Theoretical Insights into Electrochemical CO2 Reduction Mechanisms Catalyzed by Surface-Bound Nitrogen Heterocycles

We propose a novel and general reaction mechanism to explain the unique performance of nitrogen-heterocycle-promoted (photo)electrochemical CO2 reduction reactions. This mechanism is based on observations from recent computational and experimental studies of pyridinium-catalyzed CO2 reduction. Herein we report pK(a)s and standard reduction potentials of species adsorbed on GaP photoelectrodes derived from first-principles quantum chemistry computations. We show that on GaP surfaces, proton reduction or pyridinium reduction is energetically unfavorable even at very negative electrode potentials. However, it is thermodynamically favorable to convert a surface-bound pyridine into a 2e(-) reduced species such as dihydropyridine at less negative applied potentials. Intriguingly, these transient 2e(-) reduced species share a similar chemical moiety as some biological redox catalysts (e.g., NADH), and their reduction potentials are similar to the thermodynamic redox potentials that would convert CO2 to a variety of products.