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Rutile TiO2 nanorods with lengths greater than 600 nm and aspect ratios greater than ca. 16 were synthesized through a one-pot hydrothermal method using lactic acid (LA) as a structure-directing agent. Under the hydrothermal treatment at 200 °C, the LA concentration higher than 1.6 mol dm–3 and the hydrothermal time of 72 h were needed to obtain 100% rutile nanorods. The length and the width of the nanorods increased with the increasing LA concentration. The photocatalytic activity of the synthesized nanorods was evaluated for the oxygen evolution in aqueous AgNO3 solutions under ultraviolet irradiation. Calcination of the synthesized nanorods at 400 °C was required to decompose residual organic compounds on the surface and improve the oxygen evolution. The highest oxygen evolution rate was obtained with the nanorods after being calcined at 800 °C. It is worth noting that the nanorods retained their shape (aspect ratio of 8.8) at 800 °C. Selected area electron diffraction patterns indicated that the side or the end surface of the nanorods was attributable to the {110} or {111} facet, respectively. Deposition of Pt or PbO2 on the nanorods revealed that the {110} or {111} facet acted as reductive or oxidative sites. For comparison, near-spherical TiO2 nanoparticles were synthesized by a sol–gel method. Furthermore, using glycolic acid as the structure-directing agent, we synthesized small rutile TiO2 nanorods (aspect ratio of 9) and changed the shape to near-spherical (aspect ratio of 1.3) by calcining at 800 °C. Time-resolved diffuse reflectance spectra were measured to determine the lifetime of the photogenerated electrons. The photocatalytic activity of the nanorods was much lower than that of the near-spherical TiO2 nanoparticles. However, the nanorods synthesized with LA are useful as catalyst support or platforms for various applications because of their unique morphology and high heat resistance.

Photocatalysis is a promising technique for developing sustainable and environmentally friendly materials to improve indoor air quality. Visible-light-responsive TiO2 has been widely investigated but there are inconsistent results because photocatalytic properties depend strongly on synthetic methods. Herein, we synthesize TiO2 doped with 10 different metal ions (M–TiO2) by conducting a dialysis in a sol–gel method to obtain the best photocatalyst for the degradation of acetaldehyde under LED irradiation. Purification of a sol by dialysis enables us to discuss pure effects of dopants on the photocatalytic activity because impurities such as counter ions of metal salts are removed before sintering. Only Cr–, Pt–, V–, and Fe–TiO2 show photocatalytic activity and the optimal doping amounts are 0.50–1.7, 0.10, 1.0, and 0.10 atom%, respectively. Such differences in the optimal amounts can be explained in terms of the dopant ions having different valence states, suggesting the formation of oxygen vacancies. The Cr–TiO2 powder exhibits high activity even at the doping amount of 4.2 atom%. We also demonstrate that the Cr–TiO2 film prepared on a glass substrate can be used to degrade acetaldehyde by changing the film thickness and the LED intensity depending on the degree of the indoor contamination.

TiO2 nanorods have been widely investigated for various applications as catalyst support, photoanode, bioanalytical platform, and photocatalyst because of their unique properties due to the high le...

Porous titanium dioxide pellets modified with copper(II) ion (Cu-TiO2) were synthesized by sol-gel method with dialysis for photocatalytic degradation of gaseous trichloroethylene (TCE) under visible light (VL) irradiation. TCE was completely degraded by passing the gas stream (mole fractions of oxygen and TCE were 0.2 and 1.75 × 10−4, respectively) at the flow rate of 25 mL min−1 through 0.2 g of the Cu-TiO2 pellets (Cu content: 0.1 atom%) calcined at 200 °C. TCE was converted mainly to carbon dioxide, dichloroacetic acid (DCAA), and inorganic chlorine species. Relatively small quantities of pentachloroethane (PCA) and trichloroacetaldehyde (TCAH) were detected as products on the Cu-TiO2 surface. Comparison with porous TiO2 pellets under ultraviolet irradiation revealed that more chlorinated products and less carbon dioxide were formed on Cu-TiO2 under VL irradiation. The mineralization of TCE to carbon dioxide was calculated to be only ca. 30.0%. It is noted that DCAA, PCA and TCAH were accumulated on the surface and were extracted with ethyl acetate. The porous Cu-TiO2 pellets show promise as the photocatalyst acting under VL irradiation for converting TCE gas to chlorinated compounds which can be used in industries.

