Use of Biomass Glycerol as a Reducing Agent for Photocatalytic Deprotection of Pyridine N-Oxides in an Aqueous Suspension of Titanium(IV) Oxide.

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.

Ruthenium and palladium bimetallic nanoparticles achieving functional parity with a rhodium cocatalyst for TiO2-photocatalyzed ring hydrogenation of benzoic acid.

Our previous study showed that a rhodium (Rh) cocatalyst is indispensable for ring hydrogenation of benzoic acid over a titanium(IV) oxide (TiO2) photocatalyst. In this study, we explored ring hydrogenation under an Rh-free condition by using two kinds of cocatalyst that were inactive for this reaction when used solely. Cyclohexanecarboxylic acid as the ring hydrogenation product was successfully obtained when ruthenium (Ru) and palladium (Pd) were simultaneously loaded on TiO2, indicating that this bimetallic system can be used in place of an Rh cocatalyst in ring hydrogenation. The state and distribution of Ru and Pd in particles loaded on TiO2 were investigated by transmission electron microscopy, X-ray photon spectroscopy, and X-ray absorption near edge structure analysis. The functions of Ru and Pd as cocatalysts are discussed on the basis of results of characterization and activity tests. The effects of different contents of Ru and Pd in Ru-Pd/TiO2 prepared by a two-step photodeposition method on catalytic activity and the features of the reaction system were investigated in detail.

Titanium(iv) oxide having a copper co-catalyst: a new type of semihydrogenation photocatalyst working efficiently at an elevated temperature under hydrogen-free and poison-free conditions.

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 H2 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.

Photocatalytic hydrogenation of furan to tetrahydrofuran in alcoholic suspensions of metal-loaded titanium(iv) oxide without addition of hydrogen gas.

The use of metal co-catalysts broadens the application of photocatalytic reduction without the use of dihydrogen (H2) 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 H2-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.

Control of Surface Plasmon Resonance of Au/SnO2 by Modification with Ag and Cu for Photoinduced Reactions under Visible-Light Irradiation over a Wide Range.

Gold particles supported on tin(IV) oxide (0.2 wt% Au/SnO2) were modified with copper and silver by the multistep photodeposition method. Absorption around λ=550 nm, attributed to surface plasmon resonance (SPR) of Au, gradually shifted to longer wavelengths on modification with Cu and finally reached λ=620 nm at 0.8 wt% Cu. On the other hand, the absorption shifted to shorter wavelength with increasing amount of Ag and reached λ=450 nm at 0.8 wt% Ag. These Cu- and Ag-modified 0.2 wt% Au/SnO2 materials (Cu-Au/SnO2 and Ag-Au/SnO2) and 1.0 wt% 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 625 nm, 5.8% at 525 nm, and 5.1% at 450 nm, 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.

Organically modified titania having a metal catalyst: a new type of liquid-phase hydrogen-transfer photocatalyst working under visible light irradiation and H2-free conditions.

Organically modified titania having a metal catalyst (OMTC), 2,3-dihydroxynaphthalene-modified titania having palladium metal, successfully worked as a hydrogen-transfer (C[double bond, length as m-dash]C hydrogenation) photocatalyst in the presence of triethanolamine as the hydrogen source under visible light irradiation and hydrogen-free conditions.

Visible-light-induced hydrogen and oxygen formation over Pt/Au/WO3 photocatalyst utilizing two types of photoabsorption due to surface plasmon resonance and band-gap excitation.

Photocatalytic H2 and O2 formations under visible light irradiation (λ > 400 nm) are demonstrated using Pt-Au nanopaticles for the reduction site and WO3 for the oxidation site in solid-state Pt/Au/WO3.

Selective oxidation of alcohols in aqueous suspensions of rhodium ion-modified TiO2 photocatalysts under irradiation of visible light.

Photocatalytic oxidation of benzyl alcohols in aqueous suspensions of rhodium ion-modified titanium(iv) oxide (Rh(3+)/TiO2) in the presence of O2 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. Rh(3+)/TiO2 photocatalysts having various physical properties were prepared using commercially available TiO2 powders as supporting materials for Rh(3+) to investigate the effect(s) of physical properties of TiO2 on photocatalytic activities of Rh(3+)/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 Rh(3+)/TiO2 photocatalyst. The reaction mechanism was discussed on the basis of the results of photocatalytic oxidation of various p-substituted benzyl alcohol derivatives.

