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Reproduced from New J. Chem., 2023, 47, 231-237 with permission from the Royal Society of Chemistry., https://doi.org/10.1039/d2nj04361k, Ionic clathrate hydrates (ICHs) have been applied to thermal storage materials, which can be utilized as unusual thermal energy sources. In the present study, we developed a novel ICH including tri-n-butyl(cyclobutylmethyl)phosphonium bromide (P444(1c4)-Br) as an unprecedented guest species. The highest equilibrium temperature and the dissociation enthalpy of P444(1c4)-Br ICH were 279.5 ± 0.1 K and 202 ± 2 J g−1, respectively, at the mole fraction of 0.0255 ± 0.0008. The highest equilibrium temperature of P444(1c4)-Br ICH was slightly lower than that of the tetra-n-butylphosphonium bromide (P4444-Br) ICH. The dissociation enthalpy of P444(1c4)-Br ICH was larger than that of other ICHs with similar highest equilibrium temperatures. Cyclic hydrocarbon groups, together with normal, branched, saturated, and unsaturated hydrocarbon chains, can be one of the options for tuning the equilibrium temperature of ICHs.

The generation of hydrogen peroxide (H2O2) from water and dioxygen by sunlight-driven heterogeneous photocatalysis is a promising method for the artificial photosynthesis of a liquid solar fuel. We previously found that resorcinol-formaldehyde (RF) resin powders prepared by high-temperature hydrothermal synthesis act as highly active semiconductor photocatalysts for H2O2 generation. Herein, we report that RF resin powders doped with poly(3-hexylthiophene-2,5-diyl) (RF/P3HT) exhibit enhanced photocatalytic activities. The highly dispersed P3HT within the resin particles created charge transfer complexes with the conduction band of the resin via electron donation, facilitating efficient transfer of the photogenerated conduction band electrons through P3HT. This enhanced charge separation promoted efficient water oxidation and O2 reduction. The solar-to-chemical conversion efficiency for H2O2 generation on the RF/P3HT resin in water under simulated sunlight irradiation with atmospheric pressure of O2 was ∼1.0%, which is the highest efficiency reported for powder catalysts in artificial photosynthesis.

Hydrogen peroxide (H2O2) has received increasing attention as an energy carrier. To achieve a sustainable energy society, photocatalytic H2O2 splitting (H2O2 (l) → H2 (g) + O2 (g); ΔG° = + 131 kJ mol−1) is a desirable reaction for on-site H2 generation. However, this reaction has not been reported because conventional photocatalysis decomposes H2O2 by disproportionation (H2O2 (l) → H2O (l) + 1/2O2 (g); ΔG° = −117 kJ mol−1) and by promoting H2O2 reduction instead of H+ reduction. Here we report the successful example of H2O2 splitting. Visible light irradiation of a graphitic carbon nitride loaded with graphene quantum dots as co-catalysts (GQDs/g-C3N4) in a H2O2 solution containing phosphoric acid (H3PO4) produces H2. H3PO4 associates with H2O2 via hydrogen bonding, and this stabilization of H2O2 suppresses its reduction, thus promoting H+ reduction. The all-organic photosystem with H3PO4 as a stabilizer may provide a basis of photocatalytic H2O2 splitting., While H2 can serve as a renewable fuel, its large scale production, storage, and transport are challenging. Here, authors show H2O2 to serve as a potential energy carrier via the photocatalytic production of H2 from stabilized H2O2 solutions and metal-free catalysts.

Ammonia is an indispensable chemical. Photocatalytic NH3 production via dinitrogen fixation using water by sunlight illumination under ambient conditions is a promising strategy, although previousl...