There are many factors affecting TiO2 photocatalysis, such as specific surface area, crystal phase, crystallite size, and crystallinity. However, there is controversy regarding the most important factor among them. In this chapter, we prepared TiO2 by conducting dialysis in a sol–gel synthesis and investigated the photocatalytic activity. Purification of TiO2 sol by dialysis minimizes the effect of impurities originating from raw materials on the crystallization of TiO2 during the thermal treatment. As a result, we reveal that fine anatase powder with high crystallinity is the most appropriate for the degradation of 4-chlorophenol in water, whereas the crystallite size is the key factor determining the oxygen evolution from water. In the case of visible light–responsive TiO2 doped with metal ions, the photocatalytic activity is controlled by the metal ion dopants having different valence states. These factors are related to the efficiency to suppress the recombination of the photogenerated charge carriers.

We synthesized rutile TiO2 nanorods with high crystallinity, i.e., degree of crystallinity greater than ca. 90%, by using a hydrothermal method and studied the effect of the morphological change from rod shape to subsphere by the subsequent thermal treatment. The photocatalytic activity was estimated for O2 evolution from water oxidation because this reaction was hardly affected by the specific surface area of photocatalysts. Thermal desorption and gas chromatography–mass spectrometry measurements suggested the decomposition of residual glycolic acid in TiO2 nanorods at 272 and 590 °C. Observation with scanning electron microscopy and physical properties such as specific surface area and crystallite size indicated deformation of the rod shape to subsphere by the calcination above 600 °C. Time-resolved microwave conductivity measurements have revealed that the TiO2 nanorods possess the longest lifetime of photogenerated electrons among all samples. However, the O2 evolution rate of the TiO2 nanorods was mu...

Titanium dioxide doped with Cr ions (Cr–TiO2) was synthesized by a sol–gel method, with only water as the solvent, and dialysis. For Cr–TiO2 sintered at 200 °C, ca. 90% of 4-chlorophenol (4-CP) was degraded at Cr doping amounts of 0.68–1.3 atom% under visible light irradiation. At doping amounts above 1.7 atom%, the 4-CP degradation ratio increased for Cr–TiO2 sintered at 300–500 °C. X-ray photoelectron spectroscopy and X-ray absorption near edge structure measurements showed that only Cr(III) was present on the surfaces of Cr–TiO2 samples sintered at 200 or 400 °C and some of the Cr(III) in the bulk was oxidized to Cr(VI) by sintering at 400 °C. The 4-CP degradation was enhanced with increasing Cr(VI)/Cr(III) ratio in the bulk, suggesting the spatial separation of Cr(III) on the surface under visible light irradiation: electrons are excited from Cr(III) to the conduction band of TiO2 and then trapped at the doped Cr(VI) whereas the photogenerated Cr(IV) oxidizes 4-CP. Time-resolved diffuse reflectance spectroscopy was used to evaluate the lifetimes of the photogenerated electrons. The highest photocatalytic activity was obtained with ca. 0.8 atom% Cr–TiO2 sintered at 200 °C. This high activity is ascribed to the presence of oxygen vacancies, which act as electron-trapping sites.

The present paper reports the development of TiO2 photocatalysts adsorbed with squaraine dyes for the utilization of visible light. We synthesized 1,3-bis(3,3-dimethyl-2-methylene indolenine)squaraine (ISQ) and its derivatives having two COOH (ISQ-(COOH)2), COOH and COOC4H9 (ISQ-COOHCOOBu) and two COOC4H9 groups (ISQ-(COOBu)2) at both ends. We used porous TiO2 which was synthesized by a sol–gel method and possessed a specific surface area of 229 m2 g−1. ISQ without an anchoring group of COOH hardly adsorbed and the number of the adsorbed ISQ-(COOH)2 or ISQ-COOHCOOBu per gram of TiO2 was estimated to be (5.53 ± 0.74) × 10-5 or (5.93 ± 0.68) × 10-5 mol g−1, respectively. Photocatalytic degradation of methyl orange (MO) was performed on TiO2 adsorbed with ISQ-(COOH)2 or ISQ-COOHCOOBu in the range of (0.12–5.4) × 10-5 or (0.15–6.2) × 10-5 mol g−1. The degradation rate increased drastically with an increase in the adsorbed amounts of these dyes less than 1.7 × 10-5 mol g−1 but then increased gradually up to ca. 6 × 10-5 mol g−1. The electrochemical measurements revealed that the photogenerated electron was transferred from these dyes to the conduction band of TiO2. No MO was degraded on TiO2 adsorbed with ISQ-(COOH)2 in the nitrogen atmosphere under VL irradiation. We proposed the degradation mechanism of MO on TiO2 adsorbed with ISQ-(COOH)2 or ISQ-COOHCOOBu.

