Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy

The elementary steps of the electrocatalytic reduction of S2O82– using the Ru(NH3)63+/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-µm radius carbon ultramicroelectrode (UME) as the SECM tip and a 25-µm radius platinum UME as the substrate electrode. Approach curves were recorded in the positive feedback mode of SECM by reducing Ru(NH3)63+ at the tip electrode and oxidizing Ru(NH3)62+ at the substrate electrode, as a function of the tip-substrate separation and S2O82– concentration. The one-electron reaction between electrogenerated Ru(NH3)62+ and S2O82– yields the unstable S2O83•-, which rapidly dissociates to produce highly oxidizing SO4•–. Because SO4•– is such a strongly oxidizing species, it can be further reduced at both the tip or the substrate, or it can react with Ru(NH3)62+ to regenerate Ru(NH3)63+. 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 Ru(NH3)63+/2+ redox couple. Our results provide a lower limit for dissociation rate constant of S2O83•– (~ 1 × 106 s–1), as well as the rate constants for electron transfer between SO4•– and Ru(NH3)62+ (~ 1 × 109 M–1s–1) and between S2O82– and Ru(NH3)62+ (~7 × 105 M–1s–1).