Shigeru Ikeda

Photocatalytic degradation of organic compounds in aqueous systems by transition metal doped polycrystalline TiO2

Abstract Some probe catalytic photooxidation reactions with aliphatic and aromatic organic compounds having different acid strengths, i.e. methanoic acid, ethanoic acid, benzoic acid and 4-nitrophenol, were carried out in aqueous systems by using polycrystalline TiO2 powders doped with various transition metal ions (Co, Cr, Cu, Fe, Mo, V and W). The Co-doped powder showed to be more photoactive than the bare TiO2 for methanoic acid degradation while the behaviour of TiO2/Cu and TiO2/Fe was similar to that of the support. TiO2/W was the most efficient sample for the photodegradation of benzoic acid and 4-nitrophenol, TiO2 the most active powder for ethanoic acid. A tentative explanation is provided by taking into account: (i) the dissociation constants (Ka) of the different acids used as substrates; (ii) their aliphatic or aromatic nature; (iii) the points of zero charge (PZC) of the photocatalysts; (iv) their relative rate constants for photoelectron–hole recombination (kr) determined by femtosecond pump-probe diffuse reflectance spectroscopy.

Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders

The molar amounts of defective sites (Md) in several titanium(IV) oxide (TiO2) powders were determined using photoinduced reactions of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. Measurements of pH dependence of typical anatase and rutile TiO2 powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO2 in ranges of 0–0.35 V for anatase and 0–0.25 V for rutile. A linear relation of Md with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that Md could be a quantitative parameter of recombination between a photoexcited electron and a positive hole. The fact that there was no linear relation between Md and the specific surface area suggests that the surface area was not directly reflected on Md. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and Md revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO2 powders in this reaction system.

Discrimination of the active crystalline phases in anatase–rutile mixed titanium(IV) oxide photocatalysts through action spectrum analyses

The active crystalline phases in powdered titanium(IV) oxide (TiO2) photocatalysts of anatase–rutile mixtures were investigated by measuring the action spectra of the photocatalytic reactions of H2 evolution, Ag deposition and acetic acid decomposition. All of the pure anatase powders used in this study gave action spectra for the three kinds of reaction that were blue-shifted by about 30 nm from those of the pure rutile powders, being consistent with their photoabsorption behavior observed in the diffuse reflectance spectra. On the other hand, the action spectra of the mixed TiO2 powders varied, depending both on the kind of photocatalytic reaction and the content of the crystalline phases. In the case of H2 evolution, the action spectra were intermediate between those of the pure anatase and rutile phases and were shifted toward that of the rutile phase with a decrease in the content of the anatase phase; i.e., the action spectra are governed mainly by the photoabsorption of each powder. The Ag deposition and acetic acid decomposition reactions gave action spectra of the mixed TiO2 powders that were similar to those of pure rutile and anatase phases, respectively, regardless of the content of the crystalline phases; the active phase in the former was rutile while that in the latter was anatase, even when these active phases were only minor parts of the mixed powder. The results indicate that the active crystalline phase differs with the kind of photocatalytic reaction even if the same mixed TiO2 powders were used as photocatalysts.

Photochemical hydrogen evolution from aqueous triethanolamine solutions sensitized by binaphthol-modified titanium(IV) oxide under visible-light irradiation

Abstract Surface modification of titanium(IV) oxide (TiO 2 ) powders with 1,1′-binaphthalene-2,2′-diol (bn(OH) 2 ), having two phenolic hydroxyls and atropisomeric chirality, in refluxed ethanol led to the formation of a deep yellow-colored surface complex, giving visible-light absorption at a wavelength below ca. 550–600xa0nm. Quantitative analyses of the surface complex revealed that absorption intensity at 450xa0nm in diffuse reflection (DR) measurement was almost proportional to its amount and that its amount depended strongly on the amount of surface hydroxyl groups of TiO 2 but not on the crystal structure of TiO 2 , anatase, and rutile. The bn(OH) 2 complexation was also applied to platinized TiO 2 to drive photocatalytic molecular hydrogen (H 2 ) evolution from deaerated triethanolamine solutions under visible-light irradiation at a wavelength even longer than 540xa0nm. The photonic efficiency of the photocatalytic reaction at 450xa0nm was estimated to be 0.02% for bn(OH) 2 -modified TiO 2 (JRC-TIO-3) loaded with 0.1xa0wt.% of platinum. On the basis of these experimental results, the reaction mechanism, which involves visible-light excitation of the surface complex, injection of electrons from the complex to TiO 2 , and migration of the electrons to platinum deposits, where reduction of H + takes place to give H 2 , was elucidated.

