Methane dissociation and syngas formation on Ru, Os, Rh, Ir, Pd, Pt, Cu, Ag, and Au: A theoretical study

A theoretical study of methane dissociation and syngas formation on a number of transition metals M(M= Ru, Os, Rh, Ir, Pd, Pt, Cu, Ag, Au) is presented. The metal surface is simulated by a M₁₀ cluster model. Reaction energies for the steps involved are determined. The activation energies have been estimated using the analytic BOC-MP formula. The dissociation energy is shown to be an important factor determining the catalytic activity of the metal. The order of the calculated total dissociation energies (CH_4,s → C_s + 4H_s) is consistent with the order of methane conversions over the metals. In the presence of adsorbed oxygen, oxygen at metal on-top sites promotes methane dehydrogenation; oxygen at hollow sites promotes methane dehydrogenation on Pt, Cu, Ag, and Au, but shows no such effect on the other transition metals. The difference in the H₂ selectivities can be associated with the difference in the stabilities of OH on the metals. For CH_x couplings, the trend in the calculated combination energies is in agreement with experimental observation.