Reactions of methanol on TiO 2(001) single crystal surfaces

Abstract

Reactions of methanoi on TiO 2 were studied on three characteristic (001) single crystal surfaces: the Ar + -bombarded surface, the {011}-facetted surface, and the {114}-facetted surface. Methanol adsorbed on TiO 2(001) surfaces both molecularly and dissociatively at 200 K. Molecularly adsorbed methanol desorbed readily below 300 K. Surface methoxides formed by dissociative adsorption were removed via two channels. The removal of methoxides at 365 K as methanol was assigned to recombination with surface hydroxyl species; approximately 50% of surface methoxides were removed via this pathway. The remaining methoxides decomposed at higher temperatures. Methane, dimethyl ether, formaldehyde, and CO were produced from the decomposition of methoxides, with selectivities depending upon the surface structure and composition. The formation of methane was explained by deoxygenation of methoxides at surface Ti cations with a single oxygen deficit. The {;011}-facetted surface exhibited the highest selectivity for methane formation. Dimethyl ether was produced from methanol via disproportionation of pairs of methoxides coordinated to 4-fold oxygen-coordinated Ti 4+ cations. The formation of dimethyl ether was observed only on the {114}-facetted surface. The selectivity for formaldehyde formation from methanol increased in parallel with the oxygen concentration of the surface. The {114}-facetted surface showed the highest selectivity for formaldehyde formation, as it was characterized by the highest extent of surface oxidation. On the Ar +-bombarded surface, molecular methanol as well as methoxides were formed following methanol adsorption at 300 K, and CO was produced via oxidation of carbon deposited by methoxide decomposition.