Electrochemistry of aqueous pyridinium: Exploration of a key aspect of electrocatalytic reduction of CO₂ to methanol

The mechanism by which pyridinium (pyrH⁺) is reduced at a Pt electrode is a matter of recent controversy. The quasireversible cyclic voltammetric wave observed at -0.58 V vs SCE at a Pt electrode was originally proposed to correspond to reduction of pyrH⁺ to pyridinyl radical (pyrH^•). This mechanistic explanation for the observed electrochemistry seems unlikely in light of recent quantum mechanical calculations that predict a very negative reduction potential (-1.37 V vs SCE) for the formation of pyrH^•. Several other mechanisms have been proposed to account for the discrepancy in calculated and observed reduction potentials, including surface adsorption of pyrH^•, reduction of pyrH⁺ by two electrons rather than one, and reduction of the pyrH⁺ proton to a surface hydride rather than a π-based radical product. This final mechanism, which can be described as inner-sphere reduction of pyrH⁺ to form a surface hydride, is consistent with experimental observations.