On the mechanism of photocatalytic degradation of chlorinated phenols to CO2 and HCl by polyoxometalates

Photolysis of homogeneous solutions of polyoxometalates W10O324-, PW12O403-, and SiW12O404- with near visible and UV light in presence of chlorophenols, leads to complete decomposition of substrates to CO2 and HCl. At the initial stages of photooxidation, decomposition rates, in presence and absence of dioxygen, practically compete with each other. On the contrary, at the final stages of decomposition, the rate in presence of dioxygen is an order of magnitude faster than in absence of dioxygen. The main function of dioxygen seems to be the reoxidation (regeneration) of the catalyst. Several hydroxyl intermediates justify the formation of OH radicals. The hydroxyl aromatic derivatives detected are in accordance with the electrophilicity of OH radicals. Hydroxy butanedioic acid and acetic acid were detected in the three monochlorophenols, prior to mineralization.