Molecular-level insight into photocatalytic CO₂ reduction with H₂O over Au nanoparticles by interband transitions

Achieving CO₂ reduction with H₂O on metal photocatalysts and understanding the corresponding mechanisms at the molecular level are challenging. Herein, we report that quantum-sized Au nanoparticles can photocatalytically reduce CO₂ to CO with the help of H₂O by electron-hole pairs mainly originating from interband transitions. Notably, the Au photocatalyst shows a CO production rate of 4.73 mmol g^-1 h^-1 (~100% selectivity), ~2.5 times the rate during CO₂ reduction with H₂ under the same experimental conditions, under low-intensity irradiation at 420 nm. Theoretical and experimental studies reveal that the increased activity is induced by surface Au-O species formed from H₂O decomposition, which synchronously optimizes the rate-determining steps in the CO₂ reduction and H₂O oxidation reactions, lowers the energy barriers for the *CO desorption and *OOH formation, and facilitates CO and O₂ production. Our findings provide an in-depth mechanistic understanding for designing active metal photocatalysts for efficient CO₂ reduction with H₂O.