Regulating the electrocatalytic performance for nitrogen reduction reaction by tuning the N contents in Fe3@NxC20-x (x = 0~4): a DFT exploration
Abstract
The Haber-Bosch (H-B) process, which is widely used in industry to synthesize ammonia, leads to serious energy and environment-related issues. The electrochemical nitrogen reduction reaction (eNRR) is the most promising candidate to replace H-B processes because it is more energy-efficient and environmentally friendly. Atomic-level catalysts, such as single-atom and double-atom catalysts (SACs and DACs), are of great interest due to their high atomic utilization and activity. The synergy between the metal atoms and two-dimensional (2D) support not only modulates the local electronic structure of the catalyst but also controls the catalytic performance. In this article, we explored the eNRR performance of 2D Fe3@NxC20-x (x = 0~4), whose structure was based on the experimentally synthesized Ag3@C20 sheet, by means of density functional theory calculations. Through calculations, we found that the 2D Fe3@N4C16 with Fe2 site coordinated with four N is a promising eNRR catalyst: the limiting potential is as low as -0.45 V, and the competing hydrogen evolution reaction can be effectively suppressed. Our work not only confirms that the coordination environment of the metal site is crucial for the electrocatalytic activity but also provides a new guideline for designing low-cost eNRR catalysts with high efficiency.
Keywords
INTRODUCTION
Ammonia is a raw material vital to the global economy because of its wide range to be converted into fertilizers, chemicals, future fuel substitutes, and hydrogen storage[1]. The direct conversion of nitrogen in the air into ammonia is of paramount importance to both human beings and the planet’s ecosystem[2]. Yet, the Haber-Bosch (H-B) process, the main traditional process for producing ammonia in industry, requires extremely demanding conditions, such as high temperatures and pressures[3-5]. Furthermore, the hydrogen in the H-B process is derived from natural gas or methane[6], which means that the process will release large amounts of CO2[7]. Electrocatalytic nitrogen reduction reactions (eNRR) have been attracting increasing interest because of the mild reaction conditions compared to those of the H-B process; the reaction can be carried out under ambient conditions with the assistance of renewable electricity[8,9]. In addition, the source of hydrogen from water and the energy resource required for the eNRR process can be renewable, thus eliminating the negative impact of fossil fuels as the source of hydrogen and energy[1]. However, eNRR is a complex reaction involving six electron transfer processes, which is kinetically and thermodynamically more difficult compared to the hydrogen evolution reaction (HER), thus making the protons and electrons on the electrocatalyst susceptible to the production of H2 rather than NH3[10]. Therefore, it is desirable to develop catalysts for eNRR with high conversion and effective HER inhibition[11].
Many candidates could be used as eNRR electrocatalysts, for example Chen et al. used plasma-etched Ti2O3 (OV-Ti2O3) with oxygen vacancies to obtain maximum NH3 yield up to 37.24 μg·h-1·mgcat.-1 in 0.1 M HCl[12], and they proposed a mixture of titanium dioxide and effusus-derived carbon microtubes with three-dimensional cross-linked hollow tubular structures, which gave NH3 yield as high as 20.03 μg·h-1·mgcat.-1 in 0.1 M Na2SO4[13]. The single-atom catalysts (SACs)[14], which offer a new way to maximize the efficiency of metal atom use, have emerged as promising candidates[15]. The localized structures of the metal atoms allow them to exhibit excellent electronic properties and catalytic performance in a variety of chemical reactions[16]. However, the surface free energy of metals increases significantly with decreasing particle size, and metal atoms tend to aggregate[17]. Additionally, the stability of SACs in practical reactions depends on the surface configuration, the substrate, the reactants, and the limited temperature/pressure, which are not well defined and vary from one system to another[18]. The appropriate choice of substrates is a major (and in some cases a critical) factor in determining the catalytic performance of the catalyst, which should provide a large specific area to accommodate the metal sites and be able to modulate the local geometry and electronic structure of the metal atoms in the reaction, thus determining the overall catalytic performance[19]. Two-dimensional (2D) carbon[11,20], N-doped carbon[21-24], etc.[25-28], used as substrates for metal atoms as atomic catalysts, exhibit excellent efficiency toward eNRR. As an extension of SACs, double-atom catalysts (DACs) became another research hotspot due to their higher metal loading than SACs[22]. In terms of the overall reaction, DACs have multiple active sites (top and bridge) since their active centers are composed of two adjacent metal atoms, which can significantly improve the Faraday efficiency[29]. The bridge-site adsorption behavior of metal-metal atoms was found to exhibit a strong synergistic effect of modulating the electronic structure and promoting adsorption[30]. In addition, differences in surface states in different homonuclear or heteronuclear DACs can have an important impact on the selectivity and activity of the reactions[31]. Thus, both SACs and DACs may be excellent electrocatalysts if the right substrate is chosen.
