Bioinspired CO2 Reduction with Iron Porphyrin Catalysts in High Efficiency

Conversion of CO₂ to useful carbon resources as well as water splitting is the important ways to store renewable energy as chemical energy.  Both reactions are typical multi-electron ones, thus, they tend to require large overpotentials (or activation energies) unless one uses proper catalysts.  Particulary, CO₂ reduction potentially gives several reduced products, CO, HCO₂H, (CO₂H)₂, CH₃OH, etc.  CO₂ reduction also competes proton reduction.  Thus, the product selectivity is another important factor on its reaction.  Following issues are keys on the development of CO₂ reduction catalysts: (1) low overpotential, (2) high turnover frequency, (3) robustness, (4) product selectivity, (5) easy separation of product(s), and (6) use of base metal (complexes) on the elemental strategy aspect.  Carbon monoxide dehydrogenase (CODH), an enzyme in bacteria, catalyzes the mutual conversions between CO₂ and CO.  The active site of the enzyme is consisted of Ni-Fe cluster and this enzyme realizes selective CO₂ reduction to CO at very low overpotential (η < 100 mV).  We develop new catalysts to fulfill the above criteria referring the enzyme structure.  We developed iron porphyrin dimers as bio-inspired catalysts, which realized electrochemical CO₂ reduction at the lowest overpotentials and high TOF in high selectivity and robustness by choosing suitable functional groups.  In a homogeneous solution, continuous electrocatalytic reduction of CO₂ for 12 h gave a good Faradaic efficiency as well as a good product selectivity by keeping a constant current through the electrolysis and any decrease of the catalyst activity is not observed.  We will discuss the importance of the dinuclear center in the CO₂ reduction.