Studies of metal oxide clusters: Elucidating reactive sites responsible for the activity of transition metal oxide catalysts

Transition metal oxides are widely used as both catalysts and catalytic supports in industrial processes. However, the mechanisms by which these materials function as catalysts and the structure-reactivity relationships are not well understood. In particular, there is a paucity of information on the specific sites responsible for the catalytic activity of bulk surfaces. A valuable approach to identifying the active sites of transition metal oxides is to study the chemistry of gas phase metal oxide clusters. A comprehensive program is underway in our laboratory in which reactivity studies of transition metal oxide clusters are carried out using a tandem mass spectrometer system coupled to a laser vaporization source. The desired transition metal oxide cluster cations are mass-selected and then injected into a reaction cell, whereby reactions with various organic molecules are investigated. The advantage of this technique is that the reactivity of specific clusters can be probed independently of other clusters, thereby providing insight into the intermediates and mechanisms at the active sites present on transition metal oxide catalysts of which the gas-phase clusters are representative models. Hence by employing gas-phase techniques, the effect of varying the composition, stoichiometry, oxidation state, charge state, degree of coordinative saturation, and size of the metal oxide clusters on the reactivity is determined. The findings provide valuable information about reaction intermediates, reaction mechanisms, and structure-reactivity relationships. Therefore, these gas-phase studies provide an understanding of the function of transition metal oxide catalysts at the molecular level that is expected to provide knowledge that will find use in the design of more efficient catalysts. This article provides an overview of findings derived in our laboratory for reactions of group V transition metal oxide clusters, with particular emphasis on the mechanism of oxygen transfer to small organic molecules.