Previous studies revealed that excellent electrocatalytic performance for the nitrogen reduction reaction (NRR) can be obtained by loading iron onto different substrates, whether as monoatoms[32-36], dimers[22,37-39], or other sizes[40-44]. In addition, given that the iron-molybdenum cofactor (FeMoco) is thought to be the site of dinitrogen reduction[45], we focused on Mo/Fe-containing catalysts. The chosen substrate was prompted by the recent experimental preparation of the 2D material Ag3C20[46], a planar structure with a rectangular lattice containing three Ag atoms and 20 C atoms. The three metal atoms in the lattice can be considered as a single atom (M1) and a bimetallic atom (M2) site. In this work, we replaced the precious metal Ag atoms in Ag3@C20 with Mo/Fe atoms and obtained the Mo3/Fe3@C20 monolayer. Through theoretical studies of the eNRR process, we found that Mo3@C20 is not suitable for eNRR due to its strong N2 adsorption; in contrast, Fe3@C20 is a good eNRR catalyst (the limiting potential UL = -0.59 V). With the demonstration that Fe3@C20 can be used as an electrocatalyst for eNRR, we substituted the C bonded with Fe by N to tune the coordination environment of the metal sites in order to further improve the catalytic efficiency. The limiting potential was further changed to -0.45 V when the N doping of the Fe2 site was the maximum; i.e., all the four C bonded with Fe2 sites were replaced by N with the stoichiometry of Fe3@N4C16.
MATERIALS AND METHODS
All density functional theory (DFT) calculations were carried out using the Vienna ab initio Simulation Package (VASP)[47]. The Perdew, Burke, and Ernzerhof (PBE)[48] parameterization of the generalized gradient approximation (GGA) was used to describe the exchange-correlation function. The DFT-D2 dispersion correction scheme was applied for van der Waals (vdW) interactions[49]. A cut-off energy of
The adsorption energy of N2 was calculated by Eads = Etot - E* -
In the above equation, E is the DFT energy, EZPE and S are the zero-point energy and entropy[54], respectively, EZPE and TS can be obtained by VASPKIT post-processing program[55], ΔGU = -eU, ΔGpH = -kBTln10 × pH, kB is the Boltzmann constant, T is 298.15 K[56], and the pH is zero in this study. The corrected values of the gas molecules and intermediates were given in Supplementary Tables 1 and 2, respectively. The limiting potential (UL) of the entire reduction process was determined by the potential limiting step and was computed by UL = -ΔGmax/e[57].
We used the following equation for the formation energy of N doping:
where
RESULTS AND DISCUSSION
The structure and the eNRR performance of the M3@C20 (M = Mo and Fe) monolayers
After structural optimization, the structures are represented by the Fe3@C20 shown in Figure 1A with rectangular lattice constants of a = 11.14 Å and b = 7.95 Å, and a = 11.59 Å and b = 8.45 Å for Mo3@C20, respectively. Due to the different loading positions of the metal atoms, they can be characterized as single-atom (M1) and double-atom (M2) sites (the Mo-Mo/Fe-Fe bond lengths at the diatomic sites are 2.85/2.22 Å, which also demonstrates the formation of metallic bonds in the M2 sites). The results of the electron localization function (ELF: ELF values of 1.0 and 0.5, representing fully localized and free electron gases respectively, while ELF values close to 0 represent the low electron density region, Supplementary Figure 2) show that electron transfer from the Mo/Fe atoms to the C atoms, which is in line with the Bader charge analysis [Mo1 (Mo2): 1.16 (1.46) |e| and Fe1 (Fe2): 0.96 (1.34) |e|]. The covalent bond between C and C atoms (1.32~1.47 Å) can also be identified. The total density of states (TDOS) [Figure 1B and Supplementary Figure 3] reveals that there are electronic states crossing the Fermi energy level, indicating the metallicity of both Fe3@C20 and Mo3@C20, which is beneficial to electrocatalysis.
