CHARACTERIZATION OF TIO(2) PHOTOCATALYSTS USED IN TRICHLOROETHENE OXIDATION

Kinetic studies show deactivation of TiO2 catalysts during aqueous-phase and gas-phase photooxidation of trichloroethene (TCE). Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were used to examine adsorbed species on TiO2 photocatalyst surfaces after reaction, and TPD was used to determine how reactants and products adsorb on the TiO2 surface. Used and deactivated catalysts were analyzed after participating in either aqueous-phase or gas-phase photooxidation of TCE. The XPS spectra showed little difference between the surface composition of fresh TiO2 and that of a deactivated catalyst from the aqueous-phase photoreactor. Chlorine was observed only on catalysts used in the gas-phase photocatalytic decomposition of TCE. Differences due to photoreaction were observed in TPD spectra of water, carbon monoxide, and carbon dioxide. Both the total amount desorbed and the temperature of desorption of carbon monoxide and carbon dioxide were quite different for used and deactivated catalysts from the two photoreactions. Apparently strongly bound species, such as carbonates, accumulated on the surface and formed carbon monoxide upon high-temperature decomposition. Small amounts of chlorinated compounds desorbed from the used and deactivated catalysts following gas-phase photoreaction. Dichloroacetyl chloride (DCAC), a reaction intermediate, can adsorb strongly on TiO2 and readily displaces TCE. Thermally decomposed DCAC reduces the number of available adsorption sites for DCAC and TCE. An interesting low-temperature oxygen desorption peak was observed from catalysts treated with H2O2, which improves catalytic activity. This feature indicates that H2O2 is stable on TiO2 at room temperature and decomposes at 420 K.