Immobilization of highly active titanium(IV) oxide particles A novel strategy of preparation of transparent photocatalytic coatings

Titanium(IV) oxide (TiO2) powders synthesized by HyCOM (hydrothermal crystallization in organic media) method, which had been proved to exhibit ultra-high photocatalytic activity in several reaction systems, were used as starting material for fabrication of transparent TiO2 thin films. HyCOM-TiO2 powders were dispersed in aqueous sol of nitric acid to yield a TiO2 sol stable for more than 90 days. Transparent TiO2 thin films were successfully produced by dip-coating from the TiO2 sol and used for photocatalytic decomposition of malachite green (MG) in an aqueous sol under aerated conditions. These films exhibited much higher rate of MG decomposition compared with those prepared from a commercially available TiO2 sol developed for photocatalytic use (Ishihara STS-01), indicating that the excellent photocatalytic activity of original HyCOM-TiO2 particles was preserved after immobilization on glass substrates by the present method

Photocatalytic decolorization and mineralization of malachite green in an aqueous suspension of titanium(IV) oxide nano-particles under aerated conditions: correlation between some physical properties and their photocatalytic activity

Titanium(IV) oxide (TiO2) nano-particles with various physical properties, which had been prepared by hydrothermal crystallization in organic media (HyCOM) and post-calcination, were used for photocatalytic decomposition of malachite green (MG) in an aqueous suspension under aerated conditions. The amount of MG adsorbed on TiO2 ([MG]ad) increased as the surface area of HyCOM TiO2 increased. The apparent initial rate constant of pseudo zero-order kinetics, k0, for decolorization of MG in an early stage (15 min) coincided well with [MG]ad, indicating that adsorptivity is a decisive factor for the initial bleaching of MG. However, in a 1 h irradiation experiment, a HyCOM sample with a smaller [MG]ad but improved crystallinity exhibited higher decolorization activity than did HyCOM TiO2 with the largest [MG]ad, suggesting that this reaction process includes deactivation of the photocatalyst due to deposition of intermediate(s) that had been formed by degradation of MG. Longer irradiation was required for mineralization of MG. Total organic carbon (TOC) in the suspension of HyCOM TiO2 with improved crystallinity continuously decreased whereas TOC in Degussa P-25 TiO2 did not decrease any more after 5 h of irradiation, although one-third of the initial TOC still remained in the suspension

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 ε-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 ε-amino groups in L-Lys by photoexcited positive holes (h+) and the efficiency of reduction of α-CSB (produced via ε-amino group oxidation to give optically pure PCA) and ε-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 ε-CSB, gave higher OPPCA but lower SPCA, since some ε-CSB remained unreduced to give racemic PCA. In contrast, at higher SPCA, both CSB's were reduced non-selectively 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 CSB's by e-. Thus, the general tendency of plots between OPPCA and SPCA could be explained by the above-described redox-combined mechanism of photocatalysis

Correlation between some physical properties of titanium dioxide particles and their photocatalytic activity for some probe reactions in aqueous systems

Nanocrystalline anatase titanium(IV) oxide (TiO2) particles were synthesized by hydrothermal crystallization in organic media (HyCOM) followed by calcination at various temperatures up to 1273 K, and they were characterized by analysis of surface adsorption of the substrates, as well as by X-ray diffraction (XRD) and Brunauer−Emmet−Teller (BET) surface area measurements. These HyCOM TiO2 samples were used for three kinds of photocatalytic reactions:  mineralization of acetic acid (AcOH) in aerated aqueous suspensions, dehydrogenation of 2-propanol (2-PrOH) by in situ platinized powders, and silver-metal deposition from silver ions (Ag+) in deaerated aqueous suspensions of bare TiO2 samples. Dependence of the photocatalytic activities on calcination temperature (Tc) and on the amount of adsorbed substrates in each reaction and correlations with the physical properties of HyCOM TiO2 were examined. In the case of mineralization of AcOH, the activitiy of each sample was almost proportional to the amount of surface-adsorbed AcOH in the dark, and the uncalcined (as-prepared) HyCOM TiO2 showed the highest activity, which was monotonically reduced with Tc, that is, with decrease in the amount of surface-adsorbed AcOH. On the other hand, in the case of silver-metal deposition, the photocatalytic activity was enhanced by calcination at higher temperature, despite the simultaneous decrease in the amount of surface-adsorbed Ag+ in the dark. Overall, the effects of calcination on the photocatalytic activities for several reaction systems strongly suggested that photocatalytic activity depends on two significant factors, adsorbability and recombination probability, corresponding to the specific surface area and crystallinity, respectively, and that the balance of these two factors determines the Tc dependence

