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 M 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.