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 2 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.