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Three different pathways toward CO formation from adsorbed CH and O are compared by quantum-chemical density functional theory (DFT) calculations for planar and stepped Rh surfaces. The conventional pathway competes with the pathway involving a formyl (CHO) species. This holds for both types of surfaces. The barrier for carbon-oxygen bond formation for the planar surface (180 kJ/mol) is substantially higher than that for the stepped surface (90 kJ/mol). The reaction path through intermediate formyl formation competes with direct formation of CO from recombination via adsorbed C and O atoms. Calculations are used as a basis for the analysis of the overall kinetics of the methane steam reforming reaction as a function of the particle size and the metal.

We present DFT calculations of the energetics of the elementary reaction steps in the dehydrogenation of CH 4 to C on extended Rh(111) and Rh(211) surfaces. The results are compared to the energetics for the same reactions on a planar (111) surface and the edge atoms that are shared between two (111) facets of a nanorod model. The adsorption energies between comparable surfaces of the extended and nanorod models are very similar. Only C adsorbs significantly stronger on the planar surface of the nanorod model than on the extended (111) surface due to the involvement of more reactive edge atoms. Also, the reaction energies between the two types of surface models are very similar. The small differences in reaction and activation energies are largely due to small geometrical differences. In all cases, CH dissociation has the highest activation barrier. However, dissociative CH 4 adsorption is rate controlling under typical steam reforming conditions because of the entropy loss associated with methane adsorption. The barrier for CH 4 dissociation significantly decreases with a decrease of the coordination number of the surface metal atoms. Accordingly, the corrugated surfaces are predicted to be more reactive for methane dissociation than the planar ones.

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