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

Abstract

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.

A Comparison of Pyridazine and Pyridine as Electrocatalysts for the Reduction of Carbon Dioxide to Methanol

“…[12] Thereafter, in a recent experimental study, Bocarsly et al concluded that pyridinium is reduced on a Pt electrode by an innersphere reduction mechanism, including a pyridinium-bound proton to form a surface hydride. [13] These findings are in agreement with additional theoretical predictions by the group of Batista et al [14] Different to these findings, SavØant et al reported shortly after that no trace of methanol or formate could be detected upon preparative-scale electrolysis of CO 2 on the same system when using pyridinium ions as putative catalyst materials. [15] SavØant concluded that the reduction of pyridinium follows a reduction of the hydrated protons generated by rapid dissociation of the pyridinium ions, which does not lead to the formation of pyridinium radicals and catalytic CO 2 reduction.…”

“…This characteristic is in agreement with previously reported results, showing the important role of platinum in the overall reaction mechanism. [7,12,14] These results then further support the idea of a reduction mechanism including a pyridinium-activated proton on the Pt surface to form a surface hydride intermediate. [13] In recent work, Musgrave et al [19] employed quantum-chemical calculations to investigate the role and mechanism of pyridini- um-based CO 2 reduction.…”

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

“…We stress that we consider a Pt electrode to be a special case. On a Pt electrode, 1e -reduction of PyH + is favored to form adsorbed H-atoms (Pt-H*) [93][94][95][96] such that using it as a cathode introduces additional routes (e.g. H2 formation) which likely outcompete any processes catalyzed by Py.…”

“…H2 formation) which likely outcompete any processes catalyzed by Py. Therefore, surface pathways 92,94 for CO2 reduction on Pt may predominate such that the homogeneous mechanism discussed in the text that requires the production of PyH 0 becomes a minor pathway.…”

Organic, Organometallic and Bioorganic Catalysts for Electrochemical Reduction of CO2

“…expanded their studies to shed further light on the mechanism. In this study56 they concluded that pyridinium is reduced on a Pt electrode, including a pyridinium‐bound proton to form a surface hydride which was further supported by the work of Batista 57. Indeed, the study from Bocarsly and co‐workers55 created more controversy in the field and drove several studies 58, 59, 60, 61.…”