Transition metal doped TiO2: physical properties and photocatalytic behaviour

Two probe photocatalytic reactions, i.e. ethanoic acid and 4-nitrophenol photooxidation, were carried out in different experimental conditions by using suspensions of transition metal (Co, Cr, Cu, Fe, Mo, V and W) doped polycrystalline TiO2 powders in aqueous systems. A beneficial influence of the pres- ence of metal species was observed only with the samples containing copper and tungsten. In particular, the TiO2/Cu powders showed to be more photoactive of bare TiO2 for the ethanoic acid oxidation while the TiO2/W samples were more efficient for 4-nitrophenol degradation. A tentative interpretation is provided on the basis of the values of the points of zero charge of the powders and of the rate constants of recombi- nation of photogenerated electrons and holes, determined by femtosecond pump-probe diffuse reflectance spectroscopy (PP-DRS).

Photocatalytic Organic Syntheses : Selective Cyclization of Amino Acids in Aqueous Suspensions

Photocatalytic reactions occurring at semiconductor particles/solution interfaces can be applied to organic syntheses. In this review article, examples of photocatalytic syntheses of cyclic amino acids by suspended semiconductor particles, e.g., titanium(IV) oxide or cadmium(II) sulfide are introduced and interpreted. Different from the photocatalytic decomposition of pollutants under aerobic conditions, selective conversion of organic compounds can be driven by the photocatalytic reactions under deaerated conditions.

Evaluation of electron-hole recombination properties of titanium (IV) oxide particles with high photocatalytic activity

Electron-hole recombination in nano-sized titanium(IV) oxide (TiO2) particles with various physical properties, which have been shown to be highly active photocatalysts, was evaluated by quantitative analysis of reduced titanium species (Ti3+), which might be formed at crystalline defective sites in TiO2 particles through photo-irradiation in the presence of a hole scavenger under deaerated conditions. These highly active photocatalyst samples were synthesized by hydrothermal crystallization in organic media (HyCOM method) and post-calcination. The Ti3+ density decreased with increasing calcination temperature (Tc), and a linear correlation was observed between the Ti3+ density and rate constant for electron-hole recombination evaluated by femtosecond pump-probe diffuse reflection spectroscopy. Reaction rate (RAg) and the amount of silver ions (Ag+) adsorbed on TiO2 particles ([Ag+]ads) were measured for photocatalytic silver metal deposition along with oxygen formation from an aqueous Ag+ solution under deaerated conditions, and the slope of the RAgversus [Ag+]ads plot was determined. Kinetic investigation of this reaction showed that the reciprocal of the slope was approximately related to the ratio of the rates for electron-hole recombination and electron trapping (kr/ke ratio). The kr/ke ratio decreased as Tc increased, and the logarithm of the kr/ke ratio was linearly related with Ti3+ density. These two parameters were used as a measure for the recombination properties of TiO2 photocatalysts with various physical properties.

Photocatalytic redox-combined synthesis of l-pipecolinic acid from l-lysine by suspended titania particles: effect of noble metal loading on the selectivity and optical purity of the product

