Enabling Overall Water Splitting on Photocatalysts by CO-Covered Noble Metal Co-catalysts

Photocatalytic overall water splitting requires co-catalysts that efficiently promote the generation of H₂ but do not catalyze its reverse oxidation. We demonstrate that CO chemisorbed on metal co-catalysts (Rh, Pt, Pd) suppresses the back reaction while maintaining the rate of H₂ evolution. On Rh/GaN:ZnO, the highest H₂ production rates were obtained with 4–40 mbar of CO, the back reaction remaining suppressed below 7 mbar of O₂. The O₂ and H₂ evolution rates compete with CO oxidation and the back reaction. The rates of all reactions increased with increasing photon absorption. However, due to different dependencies on the rate of charge carrier generation, the selectivities for O₂ and H₂ formation increased in comparison to CO oxidation and the back reaction with increasing photon flux and/or quantum efficiency. Under optimum conditions, the impact of CO to prevent the back reaction is identical to that of a Cr₂O₃ layer covering the active metal particle