The photocatalytic generation of hydrogen peroxide from water and dioxygen (H2O + 1/2O2 → H2O2, ΔG° = +117 kJ mol–1) under sunlight is a promising strategy for the artificial photosynthesis of a liquid fuel. We had previously found that resorcinol–formaldehyde (RF) resin powders prepared by the base-catalysed high-temperature hydrothermal method act as semiconductor photocatalysts for H2O2 generation. Herein, we report that RF resins prepared by the acid-catalysed high-temperature hydrothermal method (~523 K) using common acids at pH < 4 exhibit enhanced photocatalytic activity. The base- and acid-catalysed methods both produce methylene- and methine-bridged resins consisting of π-conjugated and π-stacked benzenoid–quinoid donor–acceptor resorcinol units. The acidic conditions result in the resins with a lower bandgap (1.7 eV) and higher conductivity because the lower-degree of crosslinking creates a strongly π-stacked architecture. The irradiation of the RF-acid resins with simulated sunlight in water with atmospheric-pressure O2 generates H2O2 at a solar-to-chemical conversion efficiency of 0.7%, which is the highest efficiency ever reported for powder catalysts used in artificial photosynthesis. Low-bandgap polymers hold great potential for photocatalytic generation of hydrogen peroxide, but increasing catalytic activity remains challenging. Here, a solar-to-chemical conversion efficiency of 0.7 % is reached for a resorcinol-formaldehyde resin powder prepared via acid-catalyzed high-temperature hydrothermal synthesis.

Ammonia (NH3), which is an indispensable chemical, is produced by the Haber–Bosch process using H2 and N2 under severe reaction conditions. Although photocatalytic N2 fixation with water under ambient conditions is ideal, all previously reported catalysts show low efficiency. Here, we report that a metal-free organic semiconductor could provide a new basis for photocatalytic N2 fixation. We show that phosphorus-doped carbon nitride containing surface nitrogen vacancies (PCN-V), prepared by simple thermal condensation of the precursors under H2, produces NH3 from N2 with water under visible light irradiation. The doped P atoms promote water oxidation by the photoformed valence-band holes, and the N vacancies promote N2 reduction by the conduction-band electrons. These phenomena facilitate efficient N2 fixation with a solar-to-chemical conversion (SCC) efficiency of 0.1%, which is comparable to the average solar-to-biomass conversion efficiency of natural photosynthesis by typical plants. Thus, this metal-f...

Visible-light irradiation (λ < 600 nm) of commercially-available CdS in alcohol successfully promotes hydrogenation of azobenzene to hydrazobenzene with more than 95% selectivity. This is promoted by strong adsorption of azobenzene to the photoformed zerovalent Cd species adjacent to the surface S vacancies on CdS; this leads to efficient reduction to hydrazobenzene.

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 (ehot–). 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.

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 ( 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%) ...

Ammonia (NH3) is an essential chemical in modern society, currently manufactured via the Haber–Bosch process with H2 and N2 under extremely high pressure (>200 bar) and high-temperature conditions (>673 K). Toxic nitrate anion (NO3–) contained in wastewater is one potential nitrogen source. Selective NO3–-to-NH3 transformation via eight-electron reduction, if promoted at atmospheric pressure and room temperature, may become a powerful recycling process for NH3 production. Several photocatalytic systems have been proposed, but many of them produce nitrogen gas (N2) via five-electron reduction of NO3–. Here, we report that unmodified TiO2, when photoexcited by ultraviolet (UV) light (λ > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO3–-to-NH3 reduction with 97% selectivity. Surface defects and Lewis acid sites of TiO2 behave as reduction sites for NO3–. The surface defect selectively promotes eight-electron reduction (NH3 formation), while the Lewis acid site promotes nonselect...

Partial oxidation of cyclohexane (CHA) to cyclohexanone (CHA-one) with molecular oxygen (O2) is one of the most important reactions. Photocatalytic oxidation has been studied extensively with TiO2-based catalysts. Their CHA-one selectivities are, however, insufficient because the formed CHA-one is subsequently decomposed by photocatalysis involving the reaction with superoxide anion (O2●–) produced by one-electron reduction of O2 on TiO2. Here we report that TiO2, when hybridized with reduced graphene oxide (rGO), catalyzes photooxidation of CHA to CHA-one with enhanced activity and selectivity under UV light (λ > 300 nm). The TiO2/rGO hybrids produce CHA-one with twice the amount formed on bare TiO2 with much higher selectivity (>80%) than that on bare TiO2 (ca. 60%). The conduction band electrons photoformed on TiO2 are transferred to rGO, promoting efficient charge separation and enhanced photocatalytic cycles. The trapped electrons on rGO selectively promote two-electron reduction of O2 and suppress o...

