The True Fate of Pyridinium in the Reportedly Pyridinium-Catalyzed Carbon Dioxide Electroreduction on Platinum

Protonated pyridine (PyH⁺) has been reported to act as a peculiar and promising catalyst for the direct electroreduction of CO₂ to methanol and/or formate. Because of recent strong incentives to turn CO₂ into valuable products, this claim triggered great interest, prompting many experiments and DFT simulations. However, when performing the electrolysis in near-neutral pH electrolyte, the local pH around the platinum electrode can easily increase, leading to Py and HCO₃ ⁻ being the predominant species next to the Pt electrode instead of PyH⁺ and CO₂. Using a carefully designed electrolysis setup which overcomes the local pH shift issue, we demonstrate that protonated pyridine undergoes a complete hydrogenation into piperidine upon mild reductive conditions (near 0 V vs. RHE). The reduction of the PyH⁺ ring occurs with and without the presence of CO₂ in the electrolyte, and no sign of CO₂ electroreduction products was observed, strongly questioning that PyH⁺ acts as a catalyst for CO₂ electroreduction.