic resonance (PFG-NMR) and quasi-elastic neutron scattering (QENS) techniques and a project is underway in collaboration with University of Cambridge, UK, and the UK Catalysis Hub at Harwell, UK, to measure diffusion of selected molecules in selected frameworks to compare with MD simulations. Some initial MD simulation results investigating xylene diffusion in USY are presented in Figure 17 for ortho-, meta- and para-xylene. Figure 17 shows diffusion coefficients at temperatures between 300 K and 700 K for each isomer. The diffusion coefficients increase with temperature and indicate that diffusion of the para isomer is faster than meta or ortho isomers. Initial PFG-NMR measurements at University of Cambridge at 318 K gave a diffusion coefficient of 4.6 × 10–10 m2 s–1 which is in good agreement with MD simulated values of 5.92 × 10–10 m2 s–1 at 323 K.

6. Underpinning Research

In addition to the more applied research reported above, a key philosophy is to look at underpinning

research for the methods used to enable continuous improvement in modelling capability. This research is often carried out in collaboration with academic groups. We now proceed to discuss some of the academic collaboration Johnson Matthey has been involved in and how the underpinning knowledge is being used to improve understanding of catalytic materials.

6.1 Redox Active Oxides and Metal Support Interactions

Certain oxides provide a challenge to standard computational methods. Working with academics at Cardiff University, UK, and also via a Royal Society Industrial Fellowship at University College London (UCL), UK, we have been running academic projects looking at pragmatic ways to model ceria and other redox active metal oxides for catalytic activities. A pertinent question in this work was whether trends in metal screening studies need to go beyond standard DFT methods (Figure 18). The inclusion of the so called Hubbard U parameter has been found not only to be vital for obtaining the ‘correct’ electronic structure (105), but also to have a significant influence on obtained formation energies and, importantly for catalysis, shows that reaction energetics can be highly sensitive to the choice of U (Figure 19) (105–107).

Once a reasonable model for the support phase of the catalyst is obtained, the interaction between the nanoparticle and the support must be considered. A crucial question here is: how big are your nanoparticulate catalysts? The answer clearly has a bearing on how to consider modelling the system. For instance Figure 6 shows three separate regimes for catalysts of different sizes: smaller than 2 nm, between 2 nm and 3 nm and beyond 3 nm where the facets of

0 100 200 300 400 500 600 700 800Temperature, K

3.5

3

2.5

2

1.5

1

0.5

0

Ds,

m2 s

–1, ×

10–9

orthometapara

Fig. 17. Diffusion coefficient (Ds) as a function of temperature (T) for ortho, meta and para xylene in zeolite structure USY

Fig. 16. Two low energy sites for Cu+ identified using: (a) interatomic potentials; and (b) QM/MM methods

2.1772.8692.148 2.020

2.562

2.035

2.010

(a) (b)



the nanoparticle are sufficiently large to begin to think in bulk terms (it should be noted the 3 nm bound is a slightly arbitrary choice of bound for this regime). From Figure 7, bulk-like electronic structure can be observed even before the facets grow significantly large. This has now been confirmed by massively parallel calculations of nanoparticles (108–110). This suggests the following general regimes:

(a) <2 nm: a regime that is modulated by both electronic and geometric effects and significantly different reactivity from the bulk terminated surfaces would be expected

(b) 2–3 nm: a regime where the intrinsic electronic nature of the metal is ‘bulk-like’, but influence of transport or limited numbers of a given facet could be influencing the observed catalysis (for example

Fig. 19. (a) dependence of various properties of PdO as a function of Hubbard U parameter, compared to benchmark results. Vacancy formation energy compared to Heyd-Scuseria-Ernzerhof (HSE) calculations of Delley (106). Band gap compared to (107) and Pd4O4 compared to in-house coupled cluster calculations. The region where the difference in properties and energetics approach zero identifies the required value of U; (b) CeO2 reaction profile adapted from (105), here the acute dependence of reaction profile on the choice of U parameter is illustrated, which necessitates a deeper study of choosing U for catalyst problems (105)

0 1 2 3 4 5 6U, eV

2

1

0

–1

–2

–3

Ene

rgy,

eV

ΔE (Pd4O4)ΔE (band gap)ΔE (vacancy)

(a)

0

–1

–2

–3

–4

Ads

orpt

ion

ener

gy, e

V

CeO2 + CO CO/CeO2 CeO2–d + CO2

0.0 eV2.5 eV4.5 eV5.5 eV6.5 eV7.5 eV8.5 eV

(b)

Fig. 18. Density of states calculated using standard DFT (red) and with the Hubbard U correction (blue): (a) lanthanum oxide (La2O3), wide band gap oxides are reproduced quite well; (b) palladium (II) oxide (PdO), standard DFT predicts PdO to be a metal, the Hubbard U correction is required to obtain the correct band gap (as seen in Figure 13, this also improves the calculated energetics); (c) CeO2, whilst the overall band gap (2p–5d) is reproduced well with standard approaches, the width and location of the Ce-4f is highly sensitive to the choice of Hubbard U parameter, it is this state that is responsible for the facile redox properties of CeO2 (104)

–10 –8 –6 –4 –2 0 2 4Energy, eV

02

5

4

3

2

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0

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sity

of s

tate

s

5

4

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0D

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ty o

f sta

tes

–10 –8 –6 –4 –2 0 2 4Energy, eV

–6 –4 –2 0 2 4 6Energy, eV

50

40

30

20

10

0

Den

sity

of s

tate

s

05

05.5

(a) (b) (c)



the availability of hydrogen in the methanation reaction)

(c) >3 nm: the geometry and electronic influences are sufficiently bulk-like to use extended surfaces as ‘good’ models.

