Photocatalysis by illuminated titania: Oxidation of hydroquinone and p-benzoquinone

Photocatalytic oxidation of hydroquinonc and p-benzoquinone, the main intermediates which are formed in the process of phenol oxidation on irradiated titania, was studied. The reaction proceeds via several steps. Some of the intermediates were detected by HPLC and GC-MS: ethanedial, glycerol, and 1,2,4-trihydroxybenzene. Under illumination in the presence of TiO2, p-benzoquinone initially transforms partly to hydroquinone, partly oxidizes to unidentified intermediates. The residue, about 4% of the concentration of hydroquinone, undergoes slow oxidation together with the hydroquinone. The rate of hydroquinone photooxidation shows 1st order behaviour.