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

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.