Yasuhiro Shiraishi

Supported Au–Cu Bimetallic Alloy Nanoparticles: An Aerobic Oxidation Catalyst with Regenerable Activity by Visible‐Light Irradiation

Aerobic oxidation by a heterogeneous catalyst with molecular oxygen as an oxidant is a significant process for the synthesis of various chemicals. Gold nanoparticles supported on solid supports have been extensively studied as promising catalysts; several types of substrates, such as alcohols, aldehydes, and hydrocarbons, are oxidized at ca. 393 K. The current mission is the design of Au catalysts that exhibit high activity under milder reaction conditions (room temperature and atmospheric pressure), from the viewpoint of green chemistry. 11] One powerful approach for the design of a highly active Au catalyst is the creation of alloy particles. Several reports revealed that particles consisting of Au–Pt, Au–Pd, and Au–Ag bimetallic alloys exhibit much higher activity than monometallic Au particles. In particular, Au–Cu alloys have attracted much attention, because of their high activity and the low cost of Cu. Liu et al. reported that Au–Cu alloy particles (ca. 3 nm) supported on mesoporous silica are three times more effective for the aerobic oxidation of carbon monoxide than monometallic Au particles at room temperature. Li et al. and Pina et al. have reported that Au–Cu alloy particles (ca. 3 nm) supported on silica are 1.5 times more effective for the aerobic oxidation of alcohols than Au particles. The enhanced activity of the Au–Cu alloy is believed to be due to the efficient activation of O2 on the alloy site. The Au–Cu alloys, however, rapidly lose their activity during the reaction, because O2 oxidizes surface Cu atoms and eliminates alloying effects. The protection of surface Cu atoms from oxidation is therefore a challenge for efficient aerobic oxidation under milder conditions. Herein, we report that visible-light irradiation (l> 450 nm) of Au–Cu alloy particles during reaction suppresses the oxidation of surface Cu atoms and successfully promotes aerobic oxidation without catalyst deactivation. This is triggered through visible light absorption by the surface Au atoms owing to the resonant oscillation of free electrons coupled by light, an effect which is known as localized surface plasmon resonance (SPR). Collective oscillation of e on the surface Au atoms reduces the oxidized surface Cu atoms and maintains the Au–Cu alloying effect. Sunlight irradiation also facilitates activity regeneration and promotes aerobic oxidation at room temperature. Au–Cu alloy nanoparticles were loaded on Degussa P25 TiO2 (diameter, 24 nm; BET surface area, 57 m 2 g ; anatase/ rutile = ca. 83:17) by simultaneous deposition of Au and Cu precursors followed by reduction with H2. [20–22] TiO2 was stirred in water (pH 7) with HAuCl4 and Cu(NO3)2 at 353 K. The obtained powders were reduced with H2, affording Au1xCux/P25 catalysts as purple powders. The total amount of metals ((Au + Cu)/P25 100) was set at 1 mol%, and x denotes the amount of Cu loaded (x mol% = Cu/TiO2 100). Figure 1a shows a typical transmission electron microscopy (TEM) image of Au0.7Cu0.3/P25. Spherical metal par-

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 via 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.

Sunlight-Driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal-Free Photocatalysts†

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>400u2005nm). 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.

Spiropyran as a Selective, Sensitive, and Reproducible Cyanide Anion Receptor

A spiropyran derivative (1) behaves as a selective and sensitive cyanide anion receptor in aqueous media under UV irradiation. The receptor can be reproduced just by irradiation with visible light.

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.

BODIPY-conjugated thermoresponsive copolymer as a fluorescent thermometer based on polymer microviscosity.

A simple copolymer, poly(NIPAM-co-BODIPY), consisting of N-isopropylacrylamide (NIPAM) and boradiazaindacene (BODIPY) units, behaves as a fluorescent thermometer in water. The copolymer exhibits weak fluorescence at <23 degrees C, but the intensity increases with a rise in temperature up to 35 degrees C, enabling an accurate indication of the solution temperature at 23-35 degrees C. The heat-induced fluorescence enhancement is driven by an increase in the polymer microviscosity, associated with a phase transition of the polymer from the coil to globule state. The viscous domain formed inside the globule-state polymer suppresses the rotation of the meso-pyridinium group of the excited-state BODIPY units, resulting in heat-induced fluorescence enhancement. The polymer shows reversible fluorescence enhancement/quenching regardless of the heating/cooling process and displays high reusability with a simple recovery process.

Thermal isomerization of spiropyran to merocyanine in aqueous media and its application to colorimetric temperature indication

Thermally-induced isomerization of spiropyran derivatives in aqueous media has been studied. The colorless spirocyclic (SP) forms of spiropyran derivatives are isomerized to colored merocyanine (MC) forms even in dark conditions at elevated temperature. Equilibrium, kinetic, and deuterium experiments reveal that the thermal SP → MC isomerization is due to the stabilization of MC form by a hydrogen bonding interaction with water molecules. This leads to the ground state energy of the MC form decreasing to lower than that of the SP form, resulting in SP → MC isomerization. The thermal isomerization property is applicable to a rough indication of solution temperature. The spiropyran derivatives, when dissolved in aqueous media under irradiation of visible light with an appropriate light intensity, demonstrate an increase in MC absorbance with a rise in temperature. The absorption response occurs reversibly regardless of the heating/cooling sequence. The spiropyran derivatives therefore have a potential for colorimetric temperature indication.

Spiropyran-Conjugated Thermoresponsive Copolymer as a Colorimetric Thermometer with Linear and Reversible Color Change

A simple copolymer, poly(NIPAM-co-SP), consisting of N-isopropylacrylamide and spiropyran units, behaves as a colorimetric thermometer exhibiting temperature-responsive linear and reversible bathochromic/hypsochromic shift of the absorption spectra under UV irradiation.

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.

Thermoresponsive Copolymer Containing a Coumarin–Spiropyran Conjugate: Reusable Fluorescent Sensor for Cyanide Anion Detection in Water

A simple copolymer consisting of N-isopropylacrylamide and coumarin-conjugated spiropyran (CS) units, poly(NIPAM-co-CS), has been synthesized. This polymer enables selective fluorometric detection of cyanide anion (CN(-)) in water at room temperature. The polymer itself shows almost no fluorescence, but shows a strong blue fluorescence in the presence of CN(-) under irradiation of UV light. The fluorescence enhancement occurs via a nucleophilic interaction between CN(-) and the photoformed merocyanine form of the CS unit, leading to a localization of π-electrons on the coumarin moiety. The polymer enables accurate determination of very low levels of CN(-) (>0.5 μM). The polymer can be recovered from water by simple centrifugation at high temperature (>40 °C), due to the heat-induced aggregation of the polymer. In addition, the polymer is regenerated by simple acid treatment, and the resulting polymer is successfully reused for further CN(-) sensing without loss of sensitivity.