Steam reforming over Ni catalyst

A Study on the Suppression of Carbon Formation during

Steam Reforming Reactions over Nickel based Catalysts

D.HARSHINI

Fuel Cell Research Center, Korea Institute of Science and Technology

University of Science and Technology

Fuel Cell Research Center

Objective of This Work

• Preparation and characterization of various Ni based catalysts

• Suppression of carbon formation during SR reactions using various fuels


Outline

Chapter 1: Introduction

Chapter 2: Steam Reforming of methane over Ni-Al alloy catalysts prepared by Chemical-Vapor-Deposition method

Chapter 3: Suppression of Carbon Formation in Steam Reforming of methane by addition of Co into Ni Supported Catalysts

Chapter4: Catalytic Performance of Ni/LaAlO3 Catalysts for Propane Steam Reforming by Various Synthesis Methods

Chapter 5: Influence of Tb Doping on OSC of CeO2-ZrO2 Supports

Chapter 6: Structural characterization and Dry Reforming of Methane over rare earth metals doped CeO2-HfO2 Materials


Chapter 1 : Introduction

In order to make eco friendly environment, it is imperative to search and develop new and clean sources of energy. A promising such technology is the production of electrical power with fuel cells.

Anode reaction:H2 → 2H++2e- (1)

Cathode reaction:1/2O2 + 2H++2e- →H2O (2)

Overall reaction:H2 +1/2O2 → H2O (3)

Advantages of Fuel cells

Low/Zero Emissions

High Efficiency/Low CO2

Distributed Installation

High Quality Power

Quiet

Few moving parts


PEMFCElectric device

Fuel (hydrocarbon)

REFORMER( steam reforming, partial reform-

ing, auto thermal reforming )

High Temp. WGS(CO converter)

Preferential oxidation(CO converter)

Low Temp. WGS(CO converter)

Fuel Processor

Fuel cell systemChapter 1 : Introduction


Fuel Reforming

• Fuel Reforming for fuel cell systems– A production of H2-rich syngas from fuel for applying fuel processor..

– CnHmOp + x O2 + y H2O → aH2 + bCO2 + cCO + hydrocarbon

• CH4 Reforming

1. SRM(steam reforming of methane): a possible syngas composition is H2/CO=3 CH4 + H2O → CO + 3H2 ;ΔH˚ = 206kJ/mol 2. POX(partial oxidation of methane): a possible syngas composition is H2/CO=2 CH4 + 1/2O2 → CO + 2H2 ;ΔH˚ = -36kJ/mol

3. CO2 reforming: a possible syngas composition is H2/CO=1 CH4 + CO2 → CO + H2 ;ΔH˚ = 247kJ/mol

4. ATR(autothermal reforming of methane): POX + SRM in one reactor CH4 + xO2 + yH2O → aH2 + bCO + cCO2 + and so on , ΔH˚ = 0kJ/mol

B.C.H. Steele, Nature 400(1999) 619

Steam reforming of natural gas offers an efficient, economical, and widely used process for hydrogen production.

The efficiency of the steam reforming process is about 65% to 75%, among the other commercially available

production methods.



Challenges of Reforming Process

• Flow Rates:– High efficiency is desired at slower flow rates

» Waste less fuel and energy, less catalyst

• Cost:– Parts can be expensive, as well as precious metal coated catalysts.

• Catalyst:– Catalyst needs to withstand impurities such as sulfur, but also produce

the most amount of hydrogen possible

• Temperature:– High efficiency needs to be achieved at lower temperatures

» Less of a hazard, cost of parts is cheaper, and less energy is needed to maintain system



• Response times:– Need to wait for long time to produce hydrogen in order to start the

working of fuel cell.

• Durability:– Catalyst and Reformer need to be durable

• Coking:– Carbon deposits in the reactor

» This causes clogging in the system

Challenges of Reforming ProcessChapter 1 : Introduction


Sulfur Poisoning

o Sulfur in the feed leads to formation of hydrogen sulfide leading to deactivation of the catalyst.

o During reforming sulfur compounds will be converted into hydrogen sulfide, which is chemisorbed on transition metal surfaces by the following reaction:

H2S + Me = Me-S + H2

Chapter 1 : Introduction Deactivation of catalyst

Deactivation of Ni catalysts

Sulfur poisoning

Sintering

Phase transformation

Coke formation


Parameters leading to sintering: Reaction temperature Reaction atmosphere Catalyst composition, structure and support morphology.

SinteringChapter 1 : Introduction

Consequences of sintering: Loss in catalysts structure- Deactivation of catalysts Carbon coking Development of back pressure- Reaction shut down.


• The risk of coke formation increases with carbon number, unsaturation and aromaticity in the feed.

• Reactions leading to carbon formation during reforming:

2CO (g) → CO2 (g) + C(s) (1) Boudouard reaction

CH4 (g) → 2H2 (g) + C(s) (2) Methane decomposition

CO (g) +H2 (g) → H2O (g) + C(s) (3) CO hydrogenation

CO2 (g) +H2 (g) → 2H2O (g) + C(s) (4) CO2 hydrogenation

CnHm = olefins = coke (5)

CnHm = (CH2) n = gum (6)

Coke formationChapter 1 : Introduction


Carbon type Reaction Phenomena Critical parameters

Gum (6)Blocking of Ni

surface

Low H2O/C ratio,

absence of H2, low

temperature

Whisker Carbon (1)-(4)Breakup of

catalyst pellet

Low H2O/C ratio, high

temperature, presence of olefins, aromatics

Pyrolytic coke (5)Encapsulation ofcatalyst pellet,

deposits on tube wall

High temperature,residence time, presence

of olefins, sulfur poisoning

Chapter 1 : Introduction Coke formation

Routes to carbon formation

Whisker carbon is the principal product of carbon formation in SR reactions.


