Hirokatsu Sakamoto

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>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.

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.

Spiropyran-modified gold nanoparticles: reversible size control of aggregates by UV and visible light irradiations.

UV or visible light irradiation of gold nanoparticles (AuNPs) modified with a thiol-terminated spiropyran dye promotes reversible aggregation or dispersion of AuNPs. This is facilitated by the electrostatic repulsion/attraction between the AuNPs controlled by the ring-opening/closing photoisomerization of the surface dyes. This photochemical method successfully produces aggregates of AuNPs with tunable sizes (20-340 nm) and narrow size distributions (standard deviation <34%) in a reversible manner. In addition, the formed aggregates, even when left in the dark condition, scarcely change their sizes because the stacking interaction between the ring-opened forms of surface dyes suppresses thermal reverse isomerization and maintains the attractive force between the AuNPs.

One-Pot Synthesis of Imines from Nitroaromatics and Alcohols by Tandem Photocatalytic and Catalytic Reactions on Degussa (Evonik) P25 Titanium Dioxide

Photoirradiation (λ > 300 nm) of Degussa (Evonik) P25 TiO2, a mixture of anatase and rutile particles, in alcohols containing nitroaromatics at room temperature produces the corresponding imines with very high yields (80-96%). Other commercially available anatase or rutile TiO2 particles, however, exhibit very low yields (<30%). The imine formation involves two step reactions on the TiO2 surface: (i) photocatalytic oxidation of alcohols (aldehyde formation) and reduction of nitrobenzene (aniline formation) and (ii) condensation of the formed aldehyde and aniline on the Lewis acid sites (imine formation). The respective anatase and rutile particles were isolated from P25 TiO2 by the H2O2/NH3 and HF treatments to clarify the activity of these two step reactions. Photocatalysis experiments revealed that the active sites for photocatalytic reactions on P25 TiO2 are the rutile particles, promoting efficient reduction of nitrobenzene on the surface defects. In contrast, catalysis experiments showed that the anatase particles isolated from P25 TiO2 exhibit very high activity for condensation of aldehyde and aniline, because the number of Lewis acid sites on the particles (73 μmol g(-1)) is much higher than that of other commercially available anatase or rutile particles (<15 μmol g(-1)). The P25 TiO2 particles therefore successfully promote tandem photocatalytic and catalytic reactions on the respective rutile and anatase particles, thus producing imines with very high yields.

Photocatalytic secondary amine synthesis from azobenzenes and alcohols on TiO2 loaded with Pd nanoparticles

Photoirradiation (λ > 300 nm) of TiO2 loaded with Pd nanoparticles (ca. 2 wt%, 5 nm diameter) in water containing alcohols and azobenzene derivatives at room temperature successfully produces the corresponding secondary amines with high yields. This is facilitated via consecutive photocatalytic and catalytic reactions: (i) photocatalytic oxidation of alcohols (aldehyde formation) and reduction of azobenzenes (aniline formation); (ii) catalytic condensation of the formed aldehydes with anilines on the TiO2 surface (imine formation); and (iii) photocatalytic hydrogenation of the formed imines (secondary amine formation). This catalytic system successfully produces several kinds of secondary amines with >80% yields.

One-pot synthesis of secondary amines from alcohols and nitroarenes on TiO2 loaded with Pd nanoparticles under UV irradiation

Photoirradiation (λ > 300 nm) of TiO2 loaded with Pd nanoparticles (2 wt%, ca. 5 nm diameter) in water containing benzyl alcohol and nitrobenzene at room temperature successfully produces the corresponding secondary amine (N-benzylaniline) with 96% yield. This is achieved via three consecutive catalytic reactions: (i) photocatalytic oxidation of alcohol (aldehyde formation) and reduction of nitrobenzene (aniline formation); (ii) catalytic condensation of the formed aldehyde with aniline on the TiO2 surface (imine formation); and, (iii) photocatalytic hydrogenation of the formed imine (secondary amine formation). This catalytic system successfully produces several kinds of secondary amines, even those containing reducible substituents such as –CN, –COOH, or –CHO with >76% yields.

Photoreductive synthesis of monodispersed Au nanoparticles with citric acid as reductant and surface stabilizing reagent

Colloidal suspensions of gold nanoparticles (AuNPs) in water can be prepared by heating water containing HAuCl4 with citric acid as reductant and surface stabilizing reagent. This thermal reduction method, however, promotes aggregation of the formed AuNPs, resulting in bimodal distribution of AuNPs. Here we report that UV (254 nm) irradiation of water containing HAuCl4 with citric acid at room temperature successfully reduces HAuCl4 and produces monodispersed AuNPs, while suppressing aggregation of the formed AuNPs. Changing the intensity of UV light or the amount of citric acid added successfully produces AuNPs with tunable sizes and narrow size distributions.

Hydrogen Peroxide Production on a Carbon Nitride-Boron Nitride-Reduced Graphene Oxide Hybrid Photocatalyst under Visible Light

Photocatalytic production of hydrogen peroxide (H2O2) from earth‐abundant water and O2 is a desirable artificial photosynthesis for solar fuel production. A metal‐free hybrid photocatalyst consisting of pyromellitic diimide‐doped carbon nitride (g‐C3N4/PDI), boron nitride (BN), and reduced graphene oxide (rGO) was prepared. The g‐C3N4/PDI‐BN‐rGO catalyst, when photoirradiated in water with O2 by visible light at room temperature, efficiently produces H2O2. The photoexcited g‐C3N4/PDI moiety transfers the conduction band electrons to rGO, leading to selective production of H2O2 via two‐electron reduction of O2 on the rGO surface. In contrast, the valence‐band holes photoformed on the g‐C3N4/PDI moieties are transferred to BN, leading to efficient oxidation of water. The electron–hole separation enhanced by the incorporation of rGO and BN significantly suppresses the charge recombination and exhibits high photocatalytic activity. The solar‐to‐chemical conversion (SCC) efficiency for H2O2 production on the hybrid catalyst is 0.27 %, which is higher than the highest efficiencies obtained by overall water splitting on powdered catalysts.

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.