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

Effects of Surface Defects on Photocatalytic H₂O₂ Production by Mesoporous Graphitic Carbon Nitride under Visible Light Irradiation

Photocatalytic production of hydrogen peroxide (H₂O₂) from ethanol (EtOH) and molecular oxygen (O₂) was carried out by visible light irradiation (λ > 420 nm) of mesoporous graphitic carbon nitride (GCN) catalysts with different surface areas prepared by silica-templated thermal polymerization of cyanamide. On these catalysts, the photoformed positive hole oxidize EtOH and the conduction band electrons localized at the 1,4-positions of the melem unit promote two-electron reduction of O₂ (H₂O₂ formation). The GCN catalysts with 56 and 160 m² g^–1 surface areas exhibit higher activity for H₂O₂ production than the catalyst prepared without silica template (surface area: 10 m² g^–1), but a further increase in the surface area (228 m² g^–1) decreases the activity. In addition, the selectivity for H₂O₂ formation significantly decreases with an increase in the surface area. The mesoporous GCN with larger surface areas inherently contain a larger number of primary amine moieties at the surface of mesopores. These defects behave as the active sites for four-electron reduction of O₂, thus decreasing the H₂O₂ selectivity. Furthermore, these defects also behave as the active sites for photocatalytic decomposition of the formed H₂O₂. Consequently, the GCN catalysts with relatively large surface area but with a small number of surface defects promote relatively efficient H₂O₂ formation

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

Photocatalytic Dehalogenation of Aromatic Halides on Ta₂O₅‑Supported Pt–Pd Bimetallic Alloy Nanoparticles Activated by Visible Light

Dehalogenation of aromatic halides is one important reaction for detoxification and organic synthesis. Photocatalytic dehalogenation with alcohol, a safe hydrogen source, is one promising method; however, systems reported earlier need UV irradiation. We found that Pt–Pd bimetallic alloy nanoparticles (ca. 4 nm) supported on Ta₂O₅ (PtPd/Ta₂O₅), on absorption of visible light (λ > 450 nm), efficiently promote dehalogenation with 2-PrOH as a hydrogen source. Catalytic dehydrogenation of 2-PrOH on the alloy in the dark produces hydrogen atoms (H) on the particles. Photoexcitation of d electrons on the alloy particles by absorbing visible light produces hot electrons (e_hot^–). They efficiently reduce the adsorbed H atoms and produce hydride species (H^–) active for dehalogenation. The catalytic activity depends on the Pt/Pd mole ratio; alloy particles consisting of 70 mol % of Pt and 30 mol % of Pd exhibit the highest activity for dehalogenation

Mellitic Triimide-Doped Carbon Nitride as Sunlight-Driven Photocatalysts for Hydrogen Peroxide Production

Generating hydrogen peroxide (H₂O₂) from water and dioxygen (O₂) by photocatalysis is one ideal artificial photosynthesis for solar fuel production. Several early reported powdered photocatalysts, however, produce small amounts of H₂O₂ (<0.1 mM). We prepared graphitic carbon nitride (g-C₃N₄) doped with mellitic triimide (MTI) units by thermal condensation of melem and mellitic acid anhydride. The g-C₃N₄/MTI photocatalyst, when irradiated by visible light (λ > 420 nm) in pure water with O₂, successfully produces millimolar levels of H₂O₂ via water oxidation by valence band holes and selective two-electron reduction of O₂ by conduction band electrons. The incorporation of triply branched MTI units creates a condensed melem layer. This facilitates efficient intra- and interlayer transfer of photogenerated charge carriers and shows high electrical conductivity. The solar-to-chemical conversion efficiency for H₂O₂ production on the catalyst is 0.18%, which is higher than that of natural photosynthesis (∼0.1%) and similar to the highest values obtained by semiconductor water-splitting catalysts

Graphitic Carbon Nitride Doped with Biphenyl Diimide: Efficient Photocatalyst for Hydrogen Peroxide Production from Water and Molecular Oxygen by Sunlight

Photocatalytic hydrogen peroxide (H₂O₂) production from water and molecular oxygen (O₂) by sunlight is a promising strategy for green, safe, and sustainable H₂O₂ synthesis. We prepared graphitic carbon nitride (g-C₃N₄) doped with electron-deficient biphenyl diimide (BDI) units by a simple calcination procedure. The g-C₃N₄/BDI catalyst, when photoirradiated by visible light (λ >420 nm) in pure water with O₂, successfully promotes water oxidation by the photogenerated valence band holes and selective two-electron reduction of O₂ by the conduction band electrons, resulting in successful production of millimolar levels of H₂O₂. Electrochemical analysis, Raman spectroscopy, and ab initio calculation results revealed that, upon photoexcitation of the catalyst, the photogenerated positive holes are localized on the BDI unit while the conduction band electrons are localized on the melem unit. This spatial charge separation suppresses rapid recombination of the hole–electron pairs and facilitates efficient H₂O₂ production. The solar-to-chemical energy conversion efficiency for H₂O₂ production is 0.13%, which is comparable to that for photosynthetic plants. This metal-free photocatalysis therefore shows potential as an artificial photosynthesis for clean solar fuel production

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)

Synthesis of Au Nanoparticles with Benzoic Acid as Reductant and Surface Stabilizer Promoted Solely by UV Light

Photoreductive synthesis of colloidal gold nanoparticles (AuNPs) from Au³⁺ is one important process for nanoprocessing. Several methods have been proposed; however, there is no report of a method capable of producing AuNPs with inexpensive reagents acting as both reductant and surface stabilizer, promoted solely under photoirradiation. We found that UV irradiation of water with Au³⁺ and benzoic acid successfully produces monodispersed AuNPs, where thermal reduction does not occur in the dark condition even at elevated temperatures. Photoexcitation of a benzoate–Au³⁺ complex reduces Au³⁺ while oxidizing benzoic acid. The benzoic acid molecules are adsorbed on the AuNPs and act as surface stabilizers. Change in light intensity and benzoic acid amount successfully creates AuNPs with controllable sizes. The obtained AuNPs can easily be redispersed in an organic solvent or loaded onto a solid support by simple treatments

Carbon Nitride–Aromatic Diimide–Graphene Nanohybrids: Metal-Free Photocatalysts for Solar-to-Hydrogen Peroxide Energy Conversion with 0.2% Efficiency

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

Hot-Electron-Induced Highly Efficient O₂ Activation by Pt Nanoparticles Supported on Ta₂O₅ Driven by Visible Light

Aerobic oxidation on a heterogeneous catalyst driven by visible light (λ >400 nm) at ambient temperature is a very important reaction for green organic synthesis. A metal particles/semiconductor system, driven by charge separation via an injection of “hot electrons (e_hot^–)” from photoactivated metal particles to semiconductor, is one of the promising systems. These systems, however, suffer from low quantum yields for the reaction (<5% at 550 nm) because the Schottky barrier created at the metal/semiconductor interface suppresses the e_hot^– injection. Some metal particle systems promote aerobic oxidation via a non-e_hot^–-injection mechanism, but require high reaction temperatures (>373 K). Here we report that Pt nanoparticles (∼5 nm diameter), when supported on semiconductor Ta₂O₅, promote the reaction without e_hot^– injection at room temperature with significantly high quantum yields (∼25%). Strong Pt–Ta₂O₅ interaction increases the electron density of the Pt particles and enhances interband transition of Pt electrons by absorbing visible light. A large number of photogenerated e_hot^– directly activate O₂ on the Pt surface and produce active oxygen species, thus promoting highly efficient aerobic oxidation at room temperature