Oxidation of methanol on platinum, ruthenium and mixed Pt-M metals (M = Ru, Sn): a theoretical study

A relativistic density-functional study of the dehydrogenation of CH3OH and H2O on pure platinum, ruthenium, and mixed Pt-M (M = Ru, Sn) metals is reported. Cluster models of PtnM10-n were used to simulate the metal surfaces. Calculated adsorption energies of a series of intermediate species agree well with available experimental values. Reaction energies for the elementary steps involved are determined and their activation energies estimated by the analytic unity bond index-quadratic exponential potential (UBI-QEP) formula of Shustorovich and Sellers [Surf. Sci. Rep. 31 (1998) 1]. On pure platinum, the dehydrogenation of surface-adsorbed methanol, CH3OHs, is energetically favorable, with a hydrogen atom first stripped off the carbon end. However, the dehydrogenation of H2Os to OHs, crucial in oxidative removal of poisoning CO, has the highest activation energy (E+) among all of the reaction steps. Dehydrogenation of H2Os on pure ruthenium is considerably more favorable than on pure platinum, but the combination reaction, COs + OHs-->COOHs, for the oxidative removal of COs by OHs possesses the highest E+. On mixed Pt-M metals, the effect of hi is to promote the formation of OHs and a combined effect of platinum and M atoms favors oxidative removal of COs and formation of COOHs. The relative importance of a 'ligand effect' versus a bifunctional mechanism for oxidative removal of COs is discussed. A ligand effect plays some role in the oxidation of CO on Pt-Ru, but it is unimportant on Pt-Sn. (C) 2000 Elsevier Science B.V. All rights reserved.