Highly Efficient and Selective Hydrogenation of Nitroaromatics on Photoactivated Rutile Titanium Dioxide

We report that photoactivated rutile titanium dioxide (TiO₂) catalyzes a highly efficient and selective hydrogenation of nitroaromatics with alcohol as a hydrogen source. Photoirradiation (λ >300 nm) of rutile TiO₂ suspended in alcohol containing nitroaromatics at room temperature and atmospheric pressure produces the corresponding anilines with almost quantitative yields, whereas common anatase and P25 TiO₂ show poor activity and selectivity. The Ti³⁺ atoms located at the oxygen vacancies on the rutile surface behave as the adsorption site for nitroaromatics and the trapping site for photoformed conduction band electrons. These effects facilitate rapid and selective nitro-to-amine hydrogenation of the adsorbed nitroaromatics by the surface-trapped electrons, enabling aniline formation with significantly high quantum yields (>25% at <370 nm). The rutile TiO₂ system also facilitates chemoselective hydrogenation of nitroaromatics with reducible substituents; several kinds of functionalized anilines are successfully produced with >94% yields

Gold Nanoparticles Located at the Interface of Anatase/Rutile TiO₂ Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation

Visible-light irradiation (λ > 450 nm) of gold nanoparticles loaded on a mixture of anatase/rutile TiO₂ 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 TiO₂ 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 TiO₂. This catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of O₂ by the conduction band electrons on the surface of anatase TiO₂. This plasmonic photocatalysis is successfully promoted by sunlight exposure and enables efficient and selective aerobic oxidation of alcohols at ambient temperature

Selective Hydrogen Peroxide Formation by Titanium Dioxide Photocatalysis with Benzylic Alcohols and Molecular Oxygen in Water

Photocatalytic production of hydrogen peroxide (H₂O₂) on semiconductor catalysts with alcohol as a hydrogen source and molecular oxygen (O₂) as an oxygen source has attracted much attention as a potential method for safe H₂O₂ synthesis, because the reaction can be carried out without the use of explosive H₂/O₂ mixed gases. Early reported photocatalytic systems with aliphatic alcohol as a hydrogen source, however, produce only a few millimolar levels of H₂O₂. We found that benzylic alcohols, when used as a hydrogen source for photoreaction in water with titanium dioxide (TiO₂) photocatalyst, produce a very high concentration of H₂O₂ (ca. 40 mM). Raman spectroscopy and electron spin resonance analysis revealed that the enhanced H₂O₂ formation is due to the efficient formation of side-on coordinated peroxo species on the photoactivated TiO₂ surface, via the reaction of benzylic alcohol and O₂. The peroxo species is readily transformed to H₂O₂, thus facilitating highly efficient H₂O₂ production

Photocatalytic H₂O₂ Production from Ethanol/O₂ System Using TiO₂ Loaded with Au–Ag Bimetallic Alloy Nanoparticles

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

Platinum Nanoparticles Supported on Anatase Titanium Dioxide as Highly Active Catalysts for Aerobic Oxidation under Visible Light Irradiation

Visible light irradiation (λ >450 nm) of platinum (Pt) nanoparticles supported on anatase titanium dioxide (TiO₂) promotes efficient aerobic oxidation at room temperature. This occurs via the electronic excitation of Pt particles by visible light followed by the transfer of their electrons to anatase conduction band. The positively charged Pt particles oxidize substrates, whereas the conduction band electrons are consumed by the reduction of molecular oxygen. The activity of this photocatalysis depends on the height of Schottky barrier and the number of perimeter Pt atoms created at the Pt/anatase heterojunction, which are affected by the amount of Pt loaded and the size of Pt particles. The catalyst loaded with 2 wt % Pt, containing 3–4 nm Pt particles, creates a relatively low Schottky barrier and a relatively large number of perimeter Pt atoms and, hence, facilitates smooth Pt→anatase electron transfer, resulting in very high photocatalytic activity. This catalyst is successfully activated by sunlight and enables efficient and selective aerobic oxidation of alcohols at ambient temperature

Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g‑C₃N₄) Photocatalyst Activated by Visible Light

Photocatalytic production of hydrogen peroxide (H₂O₂) on semiconductor catalysts with alcohol as a hydrogen source and molecular oxygen (O₂) as an oxygen source is a potential method for safe H₂O₂ synthesis because the reaction can be carried out without the use of explosive H₂/O₂ mixed gases. Early reported photocatalytic systems, however, produce H₂O₂ with significantly low selectivity (∼1%). We found that visible light irradiation (λ > 420 nm) of graphitic carbon nitride (g-C₃N₄), a polymeric semiconductor, in an alcohol/water mixture with O₂ efficiently produces H₂O₂ with very high selectivity (∼90%). Raman spectroscopy and electron spin resonance analysis revealed that the high H₂O₂ selectivity is due to the efficient formation of 1,4-endoperoxide species on the g-C₃N₄ surface. This suppresses one-electron reduction of O₂ (superoxide radical formation), resulting in selective promotion of two-electron reduction of O₂ (H₂O₂ formation)