Photocatalytic degradation of pentachlorophenol on titanium dioxide particles: identification of intermediates and mechanism of reaction

The photoassisted oxidation of pentachlorophenol (PCP) in TiO_2 particle suspensions was investigated. Complete dechlorination of 47 µM PCP was achieved after 3 h of illumination at high intensity with apparent quantum efficiencies (ΦPCP, ΦCI, ΦH+, ΦH_2O_2) ranging from 1 to 3%. p-Chloranil, tetrachlorohydroquinone, H_2O_2, and o-chloranil were formed as the principal intermediates. Formate and acetate were formed as products during the latter stages of photooxidation. The mechanism for photooxidation of PCP proceeds via hydroxyl radical attack on the para position of the PCP ring to form a semiquinone radical which in turn disproportionates to yield p-chloranil and tetrachlorohydroquinone. Under high-intensity illumination, the reaction intermediates are attacked further by •OH to yield HCO_2^-, CH_3CO_2^-, CO_2, H^+, and Cl^-.