Proton-coupled electron transfer in the electrocatalysis of CO₂ reduction: prediction of sequential vs. concerted pathways using DFT

Herein we investigate computationally in detail the mechanism of the formation of the carboxylate adduct during the electroreduction of CO₂ in water catalysed by cobalt porphyrin complexes. Specifically, we address qualitatively the competition between the concerted and sequential pathways for the proton-coupled electron transfer. We use a simple methodology for accurate computation of the pKa of the neutral and anionic carboxylate intermediates, [CoP-COOH] and [CoP-COOH]⁻ (where CoP is a cobalt porphine complex), based on the isodesmic proton-exchange reaction scheme. The predicted values are used as in input for a theoretical model that describes the transition between the sequential and concerted pathways. The activation of the sequential pathway (ET-PT) that leads to the formation of the neutral [CoP-COOH] intermediate at pH ≈ 3.5 (pKa[CoP-COOH] = 3.5 ± 0.4), as predicted by the calculations, is in good agreement with the drastic increase in the faradaic efficiency of the CO₂ reduction reaction towards CO at pH = 3 compared to pH = 1, as experimentally observed. This confirms the existence of the CO₂ anionic adduct [CoP-CO₂]⁻ as a viable intermediate at pH = 3 and its crucial role for the pH dependence of the faradaic efficiency for the CO₂ reduction. The analysis also shows that when the pH is significantly higher than the pKa of the neutral carboxylate adduct, the CO₂ reduction has to go through an alternative pathway with the formation of the anionic carboxylate intermediate [CoP-COOH]⁻. It is formed through a concerted proton-electron transfer step from the anionic CO₂ adduct [CoP-CO₂]⁻ when the pH is below ∼8.6 (pKa[CoP-COOH]⁻ = 8.6 ± 0.4). At pH ≈ 8.6 and above, another decoupled ET-PT is predicted to take place, leading to the formation of a dianionic CO₂ adduct [CoP-CO₂]²⁻.