Molecular single iron site catalysts for electrochemical nitrogen fixation under ambient conditions

Abstract

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions is an attractive approach to synthesizing NH3, but remains a significant challenge due to insufficient NH3 yields and low Faraday efficiency (FE). Among studied NRR catalyst formulations, molecular catalysts with well-defined FeN4 configuration structures allow the establishment of a precise structural model for elucidating the complex multiple proton and electron transfer NRR processes competing with the undesirable hydrogen evolution reaction (HER). Inspired by biological nitrogenase, Fe sites can activate the N2 due to their strong interactions with N2. The unoccupied d orbital of Fe endows it the ideal electron acceptor and donor, which offers an attractive chemical property to facilitate NRR activity. Herein, we explore a molecular iron catalyst, i.e., tetraphenylporphyrin iron chloride (FeTPPCl) for the NRR. It exhibits promising NRR activity with the highest NH3 yield (18.28 ± 1.6 μg h−1 mg−1cat.) and FE (16.76 ± 0.9 %) at −0.3 V vs. RHE in neutral electrolytes. Importantly, 15N isotope labeling experiments confirm that the synthesized NH3 originates from the direct reduction of N2 in which 1H NMR spectroscopy and colorimetric methods were performed to quantify NH3 production. Also, operando electrochemical Raman spectroscopy studies confirm that the Fe–Cl bond breakage in the FeTPPCl catalyst is a prerequisite for initiating the NRR. Density functional theory (DFT) calculations further reveal that the active species is Fe porphyrin complex [Fe(TPP)]2− and the rate-determining step is the first hydrogenation of N2via the alternating mechanism on the [Fe0]2− sites. This work provides a new concept to use structurally defined molecular single iron catalysts to elucidate NRR mechanisms and design optimal active sites with enhanced reaction activity and selectivity for NH3 production under ambient conditions.