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 O2. Since
O2 is a stronger oxidant than CO2 and isolated
from the obvious competitive inhibition of CO2, partial
reduction of O2 leads to formation of reactive oxygen species
like O2
– 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
O2 as it can reduce O2 to benign H2O under the same conditions. Further investigations reveal that iron
porphyrins, in general, reduce CO2 selectively in the presence
of O2. The aforementioned selectivity is derived from a
500 times faster rate of reaction of CO2 with Fe(0) porphyrin
relative to O2 despite a higher driving force for the latter.
In the current environmental and economic context, there is an urgent need to develop catalytic systems to efficiently activate and transform abundant small molecules while demonstrating selectivity when multielectron processes are involved. This is especially true for catalytic production of CH 4 from CO 2 , as a limited number of active photo-or electro-catalysts have been described to date. Herein, we report the unprecedented reactivity of a molecular electrocatalyst physiadsorbed on a graphite electrode: the bioinspired [L N2S2 Ni II Fe II Cp(CO)] + (L N2S2 = 2,2′-(2,2′-bipryridine-6,6′-diyl)bis-(1,1′-diphenylethanethiolate) complex selectively and catalytically reduces CO 2 in acidic aqueous solution to produce a mixture of CH 4 and H 2 . Under optimized conditions, at pH 4, Faradaic yields of 12% and 66% for CH 4 and H 2 production (TOF CH 4 = 214 s −1 , TOF H 2 ≈ 5.1 × 10 3 s −1 ) are measured, respectively. We demonstrate that this binuclear NiFe catalyst is stable for hours under controlled potential electrolysis conditions.
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