Solvent-Dependent Mechanism of Electrochemical Oxalate Oxidation

This report describes an investigation of the role of H2O and N,N-dimethylformamide (DMF) as solvents in the electrochemical oxidation of oxalate (C2O42–). In H2O/0.1 M Na2SO4, the 2e– oxidation of C2O42– yields two CO2 molecules via a classical ECE mechanism, resulting in a single oxidation wave at 1.2 V vs Ag/AgCl. In contrast, two distinct oxidation waves are observed in DMF/0.1 M TBAP at ∼0.1 V and ∼0.7 V vs Ag/AgCl. The second wave at more positive potentials in DMF was first assigned by Maran and co-workers to the oxidation of the adduct C2O42–·CO2 formed from generation of CO2 during the first wave. Electronic interaction of C2O42– and CO2 stabilizes C2O42–, making it more difficult to oxidize than free C2O42–. Herein we present cyclic voltammetry (CV), finite difference (FD) simulations, Raman spectroscopy, and ab initio molecular dynamics (AIMD) simulations to determine the thermodynamics and kinetics of C2O42–·CO2 formation as well as the adduct structure. FD simulations of the voltammetric data provide approximate thermodynamic and kinetic parameters for C2O42–·CO2 formation in DMF as well as a new value for the diffusion coefficient of C2O42– in DMF that differs substantially from previously reported values. Raman spectroscopy directly demonstrates C2O42–·CO2 formation in DMF, while AIMD simulations predict that the solvent-dependent thermodynamics and mechanism for C2O42– oxidation can be explained by C2O42––solvent and C2O42––CO2 interactions. A byproduct of this investigation is the prediction from AIMD simulations that the C2O42–·CO2 interaction in DMF destabilizes CO2. Consistent with this finding, cyclic voltammetry demonstrates that the thermodynamic potential for CO2 reduction in DMF at a Hg electrode decreases by ∼0.3 V upon addition of C2O42– to the solution.