A novel porous graphitic carbon nitride (g-CN) substrate: prediction of metal-based π-d conjugated nanosheets toward the highly active and selective electrocatalytic nitrogen reduction reaction

The two-dimensional (2D) graphitic carbon nitride skeleton offers rich hollow sites for stably anchoring transition-metal (TM) atoms to promote single-atom catalysis, which is expected to overcome the great challenges of low activity and selectivity for ammonia synthesis resulting from the sluggish activation of inert N 2 and the competitive hydrogen evolution reaction (HER). Very recently, a novel holey graphitic carbon nitride monolayer with the C 7 N 3 stoichiometric ratio (g-C 7 N 3 ) was proposed, whose Dirac dispersion located at the Fermi level rightly provides excellent electric conductivity for achieving the high-performance nitrogen reduction reaction (NRR). Herein, first-principles calculations within the density-functional theory were performed to examine the feasibility of a single TM-atom (from Sc to Au) supported on g-C 7 N 3 as π-d conjugated single-atom catalysts (SACs) for NRR. Through a "Five-step Procedure" screening strategy, Hf, Ta, W and Re@g-C 7 N 3 were highlighted from 27 TM@g-C 7 N 3 as the best SACs for NRR with low limiting potentials of −0.06 to −0.46 V. Particularly, the two systems, Ta@g-C 7 N 3 and W@g-C 7 N 3 , possess well-HER-suppressed ability due to smaller NRR-kinetic barriers as compared with those of HER and, impressively, together with their rather low limiting potentials of −0.27/−0.27 and −0.22/−0.06 V under end-on/side-on pattern, respectively, they may exceed most NRR catalysts reported previously. Moreover, multiple-level descriptors have been developed to uncover the origins of NRR activity, among which a 3D volcano plot (screening strategies, limiting potentials, and electronic origins) shows the activity trends of NRR, achieving a fast prescreening among various candidates. This work not only accelerates the discovery of catalysts for nitrogen fixation but also contributes to broadening the understanding of single-atom catalysis. The novel porous g-C 7 N 3 offers rich hollow sites for stably anchoring transition-metal atoms to promote single-atom catalysis, which is expected to overcome the great challenges of low activity and selectivity for ammonia synthesis.