1. Effects of Oxygen on the Activation of Methane over Pt Surfaces and Clusters .
Corneliu Buda,1 Matthew Neurock,1 Cathy Chin2 and Enrique Iglesia2
1 Department of Chemical Engineering, University of Virginia, Charlottesville, VA.
2 Department of Chemical Engineering, University of California, Berkeley, CA.
January 14th 2009

2. Goals and Methods
Elucidate the mechanisms that control the activation of methane and establish the kinetics over different metal surfaces and clusters.
Explore surface morphology and the influence of oxygen coverage over methane first step dissociation.
All calculations performed using plane wave density functional theory enclosed by VASP package.

3. Previous Results

4. Influence of the Metal on the Metal and Oxygen-Assisted CH4 Activation
Carbon Binding Energy
CH4* + *  CH3* + H* Ag Pt Ru Rh
Metal Activated
CH4* + O*  CH3* + OH* Ag Pt Rh Ru
Oxygen Activated
Activation Energy (eV)
Oxygen Binding Energy

5. Influence of the Oxygen Coverage on
Methane Activation
Pt(111)
Activation Energy (eV)
Increasing
Oxygen
(Coverage)
Oxygen Binding Energy (kJ/mol)

6. Barriers for O2 Dissociation and Recombination
Pt(111)
2O* O2*
Ea (kJ/mol)
O2* O*
Coverage (ML)

7. Influence of Pt Coordination Number on CH4 Activation

8. CH4 activation over Pt-nO surfaces
Low O coverage
Medium O coverage
High O coverage
1.26 eV
0.97 eV
1.33 eV
Early TS
Late TS
Hydride Abstraction

9. CH4 activation over Rh-9O surface and Rh oxides
0.84 eV
1.35 eV
1.50 eV
C-H activation proceeds via H-abstraction rather than metal
insertion and as such is free-radical in character.
Lower band gaps will lower the barrier for C-H activation.
Rh oxide has a lower band gap than platinum oxide and has a lower barrier to activate methane.

10. Rh2O3 RhO2 H2O removal O2 adsorption 0.18 eV - 1.49 eV 0.20 eV

11. Pt55 Cluster Pt55 characteristics:
Significant Reconstruction with
Increasing O Coverage 4O 5O 6O
Gradual increase of 1O on the clean Pt55 cluster
Gradual increase of 1O to Pt55-4O cluster
Pt55 characteristics:
Surface Pt atoms have much lower coordination numbers  very reactive toward O atoms.
When O coverage is increased the cluster surface undergoes significant reconstruction forming an oxide-like surface with low reactivity toward methane.

12. Ads. En. = E(Pt-nO) – E(Pt) – n/2*E(O2)
Pt201 cluster
201 atoms in total
122 surface atoms
8 (111) faces with 19 Pt atoms
6 (100) faces with 9 Pt atoms
6 O ads. sites at 100 facet
12 O ads. sites at 111 facet
Pt + n/2*O2  Pt-nO
Ads. En. = E(Pt-nO) – E(Pt) – n/2*E(O2)

13. 1O adsorption on clean Pt201 cluster
Pt201O 111 terrace
Pt201O 111 corner
Pt201O 100 terrace
Pt201O 100 corner
-0.99
-1.18
-1.10
-1.38
fcc hcp
Pt111 surface
Pt201 terrace
The O adsorption energies on Pt201 cluster 111 facets are similar to those calculated for Pt 111 surface.

14. 1O adsorption at low O coverage
Pt201O7 111 terrace
Pt201O6 111 corner
Pt201O5 100 terrace
Pt201O6 100 corner
-0.39
-0.51
-1.15
-1.20
O atoms are placed in identical locations, with all neighbor
fcc sites previously occupied by other O atoms.

15. 1O adsorption at high O coverage
Pt201O terrace
Pt201O corner
Pt201O terrace
Pt201O corner
0.06
-1.54
-0.79
-0.94
O atoms will preferentially cover 100 facets and low coordinate Pt
atoms from 111 facets, while 111 terraces remain open for methane
dissociation (O*-Pt).
Most probable O concentration varies between 88.5% ÷ 108.2%.

16. Proposed O Adsorption Order
1 – 1c & 1e (-1.38 eV)
2 – 1c & 1e & 1t (-1.18 eV)
3 – 1e & 1t (-1.10 eV)
4 – 3t (-0.99 eV)
5 – 1e & 2t (-0.89 eV)
6 – 1e & 2t (??? eV)
c – corner; e – edge; t – terrace.

17. O adsorption determined by fcc characteristics5 6 4 2
Pt201O edge
Pt201O edge1
Pt201O terrace
Pt201O corner
0.06
-1.54
0.18
In progress
2.829Å
2.910Å
3.074Å
3.031Å
3.206Å
3.057Å
3.051Å
3.048Å
2.844Å
3.061Å
2.918Å
3.032Å

18. Coverage Effects on the Local Pt-Pt Bond Lengths in Pt 111
2.775
2.775
2.934
2.930
2.775
2.774
2.776
2.931
2.773
O- induced
Expansion
Constrained by 2D
Surface
Increasing oxygen coverage increases the lateral repulsion between O and constrains the Pt-Pt bond length.
This limits the amount of oxygen that can adsorb and weakens the oxygen binding energy thus making it more basic.

19. Coverage Effects on the Local Pt-Pt Bond Lengths in Pt201
O- induced
Expansion
2.925
2.961
2.702
2.714
2.959
2.714
3.080
3.043
Further O-Induced
Expansion
3.072

20. Pt201O108 Pt201O129 Pt201O132 88.5% O coverage 105.7% O coverage
D= Å
D= Å
D= Å
Ads. En. / O atom eV
Ads. En. / O atom eV
Ads. En. / O atom eV
3O atoms removed
from each 111 facet
3O atoms removed
from one 111 facet
full O coverage

21. Cluster stretching during O adsorption
Pt201O132
Clean Pt201
zero O coverage
Experimental
full O coverage
D=17.1 Å
D=18 Å
D=18.9 Å

22. 88.5% O Coverage Active O linked with Pt (edge)
Active O linked with Pt (terrace)
Done
In progress

23. CH4 Act. En. (eV) Pt201O108 Zero O coverage Low O coverage
High O coverage
0.82
1.23
0.80
88.5% O coverage
0.92
1.26
1.12
44.5% O coverage

24. CH4 Act. En. (eV) Pt201O129 Pt201O132 1.55 eV 0.73 eV In progress
108.2% O coverage
105.7% O coverage
In progress

25. Summary
O adsorption energies to the central atoms in the 111 terrace of Pt201 cluster are quite similar with Pt 111 surface.
Pt201 cluster elongates when O is adsorbed, but the surface maintains its initial configuration, while for smaller cluster as Pt55 it begin to take on some surface oxide-like characteristics.
Methane activation energy has a lower values on the Pt201 cluster at high O coverage than on clean cluster.

26. Future Interest

27. Increasing the number of O atoms in 111 terrace
1st layer 111  3O in terrace
2nd layer 111  7O in terrace
This artifact may be useful to simulate methane activation on bigger clusters at same, or lower, time necessary to perform the calculations.

28. Acknowledgements
BP Methane Conversion Cooperative Program