• The steam reforming catalyst is normally based on nickel. Cobalt and noble metals are also active, but are unsuitable for industrial use, considering their high cost and restricted availability.

• Supported Ni catalysts are catalytically active but suffer from deactivation by particle sintering and/or by reaction with supports, thermal deterioration (aging) of the support, and carbon deposition.

• The steam reforming catalysts can be modified in order to be more carbon resistant. Both MgO and CaO in the support material could favor the coke gasification and hence decrease the carbon formation.

Steam Reforming CatalystsChapter 1 : Introduction


• The addition of potassium, magnesia, or molybdenum to supported Ni catalysts can stabilize the reforming catalysts by inhibiting the carbon formation or by promoting the carbon gasification.

• Doping Ni catalysts with other metal, such as tin, chromium or manganese has a significant effect on the suppression of carbide formation.

• Minimizing the carbon formation is also a function of the support.

Chapter 1 : Introduction Steam Reforming Catalysts

CeO2

C

CH4

Nio

CO2


• To prepare Ni-Al powders from a mixture of Ni and Al elemental powders using AlCl3 as an activator.

• Catalytic activity of Ni-Al alloy powders for methane steam reforming.

• Test for oxidation resistance on these catalysts.

Objective of the work

Steam Reforming of methane over Ni-Al alloy catalysts prepared by

Chemical-Vapor-Deposition method

Chapter 2


Introduction to NiAl

• Ni–Al alloys have been mainly applied as anode materials for molten carbonate fuel cells (MCFCs).

• Alloying of nickel anodes with aluminum provides enhanced creep strength and sintering resistance, resulting in excellent mechanical and chemical stability during the long-term operation of the fuel cell at temperatures around 650ºC.

• The present work deals with SRM over NiAl alloy catalysts using various Ni:Al ratios.

Chapter 2


purging gas ( N2)

furnace

vent

Ni powder

AlCl3 powder

heating band

AlCl3 vaporAl powder

0 2 4 6 8 10 12 14 160

100

200

300

400

500

600

N2 purge

N2 + AlCl

3(g)

N2 purge

tem

pera

ture

(℃

)

time (hr)

gas flow rate: 400ccm

Preparation of NiAl alloy catalysts by CVD method

AlCl3(g) + Al(s) → AlCl(g) + AlCl2(g)

AlCl(g) + AlCl2(g) + Ni(s) → Ni–Al(s) + AlCl3(g)

Chapter 2

XRD and TGA results before reaction

0 200 400 600 800 1000

100

105

110

115

120

125

15wt% NiAl

9wt% NiAl

7wt% NiAl

5wt% NiAl

Wei

gh

t (%

)

Temperature (0C)

20 30 40 50 60 70 80 90

# NiAl

##

#

#

##

##

#

* Ni3Al

**

*

**

5wt% NiAl

7wt% NiAl

9wt% NiAl

15wt% NiAl

Arb

itary

Inte

nsity

2 (deg)

Formation of Ni3Al alloy for 15wt%NiAl

Increase in Al wt% increases the oxidation resistance

Chapter 2

5 Wt% NiAl = 5 Wt% of Al in 95 Wt% of Ni

SEM images of samples before reaction

5 wt% NiAl 7 wt% NiAl


Formation of Ni3Al alloy

Chapter 2

550 600 650 700 750 800 850

0

10

20

30

40

50

60

70

80

90

100

CH

4 Con

vers

ion(

%)

Temperature(oC)

5wt% NiAl 7wt% NiAl 9wt% NiAl 15wt% NiAl

550 600 650 700 750 800 850

0

50

100

150

200

250

300

5 wt% NiAl

H2yield

CO yield

Yiel

d (%

)Temperature (0C)

Effect of temperature on SRM

S/C=3 GHSV=24lg-1h-1

Chapter 2

High conversion for 5wt% NiAl. The trend for 15wt% NiAl is unlike other NiAl catalysts as the phase is different.

Long-term stability test and Test for Oxidation Re-sistance (OR)

Chapter 2

• OR test- SRM reaction was conducted with S/C ratio of 3, GHSV of 24 l/ (g of catalysts*h) and at 650 ◦C for about 2 h and then oxygen was purged into the reactor stopping CH4 flow for 1 hour. This process was continued for about 10 cycles.

LT test OR test

15 wt%of NiAl showed stable CH4 conversion for 100h.

Among all the catalysts tested for OR, 15 wt%of NiAl showed better stability.

SEM results after SRM Chapter 2


(a) (b)

Loss in structure was observed for both the catalysts.

XRD results after LT and OR testsChapter 2

After OR After LT

• 5 Wt% NiAl has lost its alloy structure after LT test. Al2O3 phase could be seen in XRD results.15 Wt% NiAl still has NiAl phase after LT test.

• Only 15 Wt% NiAl could maintain NiAl phase after OR test. All the other catalysts lost their structure.


