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 (CD 3 OH) to CO 2 . 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 CO 2 (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 CD 3 OD. Our observation that the relative fraction of desorbing CD 3 OH 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.