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Atomically dispersed single-atom catalysts are among the most attractive electrocatalysts for the CO2 reduction reaction (CRR). To elucidate the origin of the exceptional activity of atomically dispersed Fe–N–C catalyst in CRR, we have performed operando 57Fe Mössbauer spectroscopic studies on a model single-Fe-atom catalyst with a well-defined N coordination environment. Combining with operando X-ray absorption spectroscopy, the in situ-generated four pyrrolic nitrogen atom-coordinated low-spin Fe­(I) (LS FeIN4) featuring monovalent iron is identified as the reactive center for the conversion of CO2 to CO. Furthermore, density functional theory calculations reveal that the optimal binding strength of CO2 to the LS FeIN4 site, with strong orbital interactions between the singly occupied d z 2 orbital of the Fe­(I) site and the singly occupied π* orbital of [COOH] fragment, is the key factor for the excellent CRR performance.

Single-atom catalysts (SACs) have become an emerging frontier trend in the field of heterogeneous catalysis due to their high activity, selectivity and stability. SACs could greatly increase the availabilities of the active metal atoms in many catalytic reactions by reducing the size to single atom scale. Graphene-supported metal SACs have also drawn considerable attention due to the unique lattice structure and physicochemical properties of graphene, resulting in superior activity and selectivity for several chemical reactions. In this paper, we review recent progress in the fabrications, advanced characterization tools and advantages of graphene-supported metal SACs, focusing on their applications in catalytic reactions such as CO oxidation, the oxidation of benzene to phenol, hydrogen evolution reaction, methanol oxidation reaction, oxygen reduction reaction, hydrogenation and photoelectrocatalysis. We also propose the development of SACs towards industrialization in the future.

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