Titanium dioxide doped with the Pt ion (Pt–TiO2) was synthesized by a sol–gel method using only water as the solvent and conducting dialysis. The photocatalytic activity for the degradation of 4-chlorophenol (4-CP) on Pt–TiO2 was not affected by the Brunauer–Emmett–Teller specific surface area under visible light (VL) irradiation. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure measurements revealed that only the Pt(IV) ion existed in the TiO2 bulk and both Pt(II) and Pt(IV) were present near the Pt–TiO2 surface. Pt(IV) is most likely substituted in the Ti(IV) site of the TiO2 lattice because of their similar ionic sizes. Quantitative analysis of Pt(II) was performed in the XPS measurements, indicating that the amount of Pt(II) on the surface increased with an increase in the doping amount from 0.2 to 1.0 atom %. We synthesized 0.5 atom % Pt–TiO2 with various Pt(II)/Pt(IV) ratios by changing the Ti(OC3H7)4 concentration used in the sol–gel synthesis. The 4-CP conversion on Pt–TiO2 increased linearly with an increase in the Pt(II)/Pt(IV) ratios. A similar relationship was obtained with Pt–TiO2, which was prepared by a conventional sol–gel method in ethanol–water mixed solvent. Therefore, the Pt(II)/Pt(IV) ratio is a key factor affecting the photocatalytic activity of Pt–TiO2 under VL irradiation. Our results indicate that controlling the mixed valence states of the doped metal ions is a new strategy to developing highly active metal-ion-doped TiO2 under VL irradiation.

Porous platinum ion-doped TiO2 (Pt–TiO2) was prepared by a sol–gel method and demonstrated to have superior photocatalytic activity for the photodegradation of gaseous trichloroethylene (TCE) under visible light (VL) irradiation from a xenon lamp equipped with 422-nm cut-off filter. Kinetic studies were performed to clarify the effect of the doping amounts, space times, VL intensity, and mole fractions of TCE, O2, and H2O on the degradation of TCE. Under ultraviolet (UV) irradiation, the photocatalytic activity of Pt–TiO2 was the same as that of TiO2, indicating that the doped Pt ion did not act as a recombination center for the photogenerated holes and electrons. Based on the kinetic data and reaction products, we conclude that the photocatalytic degradation of TCE on Pt–TiO2 under VL irradiation proceeds similarly to TiO2 under UV irradiation. We also performed the photocatalytic degradation of TCE at the space time of 7.5 × 107 g s mol−1 in a tubular reactor packed with the Pt–TiO2 pellets which are more suitable than the Pt–TiO2 powder for the practical remediation of the contaminated gas. TCE was completely degraded, i.e. 100% conversion was achieved under VL irradiation but only a small quantity of CO2 was formed with the stoichiometric ratio of [CO2]formed/[TCE]degraded of ca. 0.33. By switching the gas stream containing TCE to humid air, more CO2 was formed, indicating that the dichloroacetates accumulated on the Pt–TiO2 surface are photodegradable to CO2 under VL irradiation.

TiO 2 modified with Cu 2+ or Cr 3+ ion (M-TiO 2 , M-grafted TiO 2 ) was synthesized by a sol-gel method using Ti(OC 3 H 7 ) 4 as a starting material or a graft method by impregnation of rutile TiO 2 . In the sol-gel method, the Brunauer-Emmett-Teller (BET) surface area of M-TiO 2 was varied by preparing with or without dialysis of TiO 2 sol (M-TiO 2 (D) or M-TiO 2 (ND), respectively). Even by using the sol-gel method, Cu 2+ whose ionic size is larger than Ti 4+ cannot be doped but is incorporated in TiO 2 due to relatively weak interaction. Dissolution of Cu 2+ ion was observed when Cu-TiO 2 (D), Cu-TiO 2 (ND) or Cu-grafted TiO 2 was immersed into the acidic solution of 0.1 mol dm −3 HCl. The photocatalytic activity for the degradation of 4-chlorophenol (4-CP) under visible light irradiation was the following order: Cu-grafted TiO 2 > Cu-TiO 2 (ND) > Cu-TiO 2 (D). We have revealed that the photocatalytic activity of Cu-grafted TiO 2 decreases linearly with an increase in the number-density of Cu 2+ ion on the TiO 2 surface. On the other hand, in the case of Cr 3+ ion which has a similar size as Ti 4+ , the photocatalytic activity increased remarkably by doping Cr 3+ in TiO 2 by the sol-gel method. An increase in the BET surface area by conducting dialysis enhanced the photocatalytic activity. This paper demonstrates from the viewpoint of the higher photocatalytic activity as well as the chemical stability in acidic conditions that Cr-doped TiO 2 prepared by the sol-gel method gives an advantage over Cu-grafted TiO 2 for the degradation of 4-CP which is one of the non-degradable organic contaminants in water.