Summary

•A series of Ni-Al catalysts were prepared by CVD method and tested for steam reforming of methane.

• XRD results of the as prepared powders shows the Ni-Al alloy phase for5, 7 and 9 NiAl catalysts where as 15NiAl has Ni3Al phase.

• TGA results of all the fresh samples shows that increase in wt% of Al increases the oxidation resistance of the samples.

• 5NiAl had highest methane conversion and also yields to the products among all the catalysts tested for SRM.


summaryChapter 2

• LT test proved that 5 NiAl lost its activity with time due to the oxidation of the catalyst while 15 NiAl, maintained the conversion and Ni-Al alloy phase during the overall reaction period.

• OR test clearly showed that the 15 NiAl with Ni3Al phase could resist the oxidation effectively.

Thus, 15NiAl prepared by this method, can withstand oxidation on long run and so can be very well used as an anode for MCFC.



• To prepare various wt% NiCo alloy catalysts supported on ZrO2,CeO2, and Al2O3.

• To test these catalysts for SRM reaction.

Suppression of carbon formation in steam reforming of methane by addition of Co into Ni supported catalysts

Korean J. Chem. Eng., 27(2), 480-486 (2010)

Chapter 3

Support

Ni Co

Ni Co

Ni Co

CH4+H2O 3H2+CO(C+2H2)

(2H++O2-)

C

O2- O2- O2-

C

O2-O2- O2-

CO or CO2

BET and ICP results before reaction

Catalyst BET SA (m2/g-1)

Actual metal loading (wt %)

Ni Co

Metal loading from AAS

analysis (wt %)

Ni Co

XRD phases identified

before reduction

NiCo(100:0)/ZrO2 35 10 0 9.8 0 NiO

NiCo(75:25)/ZrO2 33 6.6 3.3 7.1 2.7 NiO,Co3O4

NiCo(50:50)/ZrO2 32 5 5 4.5 4.8 Co1.29Ni1.71O4

NiCo(25:75)/ZrO2 34 3.3 6.6 3.1 6.8 NiO,Co3O4

NiCo(0:100)/ZrO2 32 0 10 0 9.8 Co3O4

NiCo(50:50)/TiO2 32 5 5 4.8 4.9 Co1.29Ni1.71O4

NiCo(50:50)/CeO2 50 5 5 4.7 5 Co1.29Ni1.71O4

NiCo(50:50)/Al2O3 74 5 5 4.9 5 Co1.29Ni1.71O4

Chapter 3

15 20 25 30 35 40 45 50 55 60 65 70

#

*

*

*

*

*

*

o

NiCo(50:50)/Al2O

3

NiCo(50:50)/CeO2

NiCo(50:50)/TiO2

Ni/ZrO2

NiCo(75:25)/ZrO2

NiCo(50:50)/ZrO2

NiCo(25:75)/ZrO2

Co/ZrO2

Arb

itary

Inte

nsity

(a.u

)

2(degrees) 44.0 44.2 44.4 44.6 44.8

Ni/ZrO2

NiCo(75:25)/ZrO2

NiCo(50:50)/ZrO2

NiCo(25:75)/ZrO2

Co/ZrO2

Arb

itary

Inte

nsity

(a.u

.)

2 (degree)

XRD results before reactionChapter 3

The NiCo bimetallic peak shifted from Co (44.2o) to Ni (44.5o) upon decreasing the Co content, confirming the formation of the NiCo alloy in all NiCo/ZrO2 catalysts

Effect of temperature on SRM

800 900 1000 1100 1200

0

20

40

60

80

100

CH

4 C

onve

rsio

n (%

)

Temperature (K)

Ni/ZrO2

NiCo(75:25)/ZrO2

NiCo(50:50)/ZrO2

NiCo(25:75)/ZrO2

Co/ZrO2

Chapter 3

S/C=3 GHSV=24lg-1h-1

The CH4 Conversions for all the catalysts increased with increase in the temperature.

Highest CH4 conversion was shown for Ni/ZrO2 catalyst.

Both NiCo(75:25)/ZrO2 and NiCo(50:50)/ZrO2 showed almost similar CH4 Conversion .

optimum CH4 Conversion was found at 1073K.

Effect of Cobalt AdditionTemperature =800ºC S/C=3 GHSV=24lg-1h-1

0 2 4 6 8 10 1264

66

68

70

72

74

76

78

80

82

84

86

88

CH

4 Co

nv

ers

ion

(%

)

Time (hrs)

Ni/ZrO2 NiCo(75:25)/ZrO2 NiCo(50:50)/ZrO2 NiCo(25:75)/ZrO2 Co/ZrO2

Catalysts with low Co content showed high CH4 conversions.

Oxidation of Co metal has led to loss in conversions for Co/ZrO2 and NiCo(25:75)/ZrO2 catalysts.

No carbon formation on NiCo(50:50)/ZrO2.

Among all the catalysts tested Ni:Co ratio of 50:50 was chosen to be optimum ratio for further analysis.

Catalysts Carbon formed

Ni/ZrO2 1.6

NiCo(75:25)/ZrO2 1.3

NiCo(50:50)/ZrO2 0.3

NiCo(25:75)/ZrO2 0.3

Co/ZrO2 0.3

Long term analysis

0 10 20 30 40 5070

75

80

85

90

CH

4 Con

vers

ion(

%)

Time (hrs)

Ni/ZrO2

NiCo(50:50)/ZrO2

0 2 4 6 8 10 12

60

65

70

75

80

85

90

95

NiCo(50:50)/ Al2O

3

NiCo(50:50)/ CeO2

NiCo(50:50)/ TiO2

NiCo(50:50)/ ZrO2

CH

4 C

on

vers

ion

(%)

Time (h)

Chapter 3

Loss in CH4 Conversion for Ni/ZrO2 was found due to the carbon formation.

