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(center dot)). 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(center dot). Several other mechanisms have been proposed to account for the discrepancy. in calculated and observed reduction potentials, including surface adsorption of pyrH(center dot), reduction of pyrH(+) by two electrons rather than one, and reduction of the pyrH+ proton to a surface hydride rather than a pi-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.