Photocatalysis of supported titania on micro/mesoporous zeolites for the oxidation of organic compounds in water.

摘要

Titanium oxide supported on amorphous oxides of silica and alumina, on a micro-porous zeolites of type A, X, Y, ZSM-5, and on a mesoporous molecular sieve of MCM-41 and metal-substituted MCM41s were studied for the photocatalytic oxidation of acetophenone and 4-chlorophenol in an aqueous medium. The photoactivity of the supported catalyst is strongly influenced by the method of titania loading, but less effected by the calcination temperature. All supported catalysts have a higher photo-activity than unsupported TiO{dollar}sb2{dollar} prepared in parallel. In most cases, the photocatalytic efficiency increases with the Ti content in sample, approaching a limit at {dollar}sim{dollar}20 wt % TiO{dollar}sb2.{dollar} For the ZSM-5 or Al-MCM41-supported catalysts, however, there is little dependence of the photoactivity on the Ti loading, and maximal photoactivity is achieved at low TiO{dollar}sb2{dollar} content ({dollar}{dollar} Al-MCM41 {dollar}>{dollar} siliceous MCM41 {dollar}approx{dollar} Ti-MCM41 {dollar}>{dollar} zeolite A {dollar}>{dollar} silica {dollar}>{dollar} Al-rich zeolite Y {dollar}>{dollar} unsupported {dollar}>{dollar} alumina, and ZSM-5 {dollar}>{dollar} zeolite A. Higher photoactivity of the catalyst can be well correlated with the higher crystallinity of the zeolite support in sample. Selective doping by Fe, Mn, and V into the MCM41 framework suppresses the photoactivity of the supported titanium oxide.; The physical state of the titanium oxide on the supports (and the adsorbed acetophenone on the catalysts) is characterised by XRD, adsorption and pore size analysis, IR and Raman spectroscopy, and SEM. All methods emphasize the small particle or amorphous character of the TiO{dollar}sb2.{dollar} At low loading, the titanium oxide may be formed as a film on the support, as evidenced by the shifts of TiO{dollar}sb2{dollar} Raman vibrations, especially in the case of ZSM-5 supported catalysts. At high loading, however, the formation of anatase crystallites is clearly seen by both XRD and Raman analysis. The particle size estimated from a standard XRD equation varies with support, but all particles are less than 10 nm in diameter while unsupported TiO{dollar}sb2{dollar} has a particle diameter of 24 nm. The phase transition to rutile does not occur at higher temperatures. The particle growth upon calci-nation is rather slow, and the size changes by not more than 10 nm over the range from 150 to 730{dollar}spcirc{dollar}C. On the microporous zeolite supports, the titania particles principally cover the external surface, or are formed as a separate phase on the sample, whereas on the mesoporous support, MCM41, the supported titanium oxide can exist inside the meso-pores. The adsorbed organic molecules are located inside the pores of high-silica zeolites.; Photooxidation of the model organic substrate, acetophenone, in water via either direct photolysis or TiO{dollar}sb2{dollar} photocatalysis gives hydroxylated acetophenone as the main intermediate, which can experience further oxidation. (Abstract shortened by UMI.)