Single-Atom Anchored g-C₃N₄ Monolayer as Efficient Catalysts for Nitrogen Reduction Reaction

Abstract

Electrochemical N₂ reduction reaction (NRR) is a promising approach for NH₃ production under mild conditions. Herein, the catalytic performance of 3d transition metal (TM) atoms anchored on s-triazine-based g-C₃N₄ (TM@g-C₃N₄) in NRR is systematically investigated by density functional theory (DFT) calculations. Among these TM@g-C₃N₄ systems, the V@g-C₃N₄, Cr@g-C₃N₄, Mn@g-C₃N₄, Fe@g-C₃N₄, and Co@g-C₃N₄ monolayers have lower ΔG(*NNH) values, especially the V@g-C₃N₄ monolayer has the lowest limiting potential of −0.60 V and the corresponding limiting-potential steps are ${*}^{}{\mathrm{N}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}={*}^{}\mathrm{N}\mathrm{N}\mathrm{H}$ for both alternating and distal mechanisms. For V@g-C₃N₄, the transferred charge and spin moment contributed by the anchored V atom activate N₂ molecule. The metal conductivity of V@g-C₃N₄ provides an effective guarantee for charge transfer between adsorbates and V atom during N₂ reduction reaction. After N₂ adsorption, the p-d orbital hybridization of *N₂ and V atoms can provide or receive electrons for the intermediate products, which makes the reduction process follow acceptance-donation mechanism. The results provide an important reference to design high efficiency single atom catalysts (SACs) for N₂ reduction.

1. Introduction

Ammonia (NH₃) is not only an essential substance to produce fertilizers, explosives, dyes, and pharmaceuticals, but also an important clean energy carrier [1,2,3,4]. The growing demand of NH₃ has spurred researchers to seek more efficient artificial nitrogen (N₂) fixation. Presently, large-scale production of NH₃ mainly depends on the Haber–Bosch process. However, the process not only needs to operate under harsh conditions (300–500 °C and 200–300 atm) [5], but also requires huge energy input, and simultaneously generates a large number of greenhouse gases carbon dioxide (CO₂) [6,7,8]. Considering energy consumption and environmental protection, researchers hope to find a method for converting N₂ to NH₃ using renewable energy, no polluting emissions, and mild operating conditions.

Electrochemical N₂ reduction reaction (NRR), a promising approach for sustainable NH₃ production under ambient conditions, has received extensive and increasing attention both in experiment and theoretic studies [9,10,11]. However, the strong bonding energy of N≡N triple bond (945 KJ/mol) and the weak adsorption of nonpolarized N₂ are two major challenges in the NRR process [12,13,14]. Therefore, an efficient catalyst for N₂ activation and reduction is urgent to improve NRR activity. On the other hand, the transition metal can accept the lone pair electrons of the N₂ molecule and weak the N≡N triple bond [15,16,17]. Dispersing metal atoms on suitable supporting materials can not only provide more active sites, but also regulate the electronic properties of substrate and enhance the catalytic efficiency. Two-dimensional materials (2D) are widely used as catalyst substrates for N₂ reduction due to their excellent chemical stability, thermal and electrical properties, and the ability to construct defect active sites through surface functionalization [18,19,20,21].

In recent years, transition metal single atom catalysts (SACs) constructed by anchoring single atom to two-dimensional materials have gained increasing attentions for the electrocatalysis applications and have shown excellent catalytic performances in NRR [22,23,24,25]. To avoid diffusion and agglomeration of metal atoms on the substrate, vacancies are usually constructed in the substrate to increase the stability of TM atom, or a two-dimensional material with a pore structure is used as the supporting substrate [26,27]. For example, the anchored Mo and Au on the N-doped porous carbon exhibited excellent NRR catalytic performance [28]. The Mo-embedded C₂N monolayer has been demonstrated a very high NRR catalytic activity with the onset potential (U_onset) of 0.17 V by Zhao et al. [29]. Thus, selecting the right metal atom and anchoring it to a suitable substrate with a pore structure can not only prevent the diffusion and agglomeration of the metal atom, but also higher catalytic activity can be obtained by the d-orbital electrons of metal atom inducing the π-back donation.

