Specific approaches to H2O2 production over a WO3 photocatalyst under visible light irradiation, i.e., (1) no use of a cocatalyst, (2) reaction under an O2 atmosphere, (3) reaction in a high concentration of alcohol, and (4) reaction at 60 °C, were found to be effective for rapid production of 38 mM H2O2.

The utilization of biomass glycerol would contribute to the fight against global warming by reducing greenhouse gas emissions; however, efficient utilization of glycerol in water is still a challenge. In this study, partial hydrogenation of alkynes over a titanium(IV) oxide photocatalyst with a copper cocatalyst (Cu/TiO2) was investigated in the presence of glycerol in an acetonitrile-water solvent. Glycerol worked efficiently as a hydrogen source and alkynes were converted to corresponding alkenes. Glycolic acid and formic acid, which are formed as oxidation products of glycerol, cause a decrease of recyclability due to the partial dissolution of Cu nanoparticles. The stability of the Cu/TiO2 photocatalyst is improved by the addition of NaOH, although the reaction rate decreases. A slight increase in the reaction temperature restores the reaction rate of partial hydrogenation, and preferably, the H2 yield becomes smaller. The results provide a new application of biomass glycerol in water, i.e., a hydrogen source for photocatalytic reactions.

Abstract Copper (Cu) nanoparticles (NPs) loaded on titanium(IV) oxide (TiO₂) exhibit an extinction at about λ=580 nm because of surface plasmon resonance (SPR); however, they rapidly lose their its SPR when exposed to air. In order to stabilize the Cu NPs, a multi‐step photodeposition method was used to introduce chromium, which resulted in successful synthesis of a Cr₂O₃/Cu/TiO₂ sample that exhibited extinction even when exposed to air. Transmission electron microscopy, scanning transmission electron microscopy, X‐ray photoelectron spectroscopy and UV‐vis spectroscopy were utilized to analyze the Cu/TiO₂ and Cr₂O₃/Cu/TiO₂ samples with a focus on SPR of the nanoparticles and state of the Cu species. The prepared Cr₂O₃/Cu/TiO₂ samples acted as plasmonic photocatalysts in oxidation of 2‐propanol under aerated conditions and visible light irradiation. Expandability of the Cr₂O₃/Cu/TiO₂ plasmonic photocatalyst was also investigated, and the results suggested the potential of a Cu‐based plasmonic photocatalyst working under irradiation of visible light.

Abstract Deprotection of pyridine N‐oxides under mild conditions with an inexpensive and environmentally friendly reducing reagent is an important chemical procedure. The use of biomass waste as the reducing reagent, water as the solvent and solar light as the energy source is one of the most promising approaches with minimal impact on the environment. Therefore, a TiO₂ photocatalyst and glycerol are suitable components of this type of reaction. Stoichiometric deprotection of pyridine N‐oxide (PyNO) with a minimal amount of glycerol (PyNO:glycerol= 7 : 1) was achieved, with only CO₂ being produced as the final oxidation product of glycerol. The deprotection of PyNO was thermally accelerated. Under solar light, the temperature of the reaction system increased to 40–50 °C and PyNO was also quantitatively deprotected, indicating that solar energy, i. e., UV light and thermal energy, can be effectively used. The results provide a new approach in the fields of organic chemistry and medical chemistry using biomass waste and solar light.

Cyclohexanecarboxylic acid as the ring hydrogenation product of benzoic acid was successfully obtained when Ru and Pd were simultaneously loaded on TiO₂, indicating that this bimetallic system can be used in place of an Rh cocatalyst.

Plasmonic water splitting to H₂ and O₂ with the ratio of 2 : 1 occurred continuously under irradiation of visible light when an Au/Ta₂O₅/FTO photoanode was used under an externally applied potential of 0.8 V vs. RHE to a Pt counter electrode.

Disk-shaped tungsten(VI) oxide (D-WO3) particles were synthesized according to a previously reported method consisting of pyrolysis, precipitation, and calcination, and the calcination temperature was changed (200-600 degrees C). The samples were characterized by field-emission scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, the Brunauer-Emmett-Teller single-point method, and diffuse reflectance spectroscopy. In addition, to evaluate the photocatalytic activities of the samples, the mineralization of acetic acid to carbon dioxide (CO2) was measured by loading Pt particles onto the surface of the samples by photodeposition and irradiating them in an aqueous suspension with a blue light-emitting diode. Increasing the calcination temperature was associated with several changes: the crystallites grew larger, increasing the crystallinity; the specific surface area decreased, decreasing the adsorption capacity; and the rate of the photocatalytic CO2 evolution reaction increased. Pt-loaded (0.1 wt%) D-WO3 calcined at 600 degrees C showed the highest activity with a CO2 evolution rate of 5.9 mu mol h(-1). These results indicated that improving the crystallinity of the D-WO3 samples was effective in increasing their photocatalytic activities.