Photocatalytic (>300 nm) conversion of l-(S)-lysine (l-Lys), in its neutralized aqueous solution, into l-pipecolinic acid (l-PCA) under deaerated conditions at 298 K was investigated in detail using suspended TiO2 powders (Degussa P-25, Ishihara ST-01, and HyCOM TiO2) loaded with platinum (Pt), rhodium (Rh), or palladium (Pd). A common feature of the results of experiments using a wide variety of metal-loaded TiO2 photocatalysts is that the rate of PCA formation (rPCA) was greatly reduced when higher optical purity of PCA (OPPCA), i.e., enantio excess of the l-isomer of PCA, was obtained; higher rPCA was achieved by the use of Pt-loaded TiO2 powders, while these powders gave relatively low OPPCA. Selectivity of PCA yield (SPCA), i.e., amount of PCA production based on l-Lys consumption, also tended to increase with decrease in OPPCA, giving a master curve in the plots of OPPCA versus SPCA. Among the TiO2 powders used in this study, HyCOM TiO2 showed relatively high OPPCA and SPCA but not optimum SPCA and OPPCA simultaneously. In order to interpret such relations, the mechanism of stereoselective synthesis of the l-isomer of PCA (l-PCA) was investigated using isotope-labeled α-15N-l-lysine with quantitative analysis of incorporation of 15N in PCA and ammonia (NH3), a by-product. It was observed for several photocatalysts that the 15N proportion (P15) in PCA was almost equal to OPPCA, suggesting that oxidative cleavage by photogenerated positive holes of the e-amino moiety of l-Lys gave optically pure l-PCA through retention of chirality at the α-carbon in the presumed intermediate, a cyclic Schiff base (α-CSB), which undergoes reduction by photoexcited electrons into PCA. From P15 in NH3 and PCA, the selectivity of oxidation between α and e-amino groups in l-Lys by photoexcited positive holes (h+) and the efficiency of reduction of α-CSB (produced via e-amino group oxidation to give optically pure PCA) and e-CSB (produced via α-amino group oxidation to give racemic PCA) by photoexcited electrons (e−) were calculated. The former was found to be independent of the kind of photocatalyst, especially the loaded metal, while the latter was influenced markedly only by the loaded metal. It was clarified that OPPCA and SPCA obtained for various TiO2 powders used in the present study were strongly governed by the reduction stage, i.e., the efficiency of reduction of two types of CSB. When SPCA was relatively low, photocatalysts, favoring the reduction of α-CSB rather than e-CSB, gave higher OPPCA but lower SPCA, since some e-CSB remained unreduced to give racemic PCA. In contrast, at higher SPCA, both CSBs were reduced nonselectively and OPPCA was found to be determined mainly by the selectivity in the oxidation stage. The relatively low yield of molecular hydrogen (H2) when higher SPCA was achieved is consistent with the mechanism in which H2 liberation occurs instead of the reduction of CSBs by e−. Thus, the general tendency of plots between OPPCA and SPCA could be explained by the above-described redox-combined mechanism of photocatalysis.

Development of a novel photocatalytic reaction system for oxidative decomposition of volatile organic compounds in water with enhanced aeration

Abstract A novel photocatalytic reaction system, composed of solution and gas spaces that are divided by a thin Teflon film and titanium(IV) oxide (TiO2)-coated mesh or cloth, for the treatment of contaminated aqueous solutions was developed to be operated with enhanced aeration without bubbling of air in the solution. First, the photocatalytic activities of TiO2 particles immobilized on two kinds of support material, stainless steel mesh (SSM) and fiberglass cloth (FGC), were investigated for photocatalytic oxidation of 2-propanol, as a model volatile organic compound, dissolved in aerated aqueous solution. The TiO2 particles immobilized on both support materials exhibited photocatalytic activity to oxidize 2-propanol into acetone and carbon dioxide (CO2), and the activity levels of the TiO2 particles immobilized on the two kinds of support materials were comparable. Presumably due to the presence of a small amount of metal species originating in SSM that might work as reduction catalysts, molecular hydrogen (H2) was also liberated on the TiO2-immobilized SSM. Results of analysis of weight loss after photoirradiation suggested that the stability of the TiO2-immobilized FGC was better than that of the TiO2-immobilized SSM. On the basis of these results, FGC was employed in construction of a photocatalytic reactor equipped with an oxygen (O2)-permeable Teflon membrane in order to make oxygen pass from a gas space to a solution space and to keep the surface of the immobilized TiO2 photocatalyst, facing an aqueous solution containing volatile organic compounds, saturated with dissolved O2. From the results of photocatalytic oxidative decomposition of 2-propanol, it was clarified that the surfaces of TiO2 particles could be sufficiently supplied with O2 from the gas space through the membrane to accelerate the oxidation.

Phase-boundary catalysis: a new approach in alkene epoxidation with hydrogen peroxide by zeolite loaded with alkylsilane-covered titanium oxide

An NaY zeolite, modified with partly alkylsilane-covered ntitanium oxide and located at the boundary between aqueous and organic nphases (a phase-boundary catalyst), acts as a catalyst for alkene nepoxidation without stirring or the addition of a co-solvent to drive nliquid–liquid phase transfer.