Graphitic carbon nitride (g-C₃N₄) has recently attracted great attentions because of its good physicochemical stability and superior properties [30,31]. tri-s-triazine-based g-C₃N₄ with large pore and s-triazine-based g-C₃N₄ with small pore are two common structures [32]. The uniform distribution of pore sites in both structures provide uniform nitrogen coordinators to capture metal atoms. Many studies [33,34] have shown that metal atoms anchored at tri-s-triazine-based g-C₃N₄ have excellent NRR performance. For example, Zhao et al. have found W-anchored tri-s-triazine-based g- C₃N₄ exhibits a high catalytic activity toward NRR with a limiting potential of −0.35 V [33]. Wang et al. have found that B/g-C₃N₄ can reduce N₂ to NH₃ with a lower onset potential (0.20 V) [34]. For s-triazine-based g-C₃N₄, it also has a pore structure surrounded by three nitrogen atoms and can be used as a metal-free substrate to anchor metal atoms. Hu et al. [35] have found that V@g-C₃N₄ with lying-on adsorbed N₂ pattern has the lowest limiting potential of −0.79 V. However, implicit solvation model was not used to simulate the electrolyte solution, and the origin of the catalytic activity has not been explored in detail. It is necessary to systematically study the N₂ reduction process in the electrolyte solution and explore the origin of the catalytic activity by anchoring single TM atom on s-triazine-based g-C₃N₄ (TM@g-C₃N₄).

Inspired by the above studies, the NRR catalytic behaviors of TM (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) atoms on intrinsic s-triazine-based g-C₃N₄ have been systematically investigated by density functional theory (DFT) theory. Theoretical calculations show that TM (V, Cr, Mn, Fe, Co) atoms anchored at g-C₃N₄ exhibits better NRR activities than Ru (0001) surface. V@g-C₃N₄ monolayer possesses the lowest limiting potential of −0.60 V, which is lower than that calculated by Hu et al. [35], and potential-limiting step (PLS) is *N₂-*NNH for both alternating and distal mechanisms. The charge density difference, spin electrons distribution, density of states (DOS), the variation of charge transfer, D_V-N, L_N-N, and M_tot are used to explore the origin of the excellent catalytic performance. These results provide an important reference for the research of s-triazine-based g-C₃N₄ SACs.

2. Computational Methods

DFT methods are performed using the Vienna Ab Initio Simulation Package (5.4.4, 2017, VASP Software GmbH, Vienna, Austria) [36,37]. The exchange-correlation potentials are described through the Perdew–Burke– Ernerhof (PBE) parametrization within the generalized gradient approximation (GGA) [38,39]. The DFT-D3 method is utilized to describe the weak van der Waals (vdW)-like interaction [40]. The kinetic energy cutoff of plane-wave expansion is set to 520 eV and a vacuum space of 20 Å is inserted in the z-direction to eliminate the interaction between adjacent periodic units [41]. A 3 × 3 × 1 supercell is constructed. For geometrical optimizations, a 1 × 1 × 1 Monkhorst–Pack mesh of k-points is employed to sample the first Brillouin zero. A finer 5 × 5 × 1 k-points grid is chosen for the density of states (DOS). The convergence tolerances for the force and energy are set to 0.02 eV/Å and 10⁻⁵ eV, respectively. Bader charge is used analyze electron transfer [42].

The adsorption energies (E_ads) of single TM atom and N₂ molecule on substrate are calculated by the following formula:

$${\mathrm{E}}_{\mathrm{a}\mathrm{d}\mathrm{s}}={\mathrm{E}}_{{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}@\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}_{}}-{\mathrm{E}}_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}-{\mathrm{E}}_{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}}$$

(1)

where ${\mathrm{E}}_{{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}@\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}_{}}$, ${\mathrm{E}}_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}$, and ${\mathrm{E}}_{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}}$ are the total energies of adsorbate @substrate, substrate, and an isolate adsorbate, respectively. Negative E_ads indicates that the adsorbate can be stably bond to the substrate.

The cohesive energy (E_coh) is calculated by the following formula:

$${\mathrm{E}}_{\mathrm{c}\mathrm{o}\mathrm{h}}={\mathrm{E}}_{\mathrm{T}\mathrm{M}}-{\mathrm{E}}_{\mathrm{T}\mathrm{M},\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}$$

(2)

where ${\mathrm{E}}_{\mathrm{T}\mathrm{M}}$ and ${\mathrm{E}}_{\mathrm{T}\mathrm{M},\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}$ are the energies of an isolated TM atom and bulk metal, respectively.