Figure 1. (A) Top and side views of the optimized 2D Fe3@C20 monolayer with the unit cell marked by black solid lines; (B) the partial density of states of the d orbitals of Fe atoms (red), and the DOS of the Fe3@C20 monolayer (black) with the Fermi level indicated by the red dashed line.
For the electrochemical synthesis of ammonia, the adsorption of N2 on the catalyst surface is the first step in the process. We considered two possible N2 adsorption configurations, i.e., the side-on and end-on structures, on both M1 and M2 sites, respectively. The results are displayed in Figure 2 and Supplementary Figure 4. After N2 adsorption onto Mo3@C20, the distances between N2 and Mo after structural optimization ranged from 1.95 to 2.25 Å. It is noteworthy that when N2 adsorbs to the Mo2 site in the side-on conformation, it will spontaneously shift to the end-on conformation; thus, we only considered the end-on adsorption structure. The N−N bond lengths of N2 at the Mo1 (Mo2) site in the side-on and end-on conformations are 1.21 and 1.14 (1.12) Å, respectively. The adsorption energies (Eads) of N2 at the Mo1 (Mo2) sites are all larger than 5.00 eV; such strong adsorption is not conducive to subsequent reactions or nitride releasing, and therefore Mo3@C20 is not suitable for the eNRR. In contrast, the distances between N of N2 and Fe range from 1.88 to 3.13 Å. The N−N bond lengths of the adsorbed N2 adopting side-on and end-on configurations are 1.11 (1.18) and 1.13 (1.14) Å at the Fe1 (Fe2) site, respectively, and the Eads values of N2 are -0.10 (-0.57) eV and -0.41 (-0.52) eV at the Fe1 (Fe2) site, respectively. In comparison with the 1.11 Å of N−N length of an isolated N2 molecule, the N−N length is elongated, and the adsorbed N2 is activated (also indicated by the considerable adsorption strength), except for the case of N2 adsorbing on the Fe1 site with side-on structures. The Bader charge analysis showed that the number of electrons transferred from Fe1
Figure 2. Different adsorption structures of N2 on Fe3@C20: (A) end-on and (B) side-on configurations at Fe1 sites; and (C) end-on and (D) side-on configurations at Fe2 sites, with the unit cells marked by black lines.
Before conducting the eNRR pathway test on the Fe2 site, we evaluated the stability of Fe3@C20 by carrying out first-principles molecular dynamics (FPMD) simulations at 300 K for 5 ps (with a time step of 1 fs). The result showed that both the Fe1 and Fe2 moieties in the 2D structure were well preserved during the FPMD simulations [Supplementary Figure 5], which demonstrates that Fe3@C20 has good thermodynamic stability.
The two eNRR pathways on the Fe2 site of Fe3@C20 to produce ammonia started from side-on (end-on) adsorption are shown in Figure 3A and B, which are the enzymatic (alternative) mechanism [* → *N2 → *NNH → *NHNH → *NH2NH → *NH2NH → *NH2NH2 → *NH2 → *NH3 → NH3(g)] and the consecutive (distal) mechanism [*→ *N2 → *NNH → *NNH2 → *N → *NH2 → *NH3 → NH3(g)], respectively. The main reasons for considering the above pathways can be assigned to the following two aspects: (1) N2 has different adsorption configurations; (2) the proton-electron (H+ + e-) pairs attack N atoms in different manners [whether they attack one N atom consecutively first (consecutive/distal route) or attack the two N atoms alternatively (alternating/enzymatic path)].
Figure 3. Different eNRR mechanisms on Fe3@C20 (A and B) and the free energy diagrams (C and D). Data denote the ΔG of each elementary step. eNRR: Electrochemical nitrogen reduction reaction.