Selective hydrogenation of alkynes to alkenes (semihydrogenation) without the use of a poison and H-2 is challenging because alkenes are easily hydrogenated to alkanes. In this study, a titanium (IV) oxide photocatalyst having Pd core-Cu shell nanoparticles (Pd@Cu/TiO2) was prepared by using the two-step photodeposition method, and Pd@Cu/TiO2 samples having various Cu contents were characterized by electron transmission microscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy. Thus-prepared Pd@Cu/TiO2 samples were used for photocatalytic hydrogenation of 4-octyne in alcohol and the catalytic properties were compared with those of Pd/TiO2 and Cu/TiO2. 4-Octyne was fully hydrogenated to octane over Pd/TiO2 at a high rate and 4-octyne was semihydrogenated to cis-4-octene over Cu/TiO2 at a low rate. Rapid semihydrogenation of 4-octyne was achieved over Pd(0.2 mol%)@Cu(1.0 mol%)/TiO2, indicating that the Pd core greatly activated the Cu shell that acted as reaction sites. A slight increase in the reaction temperature greatly increased the rate with a suppressed rate of H-2 evolution as the side reaction. Changes in the reaction rates of the main and side reactions are discussed on the basis of results of kinetic studies. Reusability and expandability of Pd@Cu/TiO2 in semihydrogenation are also discussed.

Since reduction and oxidation reactions simultaneously occur over a photocatalyst, a hole scavenger (a hydrogen source in the field of organic chemistry) is required to obtain a reduced product. The use of organic waste as the hole scavenger is environmentally favorable because the organic waste is degraded during the photocatalytic reduction. In this study, methylamine was used as the hole scavenger for photocatalytic reduction (hydrogenation) of nitrobenzenes over a titanium(IV) oxide photocatalyst. Corresponding anilines were obtained almost quantitatively at room temperature and under atmospheric pressure without the use of a precious metal and hydrogen gas. Nitrobenzenes having reducible functional groups such as chloro and acetyl groups were chemoselectively hydrogenated to corresponding anilines with the groups being preserved. Various parameters affecting the efficiency and the rate of the photocatalytic reaction are discussed on the basis of results of the physicochemical analyses.

Gold (Au)-modified tantalum(V) oxide (Ta2O5) prepared by using a colloid photodeposition method produced H-2 without the aid of a cocatalyst under visible light irradiation due to the sufficiently negative potential of electrons injected from Au nanoparticles into the conduction band of Ta2O5 (-0.47 V).

Photocatalytic ring hydrogenation of phenol proceeded in an aqueous suspension of palladium-loaded TiO2 at room temperature in the presence of oxalic acid as a hole scavenger, and cyclohexanone was produced in a high yield (90 %) without the use of hydrogen gas. Cyclohexanone was formed via keto-enol tautomerism of cyclohexenol. The effects of hole scavengers and solvents on the hydrogenation of phenol can be explained by the adsorption behaviour of phenol and hole scavengers in the solvents. Over a Pd-TiO2 photocatalyst, the apparent activation energy (Ea) in the ring hydrogenation of phenol and the Ea value in the hydrogenation of cyclohexanone were estimated to be 25 kJ mol(-1) and 36 kJ mol(-1), respectively, as calculated by using the Arrhenius equation. We concluded that the difference in the Ea values is attributable to the selective production of cyclohexanone and the decreased production of cyclohexanol in the ring hydrogenation of phenol over a Pd-TiO2 photocatalyst.

There is little information on the effect of the conduction band (CB) position on plasmonic hydrogen (H-2) formation under visible light irradiation over gold (Au) nanoparticles supported on semiconductors because there were no appropriate materials for which the CB position gradually changes. In this study, we analyzed the flatband potential of strontium ion (Sr2+)-doped cerium.IV) oxide (CeO2:Sr) and found that the CB position gradually shifted negatively from +0.031 V to -1.49 V vs. NHE with an increase in the Sr2+ mole fraction. Plasmonic photocatalysts consisting of Au nanoparticles, CeO2:Sr and a platinum (Pt) cocatalyst were prepared and characterized by using X-ray diffraction, UV-vis spectroscopy, and transmission electron spectroscopy. Photocatalytic reaction under visible light irradiation revealed that H-2 was produced over Au nanoparticles supported on CeO2:Sr having the CB potential of -0.61 V vs. NHE and that the negative limit of the CB position for electron injection from Au nanoparticles existed between -0.61 V and -1.49 V vs. NHE. We found that Au/CeO2:Sr plasmonic photocatalysts also produced H-2 without the aid of a Pt cocatalyst due to the sufficiently negative potential of electrons injected into the CB of CeO2:Sr.

Hydrogen (H-2)-free and poison (lead and quinoline)-free semihydrogenation of alkynes to cis-alkenes under gentle conditions is one of the challenges to be solved. In this study, a titanium(IV) oxide photocatalyst having two functions (visible light responsiveness and semihydrogenation activity) was prepared by modification with 2,3-dihydroxynaphthalene (DHN) and a copper (Cu) co-catalyst, respectively. The photocatalyst (DHN/TiO2-Cu) showed high performance for diastereoselective semihydrogenation of alkynes to cis-alkenes in water-acetonitrile solution under visible light irradiation without the use of H-2 and poisons. Alkynes having reducible functional groups were converted to the corresponding alkenes with the functional groups being preserved. The addition of water to acetonitrile changed the amount of alkynes adsorbed on the photocatalyst, which was a decisive factor determining the rate of hydrogenation. A relatively large apparent activation energy, 27 kJ mol(-1), was obtained by a kinetic study, indicating that the rate-determining step of this reaction was not an electron production process but a thermal catalytic semihydrogenation process over the Cu co-catalyst. Semihydrogenation and hydrogen evolution occurred competitively on Cu metals and the former became predominant at slightly elevated temperatures, which is discussed on the basis of the kinetic parameters of two reactions. (C) 2019 Elsevier Inc. All rights reserved.

