Development of New CO2 Reduction Catalysts and Construction of Efficient Electro-Reduction Systems

Abstract

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 TOFmax = 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 (Fe2DPPO3H2 ) 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 Fe2DPPO3H2 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.