A VIEW ON IMIDAZOLIUM ELECTROCHEMICAL REDUCTION ON PLATINUM AND GOLD ELECTRODES IN AQUEOUS SOLUTION: IMPLICATIONS FOR IMIDAZOLIUM ASSISTED CO2 REDUCTION

Abstract

Carbon dioxide mitigation in our society has implications for both the environment and energy management. Electrochemically reducing CO2 to value-added products provides an opportunity to achieve such a goal. Pyridinium-catalyzed CO2 reduction to formic acid and methanol with low overpotentials at platinum electrodes has been previously investigated. This study extends the research to another N-heterocycle, imidazolium, in a comparative fashion in order to probe the CO2 reduction mechanism. The study found that imidazolium reduces through a proton coupled electron transfer process on a Pt electrode with no involvement of the 2 position carbon or the aromatic ring.

The role of surface adsorbed hydrogen atoms in the proposed mechanism extends the study to gold electrodes, on which surface hydrides are known to be unstable. It was found for the first time that imidazolium reduction on gold has a similar proton coupled electron transfer component, and the stability of the gold surface hydride dictates the reversibility of imidazolium electrochemical reduction. A pH variation experiment found that the charge transfer kinetics increased gradually with the increase of proton concentration, consistent with the mechanism. Rotating disk electrode experiments and Nicholson-Shain diagnostics indicated a one-electron diffusive reduction of imidazolium on gold coupled with a subsequent chemical reaction. Digital simulations with a proposed proton coupled electron transfer mechanism fit well with the experimental results. Electron paramagnetic resonance spectroelectrochemistry with an in situ apparatus design observed no radical formed during imidazolium reduction on gold, which further supports the proposed proton coupled electron transfer component in the reduction.