No carbon formation on NiCo(50:50)/ZrO2 for the time period studied.

Effect of supports clearly showed that NiCo(50:50)/Al2O3 has higher CH4

conversion.

TEM analysis after LT test

Ni/ZrO2 before reaction

NiCo(50:50)/ZrO2 before reaction

Ni/ZrO2 after reaction

NiCo(50:50)/ZrO2 after reaction

Chapter 3

Carbon formation on Ni/ZrO2 catalyst.

No traces of carbon was found on NiCo(50:50)/ZrO2 catalyst


Conclusions

• NiCo catalysts of various wt% were successfully prepared. Alloys of all the bimetallic catalysts were formed after reduction.

• Oxidation of the catalysts with high Co content took place after SRM while all the other catalyst showed pure metal phase as proved from XRD results.

• SRM of all the catalysts were successfully performed. NiCo(50:50)/ZrO2 produced high CH4 conversion with no carbon formation.

• SRM of Ni/Co ratio 50:50 on various supports reveal that NiCo(50:50)/Al2O3 has given highest methane conversion of 96% with no carbon formation

Chapter 3


To prepare Ni/LaAlO3 support by various preparation methods with high surface area and low Ni particle size.

To suppress the carbon formation in steam reforming of propane(SRP) at lower S/C ratios by using Ni/LaAlO3.

To test the influence of OSC of the support on the stabilty of the catalyst on SRP.

Catalytic performance of Ni/LaAlO3 catalysts for propane steam reforming – Effect of preparation methods

Catal Lett (2012) 142:205–212

Chapter 4


Why only LaAlO3?

• LaAlO3 is thermally stable and can be used as a good support at higher temperatures.

• Utilization of the lattice oxygen in LaAlO3 effectively promotes the reaction of CHx fragments adsorbed on metallic nickel. Hence, it can prevent the formation of inactive carbon species.

Chapter 4

oNiNiNi

NiNi

OO

C3H8+3H2O

7H2+3CO

CO2

Vacancy LaAlO3 Support

Carbon species

Oxygen-ion


Preparation of Support

Aq. Soln of Al(NO3)3 +La(NO3)3

Add to soln of NH4OH with vigorous stirring

Formation of white precipitate

Washing with distilled water & ethanol

Oven drying for 24h

Calcination @ 800°C for 2h

Co-precipitation method

Chapter 4


Deposition–precipitation gives highly dispersed catalysts, with strong interaction occurring at the interface of active metal and support.

THF plays an important role as a capping agent and promotes the segregation of nickel

particles during the catalyst preparation by solvo thermal synthesis.

Impregnation is the easiest method of depositing higher wt% of metal on the surface of

the support.

Introduction to the Preparation methods usedChapter 4


LaAlO3 + NiO(ac)2 aq. soln

Deposition-precipitation method

Washing drying and calcined at 450◦C for 5h

Soln was aged for 1h at 80◦C

0.2M Na2CO3 was slowly added until

pH 10

LaAlO3 + Ni nitrate soln Washing drying and calcined at 450◦C for 5h

0.1M NH4OH was slowly added until

pH 10

Experimental

LaALO3 + Ni nitrate in aq. solnObtained powder was dried

at 120◦C for 24hSoln was heated to

remove water

THF +Ni nitrateSoln was agitated

for 1hAddition of LaAlO3

to the above soln

Washing and drying at 120◦C for 24h and calcined at 450◦C

for 5h

Solvo- thermal synthesis

Powder was then calcined at 450◦C for 5h

Impregnation method

Chapter 4


BET and crystallite size results

Catalyst BET area(m2/g)

Crsyt.size(nm)

Particle size(nm)

Solvo thermal (Ni-ST)

21.67 10.6 8-11

DP-Na2CO3

(Ni-DPS)16.7 15.3 14-16

DP-NH4OH (Ni-DPA)

14.2 18.5 17-19

Impregnation (Ni-IMP)

10.5 22.7 21-23

Ni/Al2O3 26 10.7 8-11

Ni/CeO2 19 10.4 8-11

Chapter 4


XRD and TPR results

20 30 40 50 60 70 80

o

o

o

o

o

o

*=LaAlO3

o=Ni

******

**

Ni/CeO2

Ni/Al2O

3

Ni-ST

Ni-DPS

Ni-DPA

Ni-IMP

Inte

ns

ity

(a

.u)

2(Deg)

LaAlO3

100 200 300 400 500 600 700 800 900

Arb

itra

ry In

ten

sit

y (

a.u

.)

LaAlO3

Temperature(oC)

Ni-DPS

Ni-IMP

Ni-DPA

Ni-ST

Ni/Al2O

3

Ni/CeO2

Peak at lower temperatures (<300ºC)- Reduction of free NiO (bulk) species.

Peaks at mid temperatures (300-410ºC)- Reduction of NiO species which has weak interaction with support .

Peaks at mid temperatures (470-520ºC )- Reduction of NiO species which has strong interaction with support.

