The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S2O82‒), and the strong reductant, oxalate (C2O42‒), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S2O82‒ + C2O42‒ ® 2 SO42‒ + 2 CO2 (DG0 = −490 kJ/mol). However, at room temperature, this reaction does not occur to a significant extent over the timescale of a day due to its inherently slow kinetics. We demonstrate that the S2O82‒/C2O42‒ redox reaction occurs rapidly, once initiated by the Ru(NH3)62+-mediated 1e– reduction of S2O82‒ to form S2O83•‒ at a glassy carbon electrode. Theoretically, the mediated electrochemical generation of a single molecule of S2O83•‒ is capable of initiating an autocatalytic cycle that consumes both S2O82‒ and C2O42‒ in bulk solution. Several experimental demonstrations of S2O82‒/C2O42‒ autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO2 is generated in solution for at least 10 minutes following a 30-s initiation step during which S2O83•‒ is generated. Quantitative bulk electrolysis of S2O82‒ in solutions containing excess C2O42‒ is initiated by electrogeneration of immeasurably small quantities of S2O83•‒. 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 the S2O82‒/C2O42‒ autocatalytic reaction. Preliminary evidence indicates that autocatalysis also results in a chemical traveling reaction front that propagates into the solution normal to the planar electrode surface.