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
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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
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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
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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
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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.
Chapter 1 : Introduction
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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
Chapter 1 : Introduction
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• 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
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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
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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.
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• 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
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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.
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• 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
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• 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
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• 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
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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
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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
9 wt% NiAl 15 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
5 wt% NiAl 15 wt% NiAl
(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.
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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.
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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.
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Objective of the work
• 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
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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
Objective of the work
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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Objective of the work
• 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
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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.
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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
Fuel Cell Research Center
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
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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
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
• 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
Fuel Cell Research Center
Objective of the work
• 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
Fuel Cell Research Center
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
Chapter 6 Introduction
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Chapter 6 Introduction
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
Raman
Relative OSC in the order of Pr>La>Tb>Nd>Sm>no dopant.
Formation of Solid solution
Characterization of the Supports
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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)
Characterization of the Supports
TPR
Fuel Cell Research Center
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.
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
Relative OSC of the Supports
Relative OSC in the order of CH-Pr > CH-La > CH-Tb > CH-Nd > CH-Sm > CH.
Chapter 6
Fuel Cell Research Center
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
Fuel Cell Research Center
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
Fuel Cell Research Center
• 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
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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)
Ni/CH Ni/CH-Tb Ni/CH-Sm Ni/CH-Nd Ni/CH-Pr Ni/CH-La
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
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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)
Ni/CH Ni/CH-Tb Ni/CH-Sm Ni/CH-Nd Ni/CH-Pr Ni/CH-La
(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)
Ni/CH Ni/CH-Tb Ni/CH-Sm Ni/CH-Nd Ni/CH-Pr Ni/CH-La
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
Fuel Cell Research Center
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
Fuel Cell Research Center
• 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
Fuel Cell Research Center