Photocatalytic Conversion of Nitrogen to Ammonia with Water on Surface Oxygen Vacancies of Titanium Dioxide

Ammonia (NH₃) is an essential chemical in modern society. It is currently manufactured by the Haber–Bosch process using H₂ and N₂ under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH₃ production from water and N₂ at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO₂ with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N₂, successfully produces NH₃. The active sites for N₂ reduction are the Ti³⁺ species on the oxygen vacancies. These species act as adsorption sites for N₂ and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N₂ to NH₃. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO₂ system therefore shows a potential as a new artificial photosynthesis for green NH₃ production

Noble-Metal-Free Deoxygenation of Epoxides: Titanium Dioxide as a Photocatalytically Regenerable Electron-Transfer Catalyst

Catalytic deoxygenation of epoxides into the corresponding alkenes is a very important reaction in organic synthesis. Early reported systems, however, require noble metals, high reaction temperatures (>373 K), or toxic reducing agents. Here, we report a noble-metal-free heterogeneous catalytic system driven with alcohol as a reducing agent at room temperature. Photoirradiation (λ <420 nm) of semiconductor titanium dioxide (TiO₂) with alcohol promotes efficient and selective deoxygenation of epoxides into alkenes. This noble-metal-free catalytic deoxygenation is facilitated by the combination of electron transfer from surface Ti³⁺ atoms on TiO₂ to epoxides, which promotes deoxygenation of epoxides, and photocatalytic action of TiO₂, which regenerates oxidized surface Ti atoms with alcohol as a reducing agent

Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts

Ammonia (NH₃) is an essential chemical in modern society, currently manufactured via the Haber–Bosch process with H₂ and N₂ under extremely high pressure (>200 bar) and high-temperature conditions (>673 K). Toxic nitrate anion (NO₃^–) contained in wastewater is one potential nitrogen source. Selective NO₃^–-to-NH₃ transformation via eight-electron reduction, if promoted at atmospheric pressure and room temperature, may become a powerful recycling process for NH₃ production. Several photocatalytic systems have been proposed, but many of them produce nitrogen gas (N₂) via five-electron reduction of NO₃^–. Here, we report that unmodified TiO₂, when photoexcited by ultraviolet (UV) light (λ > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO₃^–-to-NH₃ reduction with 97% selectivity. Surface defects and Lewis acid sites of TiO₂ behave as reduction sites for NO₃^–. The surface defect selectively promotes eight-electron reduction (NH₃ formation), while the Lewis acid site promotes nonselective reduction (N₂ and NH₃ formation). Therefore, the TiO₂ with a large number of surface defects and a small number of Lewis acid sites produces NH₃ with very high selectivity

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

Rutile Crystallites Isolated from Degussa (Evonik) P25 TiO₂: Highly Efficient Photocatalyst for Chemoselective Hydrogenation of Nitroaromatics

We report that the rutile crystallites, isolated from Degussa (Evonik) P25 TiO₂ by a hydrofluoric acid treatment, behave as a highly efficient photocatalyst for hydrogenation of nitroaromatics. Photoirradiation (λ >300 nm) of the isolated rutile particles with alcohol as a hydrogen source successfully promotes chemoselective hydrogenation of nitroaromatics to anilines, with an activity higher than that of commercially available rutile TiO₂. The high activity of the isolated rutile particles is due to the specific distribution of structural defects (oxygen vacancy sites) on the particles. These particles contain a relatively small number of inner defects behaving as recombination centers for photoformed electron (e^–) and positive hole (h⁺) pairs, and a relatively large number of surface defects behaving as reduction sites for nitroaromatics. Photoexcitation of the isolated particles therefore promotes efficient charge separation between e^– and h⁺, and facilitates rapid reduction of nitroaromatics adsorbed on the surface defects. This thus results in very high hydrogenation activity on the rutile particles isolated from P25 TiO₂

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

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

One-Pot Synthesis of Imines from Nitroaromatics and Alcohols by Tandem Photocatalytic and Catalytic Reactions on Degussa (Evonik) P25 Titanium Dioxide

Photoirradiation (λ > 300 nm) of Degussa (Evonik) P25 TiO₂, a mixture of anatase and rutile particles, in alcohols containing nitroaromatics at room temperature produces the corresponding imines with very high yields (80–96%). Other commercially available anatase or rutile TiO₂ particles, however, exhibit very low yields (<30%). The imine formation involves two step reactions on the TiO₂ surface: (i) photocatalytic oxidation of alcohols (aldehyde formation) and reduction of nitrobenzene (aniline formation) and (ii) condensation of the formed aldehyde and aniline on the Lewis acid sites (imine formation). The respective anatase and rutile particles were isolated from P25 TiO₂ by the H₂O₂/NH₃ and HF treatments to clarify the activity of these two step reactions. Photocatalysis experiments revealed that the active sites for photocatalytic reactions on P25 TiO₂ are the rutile particles, promoting efficient reduction of nitrobenzene on the surface defects. In contrast, catalysis experiments showed that the anatase particles isolated from P25 TiO₂ exhibit very high activity for condensation of aldehyde and aniline, because the number of Lewis acid sites on the particles (73 μmol g^–1) is much higher than that of other commercially available anatase or rutile particles (<15 μmol g^–1). The P25 TiO₂ particles therefore successfully promote tandem photocatalytic and catalytic reactions on the respective rutile and anatase particles, thus producing imines with very high yields