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

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.

Previous Results

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

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

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

Influence of Pt Coordination Number on CH4 Activation

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

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.

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

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.

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)

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 -0.92 -0.53 Pt201 terrace -0.99 -0.69 The O adsorption energies on Pt201 cluster 111 facets are similar to those calculated for Pt 111 surface.

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.

1O adsorption at high O coverage Pt201O131 111 terrace Pt201O131 111 corner Pt201O131 100 terrace Pt201O131 100 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%.

Proposed O Adsorption Order 1 – 1c & 1e 100 (-1.38 eV) 2 – 1c & 1e & 1t 111 (-1.18 eV) 3 – 1e & 1t 100 (-1.10 eV) 4 – 3t 111 (-0.99 eV) 5 – 1e & 2t 111 (-0.89 eV) 6 – 1e & 2t 111 (??? eV) c – corner; e – edge; t – terrace.

O adsorption determined by fcc characteristics 5 6 4 2 Pt201O131 111 edge Pt201O131 111 edge1 Pt201O131 111 terrace Pt201O131 111 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

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.

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

Pt201O108 Pt201O129 Pt201O132 88.5% O coverage 105.7% O coverage D=18.607 Å D=18.912 Å D=18.934 Å Ads. En. / O atom - 1.01 eV Ads. En. / O atom - 0.86 eV Ads. En. / O atom - 0.84 eV 3O atoms removed from each 111 facet 3O atoms removed from one 111 facet full O coverage

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

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

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

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

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.

Future Interest

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.

Acknowledgements BP Methane Conversion Cooperative Program