Water Adsorption and Oxidation at the Co3O4 (110) Surface

We carried out density functional theory calculations with on-site Coulomb repulsion U terms to study the interaction of water with the (110) surface of the spinel cobalt oxide, Co3O4, a widely used oxidation catalyst. This surface has two different terminations, one positively (A) and the other negatively charged (B). Dissociative water adsorption is preferred from low up to one monolayer coverage on the A termination and up to half monolayer on the B termination. On the latter, a mixed molecular and dissociated monolayer is more stable at full coverage. The computed structures are used to investigate the free-energy changes during water oxidation on both surface terminations. We find that the most difficult step of the oxygen evolution reaction is the second deprotonation to form an adsorbed O species (O*). Moreover, the A-terminated surface is more active than the B-terminated surface. Analysis of the surface electronic structure shows a larger density of cobalt states near the Fermi energy on the A termination, which stabilizes the O* species and thus reduces the overpotential.