Peak at high temperatures (650-800ºC)- reduction of the support LaAlO3.

Chapter 4


Effect of temperature and S/C ratio on SRP

600 650 700 750 80045

50

55

60

65

70

75

80

85

90

95

100

0 10 20 30 40 50 60 70 80 90 100 11070

75

80

85

90

95

100

C3H

8 Co

nv

ers

ion

(%

)

Time (h)

Ni-ST Ni-DPS

C3H

8 C

on

vers

ion

(%

)

Temperature (oC)

Ni-DPA Ni-DPS Ni-ST Ni-IMP

Spent CatalystsPropane conversion

after 24 h reaction

Amount of carbon

deposited (wt %)a

Ni-ST (S/C =2) 100 - 100 < 0.3

Ni-ST (S/C =1.75) 100 - 100 < 0.3

Ni-ST (S/C =1.5) 100 - 96 1.2

Ni-ST (S/C =1) 100 - 73 3.3

Optimum temperature of 700ºC. Highest propane conversion for Ni-ST .

Chapter 4


Long term stability test and carbon forma-tion

0 10 20 30 40 50 60 70 80 90 100 11065

70

75

80

85

90

95

100

C3H

8 C

on

vers

ion

(%)

Time(h)

Ni/Al2O

3

Ni-ST Ni/CeO

2

0 10 20 30 40 50 60 70 80 90 100 11050

55

60

65

70

75

80

85

90

H2 Y

ield

(%

)

Time (h)

Ni/Al2O

3

Ni-ST Ni/CeO

2

Ni-ST showed 100% propane conversion with no carbon formation on the surface.

Chapter 4


TEM analysis

Ni-

ST

bef

ore

reac

tion

Ni/

Al 2O

3 aft

er r

eact

ion

Ni/

Al 2O

3 bef

ore

reac

tion

N

i/C

eO2 b

efor

e re

acti

on

Ni/

CeO

2 af

ter

reac

tion

N

i-S

T a

fter

rea

ctio

n


0 200 400 600 800 10002.0

1.5

1.0

0.5

0.0

-0.5

C

B

Wei

ght l

oss

(%)

Temperature (oC)

LaAlO3

A

Test for OSC in the support

The observed weight decrease (0.16 %) in the second heat-ing stage at the temperature range of 300 - 900 ◦C corre-sponds

to the potential oxygen release properties (δ) of 0.05.

The involvement of lattice oxygen has decreased the light off temperatures for soot oxidation in case of

LaAlO3.

Chapter 4


Summary

The Ni-based catalysts supported onto LaAlO3, Al2O3, and CeO2 supports were prepared by various methods and their catalytic performances related to steam reforming of propane were examined. Catalytic activities proved to be dependent on the preparation methods employed

The superior stability of Ni/LaAlO3 (Ni-ST) originated from resistance against sintering at high temperatures and its high oxygen transport ability, as evidenced by both the sequential thermo gravimetric method and the soot oxi -dations

The Ni/LaAlO3 catalysts prepared by a solvo-thermal method exhibited the highest activity and stability. The ob-served excellent activity was attributed to the relatively well-dispersed, small Ni nano particles on LaAlO 3, induced by the solvo-thermal method.

Chapter 4



• To prepare and analyze various Tb doped CZ supports for the enhanced OSC. (Ce0.75-xZr0.25TbxO2(x=0.05,0.10,0.15,0.20))

• To test the Ni based catalysts for SRP reaction- test for high durability.

Influence of Terbium Doping on Oxygen Storage Capacity

of Ceria–Zirconia Supports: Enhanced durability of Ni catalysts for SRP

Catal Lett (2013) 143:49–57

Chapter 5

C3H8 (g) + 3H2O (g)

Ni

O2-

NiNi

carbonO2-

CO (g) or CO2 (g)

7H2 (g) + 3CO (g)

Ni

O2-

NiNicarbonO2-

CO (g) or CO2 (g)C3H8 (g) + 3H2O (g)

7H2 (g) + 3CO (g)

O2-

CeO2-ZrO2 support Tb-doped

CeO2-ZrO2 support

vs

Relatively High OSC

O2- vacancy


Chapter 5 Introduction

• Ceria has been extensively used as an active support for SR reactions due to its ability to store and release oxygen ions as well as to interconvert between its two valence states (3+and 4+).

• Pure ceria could undergo sintering at high temperatures leading to a decrease of its OSC and loss in activity during SR reactions.

• Zirconia has been found to be an excellent additive to ceria to increases the oxygen mobility of ceria and also to prevent the sintering of ceria at high temperatures.

• Doping of RE elements can enhance the OSC of Ceria-Zirconia(CZ). Tb has been used as a dopant since Tb along with Ce has redox stability (Tb3+↔Tb4+) which can improve the OSC of Tb doped CZ materials.


