Pulse radiolytic investigations of aqueous solutions of methoxybenzene cation radicals: the effect of colloidal RuO2

The formation and decay of the radical cations of 1,4-dimethoxybenzene (DMB) and 1,2,4,5-tetramethoxybenzene (TMB) were investigated by the pulse radiolysis technique in the absence and the presence of colloidal RuO2 particles. DMB+ was obtained only by Tl2+ oxidation of DMB while TMB+ was produced by oxidation of TMB using both Tl2+ and Br2-. In the absence of RuO2 both DMB+ and TMB+ decay predominantly via a second-order process, although there is a contribution of a pseudo-first-order reaction. The rate constants for these reactions are reported. RuO2 colloidal particles catalyze the decay of both TMB+ and DMB+. The reactions of TMB+ with RuO2 were found to depend on pH, pulse intensity, and colloid concentration. At pH 3-4, adsorption of TMB+ to the colloid is observed, followed by the decay of the remaining TMB+ in the bulk. At higher pHs, loading of the RuO2 colloid by positive holes takes place until equilibrium is achieved between loaded holes and TMB+ and again the remaining TMB+ decays at a later stage. The fraction of TMB+ that loads the colloidal particles increases with both pH and [RuO2]. It is also suggested that DMB+ loads the RuO2 at the pH where experiments were performed. (TMB)2 and (DMB)2 dimers (or higher oligomers) are suggested to be the final products both in the absence and presence of RuO2. No O2 is formed with the RuO2 colloid despite a favorable redox potential for water oxidation