Water Dissociation on Molybdenum and Tungsten Oxide Trimers: Roles of Oxidation State, Structure, and Microsolvation.

Here we present results of a density functional theory based systematic investigation of water adsorption and dissociation on Mo3O9/Mo3O8 and W3O9/W3O8 clusters. For fully oxidized Mo3O9 and W3O9, adsorption of a single water molecule is strongly exothermic (-0.81 and -0.96 eV, respectively), while water dissociation proceeds through similar electronic-energy activation barriers of 0.56-0.57 eV. The addition of a second water molecule on Mo3O9 and W3O9 leads to the formation of a bimolecular water cluster and thus, a hydrogen-bonded network, on the oxide. Water dissociation via a proton-relay mechanism within this hydrogen-bonded network lowers the electronic-energy dissociation barrier to 0.23 eV on Mo3O9 and 0.17 eV on W3O9, an effect that remains evident after inclusion of thermochemical corrections. Compared to this microsolvation, reduction of the oxide alone provides a considerably smaller reduction in the water-dissociation barrier. While fully oxidized molybdenum and tungsten oxide clusters exhibit nearly identical structures and energetics, the reduced Mo3O8 and W3O8 clusters adopt different ground-state geometries, giving rise to metal-dependent water activation behavior that is absent in the fully oxidized systems. Overall, the results show that water dissociation on molybdenum and tungsten oxide clusters is controlled by a complex interplay between microsolvation, oxidation state, and local geometric structure.