experimental/theoretical studies aimed at development … library/events/2006/seca/linic.pdf ·...
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Experimental/Theoretical Studies Aimed at Development of Carbon-
Tolerant Catalysts
Eranda Nikolla, Adam Halowieski, Suljo Linic
Department of Chemical EngineeringUniversity of Michigan, Ann Arbor, MI
ConclusionFirst principles approach to the development of novel catalysts
Carbon chemistry on Ni surface Can be modified by alloying
Respective rates of C-O and C-C bond formation impact the extent of carbon poisoning
Sn/Ni surface alloy is a promising carbon-tolerant catalyst
Fuel Cell TechnologyDevelopment of efficient energy conversion systemsLow temperature fuel cells (PEMFC)
CO poisoning, slow kinetics, over-potential losses
High temperature fuel cells (SOFC)
Limitation: Deactivation of Ni/YSZ anode catalystsCarbon poisoning Sulfur poisoning
O-2
Anode
CathodeLaMnO3
Ni-YSZ
YSZ electrolyte
Carbon Poisoning of Ni Monometallic Ni catalysts
Deactivate severely during steam reforming of hydrocarbons
Extended carbon structure formed: graphitic layers, nano-tubes, …
50 nm50 nm
Ni
C
ObjectivesUtilize first principles calculation (DFT) to study
Carbon chemistry over Ni surfaces
Can we understand and control carbon chemistry and therefore affect the deactivation?
Ni alloying?
Steam reforming of methane and isooctane using Ni and Ni alloys
Catalysts characterization
DFT MethodologyAccurate and efficient first-principles calculations
Adsorption energies, activation barriers, vibrationaland electronic spectra, …Model systemsNi(111) and Ni(111) alloys
Ni(111)Ni(111) alloy
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
E(e
V)
Ni (111)
CH4
CH3*, H*CH2*, 2H*
CH*, 3H*
C*, 4H*OH*, C*, 5H*
O*, C*, 6H*
CO*, 6H*
CO(g), 6H*
CO(g), 3H2(g)
H2O
Graphene-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
E(e
V)
Ni (111)
CH4
CH3*, H*CH2*, 2H*
CH*, 3H*
C*, 4H*OH*, C*, 5H*
O*, C*, 6H*
CO*, 6H*
CO(g), 6H*
CO(g), 3H2(g)
H2O
Graphene
Steam reforming over Ni(111)DFT calculated elementary step reaction energies for methane steam reforming on Ni(111)
Can be described by a mechanism of competing pathways
The C-O and C-C bond formation is important
CnHm+
H2O
Carbon Chemistry
nC* + (m+2) H* + O*
C-O CO, CO2
C-Csp2 carbon deposits
-2
-1
0
1
2
3
C atom position
4
Graphene
C-C‡C-O ‡
CO(g)
Ener
gy (
eV)
DFT: C-O and C-C bond formation on Ni(111)
Formation of C-C and C-O bonds are kinetically comparable on Ni(111)
*
* Abidil-Petersen, J. Phys. Rev. B, 2006
Graphene
C-C‡
-2
-1
0
1
2
3
4
Ener
gy (
eV)
C atom position
DFT: C-O and C-C formation on Sn/Ni(111) alloy
C and O diffusion become kinetically importantThe barrier for C-C bond formation is much higher than for C-O bond formation
CO(g)
C-O ‡
O path
C path
DFT: C-O vs. C-C formationE
nerg
y (e
V)
-2
0
2
4
C-C on Ni(111)C-O on Ni(111)
Ene
rgy
(eV
)
-2
0
2
4
C-C on Sn/Ni(111)C-O on Sn/Ni(111)
Ni(111) Sn/Ni(111)
Graphene
CO
C and O compete to react with C on Ni(111)Sn alloying increased the barrier for C-C bond formation more than for C-O bond formation
DFT: Sn/Ni formation energyCan we synthesize Sn/Ni surface alloy?DFT calculated formation energies
E = E(Sn/Ni_slab) – E(Ni_slab) – e(Sn) + e(Ni)
Sn/Ni surface alloy-2.04eV/A2
Sn/Ni bulk alloy1.67 eV/A2
Ni surface0 eV/A2
There is a thermodynamic driving force to form Sn/Ni surface alloy!
Experiments: Catalyst Synthesis
Support8%YSZ synthesis: Co-precipitation method
Ni-YSZBall milling method30 vol% Ni loadingSintering
Reduction: 30% H2/N2 at 900°C for 5 hrs
Sn impregnation Incipient wetness
0.950.90.5
0.3333330.3250.29
0.2750.2333330.166667
0.10.0750.07
0.0650.0550.050
0.5
1
0 2 4 6Distance (nm)
Nor
mal
ized
Sn/
Ni s
igna
l rat
io
EDS: Line Scan
50nm
Surface alloy
Steam reforming of methane on Ni/YSZ
0.2
0.4
0.6
0.8
1
0 50 100 150 200 250 300 350 400
Time (min)
Nor
mal
ized
Con
vers
ion
S/C=2
S/C=1
S/C=0.5
Reaction ConditionsTemp = 800°CGHSV=50,000 h-1
Reduction=5hrs at 900 °C
Steam Reforming of Methane
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
0 50 100 150 200 250 300 350 400Time (min)
Nor
mal
ized
Con
vers
ion
Sn/Ni/YSZ
Ni/YSZ
Reaction ConditionsS/C =0.5Operating Temp = 800°CGHSV = 50,000 h-1
Sn/Ni alloying results in improved catalyst stability
Steam Reforming of Isooctane
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400
time (min)
Nor
mal
ized
con
vers
ion
Ni/YSZ
Sn/Ni/YSZ
Sn/Ni alloying results in improved catalyst stability even for heavy hydrocarbon fuels
Reaction ConditionsS/C =1.5Operating Temp = 800°CGHSV = 50,000 h-1
20 30 40 50 60 70 802Θ
Inte
nsity
Pre-rxn Ni
Post-rxn Ni
Post-rxn Sn/Ni
YSZNiGraphite
XRD: Post SR of Isooctane
Graphite
No graphite was detected in the case of Sn/Ni alloySimilar results were obtained for methane steam reforming
SEM: After reaction
2μm
SEM: Fresh Ni/YSZ
2μm
SEM/TEM: Post SR of Isooctane
50 nm
TEM: After reaction
2μm
SEM: Fresh Sn/Ni/YSZ SEM: Post 1%wt Sn/Ni/YSZ50nm
TEM: After reaction
XPS: Post SR of Isooctane
Ni/YSZ 2 C peaks detectedNo Ni peak detected
851 853 855 857 859 861 863 865
Binding Energy (eV)
Inte
nsity
Ni 2p3/2 (253eV)Ni Spectrum
Sn/Ni/YSZ
Ni/YSZ
280 282 284 286 288
Binding Energy (eV)
Inte
nsity
C 1s (284.5eV)
(283.1eV)Sn/Ni/YSZ
Ni/YSZ
C Spectrum
Sn/Ni/YSZNo carbon peakStrong Ni peak signal
ConclusionFirst principles approach to the development of novel catalysts
Carbon chemistry on Ni surface Can be modified by alloying
Respective rates of C-O and C-C bond formation impact the extent of carbon poisoning
Sn/Ni surface alloy is a promising carbon-tolerant catalyst