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).