Interaction between Ruthenium Oxide Surfaces and Water Molecules. Effect of Surface Morphology and Water Coverage

RuO2 is a conducting transition metal oxide that has unique redox properties to be used as heterogeneous catalyst for oxidation reactions as well as in electrocatalysis. Furthermore, it has been reported to be an excellent catalyst for the oxygen evolution reaction, a key step for obtaining energy from water through environmentally friendly processes. In this context, a detailed knowledge of the RuO2-water interface is important for a better understanding of the electrochemical process, the water oxidation reaction and some oxidative reactions involving RuO2. Here, we use periodic boundary condition DFT (PBE-D2) calculations to analyze the influence of the surface morphology and water coverage in the adsorption energies and degree of water deprotonation. We have considered the four nonpolar ((110), (011), (100), and (001)) most relevant surfaces and three degrees of water coverage: isolated molecules, half monolayer and full monolayer. Results indicate that three effects are crucial for determining the adsorption energy and degree of deprotonation: (i) the intrinsic acidity of the unsaturated ruthenium cations and the intrinsic basicity of the O-br. centers; (ii) the presence of strong cooperative effects, already observed in the half monolayer situation of the (110) and (011) surfaces that favors 50% of deprotonation and leads to the formation of the (H3O2)(-) motif; and (iii) an increase of the surface O-br basicity by the adsorption of water molecules on Ru centers bonded to Obr groups, which is more important in the (100) and (001) surfaces.