Surface Proton Hopping and Fast-Kinetics Pathway of Water Oxidation on Co₃O₄ (001) Surface

Abstract

We propose a mechanism of water splitting on cobalt oxide surface with atomistic thermodynamic and kinetic details. The density-functional theory studies suggest that the oxidation process could proceed with several nonelectrochemical (spontaneous) intermediate steps, following the initial electrochemical hydroxyl-to-oxo conversion. More specifically, the single oxo sites Co IV ═O can hop (via surface proton/electron hopping) to form oxo pair Co IV (═O)-O-Co IV ═O, which will undergo nucleophilic attack by a water molecule and form the hydroperoxide Co III –OOH. Encounter with another oxo would generate a superoxo Co III –OO, followed by the O 2 release. Finally the addition and deprotonation of a fresh water molecule will restart the catalytic cycle by forming the hydroxyl Co III –OH at this active site. Our theoretical investigations indicate that all nonelectrochemical reactions are kinetically fast and thermodynamically downhill. This hypothesis is supported by recent in situ spectroscopic observations of surface superoxo that is stabilized by hydrogen bonding to adjacent hydroxyl group as an intermediate on fast-kinetics Co catalytic site.