Nanocrystalline Brookite-Type Titanium(IV) Oxide Photocatalysts Prepared by a Solvothermal Method: Correlation Between Their Physical Properties and Photocatalytic Activities

Nanocrystalline brookite-type titanium(IV) oxide TiO2 powder was synthesized by solvothermal treatment of oxobis(2,4-pentanedionato-O,O)titanium in a 1,2-ethanediol-water system in the presence of sodium acetate followed by hydrothermal treatment at 373 K in order to remove organic moieties contaminating the powder. The powder was calcined at various temperatures to change its physical properties and then used for three types of photocatalytic reaction: mineralization of acetic acid (AcOH) in an aerated aqueous suspension of bare TiO2 powder, evolution of molecular hydrogen from 2-propanol in an aqueous suspension of in situ platinized powder and formation of molecular oxygen (O2) from silver sulfate in a deaerated aqueous suspension of bare TiO2 powder. Dependence of the photocatalytic activities on calcination temperature (Tc) and correlations with the physical properties of brookite-type TiO2 samples were examined. In the case of mineralization of AcOH, the uncalcined brookite TiO2 sample having the largest surface area showed the highest rate of carbon dioxide evolution (RCO2), which was equal to that of representative commercial TiO2 (Degussa P-25), and RCO2 decreased monotonously with Tc, i.e., with decrease in surface area. On the other hand, in the case of O2 formation, the photocatalytic activity was enhanced by calcination at a higher temperature, despite the simultaneous decrease in surface area. Overall, the effects of calcination on the photocatalytic activities for the three reaction systems strongly suggested that photocatalytic activity of brookite-type TiO2 depends on two significant factors, adsorbability and recombination probability, corresponding to the specific surface area and crystallinity, respectively, and that the balance of these two factors determines Tc dependence

Effective Photocatalytic Reduction of Nitrate to Ammonia in an Aqueous Suspension of Metal-Loaded Titanium(IV) Oxide Particles in the Presence of Oxalic Acid

Photocatalytic reduction of nitrate ion (NO3 –) in an aqueous suspension of metal-loaded titanium(IV) oxide (TiO2) was examined in the presence of oxalic acid (OA) as a hole scavenger. Conversion of NO3 – into ammonia (NH3) competed with hydrogen liberation, and the NH3 production selectivity increased with the order of loaded metal, (Pt, Pd, Co) < (Ni, Au) < (Ag, Cu), which was attributable to the efficiency of reduction of protons by photogenerated electrons at the loaded metal, i.e., hydrogen overvoltage of the loaded metal. TiO2 powder loaded with Cu showed higher NH3 yield and selectivity as well as higher efficiency of OA consumption. TiO2 with in situ deposited Cu gave results comparable to those of a Cu pre-loaded photocatalyst

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 RAg versus [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 Reduction of Nitrobenzene to Aniline in an Aqueous Suspension of Titanium(IV) Oxide Particles in the Presence of Oxalic Acid as a Hole Scavenger and Promotive Effect of Dioxygen in the System

Nitrobenzene was effectively and selectively reduced to aniline in an acidic aqueous suspension of titanium(IV) oxide photocatalyst in the presence of oxalic acid as a hole scavenger and the aniline yield was improved in the presence of a small amount of dioxygen