Coordination Inversion of the Tetrahedrally Coordinated Ru4f Surface Complex on RuO2(100) and Its Decisive Role in the Anodic Corrosion Process

Abstract

The c(2 × 2) reconstruction of RuO2(100) leads to unusual and flexible surface functionality in the form of a tetrahedrally coordinated Ru4f surface complex with distinct chemical properties that are important for the anodic activity and stability in water electrolysis. We employ first-principles methods based on density functional theory calculations to elucidate the hybridization of this Ru4f surface species and its rich coordination chemistry. Under oxygen evolution reaction (OER) conditions, the consecutive breaking of structural Ru4f–O back bonds is shown to proceed via the back side attack of Ru4f (akin to the Walden inversion) by dissociative water adsorption and the release of two proton/electron pairs. This inversion step of the coordination environment of a surface transition metal cation is considered to be the key step in the anodic dissolution process. Oxygen evolution over Ru4f on RuO2(100)–c(2 × 2) competes with its dissolution. From ab initio thermodynamics, the inherent OER activity over Ru4f is shown to be lower than that of Ru surface sites on the unreconstructed RuO2(100), while the dissolution propensity of the Ru4f surface complex is higher. This finding calls into question the frequently stated hypothesis that higher activity is correlated with a lower stability of the active center.