Functional Role of Pyridinium during Aqueous Electrochemical Reduction of CO2 on Pt(111)

Recent breakthroughs in electrochemical studies have reported aqueous CO₂ reduction to formic acid, formaldehyde, and methanol at low overpotentials (−0.58 V versus SCE), with a Pt working electrode in acidic pyridine (Pyr) solutions. We find that CO₂ is reduced by H atoms bound to the Pt surface that are transferred as hydrides to CO₂ in a proton-coupled hydride transfer (PCHT) mechanism activated by pyridinium (PyrH⁺), CO₂ + Pt–H + PyrH⁺ + e^– → Pyr + Pt + HCO₂H. The surface-bound H atoms consumed by CO₂ reduction is replenished by the one-electron reduction of PyrH⁺ through the proton-coupled electron transfer (PCET), PyrH⁺ + Pt + e^– → Pyr + Pt–H. Pyridinium is essential to establish a high concentration of Brønsted acid in contact with CO₂ and with the Pt surface, much higher than the concentration of free protons. These findings are particularly relevant to generate fuels with a carbon-neutral footprint.