Photoactivity of titanium dioxide supported on MCM41, zeolite X, and zeolite Y

Titanium dioxide supported on microporous zeolites of type X and Y and on mesoporous molecular sieves of the MCM41 type was studied for the photocatalytic degradation of acetophenone in an aqueous medium. The photoactivity of the supported catalyst is strongly influenced by the method of titania loading, but less affected by the temperature at which the sample was calcined. The highest photoactivity among the supported catalysts is observed for a support that has a lower Si/Al ratio in the framework and relatively large pore size. The measured apparent activation energies for photocatalyzed consumption of acetophenone for a commercial TiO2 (P-25) and 10 wt % TiO2-loaded samples of zeolite A, X, Y, and Al-MCM41 are 4.2-6.7, 16.9, 13.5, 32.2 and 3.6 kJ/mol, respectively. Selective doping by Fe, Mn, and V into the framework of MCM41 suppresses the photoactivity of the supported titanium dioxide. The physical state of the titanium dioxide on the supports is characterized by XRD, adsorption and pore size analysis, IR, and Raman spectroscopy. All methods emphasize the small particle or amorphous character of the TiO2. The normal phase transition to rutile does not occur at higher temperatures on zeolite supports. In some cases, very low loadings of TiO2 appear to achieve total absorption of the light entering the reaction vessel. For the Al-rich MCM41-supported catalysts, maximal photoactivity can be achieved at low TiO2 loading (13%). It appears to absorb 100% of light entering the reaction vessel with a much lower TiO2 load than is the case with TiO2 itself. There is evidence that the crystallinity of the zeolite is an important factor in photocatalytic efficiency, but the mechanism of zeolite participation in reactions remains incompletely elucidated.