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.