Catalytic hydrolysis of dichlorodifluoromethane (CFC-12) on sol-gel-derived titania unmodified and modified with H2SO4

Catalytic hydrolysis of dichlorodifluoromethane (CFC-12) in a humid air stream was studied over pure and sulfate promoted TiO2 catalysts which were prepared by sol-gel methods. The results showed that complete conversion of CFC-12 on unmodified TiO2 was achieved at reaction temperatures higher than 340 degrees C under the employed reaction conditions, The selectivity to CO2 ([CO2](produced)/[CFC-12](converted)) varied from 0.5 to 0.88 over the range of 250-350 degrees C and CClF3 (CFC-13) was detected as the main by-product. Surface fluorination of the TiO2 catalyst during the reaction improved its activity, induced the formation of the fluorinated by-product CFC-13, and changed such properties of the catalyst as its specific surface area,pore size distribution, and crystal size. It was found that the catalytic and structural properties of TiO2 were greatly improved by sulfation. The sulfated TiO2 (TiO2/SO42-) exhibited excellent reaction activity and selectivity for CFC-12 catalytic decomposition at low reaction temperatures (190-250 degrees C) while retaining a stable structure. Complete decomposition of CFC-12 with stoichiometric production of CO2 was observed over the TiO2/SO42- catalyst at 250 degrees C under otherwise identical reaction conditions as used for pure TiO2. Results of catalyst characterization by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and N-2 sorption analysis indicated that surface sulfate species formed on the sulfated TiO2. When compared to unmodified TiO2, the sulfated TiO2 exhibited higher resistance to crystal phase transformation from anatase to rutile, higher resistance to the deleterious effects of fluorination of the catalyst, and higher specific surface area. Fluorination of the catalyst did not improve the activity of sulfated TiO2 and no CFC-13 was detected as a by-product, indicating that fluorine was not involved in the formation of reaction sites over the sulfated TiO2 catalysts. (C) 1997 Academic Press.