The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction

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 (Δ<i>G</i>⁰ = -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 <i>autocatalytic</i> 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, <i>ab initio</i> 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.