Aq. Soln of Ce(NO3)3 + Zr(NO3)2 +Tb(NO3)2

Addition of glycine (G/N ratio =0.55)

Heating the mixture to evaporate water

Spontaneous ignition due to combustion

Produced powder was calcined at 600◦C for 2h

• Ce0.75-xZr0.25TbxO2 ( x=0.05,0.10,0.15,0.20)was prepared by glycine nitrate process

Preparation of Ni based CZT catalysts

THF +Ni nitrateSoln was agitated

for 1hAddition of CZT support to the

above soln

Washing and drying at 120◦C for 24h and calcined at 450◦C

for 5h

Solvo- thermal synthesis

Chapter 5

Sample element mole%

Ce0.75Zr0.25O2 Ce Zr

74.725.3

Ce0.65Zr0.25Tb0.1O2 Ce Zr

Tb

6624.89.3

Ce0.7Zr0.25Tb0.05O2 Ce ZrTb

6924.76.5

Ce0.6Zr0.25Tb0.15O2 Ce Zr

Tb

60.223.116.5

Ce0.55Zr0.25Tb0.2O2 CeZrTb

56.123.820.1


Catalyst Crystallite size (nm)

Lattice parameter

(A)

2θ

CZ 19.06 5.383 28.69

CZT0.05 16.87 5.318 29.05

CZT0.1 15.93 5.323 29.02

CZT0.15 15.83 5.335 28.96

CZT0.2 15.57 5.315 29.07

XRD- results

No diffraction peaks of Tb, Ce, Zr oxides are observed, indicating that solid solutions with cubic fluorite structures are formed.

With increase in Tb doping till 0.15, the 2 θ values shifted to lower degree and lattice parameter has been increased, indicating that a part of Tb must be present in Tb3+ cations leading to proportional amount of oxygen vacancies.

But for Tb doping of 0.2 the lattice parameter is comparatively small, so there is a possibility that Tb4+ is high. which shows that the optimum Tb doping might be <0.2.

Catalyst BET surface area

(m2/g)

Ni Particle size

(nm)

Ni/CZ 31.2 22.6

Ni/CZT0.05 34.4 21.7

Ni/CZT0.1 31.3 22.3

Ni/CZT0.15 32.5 22.1

Ni/CZT0.2 31.8 21.5


300 400 500 600 700 800

CZCZT

0.05

CZT0.1

CZT0.15

CZT0.2

Inte

nsi

ty (

a.u

.)

Raman Shift (cm-1)

The band at 465 cm-1 is ascribed to the Raman active F2g mode of CeO2, the band of a fluorite structural material. For all the CZT samples a slight shift in the raman frequency at around 470cm-1 and absence of peaks related to Tb and Zr proves the formation of solid solutions.

The peak at 600 cm-1 was attributed to lattice defects, resulting from the formation of oxygen vacancies. The value I600/I470 reflects the abundance of the oxygen vacancy caused by the replacement of Ce atom by Tb in the mixed oxide. It can be seen that the value of I600/I470 is higher for CZT0.1.

Raman- resultsChapter 5


200 300 400 500 600 700

CZ

CZT0.2

CZT0.15

CZT0.1

CZT0.05

Arb

itrar

y in

tens

ity (a.

u)

Temperature (oC)

TPR- results

In CZT 0.05 sample, the reduction peak at around 450 ◦C is attributed to the reduction of Ce4+. The Tb4+ peak cannot be clearly seen, as there is low loading of Tb in the sample. The second reduction peak is related to and the peak at ~600 ◦C is the bulk reduction.

For CZT0.1 ,The reduction peak splitting into triplet is clearly seen in this sample. the first reduction region is due to the Tb4+ → Tb3+ reduction along with Ce4+ reduction and the bulk reduction.

In case of CZT0.15,and CZT0.2 the reduction peaks has been shifted to higher temperature when compared to CZT0.1 .

CZT0.1 sample have reduced at lower temperatures compared to other catalysts indicating higher OSC in this sample.

Chapter 5


Support reduction peaks has shifted to lower temperatures (500 ºC) after introduction of Ni- Strong metal support interaction.

Low temperature peaks at ~ 200 ºC – free Ni.

Peaks extending from 360 ~450 ºC- Ni interacting with the support.

200 300 400 500 600 700 800

Inte

nsi

ty (

a.u

)

Temperature (0C)

Ni/CZT0.2

Ni/CZT0.15

Ni/CZT0.1

Ni/CZT0.05

Ni/CZ

(b)

TPR- resultsChapter 5


The XPS spectra of Ce3d shows that Ce is in 4+ oxidation state.

For O1s spectra- peak at 528eV relates to lattice oxygen. A shoulder at 530 eV is associated to surface hydroxyl groups related to Ce3+. CZT0.1 has higher Ce3+ ions.

XPS- results

920 910 900 890 880

CZT0.2

CZT0.15

CZT0.1

CZT0.05

v

vo

v'v''v'''

uo

uu'u''

Inte

ns

ity

(a.u

)

Binding energy (eV)

Ce 3d

u'''

CZ

(a)

524 526 528 530 532 534 536 538

CZT0.2

CZ

CZT0.1

CZT0.15

CZT0.05

O 1s

Inte

nsi

ty (

a.u

)

Binding energy (eV)

(b)

Chapter 5


Tb 4d shows multi-oxidation states of Tb (Tb3+ and Tb4+).

Intensity of the peak at 150 eV is higher for CZT0.1 indicating higher amount of Tb in the 3+ oxidation state.

170 165 160 155 150 145 140

CZT0.2

CZT0.15

CZT0.1

Tb 4d

Inte

nsi

ty (

a.u

)

Binding energy (eV)

CZT0.05

(c)

XPS- resultsChapter 5


The results clearly show that the OSC is in the order of CZT0.1>CZT0.15>CZT0.2>CZT0.05>CZ>CeO2.

CZT0.1 has higher OSC.