Since photocatalytic reactions are almost consistent with the concept of green chemistry, substance conversion using photocatalysts has recently attracted the attention of researchers in the fields of organic chemistry, physical chemistry and material chemistry. We investigated photoinduced ring hydrogenation of phenol over a metal-loaded titanium(IV) oxide (TiO2) photocatalyst without the use of H-2 gas and we report here the effects of various parameters, including the type and amount of metal co-catalyst loaded on TiO2 and the kinds of solvents and hole scavengers, on the ring hydrogenation. We found that the combination of an Rh co-catalyst, water and oxalic acid resulted in the highest yield of cyclohexanol. Detailed analyses revealed that phenol was first hydrogenated to cyclohexanone via keto-enol tautomerism of cyclohexenol followed by hydrogenation of cyclohexanone to cyclohexanol and that adsorption of phenol onto Rh-TiO2 is a factor of great importance for the ring hydrogenation.

A titanium.IV) oxide photocatalyst modified with 2,3-dihydroxynaphthalene (DHN/TiO2) responds to visible light, and electrons are photogenerated to the conduction band of TiO2 under light irradiation in the presence of a suitable hole scavenger. The reducing potential of DHN/TiO2 was applied for photocatalytic conversion of a nitro group to an amino group of aromatics having other reducible groups under hydrogenfree and noble metal-free conditions at room temperature. Various nitro aromatics having vinyl, chloro, bromo, and acetyl groups were converted to amino aromatics with high yields with the reducible groups being preserved. Some parameters affecting the catalytic performance were investigated, and the reaction rate was almost determined by the amount of the substrate adsorbed on DHN/TiO2.

A copper-loaded titanium(IV) oxide photocatalyst exhibited perfect selectivity in hydrogenation of alkynes to alkenes in an alcohol solution at 298 K under hydrogen-free and poison-free conditions. A slight elevation in the reaction temperature to 323 K greatly increased the reaction rate with the selectivity being preserved and the formation of an H-2 by-product being suppressed. The apparent activation energy of 4-octyne semihydrogenation was determined to be 54 kJ mol(-1), indicating that the rate determining step of this photocatalytic reaction was not an electron production process but a thermocatalytic hydrogenation process under light irradiation.

Lindlar's catalyst, i.e., calcium carbonate-supported palladium (Pd) modified with lead, has been used for semihydrogenation of an alkynyl group in the presence of hydrogen gas (H-2). We examined hydrogenation of an alkynyl group in organosilane and hydrocarbon in methanolic suspensions of a Pd-loaded titanium(IV) oxide (Pd-TiO2) photocatalyst without the use of additives and H-2. In the photocatalytic reaction, Pd particles worked as co-catalysts for hydrogenation and alkyne hydrogenation had priority to alkene hydrogenation. Since the Pd co-catalyst was temporarily deactivated during the reaction owing to accumulation of the oxidized product(s) of methanol, the capacity of hydrogenation of the unsaturated C-C bond was limited. By optimizing the capacity and amount of alkynes, almost complete semihydrogenation of alkynes was achieved under a poison-free condition. Pd-TiO2 can be regenerated by very simple treatments, i.e., washing and drying at room temperature.

In the presence of a palladium-loaded TiO2 photocatalyst, the cleavage of benzyl phenyl ether in low-molecular-weight alcohol solvents under de-aerated conditions afforded toluene and phenol simultaneously in a 1:1 molar ratio.

We synthesized various cuboid-type tungsten trioxide (WO3) particles using a hydrothermal method followed by calcination at various temperatures between 200 and 500 degrees C. Visible-light photocatalysis of these particles was investigated for oxygen (O-2) evolution from water in the presence of sacrificial silver(I) sulfate. The calculated crystallite diameters increased with increasing calcination temperature, suggesting that the level of crystallinity increased with calcination temperature. The average specific surface area of the particles was almost the same for all calcination temperatures in the range 200-400 degrees C, but was decreased at 500 degrees C. The photocatalytic reaction rate of O-2 formation increased with increasing calcination temperature up to 400 degrees C, and then decreased as calcination temperature was further increased to 500 degrees C. We therefore concluded that the rate increased with calcination temperature up to 400 degrees C due to the increasing crystallinity, but then decreased due to the decreased specific surface area.

To investigate the effects of adding Au nanoparticles (AuNPs) to TiO2 films on the crystallization, phase transformation, and photocatalysis, films of both TiO2 and TiO2 embedded with AuNPs (Au-TiO2) with various characteristics were prepared by using the dip-coating method with preheating and post-heating treatments. The AuNPs acted as anatase nucleation agents and crystallized a lot of small anatase crystals with sizes of tens of nanometers, which suppressed the growth of anatase crystals that are large enough for them to transform into rutile crystals, resulting in repression of the transformation from anatase into rutile. The AuNPs affected the progress of the photocatalytic and adsorption reactions, resulting in improved photocatalytic activity. Of all the films we tested, the Au-TiO2 film preheated at 400 degrees C and post-heated at 400 degrees C (AT400-400), which consisted of small anatase crystals with high covalent character and high crystallinity, contained dispersed AuNPs with the smallest average crystallite size and showed the highest photocatalytic activity. This high activity resulted from the high reaction rate constants for adsorption and photocatalysis.