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.

We synthesized a pyrylium–coumarin dyad (1) and used it for colorimetric detection of cyanide anions (CN−) in aqueous media. The receptor itself exhibits a long-wavelength absorption band at 643 nm due to the strong intramolecular charge transfer (ICT) from the diethylaminocoumarin to the pyrylium moiety. Selective nucleophilic addition of CN− to the pyrylium ring triggers the ring cleavage rapidly (within 8 min). This suppresses the ICT and decreases the intensity of the 643 nm band, along with an appearance of a new shorter-wavelength band at 472 nm assigned to the π,π* transition of the diethylaminocoumarin moiety. This drastic absorption change (643 nm → 472 nm) facilitates ratiometric detection of CN− based on the intensities of these two absorption bands in the visible region and successfully detects CN− as low as 8 μM.

We synthesized an indanedione–coumarin dyad (1) and used it as an off–on type fluorescent receptor for rapid, selective, and sensitive detection of CN− in an aqueous mixture. The receptor itself shows very weak fluorescence from the coumarin moiety due to the photoinduced electron transfer (PET) from the photoexcited coumarin to the indanedione moiety. Selective nucleophilic addition of CN− to the β-carbon of the receptor suppresses the PET and results in strong fluorescence enhancement. The reaction mechanism was fully confirmed by 1H, 13C NMR, Job's plot, and FAB-MS analysis. The receptor successfully detects CN− as low as 25 μM within 20 min.

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 def...

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.

Photoexcitation (λ > 300 nm) of TiO2 loaded with Pt nanoparticles promotes selective hydrogenolysis of epoxides using alcohols as reducing agents.

Ammonia (NH3) is an essential chemical in modern society. It is currently manufactured by the Haber–Bosch process using H2 and N2 under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH3 production from water and N2 at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO2 with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N2, successfully produces NH3. The active sites for N2 reduction are the Ti3+ species on the oxygen vacancies. These species act as adsorption sites for N2 and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N2 to NH3. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO2 ...

Platinum (Pt) nanoparticles with4 nm diameter loaded on graphitic carbon nitride (g-C3N4) by reduction at 673 K behave as efficient co-catalysts for photocatalytic hydrogen evolution under visible light (λ420 nm). This is achieved by strong Pt-support interaction due to the high temperature treatment, which facilitates efficient transfer of photoformed conduction band electrons on g-C3N4 to Pt particles.

Cyanine dyes with indolium-coumarin linkages exhibit selective fluorescence enhancement for cyanide anions (CN(-)) via the nucleophilic interaction of CN(-) with their indolium carbon atoms. This facilitates rapid (detection time: 1 min) and sensitive (detection limit: 0.4 μM) sensing of CN(-) in aqueous media.

Visible light irradiation (λ > 450 nm) of Pt nanoparticles supported on Degussa P25 TiO2 (Pt/P25 catalyst) promotes efficient and selective aerobic oxidation of aniline to nitrosobenzene. This is facilicated via the interband excitation of Pt atoms by visible light followed by the transfer of activated electrons to the TiO2 conduction band. The positive charges formed on the Pt surface oxidize substrates, and the conduction band electrons reduce O2, promoting photocatalytic cycles. The high activity of Pt/P25 is due to the high electron density of Pt particles. They behave as Lewis base sites for reductive deprotonation of aniline, thus promoting efficient aniline oxidation. The Pt/P25 catalyst, when photoirradiated at a low temperature (∼283 K), suppresses subsequent condensation of aniline and the formed nitrosobenzene, successfully producing nitrosobenzene with a very high selectivity (90%).

Catalytic deoxygenation of epoxides into the corresponding alkenes is a very important reaction in organic synthesis. Early reported systems, however, require noble metals, high reaction temperatures (>373 K), or toxic reducing agents. Here, we report a noble-metal-free heterogeneous catalytic system driven with alcohol as a reducing agent at room temperature. Photoirradiation (λ

Visible light irradiation (λ450 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.

