Isotopic Probe Illuminates the Role of the Electrode Surface in Proton Coupled Hydride Transfer Electrochemical Reduction of Pyridinium on Pt(111)

Abstract

A recently proposed mechanism for electrochemical CO 2 reduction on Pt (111) catalyzed by aqueous acidic pyridine solutions suggests that the observed redox potential of ca. −600 mV vs. SCE is due to the one-electron reduction of pyridinium through proton coupled electron transfer (PCET) to form H atoms adsorbed on the Pt surface (H ads ). The initial pyridinium reduction was probed isotopically via deuterium substitution. A combined experimental and theoretical analysis found equilibrium isotope effects (EIE) due to deuterium substitution at the acidic pyridinium site. A shift in the cathodic cyclic voltammetric half wave potential of −25 mV was observed, consistent with the theoretical prediction of −40 mV based on the recently proposed reaction mechanism where pyridinium is essential to establish a high concentration of Brønsted acid in contact with the substrate CO 2 and with the Pt surface. A prefeature in the cyclic voltammogram was examined under isotopic substitution and indicated an H ads intermediate in pyridinium reduction. Theoretical prediction and observation of an EIE supported the assignment of the cathodic wave to the proposed reduction of pyridinium through PCET forming H ads and eventually H 2 on the Pt surface.

Catalysis of CO2 Electrochemical Reduction by Protonated Pyridine and Similar Molecules. Useful Lessons from a Methodological Misadventure

“…The mechanism assignment is corroborated by the simulations shown in Figure Aa′ for PyH + and 5Ab′ for AcOH. This confirms that PyH + behaves as any other weak acid and that its reduction does not go through a pyridyl radical …”

Study of Pyridine‐Mediated Electrochemical Reduction of CO2 to Methanol at High CO2 Pressure

“…In subsequent/more recent papers by the Bocarsly group another mechanism of pyridine-assisted methanol synthesis was brought forward. In agreement with [13], it was suggested [17][18][19] that the Pt electrode surface plays a key role in the multistep CO2 reduction to methanol. The first CO2 reduction step is detailed in Scheme 2.…”

“…In contrast to Scheme 1, in this scheme 5-25% of the reduction current was estimated to be responsible for the methanol synthesis (CO2 reduction) ((iii) in Scheme 2) and the remaining 75-95% for the competing hydrogen evolution reaction ((iv) in Scheme 2). 19 To our knowledge, the reports from the above two groups, 5,8,10,[17][18][19][20] which reached very different conclusions, are among the very few experimental studies attempting to elucidate the CO2-pyridnium mechanism. Other investigators focussed largely on extending the methanol production to different electrode materials 21,22 or on the electrochemistry of pyridine solutions.…”

Evaluation of Organic Hydride Donors as Reagents for the Reduction of Carbon Dioxide and Metal-Bound Formates

“…Moreover, a body of experimental results, backed by independent computational studies, have suggested that pyridine may act as an electrochemically recyclable OHD (via dihydropyridine) capable of reduction of carbon dioxide to methanol. Bocarsly et al and others reported that pyridine catalyzes the electrochemical reduction of carbon dioxide to methanol. − Theoretical studies of Musgrave, − Keith, − and Carter − ,− and their co-workers in particular implicate 1,2-dihydropyridine 6 as the active reducing agent in homogeneous solution.…”