Photocatalytic hydrogenation of furfural (FAL) was examined in 2-pentanol suspensions of TiO2 under metal-free and hydrogen-free conditions. High chemoselectivity was observed, i.e., only the carbonyl group of FAL was reduced to a hydroxymethyl group, resulting in almost quantitative conversion to furfuryl alcohol (FOL) with an apparent quantum efficiency of 17%. 2-Pentanol was simultaneously converted to 2-pentanone satisfying a high stoichiometry with FAL to FOL, indicating that double up-grading of FAL and alcohol was possible via a photocatalytic process.

There are various possibilities of co-catalyst-assisted photocatalytic reduction (CPR) over a titanium(IV) oxide (TiO2) photocatalyst, especially H-2-free and chemoselective CPR. We examined the photoinduced ring hydrogenation of aromatics having a carboxyl group over metal-loaded TiO2 under H-2-free conditions and found that the aromatics were almost quantitatively hydrogenated to the corresponding cyclohexanes having a carboxyl group when rhodium, water and oxalic acid were used as a metal co-catalyst, solvent and hole scavenger, respectively. The effects of different metal co-catalysts, solvents and hole scavengers on the ring hydrogenation were also examined. Based on the results obtained under various conditions, the light dependency and adsorption behavior of the aromatics and hole scavengers, the functions of TiO2 and the co-catalyst, and the reaction process are discussed.

The use of metal co-catalysts broadens the application of photocatalytic reduction without the use of dihydrogen (H-2) gas. We examined photocatalytic hydrogenation of furan, a representative heterocyclic compound and a compound derived from biomass, in alcoholic suspensions of metal-loaded titanium(IV) oxide (TiO2) under a H-2-free condition and we found that furan was almost quantitatively hydrogenated to tetrahydrofuran with a high apparent quantum efficiency of 37% at 360 nm when palladium was used as a co-catalyst. Effects of different metal co-catalysts, different amounts of the co-catalyst, the type of TiO2, the type of alcohol, light wavelength and reusability for furan hydrogenation were investigated. Based on the results, the functions of TiO2 and the co-catalyst and the reaction process are discussed.

By using a facile calcination method, we succeeded in the preparation of partially spherical gold particles supported on TiO2 exhibiting photoabsorption due to surface plasmon resonance at around 620 nm. Surprisingly, the selective oxidation of benzyl alcohol to benzaldehyde occurred in an aqueous suspension of thus-prepared Au/TiO2 under irradiation of light from a Xe lamp with an R60 cut-off filter (lambda > 600 nm), and the apparent quantum efficiency reached 7.2% at 625 nm and 4.2% even at 700 nm.

An organically modified titanium dioxide photocatalyst prepared by a simple impregnation method chemoselectively and almost quantitatively converted benzaldehydes having other reducible functional groups to the corresponding benzyl alcohols under visible light irradiation.

We found that plasmonic Au particles on titanium(IV) oxide (TiO2) act as a visible-light-driven photocatalyst for overall water splitting free from any additives. This is the first report showing that surface plasmon resonance (SPR) in a suspension system effectively induces overall water splitting. Modification with various types of metal nanoparticles as co-catalysts enhanced the evolution of H-2 and O-2. Among these, Ni-modified Au/TiO2 exhibited 5-times higher rates of H-2 and O-2 evolution than those of Ni-free Au/TiO2. We succeeded in designing a novel solar energy conversion system including three elemental technologies, charge separation with light harvest and an active site for O-2 evolution (plasmonic Au particles), charge transfer from Au to the active site for H-2 production (TiO2), and an active site for H-2 production (Ni cocatalyst), by taking advantage of a technique for fabricating size-controlled Au and Ni nanoparticles. Water splitting occurred in aqueous suspensions of Ni-modified Au/TiO2 even under irradiation of light through an R-62 filter.

Chemoselective reduction of benzaldehydes having other reducible functional groups without the use of a reducing agent such as metal hydrides is important because metal hydrides simultaneously produce a large amount of undesirable waste containing metal compounds. Here we report that a titanium(IV) oxide (TiO2) photocatalyst is effective for heterogeneous Meerwein-Ponndorf-Verley (MPV)-type reduction of various benzaldehydes having reducible functional groups to corresponding benzyl alcohols at room temperature under a metal-free condition. We also investigated the effects of physical properties of TiO2 samples on chemoselective reduction of p-chlorobenzaldehyde (CBAD) to p-chlorobenzyl alcohol (CBAO) and we observed a linear correlation between the amount of CBAD adsorbed and the rate of CBAO formation, suggesting that ability of TiO2 for CBAD adsorption is one of the key factors for effective MPV-type reduction of CBAD to CBAO.

p-Chlorobenzaldehyde was almost quantitatively and chemoselectively reduced to p-chlorobenzyl alcohol in alcoholic suspensions of a metal-free titanium(IV) oxide photocatalyst under hydrogen-free conditions. Since alcohols act as electron donors and hydrogen sources, this reaction can be regarded as a heterogeneous Meerwein-Ponndorf-Verley-type reduction over a photocatalyst. Excellent chemoselectivity was also observed in the intermolecular competitive reduction of benzaldehyde (BA) and chlorobenzene (CB), i.e., only BA was reduced to benzyl alcohol, with CB not being reduced.

