EXPERIMENTAL AND THEORETICAL-STUDY OF THE REACTION-MECHANISM OF THE PHOTOASSISTED CATALYTIC DEGRADATION OF TRICHLOROETHYLENE IN THE GAS-PHASE

The photoassisted catalytic degradation of trichloroethylene (TCE) in the gas phase was studied using a packed bed reactor with TiO2 pellets prepared by sol-gel techniques. A primary product of the reaction was monochloroacetic acid at 23 degrees C while TCE was completely minera lized to CO2 and HCl at 64 degrees C. Ab initio molecular orbital calc ulations served to elucidate the mechanism of the TCE degradation. Acc ording to the frontier molecular orbitals of TCE, on the catalyst surf ace the OH radical attacks preferentially at the CCl2 side of TCE, pat h A. The formation of the radical intermediate CHClCCl2(OH) was estima ted to be exothermic by -41.0 kcal mol(-1). The large stabilization en ergy is in agreement with the experimental results that TCE is degrade d very rapidly and the reaction was not dependent on the temperature o f the reaction. A Cl radical elimination from the intermediate produce s 1,2-dichloroethenol, which changes its form to monochloroacetyl chlo ride. This is the so-called keto-enol tautomerism with the large activ ation energy of 53.3 kcal mol(-1) The water assists by lowering the ac tivation barrier of the tautomerism by 21.5 kcal mol(-1). Hydrolysis o f monochloroacetyl chloride or its reaction with a OH radical produces monochloroacetic acid, the primary product pf our catalytic system. T he addition of OH radical to the CHCl side favorably occurs in the gas phase, path B, and forms a radical intermediate CHCl(OH)CCl2. Then, i t releases a Cl radical to form 2,2-dichloroethenol followed by the ta utomerism to produce dichloroacetaldehyde. The activation energy of th e reaction turned out to be as large as 64.2 kcal mol(-1). Such a high activation barrier is responsible for the previous observation that o nly 2,2-dichloroethenol was detected in the homogeneous gas phase degr adation of TCE.