Proton and Electron Transfer Control of Selectivity in Electrochemical CO ₂ Reduction: Selective Reduction of CO ₂ to CO, CH ₄ , and C ₂ H ₆ Catalyzed by the Same Iron Porphyrin

Reduction of CO₂ by ne⁻ and nH⁺ (n = 2, 4, 6, 8) leads to different products like CO, HCOOH, HCHO, CH₃OH, and CH₄. Selective reduction of CO₂ to one of these products is a major challenge. Electrochemical reduction of CO₂ by iron porphyrin complexes with a pendant pyridine in its second sphere is investigated using a series of proton sources with varying pKa in CH₃CN. Acidic proton sources like phenols yield >80% H₂ even in CO₂-saturated CH₃CN solutions. This selectivity is switched to >95% CO when weaker proton sources like H₂O or sterically hindered phenols are used. Competitive H₂ generation is suppressed at low PhOH concentrations and CO initially produced gets further reduced to produce C₂H₆ resulting in an overall 14e⁻/14H⁺ reduction of CO₂ with >80% selectivity. The C-C bond formation at a monometallic site is aided by PhOH (also RSH) insertion into an iron porphyrin carbene produced during CO₂ reduction at the iron site generating a PhOCH₃ species, which reacts with a Fe-CH₃ species produced by the reduction of a second molecule of CO₂ at the same iron center to release C₂H₆. The same catalyst with pyridine in the second sphere, immobilized on a graphite electrode for fast electron transfer, reduces CO₂ in an aqueous medium to CH₄ with more than 90% selectivity and <5% CO is produced. Hence, the selectivity of CO₂ reduction, using the same catalyst, changes from 2e⁻/2H⁺ reduction to CO to 8H⁺/8e⁻ reduction to CH₄ and 14e⁻/14H⁺ reduction to C₂H₆ by altering proton and electron delivery to the catalyst.