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

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 x 10(-6) and 1.6 x 10(-4) wt%, the rate was slowest int he absence of Ag and increased with increasing Ag content. For a Ag content of 6 x 10(-4) wt%, the degradation rate was about 50% faster than with bare TiO2, and decreased for a Ag content of 7.2 x 10(-4) wt% and beyond, eventually becoming slower than unmodified TiO2. Bare TiO2 has been compared with the 6 x 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.