Coordination Inversion of the Tetrahedrally Coordinated Ru<sub>4f</sub> Surface Complex on RuO<sub>2</sub>(100) and Its Decisive Role in the Anodic Corrosion Process
Franziska Heß, Herbert Over
Abstract
The c (2 × 2) reconstruction of RuO 2 (100) leads to unusual and flexible surface functionality in the form of a tetrahedrally coordinated Ru 4f 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 Ru 4f surface species and its rich coordination chemistry. Under oxygen evolution reaction (OER) conditions, the consecutive breaking of structural Ru 4f –O back bonds is shown to proceed via the back side attack of Ru 4f (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 Ru 4f on RuO 2 (100)– c (2 × 2) competes with its dissolution. From ab initio thermodynamics, the inherent OER activity over Ru 4f is shown to be lower than that of Ru surface sites on the unreconstructed RuO 2 (100), while the dissolution propensity of the Ru 4f 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.