In general the size of a nanoparticle has a profound influence on the observed chemistry and subsequent catalytic behaviour. This has been well documented for Au, however limited attention has been given to other metals (111, 112). In Figure 20, this point is illustrated for the activation of methane over silver. Not only does the small size increase the activity of the particle relative to the extended system, it produces a catalyst that is in a different chemical state, thereby opening up chemical routes that would not otherwise have been present.

For small clusters and nanoparticles, the combination of small size and the presence of surface oxide species opens up even more combinations of chemistry. For the case of a small nanoparticle which can be modelled explicitly, the presence of the support introduces an interfacial region that is critical for the chemistry and for the case of reducible oxides it also changes the availability of oxygen for reaction.

6.2 Metal Support Interactions

The chemical interactions between a metal particle and its metal oxide support are of critical importance

for the chemical industry and have been the focus of numerous computational modelling investigations (113–118). The typical size of these metal particles is of the order of the nanometre. At such scales the interaction of the particle with its chemical surrounding is of great importance as it impacts its stability (with public health and economic impacts) and may impact its catalytic activity as well. As a consequence, a PhD project in collaboration with UCL was initiated with the aim of using DFT computational modelling methods to investigate how transition metals such as Pd, Pt and Ni interact with an α-alumina support.

The initial investigation studied how single metal atoms adsorb on the (0001) and (1102) α-alumina surfaces. On both surfaces, the binding strength follows the trend Pt>Ni>Pd (119). When bonding to the surface, all transition metals promote a charge transfer, moving electron density from a neighbouring surface oxygen to a surface aluminium atom (Figure 21).

In order to take into account alumina’s strong affinity for water, a thermodynamic model of a different surface environment was developed. By including the chemical potential of water in the gas phase it was possible to compute the Gibbs free energy of several water coverages of α-alumina surfaces and to predict the state of the surface at a given pressure and temperature. Figure 22 presents the evolution of the Gibbs free

Reaction coordinate (gas → surface)

3

2.5

2

1.5

1

0.5

0

–0.5

–1

eV

NO*CH3*CH3O*2O*2H*

(a)

Fig. 20. Free energy diagrams for surface species present on: (a) a Ag {111} facet and (b) 13 atom cluster: solid = 423 K, dashed = 623 K, dot-dashed = 823 K. It can be seen for the Ag {111} surface only O is present at low temperature and at higher temperatures the surface is clean. However for the small cluster at 423 K, O, H, NO and CH3O are present, while at 623 K O and NO are present and at 823 K, O is still on the surface. The presence of O has significant influence on the calculated activation barriers. Eact[Ag {211}] = 2.15 eV, Eact[Ag13] = 1.57 eV, Eact[Ag13O6] = 0.74 eV; implying that O covered Ag13 is as active as Ni for the dissociation of methane (8)


NO*CH3*CH3O*2O*2H*

3

2

1

0

–1

–2

–3

eV

(b)

NO*CH3*CH3O*2O*2H*




energy of the clean, low (5 OH nm–2) and high hydrated (10 OH nm–2) states of the (0001) surface. At 3.2 kPa, the (0001) surface gradually changes from a heavy hydration state to a fully clean surface at around 900 K. On the (1102) surface, the transition between the high OH coverage and the clean surface is much more sudden without an intermediate moderate hydration state (120).

Surface hydration has a strong impact on the interaction of the transition metal with its support as the surface aluminium sites are now fully occupied by OH groups and are not available to receive electron density moved from surface oxygen atoms. Instead, upon adsorption, all three investigated transition metal atoms trigger the rupture of a surface OH group, followed by the migration of the hydrogen atom to the metal atom. This mechanism is called ‘spill-over’ and

enables a significant stabilisation of Pt and Ni atoms on the alumina surface (120).

In order to gain some insight into the catalytic activity of supported metal nanoparticles, the interactions of a carbon monoxide probe with a supported Ni6

nanocluster has been studied. CO was chosen for its relative simplicity and its wide use in catalytic processes such as steam reforming, where supported Ni catalysts are also of relevance. The (0001) α-alumina surface has a strong influence on the CO interaction with the Ni6 cluster. The favoured adsorption site on the supported Ni6 cluster is on a hollow site on the side of the particle. On that site, CO is also able to interact with a surface aluminium atom (Figure 23) (121). The CO bond is elongated (1.28 Å vs. 1.13 Å in the gas phase and 1.20 Å on a (111) Ni hexagonal close-packed (hcp) site) and its stretching frequency is lowered (1397 cm–1

vs. 1899 cm–1 on a (111) Ni hcp site). This dramatic activation of the CO molecule illustrates how, even though considered as inert, the catalyst support may play an active role in the reaction cycle.

For small nanoparticles, it is important to provide an explicit computational model to describe mechanistic aspects of sub-nanometre particles supported on oxides. Work in collaboration with David Willock, Cardiff University, has been bringing together studies on redox active supports and small metal nanoparticles in a bid to understand the metal support interaction and its influence on reactivity (122). For the probe reaction of CO oxidation, new mechanistic pathways have been opened up due to the metal/oxide interface that radically changes the rate-determining step (Figure 24) (123). The theme of metal support interaction has also seen some experimental contribution and recent work with Tsang’s group in Oxford has been published on the

200 400 600 800 1000 1200 1400 1600Temperature, K

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Sur

face

ene

rgy,

J m

–2

Fig. 22. Gibbs surface free energy at 3.2 kPa water partial pressure for the clean (blue), 5 OH nm–2 (green), and 10 OH nm–2 (red) hydration coverage of (0001)) α-Al2O3 (Adapted with permission from (120). Copyright (2003) American Chemical Society)

Fig. 23. Side view of CO on the Ni6 (0001)) α-Al2O3 system (Adapted with permission from (121). Copyright (2010) American Chemical Society)

Fig. 21. Top view of an adsorbed Pd single atom on (0001) α-Al2O3 Figure adapted from (119)



role of hydrogen spill-over influence on the reducibility of ceria supported pgm catalysts (64).