The effect of physicochemical properties such as specific surface area, crystalline phase, crystallite size, and crystallinity of TiO2 on photocatalytic water oxidation was investigated. Two types of polycrystalline TiO2 samples which have different physicochemical properties were prepared by a sol–gel method. The photocatalytic activities of the TiO2 samples for water oxidation were evaluated by the O2 evolution rate from an aqueous silver nitrate solution under ultraviolet light irradiation. Comparison of the two types of TiO2 samples revealed that the specific surface area and crystalline phase were not related to the O2 evolution rate. In contrast, a linear relationship between the crystallite sizes and the O2 evolution rates was observed for most of the TiO2 samples. Crystal face selective Pt and PbO2 depositions on the TiO2 samples were observed by photocatalytic reduction of PtCl62− and oxidation of Pb2+ ions, respectively, indicating that the oxidation and reduction sites are separated on the surface. An increase in the crystallite size of the TiO2 promotes a spatial separation of the redox sites and suppresses the electron–hole recombination, leading to the enhancement of the photocatalytic activity.

We fabricated 5,10,15,20-tetra(4-pyridyl)porphyrinatozinc(II) (ZnTPyP) fibers by utilizing surfactant-assisted method with cetyltrimethylammonium bromide (CTAB). The ZnTPyP fibers became longer as molar ratio of CTAB to ZnTPyP (γ) increased. In the case of γ ≥ 50, the Soret band of ZnTPyP monomer was observed after stirring for 1 h and was split into two peaks after the aging at 40 °C for more than 5 days. On the other hand, such split peaks were observed without the aging when the γ value was 10 or 20. These findings suggest that CTAB interacts with the ZnTPyP monomer in aqueous solution to form micelles, and the fibers are gradually constructed by self-assembling the stabilized ZnTPyP monomer. Photocatalytic activity of the ZnTPyP fibers was evaluated for the degradation of rhodamine B (RhB) in water under visible light (VL) irradiation. The highest photocatalytic activity was obtained at γ = 150–300, indicating that the fibers with 1.3–2.0 μm were the most suitable as photocatalyst. When air was bubbled in the reaction solution, the photocatalytic activity was higher than that under N2 bubbling. However, absorption peak attributed to the ZnTPyP fibers decreased under air bubbling although it remained almost constant under N2 bubbling. These results suggest that the ZnTPyP fibers can act as photocatalyst and active oxygen species such as O2•− radical might decompose the ZnTPyP fibers. Kinetic studies were performed to clarify the degradation mechanism of RhB under the experimental condition where the decomposition of the ZnTPyP fibers was negligible.

Aiming at developing a highly active photocatalyst to decompose organic compounds under visible light irradiation, we have prepared metal ion doped TiO 2 (M-TiO 2 ) by conducting dialysis in sol–gel method. Dialysis leads to the formation of mesoporous materials with surface area larger than 200 m 2 g −1 but the metal ions should be added into the TiO 2 sol at pH 3 in order to avoid their losses during dialysis. Among seven metal ions tested as dopants, Pt-TiO 2 and Cr-TiO 2 showed higher photocatalytic activity for the degradation of 4-chlorophenol under visible light irradiation. The photocatalytic activity of Cr-TiO 2 was strongly affected by the doping amount of Cr ion and the optimal value was evaluated to be 0.68–1.30 atom%. X-ray photoelectron spectroscopy analyses showed the Cr ion was present mainly in Cr(III). The Cr-TiO 2 can be used safely for a practical application to remediate environmental contamination instead of expensive Pt-TiO 2 .

