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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.

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.

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.

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.