Obtaining an explicit description of large metal nanoparticles interacting with an oxide support is still a computational challenge beyond current computing capacity. However, a simple thermodynamic screening model has been developed with the potential to predict the general behaviour of certain transition metal/oxide support combinations, in the limit of large nanoparticles. Taking inspiration from the field of electrochemistry where a chemical reaction is considered in terms of the half-reaction, we can consider the following approximations:(a) metal supported oxides can be split into two

components (metal and oxide)(b) redox catalysts can be considered analogous to

electrochemical half reactions(c) one half reaction occurs on the metal, the other on

the oxide.Taking the example of simultaneous CO oxidation

and NO reduction it can be assumed that NO reduction occurs on the metallic component while CO oxidation occurs on the oxide component. Calculating the free energy profile for a simplified reaction pathway allows firstly existing databases of metal components to be used, secondly new databases to be developed for the oxide component and finally metal/oxide pairings to be identified using calculations that are currently tractable on available resources. Figure 25 illustrates the approach being used in the example of CO and NO. Early results in this area are promising and new

methods are currently being explored to screen for novel catalyst candidates.

Whilst models can be developed either explicitly or indirectly for the metal/support interaction for the two outer-bounds of the nanoparticle regimes under investigation, there is still a significant computational challenge in simulating the intermediate regime. The following section describes work towards developing a capability that will allow the 2–3 nm regime to be tackled explicitly, starting with an electronic structure theory of the metal nanoparticle itself.

6.3 Large-scale Calculations of Metal Nanoparticles

One of the significant barriers to developing complex models of real catalysts is the limitation on system size that can be tackled explicitly. Significant progress can be made with a judicious choice of model and some prior understanding of the system in question. If one is simply interested in geometric structure then arguably, embedded atom or other parameterised potential approaches can be utilised (124, 125). However in catalysis, especially when trying to understand chemical activity, there is often interest in linking the electronic structure to reactivity. This requires methods for conducting large scale electronic structure theory calculations, for instance computer codes with favourable scaling over many computer cores (for example GPAW, Order-N Electronic Total Energy Package (ONETEP) (126, 127)) or semi-empirical approaches (128).

(a)100500

–50

–100

–150

–200–250–300–350

catalyst + CO (g)

O..CO (ts)

E

C, D

B

A

CO (ads)

CO2 (ads)

Grey: [O] = Au-OBlack: [O] = Fe-O

(b)

Fig. 24. (a) Au_10 nanoparticle adsorbed on iron (III) oxide (Fe2O3) (0001) surface, (blue: Fe, red: surface oxygen, yellow: Au, grey: carbon, green: CO oxygen); (b) reaction profile for the adsorption and reaction of CO with different surface O in the vicinity of the Au cluster. It can be seen there are a several distinct adsorption site Fe2O3, Au and interfacial, that lead to very different adsorption energies, furthermore the different reacting surface oxygens also lead to different activation barriers. (Adapted from Scott Hoh PhD thesis (123))



One part of this research is looking at exploiting the linear scaling capabilities of ONETEP through collaboration with Chris-Kriton Skylaris, Southampton University, UK. Enhanced functionality of ONETEP has been exploited to help in the simulation of metals (129) and work is now underway benchmarking speed and scaling for the adsorption of molecules such as O2, CO, OH and atomic O. The difference in adsorption energies between constrained but pre-optimised nanoparticles and ligands and between fully geometry-optimised systems of nanoparticles and ligands has been studied by looking at various sizes of cuboctahedral Pt clusters (Pt13, Pt55, Pt147) and comparing to more conventional slab models. The results show significant deviations in binding strength between the fully relaxed and constrained systems.

Another project with the University of Limpopo, South Africa, is developing parameters to describe Pd nanoparticles and their interaction with oxide supports via the semi-empirical, density-functional tight binding (DFTB) approach, which potentially allows thousands of atoms to be simulated using limited computational resources. A model system to understand the usefulness of this approach has been methane oxidation using Pd supported on titania (TiO2). This system has a number of complexities, including the metal/ceramic interface

and the propensity for the oxidation state of the surface to change during reaction. A significant challenge to utilising the DFTB approach is to find truly transferable parameters to describe all of the necessary interaction present in the simulation (130). Figure 26 illustrates geometrical structures obtained from ONETEP, GPAW and DFTB for nanoparticles that are tractable with current computational resources, the future will see this capability extended.

There are several fundamental experimental projects to augment the theoretical work on nanoparticles. The first of these involves the Nellist group from Oxford Materials, UK, who have derived experimental methods using quantitative annular dark-field scanning transmission electron microscopy (ADF-STEM)(131). Pt nanoparticle atomic coordinates have been determined for particles up to Pt943 atoms. The systems were geometry optimised using a damped MD and interatomic force-field approach, before being electronically minimised in ONETEP and studied quantum mechanically. Atomic oxygen adsorption calculations were then performed on the most exposed facet, which exhibits fcc-{111} surface symmetry and the least coordinated sites were found to be the most strongly binding, agreeing with terrace measurements from experimental literature.