Photocatalysis of a transparent aqueous WO 3 sol was investigated for the color-fading of Orange II. Dependence of the degradation rate of Orange II on pH and ionic strength of the WO 3 sol was explained in terms of the electrostatic interaction between Orange II and the WO 3 colloid particles. The degradation rate follows the first-order kinetics with respect to the concentration of Orange II. It was demonstrated that the photogenerated electron in WO 3 reduced O 2 effectively since the photochromism of the WO 3 sol observed under nitrogen atmosphere did not occur under air-saturated condition. The photocatalytic activity of WO 3 was lower than that of TiO 2 by a factor of 0.5 and was enhanced by mixing with the TiO 2 sol. The best photocatalytic performance for the degradation of Orange II was obtained by using the mixed sol with WO 3 :TiO 2 molar ratio of 1:2. Similar enhancement in the photocatalytic activity was confirmed for the degradation of 4-chlorophenol.

Kinetic studies on the photocatalytic degradation of trichloroethylene (TCE) were performed in TiO 2 or WO 3 aqueous sol which consisted of anatase nanocrystalline TiO 2 of ca. 3.9 nm or WO 3 spherical particles of ca. 17.6 nm. Photocatalytic activity of TiO 2 colloid is higher than that of WO 3 by a factor of 2.1. Kinetic analyses for both sols give a similar rate law containing γ value which represents a ratio of the reaction rate of the photogenerated electrons with O 2 against the total disappearance rate of the photogenerated electrons. Reaction temperature did not affect the TiO 2 photocatalysis whereas higher temperature increased the oxidation rate of TCE in the WO 3 sol. Such a difference in the temperature effect was explained in terms of the γ values. The photocatalytic degradation of TCE was enhanced by mixing the WO 3 and the TiO 2 sol and the best performance was obtained by using the mixed sol with WO 3 :TiO 2 molar ratio of 1:2. In the mixed sol, the similar temperature dependence as in the WO 3 sol was obtained. Reaction mechanism and the rate law are proposed to account for the obtained kinetic data.

Porous platinum-ion-doped TiO2 (Pt-TiO2) with BET surface area higher than 200 m2 g−1 was prepared by using water as solvent and conducting dialysis in a sol–gel method and demonstrated to have superior photocatalytic activity for the photodegradation of 4-chlorophenol (4-CP) under both ultraviolet and visible light (VL) irradiation. During dialysis, 76.5% of Cl− was removed and mesopores were formed in Pt-TiO2. Measurements of pore-size distribution indicated a peak at ca. 3 nm after the calcination of Pt-TiO2 at 200 °C. Optimal conditions to prepare Pt-TiO2 acting under VL were 0.5 at.% as the Pt-doping content and 200 °C as the calcination temperature in the range of 200–500 °C. Kinetic studies were performed to clarify the effect of the amount of Pt-TiO2, initial concentration of 4-CP, intensity and wavelength of the incident light, pH and temperature on the reaction rate and to obtain the rate law under VL irradiation. Formation of benzoquinone and hydroquinone as intermediates was observed. They were further photodegraded on Pt-TiO2 and 90.7% of 4-CP was completely mineralized at the VL irradiation of 390 min.

Cu-deposited TiO2 films were prepared by photoreduction of Cu(II) in the presence of sodium formate. With the initial Cu(II) concentrations more than 100 mg L−1, induction periods were observed before the transmittance decreased. Scanning electron microscopy indicated that Cu particles of 2.6 ± 0.5 μm were deposited isolatedly with much open space in the induction periods. The films prepared by changing the irradiation time within the induction periods showed a higher photocatalytic activity than a pure TiO2 for the degradation of methylene blue under the reaction condition without purging air.

TiO2 particles encapsulated in hollow silica shells with controlled shell thicknesses were fabricated. Compared to naked TiO2 without coverage by a hollow silica shell, all of these composites showed low levels of photocatalytic activity for gas-phase decomposition of acetone into CO2. The activities over these composites tended to decrease with increase in thickness of the lateral silica shell. These results can be explained by a decrease in the diffusion of acetone with increase in thickness of the silica shell, leading to hampering of supply of the substrate to the surfaces of core TiO2 particles. In contrast, for the overall photodecomposition of gaseous 2-propanol into CO2, the presence of the lateral silica shell was found to be beneficial, i.e., the composite induced a high level of activity compared to the activity of naked TiO2. The enhancement of activity for this reaction was likely to be due to condensation of the intermediate product of acetone in the void spaces, leading to an increase in the collision probability between acetone and the surface of the TiO2 core.