Computational hydrogen electrode (CHE) mode [43] is used to calculate the reaction free energy (ΔG). A solvation model is used to simulate the solution environment [44], and a relative permittivity of 80 is set for water [45]. The ΔG is evaluated as follows:

$$\mathsf{\Delta }\mathrm{G}={\mathsf{\Delta }\mathrm{E}+\mathsf{\Delta }\mathrm{Z}\mathrm{P}\mathrm{E}}_{}-\mathrm{T}\mathsf{\Delta }\mathrm{S}+\mathsf{\Delta }{\mathrm{G}}_{\mathrm{U}}+\mathsf{\Delta }{\mathrm{G}}_{\mathrm{P}\mathrm{H}}$$

(3)

where $\mathsf{\Delta }\mathrm{E}$ is the reaction energy directly obtained by DFT calculations, $\mathsf{\Delta }\mathrm{Z}\mathrm{P}\mathrm{E}$ and $\mathsf{\Delta }\mathrm{S}$ are the changes of zero-point energy (ZPE) and entropy computed from the vibrational frequencies at 298.15 K. $\mathsf{\Delta }{\mathrm{G}}_{\mathrm{U}}$ is the free energy contribution with applied potential U, ΔG_U = eU, where e and U are the number of electrons transferred and the applied electrode potential. $\mathsf{\Delta }{\mathrm{G}}_{\mathrm{p}\mathrm{H}}={\mathrm{k}}_{\mathrm{B}}\times \mathrm{p}\mathrm{H}\times \mathrm{l}\mathrm{n}10$, where pH = 0 in this work for simplicity. The limiting potential (U_L) is defined as ${\mathrm{U}}_{\mathrm{L}}=-\mathsf{\Delta }{\mathrm{G}}_{\mathrm{m}\mathrm{a}\mathrm{x}}/\mathrm{e}$.

Using the CHE model, the lowest $∆\mathrm{G}\left(\mathrm{p}\mathrm{H},\mathrm{U}\right)$ was evaluated. The $∆\mathrm{G}(\mathrm{p}\mathrm{H},\mathrm{U})$ values can be projected onto the $(\mathrm{p}\mathrm{H},\mathrm{U})$ plane of lowest Gibbs free energy, resulting in a 2D plot with pH and U descriptors to build the Pourbaix diagram [46]. The Pourbaix diagrams can provide the reliability electrocatalytic reaction results under certain conditions of pH and potential U.

3. Results and Discussion

3.1. Structural and Electronic Properties of TM@g-C₃N₄

Optimized structure of s-triazine-based g-C₃N₄ monolayer is displayed in Figure 1a. It contains two C-N bonds, the bond lengths d_C-N are 1.33 Å and 1.46 Å, respectively. The optimized lattice constant is g-C₃N₄ is 4.78 Å, which is consistent with previous theoretical results [47,48]. The symmetrical distribution of spin-up and spin-down electrons in Figure 1b indicates that the g-C₃N₄ is a non-magnetic semiconductor. TM (Sc-Cu) atom is anchored at five high-symmetry sites (Figure 1a) to investigate the stable configurations, namely, the top sites of C (T₁), pyridine N (T₂), and graphite N (T₃), hollow sites above the three pyridine N (H₁) and the triazine ring (H₂). Taking Mn atom as an example, the calculated adsorption energies (E_ads) are −5.16 eV, −6.01 eV, −4.49 eV, −6.20 eV, −3.70 eV, respectively, and the H₁ site is the most stable adsorption site with the lowest E_ads −6.20 eV. All the TM atoms have a same stable adsorption site H₁. The stable configuration of Mn atom adsorbed at H₁ site is shown in Figure 1c. The adsorption of Mn atom causes the distortion of g-C₃N₄ monolayer, but no bond breaks. The other configurations of TM@g-C₃N₄ are displayed in Figure S1.

