Insights into the carbonate effect on water oxidation over metal oxide photocatalysts/photoanodes

Photocatalytic/photoelectrochemical water splitting using metal oxide semiconductors is a promising technology for direct and simple solar-energy conversion. The addition of carbonate salts to an aqueous reaction solution has been known to promote stoichiometric O2 evolution and H2O2 production via H2O oxidation. To elucidate the effect of carbonates, density functional theory calculations are performed to study the photoinduced H2O and H2CO3 oxidation mechanisms on TiO2 and BiVO4. The oxidation reactions proceeded via peroxide intermediates, such as H2O2 for H2O, H2C2O6 for H2CO3, and H2CO4 for the coexistence of H2O and H2CO3 molecules. Regardless of the reactant and metal oxide, the free energy changes in the four proton-coupled electron-transfer (PCET) steps of the oxidation mechanism indicate that the first PCET requires the highest energy input and is the rate-limiting step. All PCET steps of the H2O oxidation, except the second one, are more endergonic than those of the H2CO3 oxidation. The H2O reactant requires a larger energy barrier at the highest energy profile, as well as at the final state, than the H2CO3 reactant. The computational results verify that the adsorbed H2CO3 molecule is easily photo-oxidized compared with the adsorbed H2O molecule, facilitating the formation of the peroxide intermediate and improving O2 evolution and H2O2 production.