Hydroxylation reactions induced by near-ultraviolet photolysis of aqueous titanium dioxide suspensions

Abstract

Aqueous suspensions of titanium dioxide (anatase) containing benzoic acid or sodium benzoate have been illuminated with near-ultraviolet light under a variety of experimental conditions. In the presence of oxygen, o-, m- and p-hydroxybenzoic acids are formed in the same isomeric distribution found for OH-radical attack on benzoic acid. When OH-radical scavengers are present the yield of o-hydroxybenzoic acid is depressed. Ratios of rate constants evaluated on the basis of competition kinetics for the OH radical are in agreement with published rate constants. In the absence of oxygen the yield of the hydroxy compound is quite low, but high yields can be restored by the addition of iron(III). In oxygenated slurries, peroxides are formed in initial yield equivalent to about half the yield of total hydroxybenzoic acids. The addition of hydrogen peroxide to the slurries makes no significant difference to the yield of o-hydroxybenzoate. The results are consistent with the generation of OH radicals via the positive holes of the TiO2 particles, followed by OH-radical attack on the aromatic ring and subsequent oxidation of the hydroxy adduct with oxygen or iron(III) to give the corresponding phenol.