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50 K

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Metal surface

CO (g) +

NO (g)

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NOM *

CO (g) +

NM * +

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0.5N 2 + O

M *

COOx * +

0.5N 2 + O

M *

CO 2 (g) +

0.5N 2 + O

M *

CO 2 (g) +

0.5N 2

AuAgCuPdPtNiRhCoRuFeReMoW

Fig. 25. Reaction profile for the NO reduction, CO oxidation, occurring on a series of metals left hand side and Ce surface, right hand side. The highlighted dashed line illustrates the interpolation between reactants and products. To a first approximation this interpolation minimises the free energy pathway and shows the region where an optimal catalyst can be found



The second experimental project is looking at methane oxidation, in order to provide data about nanoparticle surface composition under a reactive gas-phase atmosphere that in turn can be used to validate theoretical predictions. The project is being run with Professor Georg Held, University of Reading, UK, and

methods are being developed to create well defined nanoparticle samples for use in ambient pressure X-ray photoelectron spectroscopy (XPS) synchrotron beamlines (132). The ultimate aim is to bridge the ubiquitous temperature, pressure and materials gaps between surface science and real catalysts. The ambient pressure XPS allows the oxidation of different sized nanoparticles to be observed as a function of reactive atmosphere, which in turn can be correlated back to theoretical predictions. This project is ongoing and in the future will make use of the forthcoming versatile soft X-ray (VERSOX) beamline at Diamond Light Source, Harwell, UK, to develop even further new understanding of catalysis materials.

6.4 Moving Beyond the Atomic Scale

The emerging field of modelling across length scales is ideally suited to modelling catalysis since catalyst solutions rely not only on the intrinsic chemical properties of an active material, but also on the transport properties of the material at different scales. Figure 27 (133) illustrates the case of an automotive monolith, highlighting the levels that need to be modelled to develop a holistic description of the catalytic solution. A complete description of a technical solution includes describing, for example, the atomic or electronic level of matter, the porous structure of the catalyst layer (at two distinct levels: small nanopores and large macropores) and the reactor itself. Furthermore it also

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Fig. 27. Schematic illustrating some of the regimes encountered when modelling a catalyst solution, in this case a monolith reactor for emission control. Starting at the left hand side we have the intrinsic kinetics resulting from the atomic scale interactions, moving across to the meso and macroporous structure where diffusion of the reactant gases is necessary to find the active catalytic sites, finally on the right hand side the macroscale model of the reactor channel in the monolith. (Adapted from (133))

(a) (b)

(c) (d)

Fig. 26. Representations of geometrically relaxed nanoparticles, calculated using the different approaches discussed in the text. (a) Pt309 nanoparticle electrostatic potential and (b) Pt309 electron density, both calculated using ONETEP; (c) Pd321 nanoparticle calculated using the DFTB + approach; (d) 79 atom Pd/Pt core-shell with a monolayer of O adsorbed calculated using GPAW



requires the study of different time scales ranging from femtoseconds (the timescale on which the chemistry happens) to years (corresponding to the expected lifetime of the catalyst). A combination of physics, chemistry and engineering are required to generate statistical methods and engineering models, which along with chemical insight into mechanisms will provide a more complete picture of the catalytic system.

This project started through a Royal Society Industrial Fellowship held at UCL, Johnson Matthey Technology Centre and the Institute of Chemical Technology, Prague. Its aim is to look at applying the Prague group’s meso-scale methods to simulation of transport and reaction in porous catalysts (133–142). This project has been developing over the last few years and has an experimental component, which is essentially for benchmarking and validating the theoretical methods. Through the careful preparation of active catalyst it has been possible to construct layered catalysts consisting of a uniform active layer overlaid by an inert layer with well-defined morphology (thickness, porosity, particle size) that serves as a diffusion layer (Figure 28) (143, 144).

Through this approach it has been possible, firstly, to validate the computational models by simulating the transport and activity of the precise geometry prepared in the lab; and secondly, to develop a novel approach to determining the diffusivity of gases through porous media. This second aspect is important because it provides insight into the structural properties of the catalyst layer and how they influence its overall performance, as well as providing a simple relationship

for determining diffusivity that can be utilised in simpler models of an entire catalytic reactor (144). With the development of ab initio oxidation models a fully holistic ab initio description of the catalytic system is becoming closer.

7. Research Outlook

This brief review has described some of the projects run within the atomic-scale groups at Johnson Matthey. Looking to the future, the following areas appear as natural progressions. Broadly speaking there are three aspects:

Firstly, theoretical development and understanding. Continued work on model systems aims to enhance the description of nanoparticles and oxides and ultimately bring them together. The description is not just about the structure of the material, it is perhaps more importantly about using thermodynamic models to understand the state of a catalyst under reaction conditions. If one has a good model of the catalyst under reaction conditions, the reaction can be decomposed to its elementary steps and the key bond making and breaking that controls the activity and reactivity of a given material can be understood.

Secondly, further development of the correlation between a theoretical understanding of catalyst structure, activity and stability with experimental characterisation. There has undoubtedly been huge progress in this respect over the last two decades. However, specific challenges remain. For instance the combination of synthesising ideal shapes and

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Fig. 28. (a) Scanning electron micrograph depicting the inert alumina layer, active Pt/alumina layer and substrate material; (b) simulated concentration profile of CO in reconstructed porous media; (c) rate of CO consumption as a function of position in the porous media. Note the activity is purely located in the active region of the catalyst. (Adapted from (144))



structured nanoparticles for catalyst testing, with advanced characterisation techniques (for example the upcoming VERSOX beamline at Diamond) along with the detailed models from ab initio theory, provides an exciting prospect for developing a deep fundamental understanding of catalyst materials.