The thermal treatment of TiO2 pellets prepared by the sol–gel method decreased the photocatalytic activity. The activity divided by the specific surface area of the pellets for the complete mineralization of ethylene or chloroform was maximized at the firing temperature of 400°C. For the photocatalytic degradation of trichloroethylene (TCE), most of them were converted to chlorinated by-products, such as dichloroacetic acid, chloroform, and phosgene, and the stoichiometric ratio of [CO2]formed/[TCE]degraded showed a maximal value at 400°C. The electron spin resonance (ESR) spin-trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in the flow injection system indicated that firing at 400°C gave the highest signal intensity of DMPO–OH adducts. These findings indicated that the OH radical was produced most effectively on the TiO2 fired at 400°C, which would be related to the content of anatase and rutile. Concerning the formation of chlorinated by-products from TCE, more dichloroacetic acid (DCAA) were detected and less CHCl3 and COCl2 were formed at lower firing temperatures, suggesting that the branching ratio of chloroethoxy radicals to the formation of DCAA or CHCl3 and COCl2 by C–C bond scission depended on the firing temperature.

Photocatalysis of a transparent aqueous sol, which was prepared by peptizing titanium tetraisopropoxide in nitric acid for 7 days followed by dialysis was examined for the color-fading of methylene blue (MB) and Orange II and for the degradation of 4-chlorophenol (4-CP). Measurements of XRD patterns of the powders obtained by drying the sol indicated the formation of anatase. Under argon-bubbling, MB was degraded via the formation of colorless leuco form and the color of the sol returned to original by bubbling air in the dark. Reaction rates for oxidative degradation of MB under air-bubbling showed maximum at the 0.013 mol-Ti dm −3 in the sol and at pH 4.2. Reaction rates for color-fading of Orange II were higher by a factor of 6.8 than that of MB. The dependence of rates on the concentrations of MB or Orange II in the sol was discussed with the rate law. For the degradation of 4-CP in the sol under the irradiation for 6 h, 92.7% of 4-CP was completely dechlorinated and 69.5% of the initial TOC was decreased.

1,4-Dioxane in hexane as a solvent was adsorbed on TiO2 due to an electrostatic interaction. The porous TiO2 pellets (SG) prepared by sol–gel method were superior adsorbent to ST-B21 and Degussa P-25. Effects of firing temperature of the pellets and the initial concentrations of 1,4-dioxane on the adsorption percents were examined. Photocatalytic degradation of aqueous 1,4-dioxane gave 1,2-ethanediol diformate and formic acid as intermediates. Analysis of total organic carbon indicated that 1,4-dioxane was mineralized effectively in the following order: P-25 > ST-B21 > SG. The photocatalytic degradation of 1,4-dioxane adsorbed on the TiO2 pellets in air showed that ST-B21 had a higher activity than SG. These facts indicate that SG pellet acts as a good adsorbent because of its high specific surface area but the internal region of the pores is not illuminated and acts only as a support.

The photocatalytic degradation of chloroform was studied in a tubular photoreactor packed with TiO2 pellets prepared by sol–gel method. Concentrations of chloroform and CO2 in the gas stream and Cl− ion which was dissolved from the pellets into water after the photodegradation experiments were detected quantitatively. Chloroform was completely mineralized to CO2 and HCl but the conversion decreased drastically with an increase in the irradiation time. Such a decrease in the catalytic activity was attributable to the accumulation of Cl− on the TiO2 surface and was recovered by washing the pellets with water. The presence of Cu and Ca(OH)2 increased the concentration of Cl− accumulated on the surface. The deterioration of the catalytic activity was suppressed with Ca(OH)2 whereas no appreciable effect was observed with Cu. The TiO2 prepared by sol–gel method exhibited a higher catalytic activity than commercially available Degussa P-25.

