A simple synthetic method to prepare thermally stable La3+ doped mesoporous titania with highly crystallized walls and high photocatalytic activity is reported. The crystalline framework derives from the assembly of nano-anatase particles. La3+ doping is a critical factor to increase the stability and photocatalytic activity of mesoporous titania. The mesoporous materials werecharacterized by low angle and wide angle X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), N2 adsorption-desorption and UV-visible absorbance spectroscopy. Characteristic results clearly show that La3+ doped samples have highly regular mesoporous structures, and their crystallized walls are composed of nano-anatase particles. The photocatalytic activity of the prepared mesoporous titania powders was evaluated from an analysis of the photo-degradation of methyl orange. The results indicate that sample doped with 0.25 at% La3+, has the highest photocatalytic activity.

The complete photocatalytic oxidation of C2H4 with O2 into CO2 and H2O has been achieved on ultrafine powdered TiO2 photocatalysts and the addition of H2O was found to enhance the reaction. The details of the photocatalytic reaction have been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C2H4 withO2 intoCO2, CO, andH2O. The large surface area of the photocatalyst is one of the most important factors in achieving a high ef-ficiency in the photocatalytic oxidation of C2H4. The photo-formed OH species as well as Oˉ2 and Oˉ3 anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C2H4 with O2 into CO2 and H2O.

The characteristics of the fluorescence spectra of 2,5-bis(4-diethylaminophenyl)-1, 3, 4-oxadiazole (BDD) doped in Si-Ti binary oxides by the sol-gel method were investigated as a function of time in its transitions from sol to gel to xerogel. The intensity of the fluorescence spectrum decreases with the increase in the Ti content. A characteristic BDD excimer fluorescence was observed during the transition from gel to xerogel only in a mixture of tetraethyl orthosilicate (TEOS) and tetraisopropoxy titanium (TPOT) but not in pure TEOS, indicating that the presence of a tetrahedrally coordinated titanium ion species plays a significant role in the formation of the excimer. A possible model has been proposed for the entrapment process of BDD during the sol to gel to xerogel transitions.

In order to investigate the photocatalytic stability and/or lifetime of titanium oxide photocatalysts in detail, standard reference photocatalysts (JRC-TIO-2, -3, -4, and -5), of which the surface chemical and physical properties are well known, were studied for the photocatalytic decompositon of NO under a large-scale flow reaction system. In the first stage, the photocatalytic decomposition of NO proceeded with a high efficiency and continuous production of N2, O2, and N2O. However, the reaction efficiency gradually decreased and finally leveled off at constant values. These values were found to be in the order of JRC-TIO-4>-3>-5>-2. Anatase TiO2, with a large surface area and numerous OH groups, was found to exhibit a high efficiency for the decomposition of NO in the flow system. The optimal pretreatment and reaction conditions were observed at a pretreatment temperature around 573 Kunder a mixture of O2 and Ar. It was confirmed that the surface hydroxyl groups on the photocatalysts play an important role in the decomposition of NO under this flow system.

Mesoporous materials such as Ti-HMS, prepared at ambient temperature, were studied both under continuous flow and closed reaction systems for the photocatalytic decomposition of NO. The results indicate Ti-HMS to be a good photocatalyst with a continuous reactivity under prolonged irradiation. The direct decomposition of NO into N2, O2, and N2O at 275 K with a high selectivity for N2 and O2 in a closed system is noteworthy. In situ photoluminescence and EXAFS investigations show that the Ti exists in a tetrahedral coordination and is highly dispersed within the mesoporous material. The TPD studies, before and after the photocatalytic investigations, clearly indicate that NO molecules cannot be adsorb strongly on the surface of Ti-HMS photocatalysts, in contrast to the powdered TiO2 photocatalysts, where strong NO adsorption species have been observed. It is likely that the charge transfer excited state of the tetrahedrally coordinated titanium oxide species plays a significant role in the direct decomposition of NO.