The calculated adsorption energies (E_ads), energy difference between adsorption energy and cohesive energy (ΔE), total magnetic moments (M_tot), the charge (ΔQ) transferred from TM atoms to g-C₃N₄ monolayer, and the electronic structures (ES) of TM@g-C₃N₄ are shown in

Table 1. Adsorption energies (E_ads, eV), energy difference (ΔE, eV) between adsorption energy and cohesive energy, total magnetic moments (M_tot, μ_B), the charge (ΔQ, e) transferred from the TM atoms to g-C₃N₄ monolayer, and the electronic structures (ES) (SM: semi-metallic, M: metallic and SC: semiconductor).

	Eads	ΔE	Mtot	ΔQ	ES
Sc@g-C3N4	−9.55	−5.36	1.00	2.35	SM
Ti@g-C3N4	−8.26	−2.80	2.00	2.21	M
V@g-C3N4	−7.50	−2.13	4.88	1.92	M
Cr@g-C3N4	−5.97	−1.96	4.00	1.59	SC
Mn@g-C3N4	−6.20	−2.34	1.00	1.48	SC
Fe@g-C3N4	−5.70	−0.92	−0.26	1.35	M
Co@g-C3N4	−7.25	−1.89	1.00	1.02	SC
Ni@g-C3N4	−7.32	−2.42	0	0.94	SC
Cu@g-C3N4	−4.96	−1.45	1.00	0.86	SM

. The premise of good catalytic performance of SAC is that single metal atom has suitable E_ads on the substrate. The calculated E_ads values of TM@g-C₃N₄ are in the range of −9.55~−4.76 eV, which indicates that TM atoms can bind stably to the substrate. At the same time, the energy difference (ΔE) between the adsorption energy (E_ads) and the cohesive energy (E_coh) is calculated to investigate the aggregation of TM atoms on g-C₃N₄. The value of ΔE is in the range of −5.36~−0.92 eV, and a negative value of ΔE means that the adsorption of metal atoms on g-C₃N₄ is stronger than the cohesive of atoms. Hence, the aggregation of TM atoms on g-C₃N₄ can be suppressed efficiently. TM (Sc-Cu) atoms anchored to g-C₃N₄ holes have good stability.

The spin densities distribution of TM@g-C₃N₄ are shown in Figure S2. It can be seen that spin electrons are located at TM (TM= Sc, Ti, V, Cr, Mn, Fe, Co, Cu) and surrounding atoms. Compared with Figure 1b, it is obvious that the anchored TM atoms (TM= Sc, Ti, V, Cr, Mn, Fe, Co, Cu) induce the magnetic moment of TM@g-C₃N₄. V@g-C₃N₄ has the most spin densities, which is consistent with the largest moment (4.88 μ_B) in Table 1.

As is well known, the anchored TM atoms can regulate the electronic structure of substrate, and the catalytic performance of the catalyst is closely related to the electronic structure. Hence, the total density of states (TDOS) of TM@g-C₃N₄ and partial DOS (PDOS) are calculated and shown in Figure S3. It is seen that the anchored TM (TM=Sc, Ti, V, Cr, Mn, Fe, Co, Cu) atoms induce the asymmetry of spin-up and spin-down electrons of TM@g-C₃N₄, which is consistent with the spin electrons distribution in Figure S2. Second, the adsorption of Cr, Mn, Co, and Ni atoms retain the semiconductor property, but the band gap decreases from 1.57 eV to 0.38 eV, 0.46 eV, 0.58 eV, and 1.23 eV, respectively. The TDOSs of Ti@g-C₃N₄, V@g-C₃N₄, and Fe@g-C₃N₄ exhibit metallic properties and many electronic states are located near the Fermi level, which ensures a rapid transfer of charge in the reaction.

3.2. Adsorption of N₂ Molecule

It is well known that the N₂ adsorption on the catalytic is the first step to investigate the NRR performance, which reflects the sensitivity of the catalyst and determines the most favorable adsorption manner. Stable adsorption of N₂ is a prerequisite for the subsequent reaction to generate NH₃.

