Preparation of polycrystalline TiO2 photocatalysts impregnated with various transition metal ions : characterization and photocatalytic activity for the degradation of 4-nitrophenol

摘要

A set of polycrystalline TiO2 photocatalysts loaded with various ions of transition metals (Co, Cr, Cu, Fe, Mo, V, and W) were prepared by using the wet impregnation method. The samples were characterized by using some bulk and surface techniques, namely X-ray diffraction, BET specific surface area determination, scanning electron microscopy, point of zero charge determination, and femtosecond pump−probe diffuse reflectance spectroscopy (PP-DRS). The samples were employed as catalysts for 4-nitrophenol photodegradation in aqueous suspension, used as a probe reaction. The characterization results have confirmed the difficulty to find a straightforward correlation between photoactivity and single specific properties of the powders. Diffuse reflectance measurements showed a slight shift in the band gap transition to longer wavelengths and an extension of the absorption in the visible region for almost all the doped samples. SEM observation and EDX measurements indicated a similar morphology for all the particles, and a not homogeneous distribution of the metal species onto the surface of catalyst particles. The impregnated samples revealed recombination rates always higher than that of bare TiO2. The photoactivity of TiO2 was reduced by the presence of transition metal ions with the exception of W, which instead played a beneficial role. The results of femtosecond pump−probe diffuse reflectance spectroscopy appear quite in accord with the observed photocatalytic activity only for the lowest values of electron−hole recombination rate of the samples.