We report that the rutile crystallites, isolated from Degussa (Evonik) P25 TiO2 by a hydrofluoric acid treatment, behave as a highly efficient photocatalyst for hydrogenation of nitroaromatics. Photoirradiation (λ >300 nm) of the isolated rutile particles with alcohol as a hydrogen source successfully promotes chemoselective hydrogenation of nitroaromatics to anilines, with an activity higher than that of commercially available rutile TiO2. The high activity of the isolated rutile particles is due to the specific distribution of structural defects (oxygen vacancy sites) on the particles. These particles contain a relatively small number of inner defects behaving as recombination centers for photoformed electron (e–) and positive hole (h+) pairs, and a relatively large number of surface defects behaving as reduction sites for nitroaromatics. Photoexcitation of the isolated particles therefore promotes efficient charge separation between e– and h+, and facilitates rapid reduction of nitroaromatics adsorbed ...

Photocatalytic production of hydrogen peroxide (H2O2) on semiconductor catalysts with alcohol as a hydrogen source and molecular oxygen (O2) as an oxygen source has attracted much attention as 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 with aliphatic alcohol as a hydrogen source, however, produce only a few millimolar levels of H2O2. We found that benzylic alcohols, when used as a hydrogen source for photoreaction in water with titanium dioxide (TiO2) photocatalyst, produce a very high concentration of H2O2 (ca. 40 mM). Raman spectroscopy and electron spin resonance analysis revealed that the enhanced H2O2 formation is due to the efficient formation of side-on coordinated peroxo species on the photoactivated TiO2 surface, via the reaction of benzylic alcohol and O2. The peroxo species is readily transformed to H2O2, thus facilitating highly efficient H2O2 production.

Visible-light irradiation (

TiO2 loaded with Pd particles (Pd/TiO2), when photoirradiated at λ > 300 nm in alcohol containing primary amine, efficiently promotes N-monoalkylation of amine with alcohol, producing the corresponding secondary amine with almost quantitative yields. This occurs via tandem photocatalytic and catalytic reactions: (i) Pd-assisted alcohol oxidation on the photoactivated TiO2, (ii) condensation of the formed aldehyde with amine on the TiO2 surface, and (iii) hydrogenation of the formed imine by the surface H atoms on the Pd particles. The rate-determining step is the imine hydrogenation, and the reaction depends strongly on the size of the Pd particles. The catalyst with 0.3 wt % Pd, containing 2–2.5 nm Pd particles, shows the highest activity for imine hydrogenation, and smaller or larger Pd particles are inefficient. Calculations of the number of surface Pd atoms based on the cuboctahedron particle model revealed that the Pd atoms on the triangle site of Pd particles are the active site for hydrogenation. L...

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 (λ420 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.

We report that photoactivated rutile titanium dioxide (TiO2) catalyzes a highly efficient and selective hydrogenation of nitroaromatics with alcohol as a hydrogen source. Photoirradiation (λ >300 nm) of rutile TiO2 suspended in alcohol containing nitroaromatics at room temperature and atmospheric pressure produces the corresponding anilines with almost quantitative yields, whereas common anatase and P25 TiO2 show poor activity and selectivity. The Ti3+ atoms located at the oxygen vacancies on the rutile surface behave as the adsorption site for nitroaromatics and the trapping site for photoformed conduction band electrons. These effects facilitate rapid and selective nitro-to-amine hydrogenation of the adsorbed nitroaromatics by the surface-trapped electrons, enabling aniline formation with significantly high quantum yields (>25% at

Visible light irradiation (λ >450 nm) of platinum (Pt) nanoparticles supported on anatase titanium dioxide (TiO2) promotes efficient aerobic oxidation at room temperature. This occurs via the electronic excitation of Pt particles by visible light followed by the transfer of their electrons to anatase conduction band. The positively charged Pt particles oxidize substrates, whereas the conduction band electrons are consumed by the reduction of molecular oxygen. The activity of this photocatalysis depends on the height of Schottky barrier and the number of perimeter Pt atoms created at the Pt/anatase heterojunction, which are affected by the amount of Pt loaded and the size of Pt particles. The catalyst loaded with 2 wt % Pt, containing 3–4 nm Pt particles, creates a relatively low Schottky barrier and a relatively large number of perimeter Pt atoms and, hence, facilitates smooth Pt→anatase electron transfer, resulting in very high photocatalytic activity. This catalyst is successfully activated by sunlight ...

