Reactive and Inert Surface Species Observed during Methanol Oxidation over Silica-Supported Molybdenum Oxide

Abstract

The mechanism of methanol oxidation over highly dispersed 1% MoO3/SiO2 was studied using a combination of steady state and transient kinetic measurements together with in situ Fourier transform infrared (FTIR) observations of surface-adsorbed species. The main reactive intermediates were methoxide species formed on surface Mo centers. Deuterium substitution experiments showed that these species decomposed through a rate-determining C−H bond-breaking step to form the primary product of reaction, formaldehyde. The steady state data gave an apparent activation energy of 89 kJ/mol, while the oxidation rate of the surface intermediate gave an activation energy for the rate-determining step of 108 kJ/mol. The difference in these gives a heat of adsorption of −19 kJ/mol. Although the oxidation reaction occurred on the Mo centers, the methoxide species were found to migrate to the silica support by reversibly displacing mobile silica OH groups. Two kinds of methoxide species were distinguished, spectators and active intermediates. Only the active intermediates could be oxidized at temperatures lower than 300 °C, and these were the actual participants in the reaction. Quantitative FTIR measurements indicated that the number of methoxide species on the silica support was about six times larger than the number of Mo centers. Thus, the silica surface, which was otherwise inert, participated in a noninnocent manner, holding a substantial population of the reactive intermediates.