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Understanding the nature of active sites is central to controlling (electro)catalytic activity. Here we employed surface X-ray scattering coupled with density functional theory and surface-enhanced infrared absorption spectroscopy to examine the oxygen evolution reaction on RuO2 surfaces as a function of voltage. At 1.5 VRHE, our results suggest that there is an –OO group on the coordinatively unsaturated ruthenium (RuCUS) site of the (100) surface (and similarly for (110)), but adsorbed oxygen on the RuCUS site of (101). Density functional theory results indicate that the removal of –OO from the RuCUS site, which is stabilized by a hydrogen bond to a neighbouring –OH (–OO–H), could be the rate-determining step for (100) (similarly for (110)), where its reduced binding on (100) increased activity. A further reduction in binding energy on the RuCUS site of (101) resulted in a different rate-determining step (–O + H2O – (H+ + e−) → –OO–H) and decreased activity. Our study provides molecular details on the active sites, and the influence of their local coordination environment on activity. [Figure not available: see fulltext.]

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.

While the surface atomic structure of RuO2 has been well studied in ultra high vacuum, much less is known about the interaction between water and RuO2 in aqueous solution. In this work, in situ surface X-ray scattering measurements combined with density functional theory (DFT) were used to determine the surface structural changes on single-crystal RuO2(110) as a function of potential in acidic electrolyte. The redox peaks at 0.7, 1.1 and 1.4 V vs. reversible hydrogen electrode (RHE) could be attributed to surface transitions associated with the successive deprotonation of -H2O on the coordinatively unsaturated Ru sites (CUS) and hydrogen adsorbed to the bridging oxygen sites. At potentials relevant to the oxygen evolution reaction (OER), an -OO species on the Ru CUS sites was detected, which was stabilized by a neighboring -OH group on the Ru CUS or bridge site. Combining potential-dependent surface structures with their energetics from DFT led to a new OER pathway, where the deprotonation of the -OH group used to stabilize -OO was found to be rate-limiting.