The adsorption and decomposition of methanol on Pt(110)

Abstract

The adsorption of methanol on clean and oxygen dosed Pt(110) has been studied using temperature programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Methanol adsorbed in two states, a monolayer phase and a multilayer phase, which were distinguishable by both TPRS and XPS. The saturated monolayer (corresponding to 8.37 × 10 14 molecules cm −2) desorbed over a broad range of temperature (160–220 K), the only other reaction products desorbing at higher temperatures being H 2 and CO. The evolution of these gases is desorption limited and reaches a maximum upon completion of the first methanol monolayer. Analysis of TPRS and XPS peak areas demonstrated that on the clean surface approximately 10% of the adsorbed monolayer dissociated with 90% desorbing intact, in agreement with an earlier study of methanol adsorption on Pt(111) by Sexton and co-workers. Reaction of methanol with pre-adsorbed oxygen resulted in the formation of water which desorbed at 189 K. At 110 K, a substantial shift of 0.7 eV to lower binding energy in the oxygen ls peak of methanol coadsorbed with oxygen indicated a strong perturbation of the methanol molecule prior to dissociation. A model for this reaction in which deprotonation of the alcohol to form hydroxyl and methoxy species is proposed. Further heating causes recombination of the hydroxyls to produce water and decomposition of methoxy to form CO and H 2. The relation of these reactions to those occurring in the electrochemical oxidation of methanol is elucidated.