Thirdly, the possibility of going beyond catalyst materials problems to include thermodynamic modelling (using the MTData code (145)) and process modelling using tools like gPROMS (146). Furthermore, traditional materials physics problems can also be studied, for instance smart materials where the challenge lies in describing both the thermodynamic and electronic properties adequately to make material predictions. Another burgeoning area is in battery materials technology where simulation and modelling will play a significant role.

8. Conclusions

To conclude, if the last twenty years have seen growth in the capability of computational chemistry and widespread acceptance of the field as a sub-discipline of science, the next two decades hold the prospect of even greater progress. Utilisation of petascale computing, the accessibility of high performance computing and open source code development means that even larger and more complex systems are accessible and can be tackled by more people. Multiscale models that capture not only the fundamental physics and chemistry of the electronic and molecular level process, but also the macroscopic properties of the device in question will also become routine. Automation of simulations coupled with high throughput synthesis will revolutionise the way in which we discover new materials. Atomic-scale modelling and its subsequent expansion to other scales is helping Johnson Matthey be part of this computational revolution that will continue to provide increasing opportunities and will be at the heart of developing catalytic and materials science into the 21st century.

9. Acknowledgements

The work presented herein is a result of a number of internal and external collaborations. It is hard to acknowledge everyone who has been involved in the projects over the last seven years or so, however along with our experimental colleagues within Johnson Matthey, the Engineering and Physical Sciences Research Council (EPSRC) should be acknowledged

for funding of Collaborative Awards in Science and Engineering (CASE) and doctoral training centre students. Misbah Sarwar acknowledges J. L. Gavartin (Accelrys) and the Technology Strategy Board (TSB) for support in the iCatDesign project (DTI Project No: /5/MAT/6/I/H0379C). Glenn Jones acknowledges the EU for supporting the IMPRESS project (Contract NMP3-CT-2004-500635, co-funded by the European Commission in the sixth Framework Programme and European Space Agency) and the Royal Society for supporting his industrial fellowship at UCL (IF090100). Ludovic Briquet acknowledges the EU for supporting his Framework 6 Program Marie-Curie Early Stage Training studentship. Crispin Cooper gratefully acknowledges support from the EPSRC Collaborative Training Account: UCL (GR/T11364/01). Glenn Jones and Crispin Cooper also acknowledge the UK’s super computing facility for time on HECToR, which has now been superseded by ARCHER, through the UK’s HPC Materials Chemistry Consortium (EPRSC: EP/L000202).

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The Authors

Misbah Sarwar is currently a Principal Scientist at Johnson Matthey Technology Centre (JMTC), Sonning Common, UK. She has an MSci in Chemistry from University College London (UCL), UK, and a PhD from UCL and the Royal Institution (RI). Misbah joined Johnson Matthey in 2007 to work on the Technology Strategy Board (TSB) funded project iCatDesign. In April 2014 Misbah joined the Emission Control Technology research group at JMTC.

Crispin Cooper is a computational chemist working in the JMTC Emission Control research group. He has a BSc (hons) in Chemistry for Drug Discovery from the University of Bath, UK, and an EngD in Molecular Modelling and Materials Simulation from UCL. He has recently joined Johnson Matthey following an industrial postdoctoral project modelling complex metal oxide catalyst materials.

Ludovic Briquet is a Senior Scientist at JMTC, Sonning Common. He gained his PhD in Chemistry at UCL, UK, in 2010 and specialised in molecular modelling applied to various systems. His main interests however lie in surface science and catalysis.



Aniekan Ukpong is a Senior Scientist at JMTC, Pretoria, where he has been working since 2013. He completed his PhD in Physics at the University of Cape Town, South Africa, in 2008. His research focuses on the fundamental study of materials from theory to applications.

Christopher Perry originally trained as a bio-inorganic chemist. He became interested in modelling whilst lecturing at the University of the Witwatersrand, South Africa. Prior to that, he completed two postdoctoral research projects: one in nuclear magnetic resonance (NMR) spectroscopy at Manchester University, UK, and a second in organic synthesis at the University of the Witwatersrand. He has been working at Johnson Matthey since 2013 and is a Senior Scientist at JMTC, Pretoria.

Glenn Jones has extensive experience in the fields of surface science, catalysis and computational materials chemistry. He gained his PhD from the University of Cambridge, UK, before moving to the Technical University of Denmark as a post-doctoral student. He joined JMTC, Sonning Common, UK, in 2008 and was awarded a Royal Society Industrial Fellowship in 2010 which he held jointly between JMTC and UCL. He moved to Pretoria, South Africa, in 2013 to initiate JMTC’s new modelling laboratory in South Africa, where he is currently Research Manager.




In the Lab

Combining Catalyst and Reagent Design for Electrophilic AlkynylationJohnson Matthey Technology Review features new laboratory research

Jérôme Waser is an Associate Professor in the Institute of Chemical Sciences and Engineering at the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland. His research focuses on the development of new reactions based on catalysis and synthons with non-conventional reactivity.

About the Research

Alkynes are essential building blocks in synthetic and medicinal chemistry, materials science and chemical biology. Due to their linear geometry and electronic properties, they are important structural elements in supramolecular assemblies and organic materials. The unique reactivity of the triple bond also makes them ideal precursors of other functional groups, not only in a classical chemistry setting, but also for biological applications. The development of new methods to make alkynes is consequently an important fi eld of research in fundamental organic chemistry.

