CHARACTERIZATION OF TIO(2) PHOTOCATALYSTS USED IN TRICHLOROETHENE OXIDATION

Kinetic studies show deactivation of TiO2 catalysts during aqueous-pha se 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 produc ts adsorb on the TiO2 surface. Used and deactivated catalysts were ana lyzed after participating in either aqueous-phase or gas-phase photoox idation of TCE. The XPS spectra showed little difference between the s urface composition of fresh TiO2 and that of a deactivated catalyst fr om the aqueous-phase photoreactor. Chlorine was observed only on catal ysts used in the gas-phase photocatalytic decomposition of TCE. Differ ences due to photoreaction were observed in TPD spectra of water, carb on monoxide, and carbon dioxide. Both the total amount desorbed and th e temperature of desorption of carbon monoxide and carbon dioxide were quite different for used and deactivated catalysts from the two photo reactions. Apparently strongly bound species, such as carbonates, accu mulated on the surface and formed carbon monoxide upon high-temperatur e 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 st rongly on TiO2 and readily displaces TCE. Thermally decomposed DCAC re duces the number of available adsorption sites for DCAC and TCE. An in teresting low-temperature oxygen desorption peak was observed from cat alysts treated with H2O2, which improves catalytic activity. This feat ure indicates that H2O2 is stable on TiO2 at room temperature and deco mposes at 420 K.