Reduction of CO₂ in exhaust gases and its conversion to useful carbon resources are an urgent and important issue to prevent . Carbon positive processes, which consume CO₂, are the more effective way to reduce CO₂ and to be able to recycle it. We have found that the porphyrin dimers bearing two iron metal centers separated by an appropriate separation as homogeneous catalysts realize CO₂ reduction at remarkable high catalytic turnover frequencies (log TOF_max = 5.8), low overpotentials (η > 0.4 V), and high CO selectivity under neutral wet conditions, i.e. without any addition of acidic reagents.[1, 2] According to this highly efficient CO2 reduction, the performance of these dinuclear iron catalysts becomes a bench mark of CO₂ reduction catalysts. Such high catalytic activity comes from the CO₂ binding to the Fe(II) dimer, though an Fe(II) porphyrin monomer is intact in CO₂ atmosphere. The formation of these pre-associated complexes could efficiently promote the succeeding reduction of the bound CO₂-to-CO. In order to apply this kind of catalysts to aqueous heterogeneous electro-reduction system, a FTO electrode is modified with Fe porphyrin dimers bearing a phosphonic acid anchor group (Fe₂DP^PO₃H₂) on its surface.[3] Electrochemical CO₂ reduction with the resultant catalyst-modified electrode in a neutral aqueous solution shows CO and H₂ formation without any selectivity. Proton reduction could occur on the bare SnO₂ surface. The SAM modification of the unmodified FTO surface can successfully block H₂ formation by the prevention of the contact of the electrode surface from the solution. However, the low amount of the surface modified catalyst (Γ ≈ 5 x 10^-12 mol/cm²) only gave 30-40 μA/cm² in the electrolysis at -0.95 V vs. NHE (η = 0.42 V) in CO₂-saturated 0.1 M KPi solution (pH = 7). In order to increase the surface area of the electrode, meso porous SnO₂ layer modification with nano SnO₂ particles was applied to a FTO. The resultant 2-μm SnO₂ layered electrode co-modified with Fe₂DP^PO₃H₂ and SAM was applied to CO₂ reduction in 0.1 M KPi and gave 10 times higher current density (i = 1.5 mA/cm²) than that of an ordinary catalyst-modified FTO. The continuous electrolysis for 6 h showed stable current and this proved the electrode is sufficiently stable under these conditions. Since the thickness of the meso porous SnO₂ layer on a FTO can be increased up to ≈10 μm, further increase of the current density is possible. In this talk, the latest results of the cell-electrode system involving a non-precious metal anode will be discussed.

References

[1] E. A. Mohamed, Z. N. Zahran, Y. Naruta, Chem. Commun., 2015, 51, 16900-16903.

[2] Z. N. Zahran, E. A. Mohamed, Y. Naruta Sci. Rep., 2016, 6, 24533.

[3] E. A. Mohamed, Z. N. Zahran, Y. Naruta, Chem. Mater. 2017, 29, 7140–7150.