300 350 400 450 500 550 600 650 700

0

20

40

60

80

100

So

ot

Co

nve

rsio

n (

%)

Temperature (C)

CZ CZT

0.05

CZT0.1

CZT0.15

CZT0.2

CeO2

Soot

Catalyst T ½ (◦C)

CZ 460

CZT0.05 457.5

CZT0.1 418.1

CZT0.15 427.6

CZT0.2 436.02

Soot Oxidation- resultsChapter 5


Ni/CZT0.1 showed stable propane conversion for 200h.

0 25 50 75 100 125 150 175 20080

85

90

95

100

C

3H

8 C

on

ve

rsio

n (

%)

Time(h)

Ni/CZ Ni/CZT

0.05

Ni/CZT0.1

Ni/CZT0.15

Ni/CZT0.2

SRP- resultsChapter 5


Carbon formation is observed for Ni/CZ, Ni/CZT0.05 and Ni/CZT0.2

No carbon on Ni/CZT0.1 and Ni/CZT0.15.

Increase in the Ni particle size is observed in Ni/CZT0.15.

TEM- resultsChapter 5


• XRD results proves the formation of solid solution in all the samples. It also showed that increase in Tb doping till 0.15,the Tb3+ content in the samples increased, there by showing a chance of increase in OSC. presence of Ni metallic phase after reduction of catalysts.

• Raman spectroscopy results proved that the solid solutions have been formed. CZT0.1 has higher OSC

• TPR results showed that all the samples has been completely reduced below 700 ◦C. Among CZT samples,CZT0.1 have reduced at lower temperatures indicating higher OSC in this sample. Introduction of Ni decreased the reduction temperature of supports.

• Soot oxidation also proved that CZT0.1 has higher OSC compared to all other catalysts.

• Ni/CZT0.1 has stable propane conversion for 200h.while all other catalysts lost their activity due to carbon formation.

ConclusionsChapter 5



• To prepare and analyze various rare earth metals doped CH supports for the enhanced OSC. (Ce0.65M0.1Hf0.25O2(M=Tb, Sm, Nd, Pr, La))

• To test the Ni based catalysts for dry reforming of methane reaction- test for high durability.

Structural characteristics of rare earth metals doped CeO2-Hf O2 materials- Influence on OSC and dry reforming of methane

Chapter 6


Intrinsic OSC

OCe4+

O

Ce4+

O

OCe4+

Ce3+

O

2Ce4+ ↔ 2Ce3++O2-

Extrinsic OSC

OCe4+

O

Ce4+

O

2RE3+ → Ce4+ +O2-

O

Ce4+

RE3+

O

RE3+

Doping with RE elements increases the OSC of CeO2



O OO

O O

Incorporation of smaller ionic radius elements into CeO2 lattice increases the OSC

=Zr=Hf

Doping with Zr

O OO

O

Doping with Hf

IntroductionChapter 6


Strategy of Enhancing OSC in This Work

OCe4+

O

Ce4+

O

2RE3+ → Ce4+ +O2-

O

Ce4+

RE3+

O

RE3+

O OO

O

Doping with Hf

Size effect+ Extrinsic OSC



Preparation of Supports

EDTA

NH4OH

+

EDTA-NH3 Soln

Precursor SolnCe(NO3)3

HfCl4

M(NO3)3

+

+

Gel formation

pH=6, Stirring and heating

Primary powder

Calcination at 600 ◦ C for 10h

M doped CH

Oven drying at 250 ◦ C for 12h

Mixed Soln

Clear Soln

Addition of citric acid

NH4OH+

Chapter 6


Characterization of the Supports

XRD

Shift in the 2 θ values-Formation of Solid solution.

With increase in ionic radius of RE elements lattice parameter also increased.

Support BET surface area (m2/g)

Crystallite size (nm)

Lattice parameter

(Å)

2θ

CH 43.5 12.21 5.315 29.07CH-Tb 56.9 16.25 5.318 29.06CH-Sm 58.1 13.04 5.336 28.96CH-Nd

CH-Pr

CH-La

63.8

70.8

78.3

11.70

13.39

13.70

5.337

5.344

5.347

28.95

28.91

28.90

Chapter 6


Raman

Relative OSC in the order of Pr>La>Tb>Nd>Sm>no dopant.

Formation of Solid solution



Low temperature peaks at~450ºC- Surface reduction of Ce4+ to Ce3+. High temperature peak~615ºC- Bulk reduction of Ceria. The peaks has been shifted to low temperatures for M doped CH samples-

Higher OSC. CH-Pr has very low temperature reduction peak at 562.7ºC.

200 300 400 500 600 700

CH-La

CH-Pr

CH-Nd

CH-Sm

CH-Tb

CH

Arb

itra

ry In

ten

sity

(a.

u.)

Temperature (oC)


TPR


XPS

Chapter 6 Characterization of the Supports

Ce 3dO 1s

Hf 4f

Ce3d -Ce in both 3+and 4+ oxidation state

O1s - shift in the peak at 528.7 eV for CH-Pr and CH-La indicating higher OSC in these samples.

Hf 4f - Hf is in 4+ oxidation state in all samples.


Tb 4d -Tb is in both 3+ and 4+ oxidation states in CH-Tb sample. Pr 3d -Pr is in both 3+ and 4+ oxidation states in CH-Pr sample.

