Direct and Selective 8e–/8H+ Electrochemical Reduction of CO2 to CH4 in an Aqueous Medium by an Iron Porphyrin with Distal Amine Groups in the 2nd Sphere

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Abstract Reduction and valorization of CO2 to fuels and useful chemicals, using electrical energy, is an emerging area of research that addresses concerns of increasing greenhouse gas emissions as well as allowing storage of renewable electrical energy in the form of chemical fuels. The reduction of CO2 can produce several C1 compounds like CO, HCOOH, HCHO, CH3OH, and CH4. While there are several elegant molecular systems that can reduce CO2 by 2e–/2H+ to HCOOH and CO, there is a dearth of molecular catalysts that can accomplish the 8e–/8H+ reduction of CO2 to CH4 selectively. The problem lies in dissociation of partially reduced carbon species (e.g., CO, HCOOH) before the reduction can be achieved. Here, we use an iron porphyrin with four tertiary amine groups in the 2nd sphere, which can stabilize partially reduced species by hydrogen bonding as well as act as proton relays. This iron porphyrin can catalyze the reduction of CO2 to CH4 with >80% selectivity in aqueous bicarbonate solutions at room temperature. In situ surface-enhanced resonance Raman spectroscopy coupled to rotating-disk electrochemistry (SERRS-RDE) on modified graphite electrodes reveals that the reduction proceeds via the initial formation of a 2e–/2H+ reduced Fe-CO intermediate, which, unlike regular porphyrins, does not dissociate during reduction due to hydrogen-bonding stabilization, evidenced by the in-operando spectroscopic data, from the 2nd sphere amine residues. This catalyst can reduce CO2 by 8e–/8H+ to CH4 unabated in the presence of 30% O2.

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