Ammonia (NH) is an essential chemical in modern society. It is currently manufactured by the Haber-Bosch process using H and N under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH production from water and N 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 TiO with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N, successfully produces NH. The active sites for N reduction are the Ti species on the oxygen vacancies. These species act as adsorption sites for N and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N to NH. 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 TiO system therefore shows a potential as a new artificial photosynthesis for green NH production.

Visible-light irradiation (λ > 450 nm) of gold nanoparticles loaded on a mixture of anatase/rutile TiO(2) particles (Degussa, P25) promotes efficient aerobic oxidation at room temperature. The photocatalytic activity critically depends on the catalyst architecture: Au particles with <5 nm diameter located at the interface of anatase/rutile TiO(2) particles behave as the active sites for reaction. This photocatalysis is promoted via plasmon activation of the Au particles by visible light followed by consecutive electron transfer in the Au/rutile/anatase contact site. The activated Au particles transfer their conduction electrons to rutile and then to adjacent anatase TiO(2). This catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of O(2) by the conduction band electrons on the surface of anatase TiO(2). This plasmonic photocatalysis is successfully promoted by sunlight exposure and enables efficient and selective aerobic oxidation of alcohols at ambient temperature.

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Photocatalytic production of hydrogen peroxide (H 2 O 2 ) on semiconductor catalysts with alcohol as a hydrogen source and molecular oxygen (O 2 ) as an oxygen source is a potential method for safe H 2 O 2 synthesis because the reaction can be carried out without the use of explosive H 2 /O 2 mixed gases. Early reported photocatalytic systems, however, produce H 2 O 2 with significantly low selectivity (∼1%). We found that visible light irradiation (λ > 420 nm) of graphitic carbon nitride (g-C 3 N 4 ), a polymeric semiconductor, in an alcohol/water mixture with O 2 efficiently produces H 2 O 2 with very high selectivity (∼90%). Raman spectroscopy and electron spin resonance analysis revealed that the high H 2 O 2 selectivity is due to the efficient formation of 1,4-endoperoxide species on the g-C 3 N 4 surface. This suppresses one-electron reduction of O 2 (superoxide radical formation), resulting in selective promotion of two-electron reduction of O 2 (H 2 O 2 formation).

Design of green, safe, and sustainable process for the synthesis of hydrogen peroxide (H2 O2 ) is a very important subject. Early reported processes, however, require hydrogen (H2 ) and palladium-based catalysts. Herein we propose a photocatalytic process for H2 O2 synthesis driven by metal-free catalysts with earth-abundant water and molecular oxygen (O2 ) as resources under sunlight irradiation (λ>400 nm). We use graphitic carbon nitride (g-C3 N4 ) containing electron-deficient aromatic diimide units as catalysts. Incorporating the diimide units positively shifts the valence-band potential of the catalysts, while maintaining sufficient conduction-band potential for O2 reduction. Visible light irradiation of the catalysts in pure water with O2 successfully produces H2 O2 by oxidation of water by the photoformed valence-band holes and selective two-electron reduction of O2 by the conduction band electrons.

Solar-to-chemical energy conversion is a challenging subject for renewable energy storage. In the past 40 years, overall water splitting into H2 and O2 by semiconductor photocatalysis has been studied extensively; however, they need noble metals and extreme care to avoid explosion of the mixed gases. Here we report that generating hydrogen peroxide (H2O2) from water and O2 by organic semiconductor photocatalysts could provide a new basis for clean energy storage without metal and explosion risk. We found that carbon nitride-aromatic diimide-graphene nanohybrids prepared by simple hydrothermal-calcination procedure produce H2O2 from pure water and O2 under visible light (λ > 420 nm). Photoexcitation of the semiconducting carbon nitride-aromatic diimide moiety transfers their conduction band electrons to graphene and enhances charge separation. The valence band holes on the semiconducting moiety oxidize water, while the electrons on the graphene moiety promote selective two-electron reduction of O2. This metal-free system produces H2O2 with solar-to-chemical energy conversion efficiency 0.20%, comparable to the highest levels achieved by powdered water-splitting photocatalysts.

TiO2 loaded with Au–Ag bimetallic alloy particles efficiently produces H2O2 from an O2-saturated ethanol/water mixture under UV irradiation. This is achieved via the double effects created by the alloy particles. One is the efficient photocatalytic reduction of O2 on the Au atoms promoting enhanced H2O2 formation, due to the efficient separation of photoformed electron–hole pairs at the alloy/TiO2 heterojunction. Second is the suppressed photocatalytic decomposition of formed H2O2 due to the decreased adsorption of H2O2 onto the Au atoms.

Titanium dioxide with a mesoporous structure, when photoactivated in water, demonstrates an unprecedented photocatalytic activity, driven strongly by an adsorption degree of molecules onto the catalyst surface, which promotes a preferential conversion of a well-adsorbed molecule. This catalyzes a selective transformation of a well-adsorbed molecule into a less-adsorbed molecule, so-labeled "stick-and-leave" transformation, which promotes a direct hydroxylation of benzene to phenol, one of the most difficult synthetic reactions, with very high selectivity (>80%) and using water as a source of oxidant.