Kinetic probes of the mechanism of polyoxometalate-mediated photocatalyticoxidation of chlorinated organics

The present study reports the development of kinetic probes to determine th e possible role of hydroxyl radicals in the polyoxometalate-mediated photoc atalytic oxidation of aqueous chlorinated organic compounds. Photocatalytic oxidation of the model chloroaromatic 1,2-dichlorobenzene (DCB) was carrie d out in oxygenated, illuminated solutions of three contrasting polyoxometa lates (POMs), H2NaPW12O40, H4SiW12O40, or H3PMo12O40 The observed rate of D CB is pseudo-first-order in [DCB](0) for all POMs over a concentration rang e of 2-20 muM, which was modeled by Langmuir-Hinshelwood-type kinetics. The effect of the HO. scavengers Br-, 2-propanol, acetone, and acetone-d(6) on the rate of DCB oxidation was also evaluated in order to test the validity of the possible mechanisms that may account for the observed kinetic behav ior. On the basis of their ability to reduce k(exp), the effectiveness of t he scavengers is ranked as i-prOH > Br- > acetone = acetone-d(6). This cont rasts sharply with the ranking suggested by their intrinsic rate constants with HO.: Br- > i-prOH 1 acetone, acetone-d(6). This discrepancy is explain ed by the hypothesis that interfacial electron transfer, rather than HO.-me diated oxidation, accounts for the loss of DCB.