The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction

Abstract

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S₂O₈^2-), and the strong reductant, oxalate (C₂O₄^2-), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S₂O₈^2- + C₂O₄^2- → 2 SO₄^2- + 2 CO₂ (ΔG⁰ = -490 kJ/mol). However, at room temperature, this reaction does not occur to a significant extent over the time scale of a day due to its inherently slow kinetics. We demonstrate that the S₂O₈^2-/C₂O₄^2- redox reaction occurs rapidly, once initiated by the Ru(NH₃)₆²⁺-mediated 1e^- reduction of S₂O₈^2- to form S₂O₈^3•-, which rapidly undergoes bond cleavage to form SO₄^2- and the highly oxidizing radical SO₄^•-. Theoretically, the mediated electrochemical generation of a single molecule of S₂O₈^3•- can initiate an autocatalytic cycle that consumes both S₂O₈^2- and C₂O₄^2- in bulk solution. Several experimental demonstrations of S₂O₈^2-/C₂O₄^2- autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO₂ is generated in solution for at least 10 min following a 30-s initiation step. Quantitative bulk electrolysis of S₂O₈^2- in solutions containing excess C₂O₄^2- is initiated by electrogeneration of immeasurably small quantities of S₂O₈^3•-. Capture of CO₂ as BaCO₃ during electrolysis additionally confirms the autocatalytic generation of CO₂. First-principles density functional theory calculations, ab initio molecular dynamics simulations, and finite difference simulations of cyclic voltammetric responses are presented that support and provide additional insights into the initiation and mechanism of S₂O₈^2-/C₂O₄^2- autocatalysis. Preliminary evidence indicates that autocatalysis also results in a chemical traveling reaction front that propagates into the solution normal to the planar electrode surface.