Dissociation of methane on different transition metals
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Dissociation of methane on different transition metals M (M = Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag, Au) has been investigated using a quasi-relativistic density-functional method. Reaction enthalpies for the steps involved are determined. The activation energies have been estimated using the analytic BOC-MP formula. The transition metals, Ru, Rh, ..., Pt are shown to exhibit high activity in the dissociation of methane, whereas the coinage metals (Cu, Ag, Au) are very inactive. The conclusion is in agreement with experimental observations. The total dissociation enthalpy Delta H for the complete dissociation of CH4 to give surface C and H (CH4,s --> C-s + 4H(s)) can be regarded as a measure for the activity of the metal in methane dissociation. The order of the calculated Delta H's is consistent with the order of methane conversions over the metals. The dissociation of methane is also examined in the presence of adsorbed oxygen. Oxygen at on-top site promotes methane dehydrogenation. Oxygen at hollow site promotes methane dehydrogenation on Pt and the coinage metals, but is not beneficial to that on the other transition metals. (C) 1998 Elsevier Science B.V. All rights reserved.