In Situ Infrared Spectroscopic Investigations of Pyridine-Mediated CO2 Reduction on Pt Electrocatalysts

Pyridine-mediated electrochemical reduction of CO2 has attracted much attention owing to the promise of producing valuable oxygenates with high yields. However, no detectable level of methanol was observed in the pyridine-mediated CO2 electrolysis on Pt over the entire potential range investigated (−0.2 to −0.8 V vs RHE) in this study. Formate was observed at a potential below −0.6 V vs RHE in the absence and presence of pyridine, but the presence of pyridine does accelerate the rate of formate production. Numerous reaction mechanisms have been proposed on the basis of reactivity measurements, cyclic voltammetry, or computational methods; however, a direct experimental mechanistic investigation has been lacking. By employing surface-enhanced infrared absorption spectroscopy, we identified an adsorbed unidentate COOHL intermediate on Pt regardless of the presence of pyridine. Surface coverage of the COOHL intermediate relative to that of adsorbed CO appears to increase with the concentration of pyridine in the electrolyte, which is consistent with the observed production rates for formate and CO. We propose that adsorbed COOHL is a common intermediate in the formation of both formate and CO, and the presence of pyridinium promotes the formate pathway.