Effect of silver deposits on the photocatalytic activity of titanium dioxide for the removal of 2-chlorophenol in water

Abstract

The effect of Ag photodeposition on the rate of photocatalytic degradation of 2-chlorophenol (2-CP) in aqueous suspensions of Degussa P 25 titanium dioxide has been studied using HPLC. For Ag contents between 5 × 10⁻⁶ and 1.6 × 10⁻⁴ wt%, the rate was slowest in the absence of Ag and increased with increasing Ag content. For a Ag content of 6 × 10⁻⁴ wt%, the degradation rate was about 50% faster than with bare TiO₂, and decreased for a Ag content of 7.2 × 10⁻⁴ wt% and beyond, eventually becoming slower than unmodified TiO₂. Bare TiO₂ has been compared with the 6 × 10⁻⁴ wt% Ag–TiO₂ sample in terms of the variation of chlorohydroquinone (CHQ) and chloro-1,4-benzoquinone (CBQ) concentrations during 2-CP degradation, and also for the degradation of CHQ and CBQ separately. On the basis of these experiments, it is tentatively suggested that the net effect of Ag on 2-CP removal results from opposite influences, viz. the Ag⁺ cation-induced oxidation of CHQ to CBQ and the Ag⁰ particle-induced recombination of photoproduced charges, both influences being assumed to depend on the Ag particle size, itself related to the Ag content.