The adsorption energies [E_ads(*N₂)] of TM@g-C₃N₄ (TM = Sc-Cu) are illustrated in Figure 2a. For TM@g-C₃N₄ (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu), both end-on and side-on configurations can exist, while E_ads(*N₂) (−1.76~−0.58 eV) of end-on adsorption N₂ molecule are lower than those (−1.45~−0.26 eV) of side on patterns, which means that end-on patterns are more energetically favorable than the side-on patterns. Therefore, in the following studies of this paper, only the favorable end-on adsorption manner is discussed. These results indicate that the TM atoms (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) anchored at g-C₃N₄ are active sites and have strong ability to capture N₂ molecules. This result is consistent with the result calculated by Hu et al. [35] except for Ti atom.

In order to further study the adsorption of N₂ on TM@g-C₃N₄, the distance between TM atom and the N₂ molecule (D_TM-N), and N-N bond length (L_N-N) are displayed in Figure 2b. The D_TM-N (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) are in the range of 1.76~2.09 Å and the L_N-N are stretched to 1.13~1.14 Å, which indicates that the N≡N triple bonds are weakened and good for reduction reaction. For Sc@g-C₃N₄, the adsorption energies for end-on and side-on patterns are −0.01 eV and 0.52 eV, the D_Sc-N is 4.14 Å and the L_N-N is 1.11 Å, which means that Sc@g-C₃N₄ has poor NRR activity. TM@g-C₃N₄ (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) will be considered as SAC candidates in the following studies.

3.3. N₂ Electrocatalytic Reduction Reaction

The second criterion for good NRR catalyst is that it has a lower limiting potential in the N₂ reduction reaction. To evaluate the catalytic activities of TM@g-C₃N₄ (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu), the NRR mechanisms are investigated to obtain the potential-limiting step (PLS) and limiting potential.

Theoretical and experimental studies [49] on NRR show that the end-on adsorbed N₂ molecule on TM@g-C₃N₄ (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) can be reduced to NH₃ via a distal or alternating mechanism. In the distal mechanism, the proton (H⁺) continuously attacks the terminal N atom until the first NH₃ molecule is released and then the other. While for the alternating mechanism, the proton (H⁺) alternating bonds the two N atoms to release NH₃ molecule.

Previous studies suggest that the first H⁺ reaction ${*}^{}{\mathrm{N}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}={*}^{}\mathrm{N}\mathrm{N}\mathrm{H}$ is usually the potential-limiting step (PLS) for NRR [16,50]. The free energy barrier ΔG(*NNH) is calculated and displayed in Figure 3a. It can be seen that the ΔG(*NNH) values of V@g-C₃N₄ (0.60 eV), Cr@g-C₃N₄ (0.76 eV), Mn@g-C₃N₄ (0.78 eV), Fe@g-C₃N₄ (0.63 eV), and Co@g-C₃N₄ (0.71 eV) are lower than that on Ru (0001) surface. On the other hand, since the hydrogen evolution reaction (HER) is a major competitor for NRR [17,51], the calculated free energies change of H [ΔG(*H)] are shown in Figure 3b. It is seen that ΔG(*H) are positive for V@g-C₃N₄, Cr@g-C₃N₄, and Mn@g-C₃N₄, which means that H atom cannot be adsorbed on V, Cr and Mn atoms. For Fe@g-C₃N₄ and Co@g-C₃N₄, the ΔG(*H) is negative, while the values of ΔG(*N₂) are lower than ΔG(*H) and they prefer to adsorb N₂ molecule. In conclusion, V, Cr, Mn, Fe, and Co atoms anchored by g-C₃N₄ are selected as potential SACs for NRR by the second criterion.

In order to explore the whole N₂ reduction process, it is necessary to explore the PLS of the process. The PLS dominates the catalytic efficiency of the catalyst for NRR. The results show that V@g-C₃N₄ is the best catalyst with the lowest ΔG(*NNH) value 0.60 eV compared with Cr@g-C₃N₄, Mn@g-C₃N₄, Fe@g-C₃N₄, and Co@g-C₃N₄. The whole free energy diagrams of V@g-C₃N₄ during NRR process is further investigated with the favorable end-on adsorption manner.