The transfer of terminal alkynes is one of the most successful approaches for introducing triple bonds into organic molecules. This fi eld has been largely dominated by the use of acetylide anions or their equivalents as nucleophiles, due to their ease of formation. Processes such as the Sonogashira coupling and the addition of alkynes to carbonyls are highly reliable and are widely used in synthetic chemistry. Nevertheless, the drawback of this approach is that alkynes can be introduced only to the electrophilic positions of molecules . If good electrophilic alkyne synthons were available, alkyne chemistry would become even more versatile for applications in chemistry and in biology.

Waser’s group have designed new methods for the introduction of alkynes into organic molecules using transition-metal catalysis and electrophilic alkynylation reagents. The direct alkynylation of electron-rich heterocycles was developed fi rst (Scheme I). The group harnessed the unique properties of ethynylbenziodoxolone (EBX) reagents for the gold-catalysed alkynylation of indoles, pyrroles, thiophenes and furans. The cyclic hypervalent iodine

About the Researcher

• Name: Jérôme Waser

• Position: Associate Professor

• Department: Institute of Chemical Sciences and Engineering

• University: EPFL

• Street: EPFL SB ISIC LCSO, BCH 4306, Av. Forel 2

• City: Lausanne

• Post Code: 1015

• Country: Switzerland

• Email Address: jerome.waser@epfl .ch



reagent 1-[(triisopropylsilyl)ethynyl]-1,2-benziodoxol-3(1H)-one (TIPS-EBX) (Figure 1) was initially discovered by Zhdankin and although it displays an enhanced stability in the presence of transition metals, it still acts as a strong electrophilic alkyne source. The alkynylation of heterocycles with TIPS-EBX is a user-friendly method, which proceeds in open fl asks at room temperature and can tolerate a broad range of functional groups.

The developed C–H alkynylation is highly selective for the most electron-rich position of heterocycles.

To access triple bonds on other positions of aromatic rings, the group decided to use a new strategy based on a domino cyclisation-alkynylation process (Scheme I). This approach was fi rst applied to the synthesis of 3-alkynylated furans starting from allene ketones. The key for success was electronic tuning of the hypervalent reagent, with TIPS-F6-EBX being most successful. The synthesis of indoles alkynylated on the benzene ring is even more challenging, due to the much higher reactivity of the pyrrole ring. In this case, no successful C–H functionalisation method has yet been reported. Using the platinum-catalysed domino cyclisation-alkynylation of homopropargylic alkynyl pyrrole ethers, 5- or 6-alkynylated indoles could be synthesised in good yield and selectivity. Overall, the domino strategy is therefore highly complementary to C–H alkynylation.

To introduce alkynes onto C–sp3 centres, Waser’s group focused on the metal-catalysed multi-functionalisation of olefi ns. The intramolecular oxy- and amino-alkynylation of olefi ns using TIPS-EBX and a palladium(II) catalyst to give lactones and lactams at room temperature in open fl asks was developed fi rst (Scheme II). To access tetrahydrofurans and pyrrolidines, the combination of a Pd(0) catalyst and alkynyl bromides was more successful. In this case, the reaction was run at 65ºC under inert gas. Currently, the transformation cannot be made intermolecularly.

Scheme I. Gold and platinum-catalysed C–H alkynylation vs. domino cyclisation functionalisation for accessing alkynylated heterocycles

NH

RN

R1

R2S

RO

R

AuCl catalystRoom temperature, open fl asks

NH

R

SiiPr3 NR2

R1

SiiPr3

C2-pyrroles and C3-indoles48–93%

21 examples

SR

SiiPr3

C2-thiophenes48–83%

21 examples

OR

SiiPr3

C2-furans45–90%

12 examples

O

O I SiiPr3

TIPS-EBX

C–H Alkynylation: most electron-rich position

RO

R1

NR

MeO

IOF3CF3C

SiiPr3

TIPS-F6-EBXAuIII or PtII catalystRoom temperature, open fl asks

OR

SiiPr3

C3-furans53–97%

12 examples

iPr3SiR1

NR

C5 and C6-indoles31–84%

24 examples

Domino cyclisation-alkynylation: unreactive position

Fig. 1. X-ray structure of TIPS-EBX



Nevertheless the group recently reported a fi rst step in this direction by the use of an in situ tethering strategy for palladium-catalysed synthesis of vicinal aminoalcohols bearing an alkyne group starting directly from allyl amines. The use of trifl uoroacetaldehyde in its commercially available hemiacetal form as a tether played an important role in this reaction.

In conclusion, the metal-catalysed electrophilic alkynylation approach has allowed heterocyclic and aliphatic alkynes to be synthesised with high effi ciency. The obtained products are expected to be highly useful as building blocks in synthetic and medicinal chemistry, as well as in materials science.

