Regulating the spin state of single-atom doped covalent triazine frameworks for efficient nitrogen fixation.

Covalent triazine frameworks (CTFs), served as a versatile platform, can form expedient metal-N single-atom coordination sites as promising catalytic centers. To seek out excellent candidate catalysts of M/CTFs (M = Transition metal) for nitrogen reduction reaction (NRR), a "five-step" strategy involving spin states has been established for hierarchical high-throughput screening and reveals strong coordination ability of the CTFs, outstanding conductivity of the M/CTFs, effective adsorption and activation of N ₂ * attributed to the electron transfer and orbital hybridization between the M/CTFs and N ₂ *. Among the potential candidates, the Cr/CTF is screened out to be an excellent one for nitrogen fixation, which can not only inhibit hydrogen evolution reaction (HER) greatly but also has good thermodynamic stability (E _b  =  -4.40 eV), narrow band gap (E _g  = 0.03 eV), moderate adsorption energy (E _a  =  -0.84 eV), large activation energy (ΔG _N2 * = -0.71 eV) and a theoretical Faradaic efficiency of 100%. The spin state has been confirmed to be an important descriptor of catalytic activity and the two-state reactivity (TSR) is validated to exist in the NRR. Reaction mechanism with different spin states of Cr/CTF has been demonstrated to give a great impact on the nitrogen fixation, providing solid theoretical support for the design of more efficient NRR catalysts.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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