Water (Non-)Interaction with MoO₃

Molybdenum(VI) oxide (MoO₃) is used in a number of technical processes such as gas filtration and heterogeneous catalysis. In these applications, the adsorption and dissociation of water on the surface can influence the chemistry of MoO₃ and thus the course of heterogeneous reactions. We use ambient pressure X-ray photoelectron spectroscopy to study the interaction of water with a stoichiometric MoO₃ surface and a MoO₃ surface that features oxygen defects and hydroxyl groups. The experimental results are supported by density functional theory calculations. We show that on a stoichiometric MoO₃(010) surface, where Mo sites are unavailable, water adsorption is strongly disfavored. However, the introduction of surface species, which can interact with the lone pairs on the water O atom, e.g., Mo⁵⁺ atoms or surface OH groups, promotes water adsorption. Dissociation of water is favored at unsaturated Mo sites, i.e., at oxygen vacancies, while water adsorbs molecularly at hydroxyl sites.