Metal ion-modified TiO2 photocatalysts designated by combinations of various metal ions and TiO2 powders that respond to visible light and exhibit controllable photocatalytic oxidative performances were examined. The photocatalytic oxidative performances were evaluated by selective oxidation of benzyl alcohol in water in the presence of O-2 under irradiation of visible light. The TiO2 samples modified with Ru3+, Pd2+ and Rh3+ (Ru3+/TiO2, Pd2+/TiO2 and Rh3+/TiO2) exhibited high levels of photocatalytic activity. The effects of metal ions and the kind of TiO2 on photocatalytic performances of Rh3+/TiO2 and Ru3+/TiO2 were investigated in detail. (C) 2015 Elsevier B.V. All rights reserved.

Gold particles supported on tin(IV) oxide (0.2wt% Au/SnO2) were modified with copper and silver by the multistep photodeposition method. Absorption around =550nm, attributed to surface plasmon resonance (SPR) of Au, gradually shifted to longer wavelengths on modification with Cu and finally reached =620nm at 0.8wt% Cu. On the other hand, the absorption shifted to shorter wavelength with increasing amount of Ag and reached =450nm at 0.8wt% Ag. These Cu- and Ag-modified 0.2wt% Au/SnO2 materials (Cu-Au/SnO2 and Ag-Au/SnO2) and 1.0wt% Au/SnO2 were used for mineralization of formic acid to carbon dioxide in aqueous suspension under irradiation with visible light from a xenon lamp and three kinds of light-emitting diodes with different wavelengths. The reaction rates for the mineralization of formic acid over these materials depend on the wavelength of light. Apparent quantum efficiencies of Cu-Au/SnO2, Au/SnO2, and Ag-Au/SnO2 reached 5.5% at 625nm, 5.8% at 525nm, and 5.1% at 450nm, respectively. These photocatalysts can also be used for selective oxidation of alcohols to corresponding carbonyl compounds in aqueous solution under visible-light irradiation. Broad responses to visible light in formic acid mineralization and selective alcohol oxidation were achieved when the three materials were used simultaneously.

Two kinds of photoabsorption of Ag-deposited silver iodide (AgI) due to band-gap excitation of AgI and surface plasmon resonance of Ag particles each contributed to photocatalytic partial oxidation of alcohol, and a synergy effect on the reaction rate was observed when two kinds of light were simultaneously irradiated on Ag-AgI.

Photocatalytic H-2 and O-2 formations under visible light irradiation (lambda > 400 nm) are demonstrated using Pt-Au nanopaticles for the reduction site and WO3 for the oxidation site in solid-state Pt/Au/WO3.

Titanium(IV) oxide (TiO2) having both smaller and larger gold (Au) particles was successfully prepared by a multi-step (MS) photodeposition method. When 0.25 wt% Au loading per photodeposition was repeated four times, smaller and larger Au particles having average diameters of 1.4 and 13 nm, respectively, were fixed on TiO2, and the Au/TiO2 sample exhibited strong photoabsorption around 550 nm due to surface plasmon resonance (SPR) of the larger Au particles. Various Au/TiO2 samples were prepared by changing the Au loading per photodeposition and the number of photodepositions. Effects of the conditions in MS photodeposition and sample calcination on Au particle distribution and photoabsorption properties were investigated. These samples were used for hydrogen (H-2) formation from 2-propanol and mineralization of acetic acid in aqueous suspensions under irradiation of visible light. In the case of H-2 formation under deaerated conditions, the reaction rate of Au/TiO2 having both larger and smaller particles was 4 times higher than that of the Au/TiO2 sample without smaller Au particles, indicating that smaller Au particles acted effectively as a co-catalyst, that is, as reduction sites for H-2 evolution. On the other hand, in the case of mineralization of acetic acid under aerated conditions, carbon dioxide formation rates were independent of the presence of smaller Au particles, indicating that the smaller Au particles had little effect on the mineralization of acetic acid. To extend the possibility of Au/TiO2 for H-2 formation under irradiation of visible light, H-2 formation from ammonia (NH3) as biomass waste was examined under deaerated conditions; NH3 was decomposed to H-2 and nitrogen with a stoichiometric ratio of 3 : 1.

(2,3-Epoxypropyl) benzene was chemoselectively reduced to allylbenzene along with formation of ketones in alcoholic suspensions of a silver-loaded titanium(IV) oxide photocatalyst at room temperature under atmospheric pressure without the use of reducing gases, and various epoxides were also reduced to the corresponding alkenes.

Photocatalytic oxidation of benzyl alcohols in aqueous suspensions of rhodium ion-modified titanium(IV) oxide (Rh3+/TiO2) in the presence of O-2 under irradiation of visible light was examined. In the photocatalytic oxidation of benzyl alcohol, benzaldehyde was obtained in a high yield (97%) with >99% conversion of benzyl alcohol. Rh3+/TiO2 photocatalysts having various physical properties were prepared using commercially available TiO2 powders as supporting materials for Rh3+ to investigate the effect(s) of physical properties of TiO2 on photocatalytic activities of Rh3+/TiO2 for selective oxidation. Adsorption properties of benzyl alcohol, benzaldehyde and benzoic acid on TiO2 were also investigated to understand the high benzaldehyde selectivity over the Rh3+/ TiO2 photocatalyst. The reaction mechanism was discussed on the basis of the results of photocatalytic oxidation of various p-substituted benzyl alcohol derivatives.

