Unveiling Active Sites of CO₂ Reduction on Nitrogen-Coordinated and Atomically Dispersed Iron and Cobalt Catalysts

Herein, we report the exploration of understanding the reactivity and structure of atomically dispersed M-N₄ (M = Fe and Co) sites for the CO₂ reduction reaction (CO2RR). Nitrogen coordinated Fe or Co site atomically dispersed into carbons (M-N-C) containing bulk- and edge-hosted M-N₄ coordination were prepared by using Fe- or Co-doped metal-organic framework precursors, respectively, which were further studied as ideal model catalysts. Fe is intrinsically more active than Co in M-N₄ for the reduction of CO₂ to CO, in terms of a larger current density and a higher CO Faradaic efficiency (FE) (93% vs. 45%). First principle computations elucidated that the edge-hosted M-N₂₊₂-C₈ moieties bridging two adjacent armchair-like graphitic layers is the active sites for the CO2RR. They are much more active than previously proposed bulk-hosted M-N₄-C₁₀ moieties embedded compactly in a graphitic layer. During the CO2RR, when the dissociation of∗COOH occurs on the M-N₂₊₂-C₈, the metal atom is the site for the adsorption of∗CO and the carbon atom with a dangling bond next to an adjacent N is the other active center to bond∗OH. In particular, on the Fe-N₂₊₂-C₈ sites, the CO2RR is more favorable over the hydrogen evolution reaction, thus resulting in a remarkable FE.