Visible-light irradiation (λ450 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 with5 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.

TiO2 loaded with Au–Ag bimetallic alloy particles efficiently produces H2O2 from an O2-saturated ethanol/water mixture under UV irradiation. This is achieved via the double effects created by the alloy particles. One is the efficient photocatalytic reduction of O2 on the Au atoms promoting enhanced H2O2 formation, due to the efficient separation of photoformed electron–hole pairs at the alloy/TiO2 heterojunction. Second is the suppressed photocatalytic decomposition of formed H2O2 due to the decreased adsorption of H2O2 onto the Au atoms.

Photocatalytic oxidation of cyclohexane (CHA) with molecular oxygen (O2) was carried out using WO3 loaded with Pt nanoparticles (Pt/WO3) under irradiation of visible light (λ > 420 nm). The Pt/WO3 catalysts promote oxidation of CHA and give rise to partial oxidation products such as cyclohexanol (CHA-ol) and cyclohexanone (CHA-one) with ca. 93% selectivity. Among the catalysts, Pt/WO3 with 0.2 wt% Pt shows the highest activity. The high selectivity for partial oxidation on Pt/WO3 is because subsequent photocatalytic decomposition of partial oxidation products is suppressed. In the Pt/WO3 system, the photoformed electrons on the conduction band of WO3 are consumed by a multi-electron reduction of O2 on the Pt particles (formation of H2O and H2O2), where a single-electron reduction of O2 is unfavored. This suppresses the formation of a superoxide anion (O2˙−) that promotes decomposition of CHA-ol and CHA-one and, hence, results in selective formation of partial oxidation products.

Cr/Ti/Si ternary mixed oxides (CrTiSi) were prepared by a hydrolysis of tetraethyl orthosilicate (TEOS), Cr(NO3)3, and titanium tetraisopropoxide (TTIP) followed by calcination. These oxides were used as catalysts for partial oxidation of cyclohexane with molecular oxygen under irradiation of visible light (λ > 400 nm). The CrTiSi catalysts produce cyclohexanol and cyclohexanone with high selectivity (>91%) and show much higher activity than the CrSi binary oxides. Among them, the catalyst with equimolar amounts of Cr and Ti shows the highest activity. Visible-light irradiation of the CrSi catalyst promotes a reduction of tetrahedrally coordinated Cr oxide species (Td6+) and produces the excited state (Td4+*), which behaves as an active site for oxidation. In contrast, the CrTiSi catalyst contains tetrahedrally coordinated Cr and Ti oxide species that are connected through the Cr–O–Ti bond. The photoformed Td4+* species is strongly stabilized due to delocalization of excited electrons on the Cr–O–Ti speci...

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.

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.

Cross-linked polymer nanocapsules (PC) encapsulating an iron tris(bipyridine) complex, Fe(bpy)32+@PC, have been synthesized by a ship-in-a-bottle method. This was used as a photosensitizer for the decomposition of organic pollutant in water under visible light irradiation with molecular oxygen. The Fe(bpy)32+@PC promotes efficient decomposition of organic pollutant, whereas noncapsulated Fe(bpy)32+ is inactive. The enhanced sensitization activity of Fe(bpy)32+ within PC is due to the interaction with the PC wall. This lengthens the lifetime of the excited state Fe(bpy)32+ and promotes efficient production of hydroxyl radicals. The sensitization activity of Fe(bpy)32+@PC depends on the thickness of the PC wall. The Fe(bpy)32+@PC with an appropriate PC wall thickness allows accumulation of pollutant within the PC and, hence, promotes efficient decomposition of pollutant.

