Investigation of Iron Based Di-Atomic Catalysts to Identify Active Sites for Coupled Reduction Reactions of CO2 and N2
Authors: Santra Saswati , Streibel Verena , Sharp Ian D. 
Published: February 2024

Urea is a valuable chemical feed stock that is commonly used in fertilizers and other industrial applications, including for the production of plastics and resins. Conventionally, industrial urea is produced through the reaction of ammonia and carbon dioxide under high pressure and temperature, resulting in intensive energy inputs and inefficiencies. In contrast, electrochemical synthesis of urea under mild conditions has been shown to be possible by coupled electrochemical reduction of CO2 (CO2RR) and N2 (NRR)1. While finding active and efficient catalysts remains a major challenge, rational design can be advanced by identification of the favourable atomic arrangements that facilitate C-N bond formation. For this purpose, carbon-supported and nitrogen-coordinated metal single atom catalysts (SACs) are a promising class of electrocatalysts2. Given their well-defined atomic structure, these material systems are valuable model systems for identifying active sites for specific reactions.

Cu and Ni are well-known as efficient CO2RR catalysts3,4, whereas Fe-based catalysts are traditionally used for N2 fixation to produce NH3 via the thermochemical Haber-Bosch method5. This background knowledge served as motivation for fabrication of two types of di-atomic catalysts (DACs), containing Cu-Fe and Ni-Fe components anchored in an electrochemically inert carbon framework. A wet-chemical method was used for synthesizing these materials. While no crystalline metallic reflections were observed in X-ray diffraction patterns, X-ray photoelectron spectroscopy confirmed the presence of constituent metals along with lattice N, O, and C. The X-ray absorption near edge structure (XANES) measurements of Fe, Ni and Cu edges show significantly different spectra when compared to corresponding metal foils and oxides. Importantly, a nitrogen-coordinated ‘single atom’ morphology with inter-metal interactions is confirmed by extended X-ray absorption fine structure (EXAFS). Electrochemical reduction reactions in the presence of CO2 and N2 showed formation of 2e- CO2RR products, with CO dominant for the case of Ni-Fe DACs. In contrast, for the case of Cu-Fe DACs, formic acid generation was significantly more pronounced than CO. For the case of isolated NRR, NH3 was observed for both DACs. The production of urea, however, was only observed for Cu-Fe DACs. While mechanistic investigations are underway, the primary difference in these activities is hypothesized to be due to formation of different intermediates and reaction paths followed during CO2RR. Previous studies6 have suggested that the choice of reaction path, CO2→*OCHO→formic acid or CO2→*COOH→CO, could be dependent on oxophilicity of the reaction site. Our observations indicate that C-N coupling is promoted by oxophilic CO2RR catalysts. Since the direct identification of active sites for coupled CO2RR and NRR has rarely been reported using simple ‘single atom’ based catalyst systems, our work opens up a new direction for designing catalysts capable of C-N coupling for the efficient generation of the valuable chemical, urea, under ambient conditions.

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