Titanium(IV) oxide (TiO2) having both small platinum (Pt) nanoparticles and large gold (Au) particles without alloying and nanoparticle coagulation was successfully prepared by the combination of traditional photodeposition of Pt and subsequent Au colloid photodeposition onto TiO2-Pt. The Au/TiO2 and Au/TiO2-Pt samples exhibiting strong photoabsorption due to surface plasmon resonance (SPR) of supported Au particles were used for the reduction of hexavalent chromium (Cr6+ in Cr2O72-) in aqueous suspensions under irradiation of visible light from a green light emitting diode (LED). These SPR-type photocatalysts reduced Cr6+ and evolved dioxygen (O-2) with O-2-reduced Cr6+ ratio of 3:4 until consumption of Cr6+, indicating that these SPR-type photocatalysts had the ability to oxidize water (H2O) utilizing visible light (lambda = 540 nm) and that a photocatalytic reaction (2Cr(2)O(7)(2-) + 16H(+) -> 4Cr(3+) + 3O(2) + 8H(2)O) occurred. The Au/TiO2 Pt sample exhibited a reaction rate about twice larger than that of Pt-free Au/TiO2, and the apparent quantum yield reached 1.0% at 550 nm and 0.47% even at 700 nm, indicating that functionalization of Au/TiO2 was successfully achieved by introduction of a Pt cocatalyst. This reaction can be used as a test reaction for evaluation of O-2 evolution ability of photocatalysts because this reaction does not induce irreversible changes in photocatalysts.

Gold (Au) nanoparticles supported on titania (TiO2) and ceria have been applied to a visible-light-responding plasmonic photocatalyst and functionalization of Au/TiO2 photocatalysts with cocatalysts were achieved by separate loading of Au nanoparticles and cocatalyst nanoparticles without alloying and nanoparticle coagulation. Here we report effectiveness of another functionalization by formation of a core-shell structure in photocatalytic reaction induced by SPR of Au/TiO2. Newly functionalized core-shell gold-palladium particles supported on TiO2 (Au@Pd/TiO2) were successfully prepared by using a two-step photodeposition method, and Au@Pd/TiO2 with a shell thickness of 1.0 nm quantitatively converted chlorobenzene and 2-propanol to benzene and acetone under irradiation of visible light. Thickness control of the Pd shell was very important for both a satisfactory cocatalyst effect and absorption due to SPR of Au nanoparticles. The results of this study provide useful information for design of functionalization of plasmonic photocatalysts by formation of a core-shell structure.

Copper(II) oxide (CuO)-modified indium oxide (In2O3) prepared by physical mixing efficiently mineralized acetone under irradiation of light from a blue LED. The reaction rate increased with additional irradiation of light from a red LED, though no reactions occurred over CuO and In2O3 under irradiation of the red light, indicating that band-gap excitation of CuO contributed to photocatalytic reaction over CuO-In2O3. The effect of CuO modification was also observed in action spectra with and without irradiation of the red light.

Titanium(IV) oxide (TiO2) having both small platinum (Pt) nanoparticles and large gold (Au) particles without alloying and nanoparticle coagulation was successfully prepared by the combination of traditional photodeposition of Pt in the presence of a hole scavenger (PH) and subsequent Au colloid photodeposition in the presence of a hole scavenger (CPH) onto TiO2-Pt. Au particles having an average diameter of 13 nm were fixed on both TiO2 and TiO2-Pt samples without change in the original size of Au particles, and the Au/TiO2 and Au/TiO2 Pt samples exhibited strong photoabsorption around 550 nm as a result of surface plasmon resonance (SPR) of Au to which the large size of Au particles was attributed. Bare TiO2, TiO2-Pt, Au/TiO2, and Au/TiO2-Pt samples were used for photoinduced hydrogen (H-2) formation from 2-propanol in aqueous solutions under irradiation of visible light. The first two samples yielded no H-2 because of no response to visible light, but the latter two formed H-2, indicating that SPR photoabsorption of supported Au particles contributed to the H-2 evolution under irradiation of visible light. The H-2 formation rate of the Au/TiO2-Pt sample was similar to 7-times larger than that of the Pt-free Au/TiO2 sample, indicating that Pt nanoparticles loaded on TiO2 acted effectively as a cocatalyst, that is, as reduction sites for H-2 evolution. The combination of the PH and CPH methods was effective for preparation of Au/TiO2 having other metal cocatalysts (M) including Au, that is, Au/TiO2 Au, and H-2 evolution rates decreased in the order of M; Pt > Pd > Ru > Rh > Au > Ag > Cu > Jr. An inverse correlation between the rate and the hydrogen overvoltage (HOV) of M, that is, it was observed that the higher the HOV, the more difficult it is to reduce protons by photogenerated electrons. Since the amounts of Au and M loaded on TiO2 were changed independently, the effects on photoabsorption and the rate of H-2 evolution were examined. A linear correlation was observed between rate and light absorption due to SPR, suggesting that SPR photoabsorption by Au particles was one of the important factors determining the rate of the H-2 evolution.

A functionalized plasmonic Au/TiO2 photocatalyst with an Ag co-catalyst was successfully prepared by the combination of two types of photodeposition methods, and it quantitatively converted nitrobenzene and 2-propanol to aniline and acetone under irradiation of visible light.

Nearly second-order (1.87th) light-intensity dependence was observed for photocatalytic oxidation of formic acid in aqueous solutions induced by visible-light surface-plasmon resonance absorption of gold nanoparticles loaded on titanium(IV) oxide prepared by the colloid-photodeposition method using a hole scavenger, indicating that the photocatalytic reaction proceeds through a non-linear process.