Selected PublicationsU. Orcel and J. Waser, Angew. Chem. Int. Ed., 2015, 54,

(17), 5250

Y. Li and J. Waser, Angew. Chem. Int. Ed., 2015, 54, (18), 5438

Y. Li, J. P. Brand and J. Waser, Angew. Chem. Int. Ed., 2013, 52, (26), 6743

J. P. Brand and J. Waser, Chem. Soc. Rev., 2012, 41, (11), 4165

S. Nicolai, C. Piemontesi and J. Waser, Angew. Chem. Int. Ed., 2011, 50, (20), 4680

S. Nicolai and J. Waser, Org. Lett., 2011, 13, (23), 6324

J. P. Brand and J. Waser, Angew. Chem. Int. Ed., 2010, 49, (40), 7304

S. Nicolai, S. Erard, D. Fernández González and J. Waser, Org. Lett., 2010, 12, (2), 384

J. P. Brand, J. Charpentier and J. Waser, Angew. Chem. Int. Ed., 2009, 48, (49), 9346

R3

R3

R4

O

R1

R

N Boc

R1

R

N

R3

RR2OF3C

22 examples55–92%

20 examples51–86%

28 examples51–98%

Br RO

O I SiiPr3

TIPS-EBX

XHR2R3

R1

Pd0 catalyst65ºC, N2

PdII catalystRoom temperature,

air

R3O Me SiiPr3

OR3

O

R2

SiiPr3

OXR3 SiiPr3NTs

R2

10 examples43–87%

10 examples69–83%

20 examples50–89%

Scheme II. Palladium-catalysed olefi n oxy- and amino-alkynylation

Y

EMISSION CONTROL TECHNOLOGIESIncreased NO2 Concentration in the Diesel Engine Exhaust for Improved Ag/Al2O3 Catalyst NH3-SCR ActivityW. Wang, J. M. Herreros, A. Tsolakis and A. P. E. York, Chem. Eng. J., 2015, 270, 582

Fast-SCR was investigated for NOx reduction in internal combustion engines. H2 addition was found to increase NO2 formation over a Ag/Al2O3 catalyst. This led to an improved NH3-SCR activity at low temperature. It is concluded that NO2 formation before the Ag/Al2O3 catalyst either in the engine or a Pt/Al2O3 based DOC will improve SCR performance. This NO2 promotion effect was less at higher temperatures.

Thermochemical Recovery Technology for Improved Modern Engine Fuel Economy – Part 1: Analysis of a Prototype Exhaust Gas Fuel ReformerD. Fennell, J. Herreros, A. Tsolakis, K. Cockle, J. Pignon and P. Millington, RSC Adv., 2015, 5, (44), 35252

Reformed exhaust gas recirculation (REGR) provides H2 to the combustion process to recover heat from exhaust and improve fuel conversion effi ciency. A full scale prototype reformer for gasoline direct injection engines is presented and its performance is assessed. The performance is better at higher temperatures with a decline in performance at lower exhaust temperature. The reformate quality is also dependent on process temperature and reactant composition.

FINE CHEMICALSPalladium-Catalyzed ɑ-Arylation Reactions in Total SynthesisS. T. Sivanandan, A. Shaji, I. Ibnusaud, C. C. C. Johansson Seechurn and T. J. Colacot, Eur. J. Org. Chem., 2015, (1), 38

New methods for synthesising natural products and active pharmaceutical ingredients have been explored using palladium-catalysed ɑ-arylation of carbonyl compounds. The advantages of this particular method are an increase in the overall yield, an improved synthesis scope and a reduction in the number of steps.

The signifi cance of palladium-catalysed ɑ-arylation methods are discussed and a number of case studies have been included.

The Effects of 1-pentyne Hydrogenation on the Atomic Structures of Size-selected AuN and PdN (N = 923 and 2057) NanoclustersK.-J. Hu, S. R. Plant, P. R. Ellis, C. M. Brown, P. T. Bishop and R. E. Palmer, Phys. Chem. Chem. Phys., 2014, 16, (48), 26631

The variation in atomic structures of size-selected Au and Pd nanoclusters (containing 923 and 2057 atoms) supported on amorphous carbon fi lms before and after being exposed to the vapour-phase hydrogenation of 1-pentyne was studied. The populations of the nanoclusters were studied at atomic resolution before and after the reaction using an aberration-corrected high-angle annular dark fi eld (HAADF) scanning transmission electron microscopy (STEM). The atomic structures of the observed nanoclusters were determined by comparing the multi-slice HAADF-STEM and experimental images for a full range of cluster orientations. The results show that Au nanoclusters consisting of 923 ± 20 and 2057 ± 45 atoms are robust and exhibit high structural stability. A big proportion of Pd923 ± 26 nanoclusters, on the other hand, appear to be amorphous before the treatment and after the reaction were found to exhibit high symmetry structures which suggests the reduction of oxidised Pd nanoclusters in reaction conditions.

FINE CHEMICALS: CATALYSIS AND CHIRAL TECHNOLOGIESStereoselective Synthesis of the Halaven C14-C26 Fragment from D-Quinic Acid: Crystallization-Induced Diastereoselective Transformation of an α-Methyl NitrileF. Belanger, C. E. Chase, A. Endo, F. G. Fang, J. Li, S. R. Mathieu, A. Z. Wilcoxen and H. Zhang, Angew. Chem. Int. Ed., 2015, 54, (17), 5108

A series of substrate controlled stereoselective reactions with crystalline intermediates was carried out via an α-methyl nitrile to produce a C14–C26 fragment of halichondrin B/Halaven. The synthesis does not require

Johnson Matthey HighlightsA selection of recent publications by Johnson Matthey R&D staff and collaborators





chromatography and relies entirely on crystallisation for quality control. D-quinic acid is the starting material, providing all four chiral centres, and is readily available. Raw material cost and waste are both reduced by the present synthesis.

A Halogen- and Hydrogen-Bonding [2]Catenane for Anion Recognition and Sensing J. M. Mercurio, A. Caballero, J. Cookson and P. D. Beer, RSC Adv., 2015, 5, (12), 9298

Halogen bonding has been little explored outside the areas of solid state crystal engineering. A novel halogen bonding rotaxane structure was prepared for use in anion recognition and exhibits good anion recognition and sensing properties. An ion templated Grubbs’ II-catalysed RCM clipping mechanical bond forming was used to synthesise the structure which contains both halogen- and hydrogen-bonding macrocyclic components. 1H NMR spectroscopy and fl uorescence titration experiments were carried out and showed that the new structure can strongly associate with acetate and dihydrogen phosphate.

