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Lu et al. J Mater Inf 2024;4:31 https://dx.doi.org/10.20517/jmi.2024.65 Page 7 of 16

activation (except for the Zn system, where minimal activation is observed). The side-on mode typically
results in greater N activation due to simultaneous interactions between both nitrogen atoms and the active
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metal center. To further elucidate the mechanism of N activation at the metal active sites, Fe@C N-NCM
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and Nb@C N-NCM, which have been identified as the most promising NRR catalysts in subsequent studies,
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were selected for detailed analysis using charge density difference, Bader charge, and projected density of
states (PDOS) calculations. In both systems, the N molecules adopt the side-on configuration. As shown in
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Figure 4A and B, significant electron transfer occurs between the metal atoms and the adsorbed N
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molecules, resulting in charge accumulation at the TM-N interface and depletion along the N≡N bond. This
electron redistribution is consistent with the classical “donation-acceptance” mechanism, where the empty d
orbitals of metal atoms accept electron density from the lone pairs of N , while simultaneously
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back-donating electrons from their d orbitals into the N anti-bonding orbitals, thereby weakening the N≡N
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bond. This results in an N≡N bond elongation to 1.16 Å for Fe and 1.19 Å for Nb, compared to 1.10 Å for
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free N . Bader charge analysis further confirms this interaction, with Fe and Nb donating 0.37 e and 0.58 e
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to N , respectively. Additionally, PDOS analysis[Figure 4C and D] reveals significant overlap between the d
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orbitals of the TM atoms and the 2p orbitals of N near the Fermi level. This strong orbital hybridization
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reveals the robust interaction between the metal centers and the adsorbed N , which is crucial for efficient
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N activation and subsequent hydrogenation steps required for NRR. Based on the first screening criteria,
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five metals (Sc, Zn, Pd, Cd, Hg) were excluded due to weak N binding. The remaining 22 TMs (TM = Ti, V,
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Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Ag, Hf, Ta, W, Re, Os, Ir, Pt) correspond to 37 distinct N
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adsorption systems, comprising 22 end-on and 15 side-on configurations.
Screening of the first and last protonation steps
After the initial screening based on N adsorption, the ΔG values for the first and last protonation steps were
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calculated for 37 N adsorption systems to refine the catalyst selection. As shown in Figure 5, eleven systems
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in the yellow-shaded region meet the second and third screening criteria (ΔG(*N →*NNH) < 0.55 eV and
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ΔG(*NH →*NH ) < 0.55 eV). Of these, nine TM@C N-NCM systems (TM = Ti, V, Fe, Zr, Nb, Ta, W, Os,
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3
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Ir) exhibit side-on N adsorption; one (Pt@C N-NCM) shows end-on adsorption, and Mn@C N-NCM
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supports both adsorption modes. These eleven systems were the final candidates screened for
comprehensive evaluation of their NRR catalytic performance. It should be noted that our earlier work
involving analogous structures has shown that the impact of solvation on the ΔG of key reaction steps is
within 0.15 eV . Therefore, the solvation effect is not considered in this study.
[47]
Full reaction pathways and selectivity
U , which corresponds to the maximum free energy change (ΔG ) in the PDS, is commonly used to
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evaluate the catalytic activity of NRR catalysts. To accurately determine U , full-pathway calculations were
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performed for the eleven systems identified in the screening process. For catalysts with side-on adsorption
(TM = Ti, V, Fe, Zr, Nb, Ta, W, Os, Ir), both enzymatic and consecutive pathways were analyzed. The distal
and alternating pathways were evaluated for the Pt system due to the end-on adsorption, while Mn, capable
of both adsorption modes, was assessed across all four pathways. The U values and corresponding PDS for
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each system are illustrated in Figure 6A. The full reaction pathway for Nb@C N-NCM, which demonstrated
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the best catalytic performance, is presented in Figure 6B, while the pathways for the remaining systems are
provided in Supplementary Figure 2. Generally, the protonation steps across all pathways exhibit a
characteristic increasing-decreasing energy trend. Seven systems exhibit the PDS in the first protonation
step, with corresponding U values of -0.46 V for V, -0.42 V for Mn, -0.30 V for Fe, -0.29 V for Nb, -0.47 V
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for Ta, -0.44 V for W, -0.53 V for Os, -0.51 V for Ir, and -0.44 V for Pt. Among these, Nb@C N-NCM
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demonstrated the lowest U , indicating the highest catalytic activity. In the cases of Zr@C N-NCM and
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Ta@C N-NCM, the PDS is located in the last step of the consecutive pathway, with U values of -0.48 V and
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-0.47 V, respectively. For Ti@C N-NCM, the PDS occurs during the third step of the enzymatic pathway
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