Daijiro Tsukamoto

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 TiO(2) 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 TiO(2) 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 TiO(2). This catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of O(2) by the conduction band electrons on the surface of anatase TiO(2). This plasmonic photocatalysis is successfully promoted by sunlight exposure and enables efficient and selective aerobic oxidation of alcohols at ambient temperature.

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-

Selective Photocatalytic Oxidation of Alcohols to Aldehydes in Water by TiO2 Partially Coated with WO3

Semiconductor TiO(2) particles loaded with WO(3) (WO(3)/TiO(2)), synthesized by impregnation of tungstic acid followed by calcination, were used for photocatalytic oxidation of alcohols in water with molecular oxygen under irradiation at λ>350 nm. The WO(3)/TiO(2) catalysts promote selective oxidation of alcohols to aldehydes and show higher catalytic activity than pure TiO(2). In particular, a catalyst loading 7.6 wt % WO(3) led to higher aldehyde selectivity than previously reported photocatalytic systems. The high aldehyde selectivity arises because subsequent photocatalytic decomposition of the formed aldehyde is suppressed on the catalyst. The TiO(2) surface of the catalyst, which is active for oxidation, is partially coated by the WO(3) layer, which leads to a decrease in the amount of formed aldehyde adsorbed on the TiO(2) surface. This suppresses subsequent decomposition of the aldehyde on the TiO(2) surface and results in high aldehyde selectivity. The WO(3)/TiO(2) catalyst can selectively oxidize various aromatic alcohols and is reusable without loss of catalytic activity or selectivity.

Selective photocatalytic transformations on microporous titanosilicate ETS-10 driven by size and polarity of molecules.

Photocatalytic activity of microporous titanosilicate ETS-10 has been studied in water. The photoactivated ETS-10 shows catalytic activity driven by size and polarity of substrates. ETS-10 efficiently catalyzes a conversion of substrates with a size larger than the pore diameter of ETS-10. In contrast, the reactivity of small substrates depends strongly on substrate polarity; less polar substrates show higher reactivity on ETS-10. Electron spin resonance analysis reveals that large substrates or less polar substrates scarcely diffuse inside the highly polarized micropores of ETS-10 and, hence, react efficiently with hydroxyl radicals (*OH) formed on titanol (Ti-OH) groups exposed on the external surface of ETS-10. In contrast, small polar substrates diffuse easily inside the micropores of ETS-10 and scarcely react with *OH, resulting in low reactivity. The photocatalytic activity of ETS-10 is successfully applicable to selective transformations of large reactants or less polar reactants to small polar products, enabling highly selective dehalogenation and hydroxylation of aromatics.

Selective side-chain oxidation of alkyl-substituted aromatics on TiO2 partially coated with WO3 as a photocatalyst

TiO2 particles partially coated with WO3 (WO3/TiO2) were used as photocatalysts for selective oxidation of toluene to benzaldehyde with molecular oxygen (O2) under photoirradiation at λ > 300 nm. The catalysts loaded with 8–10 wt% WO3 exhibit very high oxidation activity and produce benzaldehyde with ca. 50% selectivity, which is the highest selectivity among the photocatalytic systems reported so far. This is achieved because subsequent photocatalytic decomposition of the formed benzaldehyde is suppressed on the catalysts. On the photoactivated WO3/TiO2, the exposed TiO2 surface behaves as the oxidation site, whereas the WO3 surface is much less active for oxidation. The WO3 loading decreases the amount of benzaldehyde adsorbed onto the TiO2 surface. This thus suppresses photocatalytic decomposition of benzaldehyde on the TiO2 surface, resulting in high benzaldehyde selectivity. The WO3/TiO2 catalyst successfully promotes side-chain oxidation of several kinds of alkyl-substituted aromatics and selectively produces aldehydes and ketones.

Highly Efficient Methyl Ketone Synthesis by Water-Assisted C−C Coupling between Olefins and Photoactivated Acetone

Photoirradiation of an acetone/water mixture containing olefins affords the corresponding methyl ketones highly efficiently via a water-assisted C-C coupling between acetonyl radical and olefins.

Highly Efficient Methyl Ketone Synthesis with Photoactivated Acetone and Olefins Assisted by Mg(II)-Exchanged Zeolite Y

We previously found that photoirradiation of an acetone/water mixture containing olefins affords the corresponding methyl ketones highly efficiently and selectively via a water-assisted C-C coupling between the acetonyl radical and olefins (Org. Lett. 2008, 10, 3117-3120). The reaction proceeds at room temperature without any additives and has a potential to be a powerful method for methyl ketone synthesis. Here we report that an addition of Mg(2+)-exchanged zeolite Y (MgY) to the above photoreaction system accelerates the methyl ketone formation, while maintaining high selectivity. Ab initio molecular orbital calculation reveals that the accelerated methyl ketone formation is due to the electrostatic interaction between Mg(2+) and excited-state acetone. This leads to a charge polarization of the carbonyl moiety of excited-state acetone and accelerates the hydrogen abstraction from ground-state acetone (acetonyl radical formation). This promotes efficient addition of the acetonyl radical to olefins, resulting in methyl ketone formation enhancement. Adsorption experiments reveal that accumulation of olefins inside the zeolite pore also affects efficient radical addition to olefins. The present process successfully produces various methyl ketones with very high yield, and the MgY recovered can be reused for further reaction without loss of activity.