Pt–Cu Bimetallic Alloy Nanoparticles Supported on Anatase TiO₂: Highly Active Catalysts for Aerobic Oxidation Driven by Visible Light

Visible light irradiation (λ > 450 nm) of Pt–Cu bimetallic alloy nanoparticles (∼3–5 nm) supported on anatase TiO₂ efficiently promotes aerobic oxidation. This is facilicated <i>via</i> the interband excitation of Pt atoms by visible light followed by the transfer of activated electrons to the anatase conduction band. The positive charges formed on the nanoparticles oxidize substrates, and the conduction band electrons reduce molecular oxygen, promoting photocatalytic cycles. The apparent quantum yield for the reaction on the Pt–Cu alloy catalyst is ∼17% under irradiation of 550 nm monochromatic light, which is much higher than that obtained on the monometallic Pt catalyst (∼7%). Cu alloying with Pt decreases the work function of nanoparticles and decreases the height of the Schottky barrier created at the nanoparticle/anatase heterojunction. This promotes efficient electron transfer from the photoactivated nanoparticles to anatase, resulting in enhanced photocatalytic activity. The Pt–Cu alloy catalyst is successfully activated by sunlight and enables efficient and selective aerobic oxidation of alcohols at ambient temperature

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

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

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

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)

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