Rapid production of 38 mM H2O2 in an alcoholic suspension of a WO3 photocatalyst under visible light.

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.

Preparation of Au/CeO2 exhibiting strong surface plasmon resonance effective for selective or chemoselective oxidation of alcohols to aldehydes or ketones in aqueous suspensions under irradiation by green light.

Au/CeO(2) 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/CeO(2) 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/CeO(2) 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 × 10(12) (g-Au/CeO(2))(-1) for MS photodeposition and 59 nm and 4.8 × 10(12) (g-Au/CeO(2))(-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/CeO(2) 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/CeO(2) samples, which was weak and was saturated even at around 0.5 wt %. Due to the strong photoabsorption, the MS-Au/CeO(2) samples exhibited reaction rates approximately twice larger than those of SS-Au/CeO(2) samples with the same Au contents, and apparent quantum efficiency of MS-Au/CeO(2) 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/CeO(2) 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/CeO(2) photocatalyst exhibited high chemoselectivity toward the hydroxyl group of alcohol, i.e, the Au/CeO(2) photocatalyst almost quantitatively converted aminobenzyl alcohol to aminobenzaldehyde with 99% yield.

Gold-titanium(IV) oxide plasmonic photocatalysts prepared by a colloid-photodeposition method: correlation between physical properties and photocatalytic activities.

Colloidal gold (Au) nanoparticles were prepared and successfully loaded on titanium(IV) oxide (TiO(2)) without change in the original particle size using a method of colloid photodeposition operated in the presence of a hole scavenger (CPH). The prepared Au nanoparticles supported on TiO(2) 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/TiO(2) 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 TiO(2) supports without change in the original size independent of the TiO(2) phase. The rate of CO(2) formation also increased linearly with increase in the external surface area of Au. Interestingly, the TiO(2) supports showed different slopes of the plots. The slope is important for selection of TiO(2) as a material supporting colloidal Au nanoparticles.

Photocatalytic reduction of nitrite to dinitrogen in aqueous suspensions of metal-loaded titanium(IV) oxide in the presence of a hole scavenger: an ensemble effect of silver and palladium co-catalysts.

Nitrite (NO(2)(-)) was photocatalytically reduced to dinitrogen (N(2)) in an aqueous suspension of two kinds of titanium(iv) oxide particles loaded with palladium and silver (Pd-TiO(2) and Ag-TiO(2)) at pH 8 under irradiation of UV light in the presence of sodium oxalate as a hole scavenger. The two metal-loaded TiO(2) photocatalysts had different roles in conversion of NO(2)(-) to N(2) and worked in an effective ensemble without conflict: (1) Pd-TiO(2) induced photocatalytic disproportionation of NO(2)(-) to N(2) and nitrate (NO(3)(-)) and (2) Ag-TiO(2) selectively reduced the thus-formed NO(3)(-) back to NO(2)(-) (partially to N(2)) with oxalate acting as a hole scavenger. When Pd-TiO(2) was used alone for NO(3)(-) reduction in the presence of sodium oxalate, Pd-TiO(2) induced fruitless photocatalytic decomposition of oxalate to carbon dioxide and dihydrogen. The presence of Ag-TiO(2) suppressed the fruitless decomposition of oxalate by Pd-TiO(2) because Ag-TiO(2) continuously provided NO(2)(-) in the reaction system using oxalate as a hole scavenger and Pd-TiO(2) therefore only worked as a photocatalyst for disproportionation of NO(2)(-) to N(2) and NO(3)(-) as it did when used alone.

Photocatalytic reduction of nitrobenzenes to aminobenzenes in aqueous suspensions of titanium(IV) oxide in the presence of hole scavengers under deaerated and aerated conditions.

