Control of reactivity in C-H bond breaking reactions on oxide catalysts: Methanol oxidation on supported molybdenum oxide

Oxidation of organic substrates on metal oxide catalysts can be viewed as entailing a transfer of electrons from the organic moeity to the catalytic center, and is expected to involve the electron-accepting levels in the metal center. This investigation deals with the study of the electron-transfer processes associated with the C-H bond breaking reaction in adsorbed methoxide species in the course of methanol oxidation on supported MoO3. It is shown that the activity of a series of catalysts duly correlates with the unoccupied density of electronic states of the parent metal oxides. The unoccupied levels are probed with near-edge X-ray absorption spectroscopy (NEXAFS). The particular system chosen for study is that of methanol oxidation on supported molybdenum oxide, a reaction in which C-H bond breaking is rate-determining. The findings of this study are in good agreement with the known facts about the reaction and allow a deeper understanding of various reported effects. For example, the positive relationships between the catalytic activity and the decreasing electronegativity of the support ion, the increasing reducibility of the dispersed molybdenum centers, and the increasing size of the molybdenum ensembles can all be explained by the increasing ease of accommodation of charge by the metal centers. The findings in this study should be of importance in understanding other C-H bond breaking reactions and multicomponent catalyst systems.