Surface Orientation Dependent Water Dissociation on Rutile Ruthenium Dioxide

Rutile RuO₂ is a highly active catalyst for a number of (electro)chemical reactions in aqueous solutions or in humid environments. However, the study of the interaction of RuO₂ surfaces with water has been confined largely to the ultrahigh vacuum environment and to the thermodynamically stable (110) surface. In this work, we combine ambient-pressure X-ray photoelectron spectroscopy, in situ surface diffraction, and density functional theory calculations to investigate how four different facets of RuO₂ interact with water under humid and electrochemical environments. The vacant coordinatively unsaturated Ru site (CUS) allows for the adsorption and dissociation of water molecules. Different surfaces exhibit unique binding energetics for -H₂O and -OH and can allow for different degrees of hydrogen bonding between the adsorbates. Consequently, the degree of water dissociation is found to be sensitive to the surface crystallographic orientation - being maximum for the (101) surface, followed by the (110), (001) and (100) surfaces. This study identifies crystallographic orientation as an important parameter to tune not only the density of active sites but also the energetics for water dissociation; this finding is of great significance for many catalytic reactions, where water is a key reactant, or product.