Titanium Dioxide/Reduced Graphene Oxide Hybrid Photocatalysts for Efficient and Selective Partial Oxidation of Cyclohexane

Partial oxidation of cyclohexane (CHA) to cyclohexanone (CHA-one) with molecular oxygen (O₂) is one of the most important reactions. Photocatalytic oxidation has been studied extensively with TiO₂-based catalysts. Their CHA-one selectivities are, however, insufficient because the formed CHA-one is subsequently decomposed by photocatalysis involving the reaction with superoxide anion (O₂^●–) produced by one-electron reduction of O₂ on TiO₂. Here we report that TiO₂, when hybridized with reduced graphene oxide (rGO), catalyzes photooxidation of CHA to CHA-one with enhanced activity and selectivity under UV light (λ > 300 nm). The TiO₂/rGO hybrids produce CHA-one with twice the amount formed on bare TiO₂ with much higher selectivity (>80%) than that on bare TiO₂ (ca. 60%). The conduction band electrons photoformed on TiO₂ are transferred to rGO, promoting efficient charge separation and enhanced photocatalytic cycles. The trapped electrons on rGO selectively promote two-electron reduction of O₂ and suppress one-electron reduction. This inhibits the formation of O₂^●–, which promotes photocatalytic decomposition of the CHA-one formed. These properties of rGO therefore facilitate efficient and selective formation of CHA-one on the hybrid catalyst

Au Nanoparticles Supported on BiVO₄: Effective Inorganic Photocatalysts for H₂O₂ Production from Water and O₂ under Visible Light

The design of a safe and sustainable process for the synthesis of hydrogen peroxide (H₂O₂) is a very important subject from the viewpoint of green chemistry. Photocatalytic H₂O₂ production with earth-abundant water and molecular oxygen (O₂) as resources is an ideal process. A successful system based on an organic semiconductor has been proposed; however, it suffers from poor photostability. Here we report an inorganic photocatalyst for H₂O₂ synthesis. Visible light irradiation (λ >420 nm) of the semiconductor BiVO₄ loaded with Au nanoparticles (Au/BiVO₄) in pure water with O₂ successfully produces H₂O₂. The bottom of the BiVO₄ conduction band (0.02 V vs NHE, pH 0) is more positive than the one-electron reduction potential of O₂ (−0.13 V) while more negative than the two-electron reduction potential of O₂ (0.68 V). This thus suppresses one-electron reduction of O₂ and selectively promotes two-electron reduction of O₂, resulting in efficient H₂O₂ formation

Nitrogen Fixation with Water on Carbon-Nitride-Based Metal-Free Photocatalysts with 0.1% Solar-to-Ammonia Energy Conversion Efficiency

Ammonia (NH₃), which is an indispensable chemical, is produced by the Haber–Bosch process using H₂ and N₂ under severe reaction conditions. Although photocatalytic N₂ fixation with water under ambient conditions is ideal, all previously reported catalysts show low efficiency. Here, we report that a metal-free organic semiconductor could provide a new basis for photocatalytic N₂ fixation. We show that phosphorus-doped carbon nitride containing surface nitrogen vacancies (PCN-V), prepared by simple thermal condensation of the precursors under H₂, produces NH₃ from N₂ with water under visible light irradiation. The doped P atoms promote water oxidation by the photoformed valence-band holes, and the N vacancies promote N₂ reduction by the conduction-band electrons. These phenomena facilitate efficient N₂ fixation with a solar-to-chemical conversion (SCC) efficiency of 0.1%, which is comparable to the average solar-to-biomass conversion efficiency of natural photosynthesis by typical plants. Thus, this metal-free catalyst shows considerable potential as a new method of artificial photosynthesis