Oxidation of Methanol on Oxygen Covered Ptn/Ru(0001) Layers

Thin pseudomorphic as well as thick "Pt(111)- like" Pt/Ru(0001) bimetallic substrates have been used to study the oxidation of methanol (CD3OH) to CO2. Specifically, molecular (cold deposited) and ordered atomic (annealed) oxygen precovered Pt-n/Ru(0001) layers with n = 1-15 ML have been studied. Thereby, the thickness of the Pt films primarily influences the adsorption probability of O-2 and much less the energy barriers for methanol reactions. In particular, no indication of a strain effect could be detected, and the reactions of methanol on Pt-n/Ru(0001) multilayer films closely follow the known reaction scheme on Pt(111) substrates: On O-2/Pt-n/Ru(0001), methanol is directly oxidized to formate (DCOO) and eventually desorbs as CO2 (300 K). The formate producing reaction is thereby promoted by dissociating O-2 molecules (130 K) and, at higher temperatures (130-200 K), by disordered atomic oxygen. This is in contrast to methanol postadsorbed onto ordered atomic oxygen covered Pt-n/Ru(0001) layers, which exhibit formaldehyde formation and decomposition to CO and H. Interestingly, during methanol dissociation at 160-200 K, a remarkable fraction of methanol molecules experiences an H <-> D exchange of the hydroxyl group hydrogen, leading to the dominant desorption of CD3OD. Our observation that the relative fraction of desorbing CD3OH is exceptionally low implies a bold primary isotope effect, which we attribute to the dissimilar zero point energies associated with OH and OD vibrations of methanol.