Water (Non-)Interaction with MoO.

Molybdenum(VI) oxide (MoO) 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 MoO and thus the course of heterogeneous reactions. We use ambient pressure X-ray photoelectron spectroscopy to study the interaction of water with a stoichiometric MoO surface and a MoO surface that features oxygen defects and hydroxyl groups. The experimental results are supported by density functional theory calculations. We show that on a stoichiometric MoO(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., Mo 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.