Yoshiya Kera

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.

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.

Mesoporous electrodes having tight agglomeration of single-phase anatase TiO2 nanocrystallites: Application to dye-sensitized solar cells

Abstract Single-phase anatase nanocrystalline HyCOM-TiO2 (Hydrothermal Crystallization in Organic Media) to label this method was synthesized by high-temperature hydrolysis of titanium tetrabutoxide in toluene. The resulting HyCOM-TiO2 nanocrystallites were found to be covered by n-butoxide, yielding mesoporous, transparent anatase films with a narrow pore size distribution and good electron transport characteristic when sintered at 350–550°C on optically transparent conducting glass. Dye-sensitized solar cells made of the Ru-dye-adsorbed mesoporous HyCOM films as photoanodes achieved better photo-energy conversion efficiency as compared to those prepared using commercially available Degussa P25 films.

Photocatalytic oxidation of nitrogen monoxide over titanium(IV) oxide nanocrystals large size areas

Abstract The titania prepared by high-temperature hydrolysis of titanium alkoxides in a hydrocarbon solvent is highly active for the photocatalytic oxidation of NO to NO 2 and NO 3 . The photocatalytic oxidation activity decreases with an increase in post-calcination temperature of the titania. The surface area and crystal size of the titania make some contributions to the photocatalytic oxidation activity. IR spectra show that ultraviolet (UV) irradiation of the titania promotes the formation of nitrate species. The data of ESR and IR lead to the conclusions that NO, when irradiating the titania with UV rays, reacts with O 2 − to form NO 2 and NO 3 − , but the radicals generated as a result of secondary products of O − make no contribution to the photocatalytic oxidation. In addition, the concentration of free electron and O 2 − decreases significantly with an increase in post-calcination temperature of the titania.

Photocatalytic oxidation of nitrogen oxide over titania–zeolite composite catalyst to remove nitrogen oxides in the atmosphere

The photooxidation of NO with oxygen over Hycom TiO2 and zeolite (A and Y form zeolite: TiO2-AZ and TiO2-YZ) composite catalysts was studied to remove NOx in the atmosphere. The photocatalytic oxidation activity of the titania in the composite catalyst in a proportion of AZ:TiO2=3:7 is about three times larger than that in the bare titania. The adsorption behaviors of NO and NO2 for the bare titania sample obey Langmuir adsorption equations of NO and NO2, respectively. In the titania–zeolite composite catalysts, the adsorption data indicate the increase in the amount of NO adsorption on the TiO2 phase and the decrease in the amount of NO2 adsorption, compared with the bare titania. The acceleration of NO photooxidation rate, resulting from the increase in the amount of NO adsorbed and the decrease in the amount of NO2 adsorbed, thus occurs on the TiO2 phase. IR spectra, when irradiating the catalysts with UV, showed the immediate formation of nitrate and NO2 species on the catalyst. The results lead to the conclusion that the zeolites promote the photocatalytic oxidation of NO over the titania.

Novel synthesis of microcrystalline titanium(IV) oxide having high thermal stability and ultra-high photocatalytic activity : thermal decomposition of titanium(IV) alkoxide in organic solvents

Thermal decomposition of titanium(IV) tetra-tert-butoxide (TTB) in inert organic solvents at 573 K yielded microcrystalline anatase (titanium(IV) oxide, TiO2) powders with a crystallite size of ca. 9 nm and a surface area of <100 m2 g-1. Primary and secondary alkoxides of titanium(IV), however, were not decomposed under similar conditions, indicating that the thermal stability of C-O bonds in the alkoxides was a decisive factor for their decomposition. The TiO2 prepared from TTB by this manner was thermally stable upon calcination in air and retained high surface area of ca. 100 m2 g-1 even after calcination at 823 K. The as-prepared TiO2 powders, without calcination, exhibited much higher rate of carbon dioxide formation than any other active photocatalysts such as Degussa P-25 and Ishihara ST-01 in the photocatalytic mineralization of acetic acid in aerated aqueous solutions. The higher activity of the present TiO2 photocatalysts is attributed to both high crystallinity and large surface of the present product. The calcination of the as-prepared TiO2 in air reduced the photocatalytic activity, but it was still higher than the other commercially available TiO2s.

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.

Synthesis of titanium(IV) oxide of ultra-high photocatalytic activity : high-temperature hydrolysis of titanium alkoxides with water liberated homogeneously from solvent alcohols

Abstract Thermal treatment of titanium(IV) alkoxides dissolved in alcohols at temperature of 523–573 K under autogenous pressure yielded microcrystalline anatase titanium(IV) oxide (TiO 2 ) with diameter of 11–31 nm and surface area of 42–138 m 2 g −1 . Similar treatment in hydrocarbons such as toluene gave no solid products, indicating that both hydrolysis of alkoxides with water liberated homogeneously from solvent alcohols and crystallization of anatase phase occurred simultaneously. These TiO 2 s were thermally stable, e.g., BET surface area of TiO 2 synthesized at 573 K, 63 m 2 g −1 , was reduced only 30% (45 m 2 g −1 ) even by calcination at 973 K. Their photocatalytic activity was examined in mineralization of acetic acid in aqueous solutions under aerated conditions and dehydrogenation of 2-propanol under deaerated conditions; almost all the samples showed the activity more than twice higher than those of representative active photocatalysts, Degussa P-25 and Ishihara ST-01. The superior activity of the present TiO 2 photocatalysts was attributable to compatibility of high crystallinity and large surface area owing to the newly developed synthetic procedure.

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

Abstract Titanium(IV) oxide (TiO 2 ) 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 TiO 2 thin films. HyCOM-TiO 2 powders were dispersed in aqueous sol of nitric acid to yield a TiO 2 sol stable for more than 90 days. Transparent TiO 2 thin films were successfully produced by dip-coating from the TiO 2 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 TiO 2 sol developed for photocatalytic use (Ishihara STS-01), indicating that the excellent photocatalytic activity of original HyCOM-TiO 2 particles was preserved after immobilization on glass substrates by the present method.

TITANIUM(IV) OXIDE PHOTOCATALYST OF ULTRA-HIGH ACTIVITY FOR SELECTIVE N-CYCLIZATION OF AN AMINO ACID IN AQUEOUS SUSPENSIONS

Abstract Titanium(IV) oxide (TiO 2 ) powder was prepared by the high-temperature hydrolysis of titanium(IV) tetrabutoxide in toluene as anatase crystallites of average diameter ≈ 15 nm. They were platinized by impregnation from aqueous chloroplatinic acid solution followed by hydrogen reduction. The catalyst was suspended in an aqueous L-lysine (Lys) solution and photoirradiated under argon at ambient temperature to obtain L-pipecolinic acid (PCA). Among several platinized commercial TiO 2 powders, the present TiO 2 showed the highest photocatalytic activity for both Lys consumption and PCA production. This result is attributed to both the higher activity owing to the larger surface area with lesser defects and the higher dispersion of platinum deposits acting as an efficient reduction site for a Schiff base intermediate into PCA.