Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy

The elementary steps of the electrocatalytic reduction of S2O82- using the Ru(NH3)(6)(3+/2+) redox couple were investigated using scanning electrochemical microscopy (SECM) and steady-state voltammetry (SSV). SECM investigations were carried out in a 0.1 M KCl solution using a 3.5 mu m radius carbon ultramicroelectrode (UME) as the SECM tip and a 25 mu m radius platinum UME as the substrate electrode. Approach curves were recorded in the positive feedback mode of SECM by reducing Ru(NH3)(6)(3+) at the tip electrode and oxidizing Ru(NH3)(6)(2+) at the substrate electrode, as a function of the tip-substrate separation and S2O82- concentration. The one-electron reaction between electrogenerated Ru(NH3)(6)(2+) and S2O82- yields the unstable S2O83 center dot-, which rapidly dissociates to produce highly oxidizing SO4 center dot-. Because SO4 center dot- is such a strongly oxidizing species, it can be further reduced at both the tip and the substrate, or it can react with Ru(NH3)(6)(2+) to regenerate Ru(NH3)(6)(3+). SECM approach curves display a complex dependence on the tip-substrate distance, d, due to redox mediation reactions at both the tip and the substrate. Finite element method (FEM) simulations of both SECM approach curves and SSV confirm a previously proposed mechanism for the mediated reduction of S2O82- using the Ru(NH3)(6)(3+/2+) redox couple. Our results provide a lower limit for dissociation rate constant of S2O83 center dot- (similar to 1 x 10(6) s(-1)), as well as the rate constants for electron transfer between SO4 center dot- and Ru(NH3)(6)(2+) (similar to 1 x 10(9) M-1 s(-1)) and between S2O82- and Ru(NH3)(6)(2+) (similar to 7 x 10(5) M-1 s(-1)).