Au/CeO2 samples with various Au contents were prepared by the multistep (MS) photodeposition method. Their properties including Au particle size, particle dispersion, and photoabsorption were investigated and compared with properties of samples prepared by using the single-step (SS) photodeposition method. The MS- and SS-Au/CeO2 samples were used for selective oxidation of benzyl alcohols to corresponding benzaldehydes in aqueous suspensions under irradiation by visible light from a green LED, and the correlations between reaction rates and physical properties of the MS- and SS-Au/CeO2 samples were investigated. Difference in the two photodeposition methods was reflected in the average size and number of Au nanoparticles, for example, 92 nm and 1.3 x 10(12) (g-Au/CeO2)(-1) for MS photodeposition and 59 nm and 4.8 x 10(12) (g-Au/CeO2)(-1) for SS photodeposition in the case of 1.0 wt % Au samples. Fixation of larger Au particles resulted in strong photoabsorption of the MS-Au/CeO2 samples at around 550 nm due to the surface plasmon resonance, and the Kubelka-Munk function of the photoabsorption linearly increased with increase in Au content up to 2.0 wt %, in contrast to the photoabsorption of SS-Au/CeO2 samples, which was weak and was saturated even at around 0.5 wt %. Due to the strong photoabsorption, the MS-Au/CeO2 samples exhibited reaction rates approximately twice larger than those of SS-Au/CeO2 samples with the same Au contents, and apparent quantum efficiency of MS-Au/CeO2 reached 4.9% at 0.4 mW cm(-2). Linear correlations were observed between reaction rates (r) and surface area of Au nanoparticles (S) in both MS- and SS-Au/CeO2, samples, though the two slopes of r versus S plots were different, suggesting that oxidation of benzyl alcohol occurred on the Au surface and that S was one of the important factors controlling the reaction rate. Photocatalytic oxidation of benzyl alcohol having an amino group revealed that the Au/CeO2 photocatalyst exhibited high chemoselectivity toward the hydroxyl group of alcohol, i.e, the Au/CeO2 photocatalyst almost quantitatively converted aminobenzyl alcohol to aminobenzaldehyde with 99% yield.

Colloidal gold (Au) nanoparticles were prepared and successfully loaded on titanium(IV) oxide (TiO2) without change in the original particle size using a method of colloid photo-deposition operated in the presence of a hole scavenger (CPH). The prepared Au nanoparticles supported on TiO2 showed strong photoabsorption at around 550 nm due to surface plasmon resonance (SPR) of Au and exhibited a photocatalytic activity in mineralization of formic acid in aqueous suspensions under irradiation of visible light (>ca. 520 nm). A linear correlation between photocatalytic activity and the amount of Au loaded, that is, the number of Au nanoparticles, was observed, indicating that the activity of Au/TiO2 plasmonic photocatalysts can be controlled simply by the amount of Au loading using the CPH method and that the external surface area of Au nanoparticles is a decisive factor in mineralization of formic acid under visible light irradiation. Very high reaction rates were obtained in samples with 5 wt % Au or more, although the rate tended to be saturated. The CPH method can be widely applied for loading of Au nanoparticles on various TiO2 supports without change in the original size independent of the TiO2 phase. The rate of CO2 formation also increased linearly with increase in the external surface area of Au. Interestingly, the TiO2 supports showed different slopes of the plots. The slope is important for selection of TiO2 as a material supporting colloidal Au nanoparticles.

Au/TiO2 samples exhibiting stronger photoabsorption at around 550 nm due to surface plasmon resonance were prepared by using a multi-step photodeposition method and the samples exhibited higher levels of activity for H-2 production from various compounds such as 2-propanol, ethanol and ammonia in aqueous suspensions under visible light irradiation.

Gold and copper nanoparticles supported on cerium(IV) oxide (CeO(2)) powder, which were prepared using a simple multistep photodeposition method, showed strong absorptions at around lambda = 620 nm attributed to the surface plasmon resonance of Cu-Au nanoparticles and exhibited a higher level of activity than that of Au/CeO(2) in the mineralization of organic acids (formic acid and acetic acid) in aqueous suspensions under light irradiation (lambda = 640 nm) from a red light-emitting diode. The action spectrum in the mineralization of formic acid indicates that this mineralization involved a photoinduced step by the SPR of Cu-Au supported on CeO(2). The apparent quantum efficiency reached 7.5% at lambda = 625 nm and 5.2% even at lambda = 700 nm.

Gold (Au) nanoparticles supported on cerium(IV) oxide (CeO2) were prepared by the photodeposition method. These samples showed strong absorption at around 550 nm due to localized surface plasmon resonance (LSPR) of Au. These Au/CeO2 samples were used for mineralization of organic acids (formic acid, oxalic acid and acetic acid) in the aqueous suspensions under irradiation of visible light (>ca. 520 nm) and three acids were stoichiometrically decomposed to carbon dioxide. Apparent activation energy for mineralization of formic acid was very small (2.4 kJ mol(-1)) compared with those by thermocatalytic reactions, and the action spectrum was in good agreement with the photoabsorption spectrum, indicating that the rate-determining step in mineralization of organic acids in the irradiated Au/CeO2 system was different from the thermal activation process and that this mineralization involved a photoinduced step by LSPR of Au supported on CeO2. When a green light-emitting diode (center wavelength = 530 nm) was used as the light source of visible light, mineralization of formic acid also occurred. Apparent quantum efficiency of formic acid mineralization increased with decrease in the intensity of the green light and reached 4.7% at 0.4 mW cm(-2). (C) 2011 Elsevier B.V. All rights reserved.