This paper proposes a functionally graded Pd/γ-alumina composite membrane fabricated by a novel method for electroless plating by combining supercritical fluid technology and Pd electroless plating in a hybrid technique. The electroless plating reactions on catalyzed nanoporous γ-alumina surfaces were carried out in an emulsion of supercritical carbon dioxide (sc-CO 2 ) and an electroless plating solution with fluorinated surfactant F(CF(CH 3 )CF 2 O) 3 CF(CF 3 )COO(CH 2 CH 2 O)CH 3 . Pd/γ-alumina graded membrane produced by electroless plating with sc-CO 2 had thicker Pd/γ-alumina graded layer and smoother surface than conventional electroless plating. In electroless plating with sc-CO 2 , the penetration into the nanoporous γ-alumina pores was dominant reaction because the deposited Pd particles were smaller and, moreover, the electroless plating media with dispersed CO 2 has lower viscosity than conventional electroless plating. The Pd membrane formed in the emulsion of sc-CO 2 was about 1 μm thickness and composed of a composite membrane of 0.45 mol m −2 min −1 as hydrogen flux, while 0.30 mol m −2 min −1 of the Pd membrane formed by conventional electroless plating.

Photocatalytic activity of TiO 2 particles embedded in mesoporous silica (TiO 2 @MPS), prepared by surfactant-templating method with a TiO 2 colloidal suspension, has been studied in water with molecular oxygen. Photocatalytic reactions of several kinds of aromatic molecules with TiO 2 @MPS reveal that less polar substrates show high reactivity, while polar substrates show much lower reactivity. Electron spin resonance analysis with spin-trapping reagents reveals that less polar substrates diffuse easily into the pore of the catalysts and react efficiently with short-lived hydroxyl ( OH) radicals formed at the surface of inner TiO 2 particles, resulting in a high reactivity. In contrast, polar substrates cannot diffuse into the pores and hence, shows a low reactivity. The photocatalytic activity of TiO 2 @MPS driven by the substrate polarity is applicable to selective transformation of a less polar reactant to a polar product, such as hydroxylation of benzene to phenol with high selectivity (>69%).

This article reviews recent advances in selective organic transformations, both in gas and liquid media, using titanium oxide-based photocatalysts. Several photocatalytic reactions, such as oxidation, reduction, and coupling reactions, proceed highly efficiently and selectively without requiring harmful and dangerous chemical reagents and without harmful byproducts. In addition, multistep processes usually required for conventional synthesis of various kinds of valuable compounds can be simplified to a one-pot reaction when in photocatalytic systems. Photocatalytic transformations will therefore play a very important role for organic synthesis in an economically and environmentally friendly way. This review article demonstrates that titanium oxide-based photocatalysts have a great potential as a versatile tool in “green” organic synthesis.

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.

Photocatalytic oxidation of neat cyclohexane (CHA) with molecular oxygen was carried out using hydrophobically modified Cr-containing mesoporous silica under visible light. Two catalysts (Cr/MOS and Cr/MOS-cal) containing highly dispersed chromate species were synthesized by Cr impregnation onto mesoporous organosilica (MOS) prepared with tetraethyl orthosilicate (TEOS) and 1,2-bis(triethoxysilyl)ethane (BTESE). Cr/MOS was synthesized by Cr impregnation onto MOS followed by calcination, and Cr/MOS-cal was synthesized by calcination of MOS, Cr impregnation, and calcination. Both catalysts promoted partial CHA oxidation with high selectivity (>86%). Cr/MOS-cal synthesized with 10% BTESE showed the highest catalytic activity, due mainly to the higher surface hydrophobicity, which accelerated access of CHA to the chromate species. ESR analysis revealed that the tetrahedrally coordinated chromate species ( T d 6 + ) on Cr/MOS-cal were photoexcited and formed highly reduced T d 4 + ∗ species with high catalytic activity. This also contributed to the high catalytic activity of Cr/MOS-cal.

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.

Aerobic oxidation on a heterogeneous catalyst driven by visible light (λ400 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.

