Catalysis and Inhibition in the Electrochemical Reduction of CO2 on Platinum in the Presence of Protonated Pyridine. New Insights into Mechanisms and Products

In the framework of modern energy challenges, the reduction of CO₂ into fuels calls for electrogenerated low-valent transition metal complexes catalysts designed with considerable ingenuity and sophistication. For this reason, the report that a molecule as simple as protonated pyridine (PyH⁺) could catalyze the formation of methanol from the reduction of CO₂ on a platinum electrode triggered great interest and excitement. Further investigations revealed that no methanol is produced. It appears that CO₂ is not really reduced but rather participates, on the basis of its aquation into carbonic acid, in hydrogen evolution. Actually, the situation is not that straightforward, as revealed by scrutinizing what happens at the platinum electrode surface. The present study confirms the lack of methanol formation upon bulk electrolysis of PyH⁺ solutions at Pt and provides a detailed account of the Faradaic yield for H₂ production as a function of the electrode potential, but the main finding is that CO₂ reduction is accompanied by a strong inhibition of the electrode process taking place when it is carried out in the presence of acids such as PyH⁺ and AcOH. Cyclic voltammetry and in situ infrared spectroscopy were closely combined to investigate and understand the nature and consequences of the inhibition process. Constant comparison between the two acids was required to decipher the course of the reaction owing to the fact that the IR responses are perturbed by PyH⁺ adsorption. It finally appears that inhibition is caused by the reduction of CO₂ into CO, whose high affinity with platinum triggers the formation of a Pt–CO film that prevents the reaction process. Thus, a paradoxical situation develops in which the high affinity of Pt for CO helps to decrease the overpotential for the reduction of CO₂ and therefore blocks the electrode, preventing the reaction process.