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

Sn/Ni CharacterizationSTEM/EDS experiments

50nm 50nm

Blue - NiYellow - Sn

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

Acknowledgments

Department of Energy (DOE-NETL)DE-FC26-05NT42516

Johannes SchwankKai Sun