We have found that the use of sulfuric acid in the peptization process of sol–gel method produced SO 4 2− -incorporated TiO 2 which is mechanically strong by firing at low temperature such as 200°C. The synthesized TiO 2 has larger specific surface area and retards the phase transition from anatase to rutile compared with that prepared from the peptization with nitric acid. The S-content of the TiO 2 fired at 200°C was 1.52 wt.%; the value has a maximum of 2.74 wt.% at 400°C. The XPS measurements indicate that S exists mainly as SO 4 2− . The TiO 2 fired at 400°C showed the highest photocatalytic activity for ethylene degradation. Especially, we would like to emphasize the TiO 2 sintered at 200°C because they are extremely hard in spite of such a low sintering temperature. This photocatalyst may provide a great opportunity for extensive applications as self-supporting membranes.

The photocatalytic degradation of trichloroethylene and tetrachloroethylene in the gas phase, when porous TiO2 pellets were used, produced such undesirable chlorinated byproducts as chloroform (CHCl3) and carbon tetrachloride (CCl4). The formation of byproducts was reduced by increasing oxygen mole fractions in the feed gas stream and/or using the TiO2 pellets fired at a lower temperature. Theoretical calculations indicate that the Cl-radical-initiated reaction produces these byproducts. On the basis of the experimental and the theoretical results, a method to prevent the formation of the chlorinated byproducts is discussed.

The effects of alkoxides, solvents, and the peptization and dialysis procedures in the sol-gel synthesis of TiO2 on the degradation rates of ethylene were investigated.

The degradation of trichloroethylene (TCE, Cl2C=CHCl) and tetrachloroethylene (PCE, Cl2C=CCI2) in a gas stream from a soil vapor extraction (SVE) well was demonstrated with an annular photocatalytic reactor packed with porous TiO2 pellets in a field trial at the Savannah River Site in Aiken, SC. The TiO2 pellets were prepared using a sol‐gel method. The experiments were performed at 55 to 60°C using space times of 108 to 1010 g • s • mol‐1 for TCE and PCE. Chloroform (CHCl3) and carbon tetrachloride (CCl4) were detected as minor products from side reactions. On a molar basis, the amounts CCl4 and CHCl3 produced were about 2 and 0.2% of the reactants, respectively.

The photocatalytic degradation of chlorinated ethanes was studied in a tubular photoreactor packed with TiO(2) pellets prepared by sol-gel method. The steady-state condition was not obtained, but the deterioration in the photocatalytic activity was observed during the irradiation. Effects of mole fractions of water vapor, O(2), and C(2)H(5)Cl or C(2)H(4)Cl(2) and reaction temperature on the photodegradation of C(2)H(5)Cl or C(2)H(4)Cl(2) were examined, and these data were compared with those obtained by the photodegradation of chlorinated ethylenes. On the basis of the products detected with and without oxygen in the reactant's gas stream, we proposed the degradation mechanism. Measurement of diffuse reflectance infrared Fourier transform spectroscopy of pyridine adsorbed on the catalysts showed that decrease in the conversion for the photodegradation of C(2)H(5)Cl was attributable to the formation of Bronsted acid sites. Comparison of the data obtained with the TiO(2) and the sulfated TiO(2) (SO(4)(2-)/TiO(2)) pellets indicated that the photodegradation of C(2)H(5)Cl was suppressed by the presence of the Bronsted sites, but that of trichloroethylene was not affected. Such a difference is attributable to the adsorption process of these reactants on the acid sites on the catalyst surface.

A recirculating system of aqueous trichloroethylene (TCE) solutions through the packed bed reactor with TiO 2 pellets has been developed in order to mineralize TCE without difficulties for filtration and recovery of catalyst. The TiO 2 pellets prepared by sol–gel method have photocatalytic activity similar to commercially available PC-101 and PC-102 in the powder form and to ST-B11 pellets. In batch experiments with TiO 2 powders, Degussa P-25 is the most active photocatalyst, which indicates that specific surface area is not an important factor controlling the photocatalytic activity in aqueous solutions. The degradation rates of TCE in the recirculating system with TiO 2 pellets decreased in the presence of H 2 O 2 , while were remarkably accelerated by adding S 2 O 8 2− . The presence of S 2 O 8 2− ions more than 0.01 mol dm −3 completely suppressed hole-electron recombination and mineralized 50 ppm TCE with the 2 h irradiation. In a reactor without TiO 2 photocatalysts, TCE was photodegraded by SO 4 − radicals which produced by photodissociation of S 2 O 8 2− . The degradation rates increased with increase of the initial S 2 O 8 2− concentration. However, TCE was not mineralized but converted to intermediates which were slowly degraded to Cl − by continuing the irradiation.