Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy.

The elementary steps of the electrocatalytic reduction of SO using the Ru(NH) 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(NH) at the tip electrode and oxidizing Ru(NH) at the substrate electrode, as a function of the tip-substrate separation and SO concentration. The one-electron reaction between electrogenerated Ru(NH) and SO yields the unstable SO, which rapidly dissociates to produce highly oxidizing SO. Because SO is such a strongly oxidizing species, it can be further reduced at the tip and the substrate, or it can react with Ru(NH) to regenerate Ru(NH). SECM approach curves display a complex dependence on the tip-substrate distance, , 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 SO using the Ru(NH) redox couple. Our results provide a lower limit for dissociation rate constant of SO (∼1 × 10 s), as well as the rate constants for electron transfer between SO and Ru(NH) (∼1 × 10 M s) and between SO and Ru(NH) (∼7 × 10 M s).