Photocatalytic properties of titanium based mixed oxides prepared by sol-gel and gas-phase synthesis

Abstract

Titanium based photocatalyst was prepared by the sol-gel technique and the gas-phase reaction. The photoactivity of prepared photocatalysts was investigated by the decomposition of trichloroethylene as a model reaction. X-ray diffraction, BET, SEM or TEM photographs, FT-IR, UV/visible absorption, ESR, and photoluminescence(PL) spectroscopy were used to characterize all prepared catalysts. It was tried to determine the priority in parameters affecting the photoactivity of titanium based photocatalyst. For nanophase $TiO_2$ prepared by the sol-gel method, it was found that the heat treatment at high temperature without forming rutile phase is a key to the high photocatalytic activity due to the increase of crystallinity and the reduction of bulk defects. From the analysis of ESR and PL spectra, it was clear that the surface active site like $O^-$ is produced and leads the enhancement of photoactivity with increasing the calcination temperature. The crystallite size of titania particle was controlled at a fixed particle size and surface area by the spray pyrolysis. The photoactivity of non-porous titania particles increased monotonically with increasing the crystallite size. In further investigation on the photoactivity of nanometer size titania prepared by the gas-phase pyrolysis of titanium-tetra-isopropoxide, it was confirmed that the crystallinity is more important than the surface area to enhance the photoactivity for TCE decomposition regardless of the preparation method. The photoactivity of nano-size titania prepared by the gas-phase oxidation of $TiCl_4$ is proportional to the relative density of surface OH groups regardless of the content of rutile phase. How much surface OH groups exist on the surface was the primary factor for determining the photoactivity of nano-size titania particles. It was also found that the direct preparation of anatase phase of large crystallite size without any further post heat treatment is crucible to get a high photoacti...