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 CO 2 by n e – and n H + ( n = 2, 4, 6, 8) leads to different products like CO, HCOOH, HCHO, CH 3 OH, and CH 4 . Selective reduction of CO 2 to one of these products is a major challenge. Electrochemical reduction of CO 2 by iron porphyrin complexes with a pendant pyridine in its second sphere is investigated using a series of proton sources with varying p K a in CH 3 CN. Acidic proton sources like phenols yield >80% H 2 even in CO 2 -saturated CH 3 CN solutions. This selectivity is switched to >95% CO when weaker proton sources like H 2 O or sterically hindered phenols are used. Competitive H 2 generation is suppressed at low PhOH concentrations and CO initially produced gets further reduced to produce C 2 H 6 resulting in an overall 14e – /14H + reduction of CO 2 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 CO 2 reduction at the iron site generating a PhOCH 3 species, which reacts with a Fe-CH 3 species produced by the reduction of a second molecule of CO 2 at the same iron center to release C 2 H 6 . The same catalyst with pyridine in the second sphere, immobilized on a graphite electrode for fast electron transfer, reduces CO 2 in an aqueous medium to CH 4 with more than 90% selectivity and <5% CO is produced. Hence, the selectivity of CO 2 reduction, using the same catalyst, changes from 2e – /2H + reduction to CO to 8H + /8e – reduction to CH 4 and 14e – /14H + reduction to C 2 H 6 by altering proton and electron delivery to the catalyst.