Water reactivity with tungsten oxides: H ₂ production and kinetic traps

In a recent mass spectrometry/photoelectron spectroscopy study on the reactions between W ₂ O _y ^- (y=2-6) and water, Jarrold and co-workers [J. Chem. Phys. 130, 124314 (2009)] observed interesting differences in the reactivity of the different cluster ions. Particularly noteworthy is the observation that the only product with the incorporation of hydrogens is a single peak corresponding to W ₂ O ₆ H ₂ ^-. As reactions between metal oxide clusters and small molecules such as water have high potential for catalytic applications, we carried out a careful study to obtain a mechanistic understanding of this observed reactivity. Using electronic structure calculations, we identified and characterized multiple modes of reactivity between unsaturated tungsten oxide clusters [W ₂ O _y ^- (y=4-6)] and water. By calculating the free energy corrected reaction profiles, our results provide an explanation for the formation of W ₂ O ₆ H ₂ ^-. We propose a mechanism in which water reacts with a metal oxide cluster and eliminates H ₂. The results from our calculations show that this is nearly a barrierless process for all suboxide clusters with the exception of W ₂ O ₅ ^-.