Photocatalytic activity of cerium(IV) oxide (CeO2) particles in decomposition of acetic acid in aqueous suspension was examined under irradiation of visible light. Acetic acid was decomposed to carbon dioxide (CO2) and the rate of CO2 formation was increased by loading of 0.1 wt% cocatalysts (Pt, Au, Rh, Ru, Pd, Ag, Cu, and Ir) on CeO2 by photodeposition. Cocatalysts showed almost the same levels of enhancement of CO2 evolution, i.e., Ag/CeO2 and Cu/CeO2 exhibited rates similar to that of Pt/CeO2. The action spectrum revealed that decomposition of acetic acid was induced by photoabsorption of CeO2.

Gold nanoparticles supported on cerium(IV) oxide powder showing strong absorption at around 550 nm due to surface plasmon resonance oxidized aromatic alcohols to corresponding aldehydes almost quantitatively in an aqueous suspension under irradiation of green light.

Gold nanoparticles supported on cerium(IV) oxide powder showed strong absorption at around 550 nm due to localized surface plasmon resonance of Au and exhibited a higher level of activity than other Au-supporting samples in mineralization of organic acids in aqueous suspensions under irradiation of visible light (>ca. 520 nm).

本研究では，金粒子の粒径，金属酸化物上への接合状態を制御すること，また，異種金属（助触媒）を巧みに組み合わせることにより，光応答性制御による広範囲な可視光吸収の達成を目指す．交付申請書の「研究実施計画」には「金プラズモニック光触媒による水分解反応」を記載した．プラズモニック光触媒を用いた水の分解反応において，水の酸化反応による酸素生成反応の活性の低さが課題となっていた．当該年度は水の酸化反応に着目し，進めた． 【具体的内容】 金粒子を様々な方法（光析出法，析出沈殿法，コロイド光電着法）で酸化チタン上に担持し，さらに光析出法を用いて遷移金属種（クロム，鉄，ニッケル等）を担持することで金コア－遷移金属種シェルを作製した．この中でクロム種を修飾した金コア－酸化クロム種シェル担持酸化チタンを水の酸化反応に用いたところ，金単独のサンプルよりも高い活性を示し，担持量を制御することで，約2.5倍の活性となった．この反応中におけるクロム種の役割については現在調査中である．参考文献を基に酸化クロム種の役割を正孔移動層もしくは電子移動層であることを予想した．この場合の酸化反応サイトを探索する実験を含め，現在，電気化学測定を実施することで役割の理解を行っている． 【意義・重要性】 以上の結果より，金粒子上に適切な酸化物層を形成することにより，酸化反応活性が上昇した．これにともない水分解反応活性の上昇が期待できる．しかし，水分解反応には還元反応サイトの助触媒も必要であり，調製法を工夫する必要があるが，これまでの知見を駆使することで可能であると予想する．

可視光に応答する光触媒の合成に関する研究が精力的に行われている．これまで，申請者らは，Auナノ粒子の表面プラズモン共鳴（SPR）を利用した光触媒反応を実施した．本研究ではAuに対し，異種元素（AgやCu）を複合することで光学特性制御および応答波長制御を検討した．550 nm付近に吸収トップを示すAu粒子に対し，Agを導入することで短波長化，Cuを導入することで長波長化が起こり，それぞれ420 nm，620 nmまで光学特性が変化した．また，それぞれの材料が有機化合物の無機化反応や有機化合物の選択酸化反応に活性を示した．

本研究では，太陽光エネルギーを最大限に利用した光触媒の構築のために，金（Au）の表面プラズモン共鳴（SPR）を可視光光触媒反応への応用を目的としている．報告者はこれまでに、AuのSPR誘起型光触媒の光触媒反応を積極的に検討してきた．種々の化学反応への応用，それぞれの化学反応に適応した材料の創出を考えた．交付申請書の「研究実施計画」には「まず，Au粒子担持酸化物を調製し，SPR光触媒の種々の化学反応への応用について検討する．」と記載した． ＜具体的内容＞ 酸化反応（水の酸化）と還元反応（プロトンからの水素生成）をそれぞれ達成することができる新たなSPR光触媒系としてPt/Au/WO3の合成に成功し，その成果を第114回触媒討論会で発表した．さらに，論文として，J. Am. Chem. Soc.に掲載された．他方で，多層構造を有する金属ナノ粒子内包型光触媒の創製を目指したが，金属－酸化物内包構造には至っていない．その前段階として，上記に示した金属－金属内包構造を達成することができ，Catal. Sci. Technol.への論文掲載に至った． ＜意義・重要性＞ 以上の結果より、はじめのPt/Au/WO3においてはAuのSPR粒子とWO3のバンドギャップ励起を用いた光触媒を創生することができた．この光触媒ではAu粒子状にPt粒子を担持したPt/Au粒子上でプロトンの還元反応，WO3上で水の酸化反応が起こっていると予想される．これにより，これまで問題となっていたSPR光触媒の低活性をWO3の半導体光触媒で補えたといえる．この研究中に両者の光機能を補助光照射アクションスペクトルにより達成することができた．また，Au上へのPt粒子の生成は今後，申請者が目指している多層構造を有するSPR光触媒の創生につながるといえる．