Water Adsorption on MO2 (M = Ti, Ru, and Ir) Surfaces. Importance of Octahedral Distortion and Cooperative Effects.

Abstract

Understanding metal oxide MO2 (M = Ti, Ru, and Ir)–water interfaces is essential to assess the catalytic behavior of these materials. The present study analyzes the H2O–MO2 interactions at the most abundant (110) and (011) surfaces, at two different water coverages: isolated water molecules and full monolayer, by means of Perdew–Burke–Ernzerhof-D2 static calculations and ab initio molecular dynamics (AIMD) simulations. Results indicate that adsorption preferably occurs in its molecular form on (110)-TiO2 and in its dissociative form on (110)-RuO2 and (110)-IrO2. The opposite trend is observed at the (011) facet. This different behavior is related to the kind of octahedral distortion observed in the bulk of these materials (tetragonal elongation for TiO2 and tetragonal compression for RuO2 and IrO2) and to the different nature of the vacant sites created, axial on (110) and equatorial on (011). For the monolayer, additional effects such as cooperative H-bond interactions and cooperative adsorption come into play in determining the degree of deprotonation. For TiO2, AIMD indicates that the water monolayer is fully undissociated at both (110) and (011) surfaces, whereas for RuO2, water monolayer exhibits a 50% dissociation, the formation of H3O2– motifs being essential. Finally, on (110)-IrO2, the main monolayer configuration is the fully dissociated one, whereas on (011)-IrO2, it exhibits a degree of dissociation that ranges between 50 and 75%. Overall, the present study shows that the degree of water dissociation results from a delicate balance between the H2O–MO2 intrinsic interaction and cooperative hydrogen bonding and adsorption effects.