Characterization of the SupportsChapter 6

Tb 4d Pr 3d

XPS


Sm 3d - Sm in 3+ oxidation state in CH-Sm sample.

Nd 3d- Nd in 3+ oxidation state in CH-Nd sample.

La 3d- La in 3+ oxidation state in CH-La sample

Characterization of the SupportsChapter 6

970 975 980 985 990

Inte

nsi

ty (

a.u

)

Binding energy (eV)

Nd 3d 5/2 Nd 3d(b)

860 855 850 845 840 835 830

La 3d3/2 La 3d

5/2

Inte

nsi

ty (

a.u

)

Binding energy (eV)

La 3d(c)

1130 1120 1110 1100 1090 1080 1070

Inte

nsi

ty (

a.u

)

Binding energy (eV)

Sm 3d5/2

Sm 3d3/2

Sm 3d(a)XPS


Relative OSC of the Supports

Relative OSC in the order of CH-Pr > CH-La > CH-Tb > CH-Nd > CH-Sm > CH.

Chapter 6


Characterization of the Ni-based Catalysts

XRD

Catalyst Ni Wt%

Ni/CH 10.6

Ni/CH-Tb 9.74

Ni/CH-Sm 9.68

Ni/CH-Nd 9.69

Ni/CH-Pr 9.75

Ni/CH-La 9.72

Support Catalyst10wt% Ni

450ºC/5h

Chapter 6

20 30 40 50 60 70

** Ni/CH-La

Ni/CH-Pr

Ni/CH-Nd

Ni/CH-Sm

Ni/CH-Tb

Ni/CH

Arb

ita

ry In

ten

sit

y (

a.u

.)

2(deg)

Presence of metallic Ni phase after reduction


200 300 400 500 600 700 800

NiCHLa

NiCHPr

NiCHNd

NiCHSm

Arb

itra

ry In

ten

sit

y (

a.u

.)

Temperature (oC)

NiCH

NiCHTb

Characterization of the as-synthesized catalysts

The peak ranging from 300 ºC -450ºC relates to Ni species interacting with the support.

Support peaks centered at 600 ºC shifted into low temperatures due to strong Ni-Supports interactions.

Only little free Ni species were found at < 300 oC

Chapter 6

TPR


• Effect of temperature :600-900 oC• GHSV:23,050 h-1

• CH4:CO2 =1:1

• Reduction with 30 ml H2 at 700 oC for 2h prior to reaction

• Flow rate of CH4+CO2:50 sccm

• Flow rate of N2:150 sccm

Dry reforming of Methane

CH4 + CO2 2CO + 2H2

Chapter 6

Reaction conditions


600 650 700 750 800 850 90010

20

30

40

50

60

70

80

90

Temperature (oC)

CH

4 Co

nv

ers

ion

(%

)

Ni/CH Ni/CH-Tb Ni/CH-Sm Ni/CH-Nd Ni/CH-Pr Ni/CH-La

600 650 700 750 800 850 900

20

30

40

50

60

70

80

90

100

CO

2 C

on

ve

rsio

n (

%)

Temperature (oC)


The CH4 conversions increased in the order of Ni/CH-Pr > Ni/CH-La > Ni/CH-Tb > Ni/CH-Nd > Ni/CH-Sm > Ni/CH.

The observed high CO2 conversions over the materials suggest that RWGS reactions (CO2 + H2 CO + H2O) occurred simultaneously.

XRD studies following the reforming showed no carbon formation.

Chapter 6 Dry reforming of Methane- Effect of Temperature


Chapter 6 Dry reforming of Methane- LT test

0 20 40 60 80 100 120 140 16010

20

30

40

50

60

70

80

90

CH

4 Co

nv

ers

ion

(%

)

Time (h)


(a)

0 20 40 60 80 100 120 140 16080

85

90

95

100

CO

2 Co

nv

ers

ion

(%

)

Time (h)

Ni/CH Ni/CH-Tb Ni/CH-Sm NI/CH-Nd Ni/CH-Pr Ni/CH-La

(b)

0 20 40 60 80 100 120 140 1600.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Time (h)


H2/C

O r

ati

o

(c)

The stability in the order of Ni/CH-Pr > Ni/CH-La > Ni/CH-Tb > Ni/CH-Nd > Ni/CH-Sm > Ni/CH.

Ni/CH-Pr, Ni/CH-La and Ni/CH-Tb maintained their conversions at 54%, 51%, and 49%, respectively

CH4:CO2 =3:1


NO carbon formation on Ni/CH-Pr, Ni/CH-La and Ni/CH-Tb catalysts.

(c)(b)

(e) (f)(d)

(a)

Dry reforming of MethaneTEM

Chapter 6


• Ce0.65Hf0.25M0.1O2(M=Tb, Sm, Nd, Pr and La) were successfully prepared by EDTA-citrate method.

• XRD and raman results clearly show that formation of homogenous solid solution.

• CH-Pr, CH-La and CH-Tb had higher OSC compared to other M doped CH samples as confirmed from raman, TPR and XPS analyses.

• Soot oxidation results showed that the OSC is in the order of CH-Pr > CH-La > CH-Tb > CH-Nd > CH-Sm >CH.

• Ni/CH-Pr illustrated high and stable activity with no carbon formation for 200h for dry reforming of methane due to the high OSC of the support.

ConclusionsChapter 6


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Steam reforming over Ni catalyst

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