The NRR free energy diagrams of V@g-C₃N₄ are displayed in Figure 4. Optimized adsorption structures of intermediate via the alternating and distal pathways are depicted in Figure S4. The free energy diagrams via alternating and distal mechanisms are very similar. During N₂ reduction process, the N₂ adsorption and protonation into *NNH along the two mechanisms are same. The adsorption process of $*+{\mathrm{N}}_2={*}^{}{\mathrm{N}}_2$ is downhill with ΔG value −0.89 eV, which shows that V@g-C₃N₄ can spontaneously adsorb the N₂ molecule. The step of ${*}^{}{\mathrm{N}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}={*}^{}\mathrm{N}\mathrm{N}\mathrm{H}$ is uphill with ΔG value 0.60 eV, which means that the protonation process is endothermic. The second proton-electron may bond with another N atom of *N₂ molecule to generate *NHNH (alternating mechanism) or the same N atom (distal mechanism) to generate *NNH₂, and the hydrogenation in both ways are both uphill with ΔG value 0.48 eV and 0.13 eV, respectively. In the following steps of two mechanisms, ΔG values are all negative. For alternating mechanism in Figure 4a, the calculated ΔG values are −0.89, 0.60, 0.48, −0.44, −0.12, −1.81 eV, and −0.27 eV, respectively. The first protonation step has the highest ΔG values and is confirmed to be the PLS with an uphill value 0.60 eV. When the U_L is applied to −0.60 V, all the reaction steps are exothermic, therefore, the limiting potential is −0.60 V. For distal mechanism in Figure 4b, the ΔG values are −0.89, 0.60, 0.13, −0.77, −0.55, −0.70, and −0.27 eV, respectively, and it is obvious that it has the same PLS and U_L as the alternating mechanism. The calculated limiting potentials of V@g-C₃N₄ are lower than those calculated by Hu et al., where the limiting potential of V@g-C₃N₄ with lying-on adsorbed N₂ is −0.79 V, while that of standing-on pattern is −0.93 V [35]. The final desorption of the second *NH₃ molecule requires the absorption energy of 1.16 eV, and it can also be further protonated to form NH₄⁺ [52].

Pourbaix diagrams can provide an effective guidance for the electrocatalytic reactions, which will not be discussed in this article.

3.4. Origin of Catalytic Activity

To explore the excellent catalytic efficiency of V@g-C₃N₄, the electronic properties, charge density difference, and spin electrons distribution are investigated. The charge density difference is shown in Figure 5a. It can be seen that the electron densities of the three N atoms anchoring the V atom increases, and Bader charge analysis finds that the V atom loses 1.92 e, which makes the V atom be a good active site for trapping N₂ molecule. Figure 5b shows that the spin density is mainly located at the V atom, which makes V@g-C₃N₄ possess a total spin moment of 4.88 μ_B. These results prove the conclusion that the magnetism of the catalyst can increase its activity. The density of states (DOS) is presented in Figure 5c. Different from the intrinsic s-triazine-based g-C₃N₄ monolayer of direct bandgap semiconductor in Figure 1b [47,48], V@g-C₃N₄ exhibits a metallic property owing to the V atom. The spin moment is vital to activate N₂ molecule, and the excellent electrical conductivity is essential to ensure good charge transfer for efficient electroreduction reaction [48,53]. Both of them play an important role in the outstanding NRR catalytic performance of V@g-C₃N₄.

In order to further explore the understanding NRR catalytic performance of V@g-C₃N₄, the charge density difference and spin electrons distribution after N₂ adsorption are calculated. As shown in Figure 6a, the electrons accumulate near the *N₂ molecule, and it is confirmed by Bader charge analysis that 0.42 e transferred from V atom to the N₂ molecule. The spin electrons distribution in Figure 6b reveals that after N₂ adsorption, some spin moment on V atom can be transferred to the N₂ molecule, so that the N₂/V@g-C₃N₄ only possesses a magnetic moment 1.00 μ_B. The change in charge and spin magnetic moment drives its further activation and reactions. The DOS of N₂/V@g-C₃N₄ are displayed in Figure 6c. Obvious overlap between the *N₂-2p and V-3d orbitals around the Fermi energy happens, in which the empty d orbitals of V can accept the lone-pair electrons in N₂, at the same time, the occupied d orbitals can donate electrons to the antibonding orbitals of N₂, and this process follows acceptance-donation mechanism.

