Electrochemical Reduction of CO₂ to CH₃OH Catalyzed by an Iron Porphyrinoid

Abstract

Designing catalysts for the selective reduction of CO₂, resulting in products having commercial value, is an important area of contemporary research. Several molecular catalysts have been reported to facilitate the reduction of CO₂ (both electrochemical and photochemical) to yield 2e^-/2H⁺ electron-reduced products, CO and HCOOH, and selective reduction of CO₂ beyond 2e^-/2H⁺ is rare. This is partly because the factors that control the selectivity of CO₂ reduction beyond 2e^- are not yet understood. An iron chlorin complex with a pendent amine functionality in its second sphere, known to selectively catalyze CO₂RR to HCOOH with a very low overpotential from its formal Fe(I) state, can catalyze CO₂RR from its formal Fe(0) state by 6e^-/6H⁺, forming CH₃OH as a major product with a Faradaic yield of ∼50%. Mechanistic investigations using in situ spectro-electrochemistry indicate that the reactivity of a low-spin d⁷ Fe^I-COOH intermediate species generated during CO₂RR is crucial in determining the product selectivity of this reaction. In weakly acidic conditions, C-protonation of this Fe^I-COOH species, which is also chemically prepared and spectroscopically characterized, leads to HCOOH. The O-protonation, leading to C-OH bond cleavage and eventually to CH₃OH, is ∼3 kcal/mol higher in energy and can be achieved in more acidic solutions. Hydrogen bonding to the pendent amine in the catalyst stabilizes reactive intermediates formed in the CO₂RR and enables 6e^-/6H⁺ reduction of CO₂ to CH₃OH.