Reduction of CO2 to CO by an Iron Porphyrin Catalyst in the Presence of Oxygen

Reduction of CO2 to value-added chemicals is a logical way of fixing the rising levels of CO2.( )Activation and reduction of CO2 requires low-valent transition metals as catalysts. A major challenge in this chemistry is sensitivity of these low-valent metal sites to more abundant O-2. Since O-2 is a stronger oxidant than CO2 and isolated from the obvious competitive inhibition of CO2, partial reduction of O-2 leads to formation of reactive oxygen species like O-2(-) and H2O2, which are deleterious to the catalyst itself. An iron porphyrin complex appended with four ferrocene groups in its distal site is demonstrated to reduce CO2 unabated in the presence of O-2 as it can reduce O-2 to benign H2O under the same conditions. Further investigations reveal that iron porphyrins, in general, reduce CO2 selectively in the presence of O-2. The aforementioned selectivity is derived from a 500 times faster rate of reaction of CO2 with Fe(0) porphyrin relative to O-2 despite a higher driving force for the latter.