Formation of Formate in the Deep Oxidation of Methanol on Pt(111) under UHV Condition Studied by IRAS

Abstract

The reaction of methanol with oxygen on Pt(111) under ultrahigh vacuum condition has been investigated by infrared reflection absorption spectroscopy (IRAS). Molecular and atomic states of adsorbed oxygen existed on the Pt(111) surface, and the adsorption of oxygen at 100 K gave molecularly adsorbed oxygen that dissociated to adatoms at ∼160 K. Formate species, which gave the symmetric C−O stretching band at 1327 cm-1, was observed when methanol-adsorbed Pt(111) surface, which was precovered by molecular oxygen, was heated to 160 K. The IRAS band of formate was verified by comparison with the band of the formate derived from formic acid on the Pt(111) surface and by the isotope shift of the band observed by using deuterated methanol. The amount of produced formate was proportional to the amount of the molecularly adsorbed oxygen. The formate band disappeared at 270 K, at which temperature desorption of CO2 and H2 was observed on temperature-programmed desorption spectra. Molecular oxygen on the Pt(111) surface dissociated at 300 K and the produced adatoms did not oxidize methanol, but the methanol was decomposed to CO and H2. Molecularly adsorbed oxygen is thus considered to be responsible for the oxidation of methanol to formate.