Photocatalytic reduction of nitrobenzenes to corresponding aminobenzenes in aqueous suspensions of titanium(IV) oxide (TiO(2)) containing hole scavengers under various conditions was examined. In photocatalytic reduction of m-nitrobenzenesulfonic acid (m-NBS) in the presence of formic acid (FA) under deaerated conditions, m-aminobenzenesulfonic acid (m-ABS) was produced almost quantitatively in acidic suspensions and high efficiency (>99%) in FA utilization as a hole scavenger was achieved. No re-oxidation of m-ABS occurred in acidic conditions both in the presence and absence of FA. The high yield of m-ABS was explained by strong ability of FA as a hole scavenger and possible repulsion of the reduced functional group (ammonium group, -NH(3)(+)) from the protonated, i.e., positively charged TiO(2) surface in acidic suspensions avoiding re-oxidation of m-ABS. Using TiO(2) samples of various physical properties, which had been synthesized by a solvothermal method and post-calcination at various temperatures, effects of physical properties of the TiO(2) samples on m-ABS yield were also investigated. A linear correlation between the amount of m-NBS adsorbed and the m-ABS yield was observed, suggesting that ability of TiO(2) for m-NBS adsorption is one of the key factors for effective photocatalytic reduction of m-NBS to m-ABS. This photocatalytic system can be applied for reduction of aminonitrobenzenes to corresponding diaminobenzenes (DAB) in the presence of oxalic acid as a hole scavenger. High yields of m-ABS and DAB were achieved even when the reactions were performed in the presence of oxygen.

Photocatalytic disproportionation of nitrite to dinitrogen and nitrate in an aqueous suspension of metal-loaded titanium(IV) oxide nanoparticles.

Photocatalytic reaction of a nitrite ion in aqueous suspensions of bare and metal-loaded TiO(2) particles was examined without electron and hole scavengers under irradiation of UV light. In the bare TiO(2) system, disproportionation of NO(2)(-) to N(2) (or N(2)O) and NO(3)(-) with nitrogen balance (NB) and redox balance (ROB) close to unity within experimental errors was observed, although the reaction was slow. Palladium (Pd)-loaded TiO(2) particles exhibited an extraordinarily large rate of disproportionation of NO(2)(-) in their aqueous suspension, i.e. NO(2)(-) was almost completely converted to N(2) (or N(2)O) and NO(3)(-) even after only 3 h of photoirradiation, both the values of NB and ROB being close to unity. This result suggests that Pd loaded on TiO(2) particles acted as storage sites for photogenerated electrons and effectively transferred the electrons to NO(2)(-) and, therefore, that the reduction process in the photocatalytic disproportionation of NO(2)(-) was accelerated by Pd loaded on TiO(2). Effects of the amount of Pd and pH of the suspension on the reaction rate were also examined.

Synthesis of Disk-Shaped Tungsten(VI) Oxide Particles with Various Physical Properties for Mineralization of Acetic Acid in Water Under Irradiation of Visible Light.

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 °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 °C showed the highest activity with a CO2 evolution rate of 5.9 µmol h-1. These results indicated that improving the crystallinity of the D-WO3 samples was effective in increasing their photocatalytic activities.

Correction: A silver-manganese dual co-catalyst for selective reduction of carbon dioxide into carbon monoxide over a potassium hexatitanate photocatalyst with water.

Correction for 'A silver-manganese dual co-catalyst for selective reduction of carbon dioxide into carbon monoxide over a potassium hexatitanate photocatalyst with water' by Xing Zhu et al., Chem. Commun., 2019, 55, 13514-13517.

A silver-manganese dual co-catalyst for selective reduction of carbon dioxide into carbon monoxide over a potassium hexatitanate photocatalyst with water.

A Ag-Mn dual co-catalyst deposited on a K2Ti6O13 photocatalyst significantly enhances the photocatalytic CO2 reduction into CO with an extremely high selectivity of 98% by using H2O as an electron donor, owing to the properties of Ag and MnOx species for promoting CO and O2 formation, respectively.

Photocatalytic chemoselective cleavage of C-O bonds under hydrogen gas- and acid-free conditions.

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.

A very simple method for the preparation of Au/TiO2 plasmonic photocatalysts working under irradiation of visible light in the range of 600-700 nm.

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 (λ > 600 nm), and the apparent quantum efficiency reached 7.2% at 625 nm and 4.2% even at 700 nm.

Visible light-induced heterogeneous Meerwein-Ponndorf-Verley-type reduction of an aldehyde group over an organically modified titanium dioxide photocatalyst.

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.