NEW BUSINESSES: FUEL CELLSPlatinum-carbide Interactions: Core-shells for Catalytic UseJ. L. R. Yates, G. H. Spikes and G. Jones, Phys. Chem. Chem. Phys., 2015, 17, (6), 4250

Five carbides (TiC, NbC, TaC, WC and SiC) were investigated using density functional theory with the aim of determining their suitability as core-shell components in fuel cell applications. The fcc forms of the carbides were compared with hexagonal close-packed (hcp) WC and zinc blende SiC and the latter was found to support Pt overlayers on surfaces, therefore, showing potential for full Pt encapsulation. The transition metal surface resonances (TMSRs) play a vital role during the adsorption of Pt on fcc (111) carbide surfaces and fcc (100) was found to be adverse towards Pt adsorption. Reduced oxygen adsorption energies was displayed by several Pt-WC surfaces during the oxygen adsorption study; the authors conclude that ORR activity should be promoted or maintained with respect to nanoparticulate Pt catalysts.

PROCESS TECHNOLOGIESDual Doping Effects (Site Blockage and Electronic Promotion) Imposed by Adatoms on Pd Nanocrystals for Catalytic Hydrogen ProductionS. Jones, S. M. Fairclough, M. Gordon-Brown, W. Zheng, A. Kolpin, B. Pang, W. C. H. Kuo, J. M. Smith and S. C. E. Tsang, Chem. Commun., 2015, 51, (1), 46

Additives based on polymer or metal adatoms can modify the electronic structure of metal nanoparticles but greater understanding of atomic level effects

is needed to rationally design better catalysts by surface tailoring. Electronic and geometric effects of various metals on unsupported Pd nanocrystals were investigated using the decomposition of HCOOH to H2 and CO2 as a probe reaction. Bi was found to occupy high index sites causing a decrease in HCOOH dehydration, Te occupies terrace sites which reduces the dehydrogenation rate while Ag induced strong electronic effects and increased the activity of the Pd surface sites. Ag and Bi were concluded to be the most effective additives for a surface reaction, while Te should be added at corner sites to promote the desired reaction route.

Surfactant Mediated CO2 Adsorption: The Role of the Coimpregnation SpeciesC. M. Starkie, A. Amieiro-Fonseca, S. P. Rigby, T. C. Drage and E. H. Lester, Energy Procedia, 2014, 63, 2323

Carbon capture and storage requires novel, second generation adsorbent systems to potentially reduce costs associated with this technology. Solid supported amines have been investigated. These consist of basic amines either tethered or impregnated on silica or alumina and co-impregnated with surfactant additives. The mechanisms of adsorption of these systems were studied and they were shown to have 55% improved working capacity relative to single component systems. Triethanolamine and sodium dodecylsulfate produced the best adsorbent properties.

The Synergistic Effect in the Fe-Co Bimetallic Catalyst System for the Growth of Carbon Nanotube ForestsD. Hardeman, S. Esconjauregui, R. Cartwright, S. Bhardwaj, L. D’Arsié, D. Oakes, J. Clark, C. Cepek, C. Ducati and J. Robertson, J. Appl. Phys., 2015, 117, (4), 044308


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Reproduced by permission of the PCCP Owner Societies from J. L. R. Yates, G. H. Spikes and G. Jones, Phys. Chem. Chem. Phys., 2015, 17, (6), 4250


The growth of multi-walled carbon nanotube forests using an active bimetallic Fe-Co catalyst was investigated. When this bimetallic catalyst system was compared to pure Fe or Co a synergistic effect is observed. The height of the forests was considerably increased and an improvement of the homogeneity in the as-grown nanotubes was found. The catalyst system was characterised using energy dispersive spectroscopy and in situ X-ray photoelectron spectroscopy. The authors conclude that the growth rate of the nanotubes is greatly improved in the presence of Fe and Co.

TEM Characterization of Simultaneous Triple Ion Implanted ODS Fe12CrV. de Castro, M. Briceno, S. Lozano-Perez, P. Trocellier, S. G. Roberts and R. Pareja, J. Nucl. Mater., 2014, 455, (1–3), 157

The performance of oxide dispersion strengthened (ODS) ferritic/martensitic steels under irradiation is studied. This is essential in the design of advanced fusion reactors. Transmission electron microscopy was used to characterise a simultaneous triple ion implanted ODS Fe12Cr steel with the aim of investigating the impact of irradiation on the grain and dislocation structures, oxide nanoparticles and other secondary phases present in

steel. The ODS steel was irradiated simultaneously with Fe8+, He+ and H+ at room temperature to a damage of 4.4 dpa at the Joint Accelerators for Nanosciences and NUclear Simulation (JANNUS) Saclay facility. The authors concluded that ODS nanoparticles are very stable under these irradiation conditions.

An Experimental Investigation of Biodiesel Steam ReformingS. Martin, G. Kraaij, T. Ascher, D. Wails and A. Wörner, Intl. J. Hydrogen Energy, 2015, 40, (1), 95

The optimum operating conditions of a proprietary precious metal based catalyst for biodiesel steam reforming was investigated with the aim of preventing catalyst deactivation. Different operating conditions include varying the temperature from 600ºC to 800ºC, applying different pressure from 1 bar to 5 bar and altering the molar steam-to-carbon ratio from 3 to 5. Coke formation and sintering have been determined as the main deactivation mechanisms. The authors conclude that coking can be reduced by using low feed fl ow rates (31 g h–1 cm–2) and a relatively high catalyst inlet temperature (>750ºC).


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Johnson Matthey Technology Review is Johnson Matthey’s international journal of research exploring science and technology in industrial applications


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