The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction

• Published in: Journal of the American Chemical Society 2024-09, Vol.146 (36), p.25088-25100
• Main Authors: Janusz, Jordyn N., Beeler, Joshua A., Hosseini, Seyyedamirhossein, Tanwar, Mayank, Zeng, Rui, Wang, Hongsen, Abruña, Héctor D., Neurock, Matthew, White, Henry S.
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c08080

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 (ΔG 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)6 2+-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.