To further explore the charge transferred among intermediate adsorbent, the variations of the charge transferred between them are studied. Based on the previous studies [54,55], N_xH_y/V@g-C₃N₄ is divided into three parts (Figure S5): g-C₃N₄ substrate without V and the three N atoms bonded with it (moiety 1), V-3N (moiety 2), and the adsorbed intermediate N_xH_y (moiety 3). The charge variations along distal and alternating mechanisms are shown in Figure 7 (the charge variation is the charge difference of each moiety between the present step and the previous step). For N₂ adsorption on V@g-C₃N₄ monolayer in Figure 7a, V-3N offers 0.42 e and 0.45 e to *N₂ and g-C₃N₄, respectively. During the protonation process ${*}^{}{\mathrm{N}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}={*}^{}\mathrm{N}\mathrm{N}\mathrm{H}$, V-3N and g-C₃N₄ substrate provide 0.15 e and 0.19 e, respectively. However, during the second protonation process ${*}^{}\mathrm{N}\mathrm{N}\mathrm{H}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}={*}^{}{\mathrm{N}\mathrm{N}\mathrm{H}}_2$, V-3N gains 0.52 e from the substrate and *NNH₂. In the following hydrogenation and reduction steps, obvious charge fluctuation occurs for the three moieties, which means that V-3N is the charge transferred medium between N_xH_y and g-C₃N₄ substrate, providing or receiving electrons to the adsorbed intermediate. The above results once again prove that the excellent catalytic performance of V@g-C₃N₄ in NRR process follows the acceptance-donation mechanism. The charge variation in the alternating mechanism shown in Figure 7b is similar to those the distal mechanism.

In order to explore the change of adsorbates configuration and magnetic properties during the reduction of N₂, the V-N distance (D_V-N), N-N bond length (L_N-N), and the total magnetic moments (M_tot) are examined and displayed in Figure 8. In Figure 8a, D_V-N along distal pathway decreases during protonation and reaches a minimum when the first NH₃ molecule is released, and then continuously increases. While in alternating mechanism, when the proton (H⁺) attacks the distal N atom, D_V-N decreases, and when it attacks the N atom bonded to V, D_V-N increases. D_V-N increases in a zigzagging fluctuation way during the whole reduction process. For L_N-N, as shown in Figure 8b, it is stretched continuously until the first NH₃ is released for both distal and alternating mechanisms. Figure 8c shows the magnetic moments of the N_xH_y, and it can be seen that the protonation process makes the M_tot show zigzag fluctuations for both mechanisms. These results provide strong evidence for the excellent catalytic performance of V@g-C₃N₄.

4. Conclusions

In this paper, the catalytic performance of 3d TM atoms anchored on s-triazine-based g-C₃N₄ (TM@g-C₃N₄) for N₂ reduction is systematically investigated. The results show that N₂ molecules can be stably adsorbed on TM@g-C₃N₄ (TM=Ti, V, Cr, Mn, Fe, Co, Ni, Cu), and the end-on modes are more energetically favorable than the side-on modes. The ΔG(*NNH) values indicate that TM@g-C₃N₄ (TM = V, Cr, Mn, Fe, Co) are potential NRR candidates with lower ΔG(*NNH) value than that on Ru (0001) surface. The free energy diagrams show that V@g-C₃N₄ exhibits the highly catalytic performance for NRR with a limiting potential −0.60 V, and the PLS is the first protonation of *N₂ for both alternating and distal mechanisms. In order to explore the origin of excellent NRR performance of V@g-C₃N₄, the charge density difference, spin electrons distribution and DOS plots before and after N₂ adsorption, and their corresponding charge variation of adsorbates during N₂ reduction reaction are discussed. First, for V@g-C₃N₄, V atom as an active site provide 1.92 e and has the ability to trap N₂ molecule. A spin moment of 4.88 μ_B of V@g-C₃N₄ contributed by V atom can activate N₂ molecule. Second, after N₂ adsorption, the strong interaction between *N₂-2p and V-3d orbitals ensures the charge transferred during N₂ reduction reaction. Third, the charge variation of moieties indicate that V-3N acts as a medium between adsorbates N_xH_y and g-C₃N₄ substrate, which ensures the efficient reduction of N₂. The D_V-N, L_N-N and M_tot of N_xH_y/V@g-C₃N₄ are calculated to illustrate the change of adsorbate configurations and magnetic properties during the N₂ reduction reactions, which provides an important evidence for the excellent catalytic performance of SACs TM@g-C₃N₄.