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Epoxides are significant intermediates for the manufacture of pharmaceuticals and epoxy resins. In this study, we develop a Br−/BrO− mediated photoelectrochemical epoxidation system on α-Fe2O3. High selectivity (up to >99%) and faradaic efficiency (up to 82 ± 4%) for the epoxidation of a wide range of alkenes are achieved, with water as oxygen source, which are far beyond the most reported electrochemical and photoelectrochemical epoxidation performances. Further, we can verify that the epoxidation reaction is mediated by Br−/BrO− route, in which Br− is oxidized non-radically to BrO− by an oxygen atom transfer pathway on α-Fe2O3, and the formed BrO− in turn transfers its oxygen atom to the alkenes. The non-radical mediated characteristic and the favorable thermodynamics of the oxygen atom transfer process make the epoxidation reactions very efficient. We believe that this photoelectrochemical Br−/BrO−-mediated epoxidation provides a promising strategy for value-added production of epoxides and hydrogen. Alkene epoxidation, whose products are important intermediates for the manufacture of pharmaceuticals and epoxy resins, has attracted extensive attention. Unlike the conventional electrochemical Br−/Br2-mediated epoxidation, here the authors report a photoelectrochemical Br−/BrO−- mediated system to achieve efficient epoxidation performance on α-Fe2O3. [ABSTRACT FROM AUTHOR]

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. While plasmonic metals show some activity for photocatalysis, they often must be paired with semiconductors or sacrificial reagents. Here, authors find quantum-sized Au nanoparticles can achieve photocatalytic CO2-to-CO conversion with H2O by charge carriers generated from interband transitions. [ABSTRACT FROM AUTHOR]