Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy

The elementary steps of the electrocatalytic reduction of S₂O₈^2- 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 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 S₂O₈^2- concentration. The one-electron reaction between electrogenerated Ru(NH₃)₆²⁺ and S₂O₈^2- yields the unstable S₂O₈^3•-, 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 and 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 S₂O₈^2- using the Ru(NH₃)₆^3+/2+ redox couple. Our results provide a lower limit for dissociation rate constant of S₂O₈^3•- (∼1 × 10⁶ s^-1), as well as the rate constants for electron transfer between SO₄^•- and Ru(NH₃)₆²⁺ (∼1 × 10⁹ M^-1 s^-1) and between S₂O₈^2- and Ru(NH₃)₆²⁺ (∼7 × 10⁵ M^-1 s^-1).