We previously found that Ti-containing mesoporous silica (T-S) with isolated and tetrahedrally coordinated Ti-oxide species, when photoactivated in acetonitrile with molecular oxygen (O(2)), catalyzes highly selective epoxidation of olefins (Chem. Commun. 2005, 5977). The system showed the highest epoxide selectivity among the photocatalytic systems proposed so far, but showed insufficient olefin conversion. In the present work, we have employed Ti-containing mesoporous organosilicas (T-OS), synthesized by a surfactant-templating method with an organosilane precursor, as the photocatalyst and have studied the effects on the olefin conversion and the epoxide selectivity. The T-OS catalysts demonstrate the same high epoxide selectivity as does T-S, but scarcely improve the olefin conversion. Photoluminescence measurement reveals that the T-OS catalysts with high surface hydrophobicity enhance the access of hydrophobic olefins to the photoexcited Ti-oxide species as expected, but destabilize the excited species themselves. ESR analysis demonstrates that the T-OS catalysts also destabilize the active oxygen radical (O(3)(*-)), a crucial oxidant for olefin epoxidation, formed on the excited Ti-oxide species. These destabilizations counteract the enhanced olefin access to the excited species, resulting in almost no improvement in olefin conversion. Through detailed analyses, we have summarized the changes in photocatalytic properties of the Ti-oxide species, associated with the organic modification of the catalyst.

Photocatalytic oxidation of olefins on Cr-containing silica with molecular oxygen by visible light irradiation (lambda > 400 nm) has been investigated. Cr-SiO(2) catalyst prepared by a conventional sol-gel method, containing highly dispersed chromate species, catalyzes efficient olefin oxidation with very high selectivity for partially oxidized products (>90%), whereas semiconductor TiO(2) promotes complete decomposition (CO(2) production). The Cr-SiO(2) catalyst shows much higher activity than Cr/SiO(2) prepared by an impregnation method or Cr proportional variant MCM-41 prepared by a templating method. ESR analysis reveals that photoirradiation of the chromate species with a tetrahedral coordination (T(d)(6+)) on Cr/SiO(2) and Cr proportional variant MCM-41 catalysts leads to the formation of excited state T(d)(5+) species (T(d)(5+*)), while irradiation to T(d)(6+) on Cr-SiO(2) produces T(d)(4+*) species. This can be explained by a homogeneous T(d)(6+) arrangement with Si species on the Cr-SiO(2) catalyst. On the strongly reduced T(d)(4+*), olefins are strongly attracted by an electron and/or proton donation, resulting in high oxidation activity. The Cr-SiO(2) catalyst is applicable to partial oxidation of various aliphatic and aromatic olefins with very high selectivity, and does not promote undesirable dimerization. The obtained findings suggest a potential use of Cr-SiO(2) as an efficient and recyclable heterogeneous photocatalyst for partial oxidation of olefins.

S -Methylsulfonium salts, obtained by a new desulfurization process for light oil based on methylation of sulfur-containing compounds using CH 3 I and AgBF 4 , were used as a methyl-transfer agent for transmethylation of 2-methyl-2-octene. The sulfonium salts obtained from light oil showed higher transmethylation activity than the salts of dibenzothiophene (DBT) and benzothiophene (BT), used as model sulfonium salt. The salts obtained from light oil are derived from highly alkyl-substituted DBTs and BTs, of low nucleophilicity (low electron density on the sulfur atom), and release the methyl group easily, thus giving the higher transmethylation activity. The product distribution was almost the same as that obtained by the model sulfonium salts, suggesting that the salts obtained from light oil are a potential effective methyl-transfer agent.

TiP 2 O 7 carbon composite photocatalyst was successfully prepared by using ion-exchanged resin (C467) containing amino phosphate by metal ion-exchanged carbothermal reduction (MIER-CTR) method using TiCl 3 and TiCl 4 . During the carbonization process in nitrogen, the pre-oxidation (300–350 °C) in air is essential for producing homogeneously dispersed TiP 2 O 7 on the carbon matrix. In the absence of pre-oxidation, the resin was melted. The carbonization temperature 500 °C was found to be suitable for producing single phase TiP 2 O 7 with higher yields. Powder X-ray diffraction (XRD) and Raman spectroscopic results suggest the formation of TiP 2 O 7 , while X-ray diffraction results reveal that the crystallite size was less than 35 nm. UV-Vis studies show that the band gap of TiP 2 O 7 was 3.32 eV. The TiP 2 O 7 carbon composite catalyst was applied for the photocatalytic decomposition of 2-propanol at 30 °C using a mercury lamp (365 nm).