The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction.

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S 2 O 8 2- ), and the strong reductant, oxalate (C 2 O 4 2- ), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S 2 O 8 2- + C 2 O 4 2- → 2 SO 4 2- + 2 CO 2 (Δ G 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 S 2 O 8 2- /C 2 O 4 2- redox reaction occurs rapidly, once initiated by the Ru(NH 3 ) 6 2+ -mediated 1e - reduction of S 2 O 8 2- to form S 2 O 8 3•- , which rapidly undergoes bond cleavage to form SO 4 2- and the highly oxidizing radical SO 4 •- . Theoretically, the mediated electrochemical generation of a single molecule of S 2 O 8 3•- can initiate an autocatalytic cycle that consumes both S 2 O 8 2- and C 2 O 4 2- in bulk solution. Several experimental demonstrations of S 2 O 8 2- /C 2 O 4 2- autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO 2 is generated in solution for at least 10 min following a 30-s initiation step. Quantitative bulk electrolysis of S 2 O 8 2- in solutions containing excess C 2 O 4 2- is initiated by electrogeneration of immeasurably small quantities of S 2 O 8 3•- . Capture of CO 2 as BaCO 3 during electrolysis additionally confirms the autocatalytic generation of CO 2 . 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 S 2 O 8 2- /C 2 O 4 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.