Following the above mechanisms, the structural illustrations of each intermediate and free energy diagram are given in Figure 3. The results show that the change in free energy is preferred along the distal pathway. In detail, starting from the N2 adsorbing with side-on configurations, the free energy change (ΔG) follows the sequence of -0.09, 0.64, 0.00, -0.12, -0.04, -1.82, 0.07, and 0.53 (-0.09, 0.64, 0.50, -1.05, -0.57, -0.87, 0.07, and 0.53) eV for each elementary step along the enzymatic (consecutive) route; while starting from the end-on adsorption of N2, ΔG goes through -0.09, 0.59, -0.06, -0.20, 0.14, -1.83, 0.08, and 0.53 (-0.09, 0.59, -0.04,
The HER, requiring proton-electron pairs to proceed with the reaction, is the primary competitor against the eNRR. Therefore, we calculated the adsorption energy of *H and found Eads of *H (-0.30 eV) > Eads of *N2 (-0.57/-0.52 eV) on the Fe2 site. In addition, to exclude the possibility that the Fe1 site may also proceed HER, lowering the selectivity towards eNRR, we also calculated Eads of *H on the Fe1 site (0.16 eV). All results suggest that HER would be effectively suppressed over the Fe3@C20.
The geometry and the eNRR performance of the Fe3@NxC20-x (x = 1~4)
Previous studies showed that the catalytic performance of catalysts is associated with the coordination environment of the active site. Thus, 2D Fe3@NxC20-x (x ≤ 4) materials were constructed by replacing Fe2-bonded C with N, and all the optimized structures remain well with the initial plane [Supplementary Figure 6]. The formation energy of Fe3@NxC20-x decreases from -0.20 eV at x = 1 to -0.65 eV at x = 4
The adsorption energies of N2 with side-on configurations on Fe2 sites of most N-substituted monolayers are also stronger than those with end-on structures, and the adsorption strength decreases as the content of N increases, as demonstrated by the data in Supplementary Table 4: the Eads values of the side/end-on adsorption configurations on Fe2 sites of 2D Fe3@NxC20-x (x ≤ 4) monolayers are -0.62/-0.55 eV for
Figure 4. Top and side views of N2 adsorbed at the Fe2 site of Fe3@N4C16 with the side-on configuration (A); the end-on configuration (B) with the unit cells marked by black lines; ΔG for the *NN → *NNH step of the energy preferred path for the Fe3@NxC20-x(x = 0~4) materials and the black dashed line indicates the free energy change value for Fe3@C20 (C) and the eNRR free energy diagram over the Fe2 site of Fe3@N4C16 (D). eNRR: Electrochemical nitrogen reduction reaction.
On the Fe3@N4C16, the free energy change for each elementary step along the enzymatic mechanism was 0.21, 0.45, -0.07, 0.01, 0.09, -2.12, 0.19, and 0.40 eV, and along the consecutive mechanism, it was 0.21, 0.45, 1.08, -1.43, -0.36, -1.38, 0.19 and 0.40 eV, respectively [Figure 4D]. Notably, although the ΔG of nitrogen adsorption is a positive value (0.21 eV), the adsorption energy before correction is negative (-0.20 eV), and the elongation of the N−N bond (1.17 Å) further suggests that the nitrogen was activated. However, in terms of the overall reaction, this small positive value is smaller than the free energy change in the first step of hydrogenation, i.e., *NN → *NNH (0.45 eV). As for the eNRR on the Fe2 site of Fe3@N4C16, the step *NN → *NNH becomes the PDS in the enzymatic mechanism, possessing a free energy change value of 0.45 eV, which is smaller than the values on Fe2Mo@C6N6 (0.53 eV)[60] and Fe/Mo-N-C (0.63 eV)[61]. Furthermore, the free energy change of releasing the second NH3(g) (0.40 eV) is also smaller than ΔG of the PDS
In addition, since the 2D Fe3@NxC20-x (x ≤ 4) includes both Fe1 and Fe2 sites, to make sure that the Fe2 site is the active site for eNRR to proceed, we also calculated the adsorption energy of N2 at the Fe1 site for comparison [Supplementary Table 4]. The adsorption energies of N2 for the side-on/end-on adsorption configuration at the Fe1 site are -0.11/-0.43 eV for Fe3@N1C19, -0.06/-0.39 eV for Fe3@N2C18-I, -0.08/-0.47 eV for Fe3@N2C18-II, and -0.17/-0.43 eV for Fe3@N2C18-III. For x = 1, 2, it indicates that N2 prefers to adsorb at Fe2 sites, while N2 may prefer to adsorb on the Fe1 site of Fe3@N3C17 (-0.08/-0.58 eV) and Fe3@N4C16 (0.15/
All the above calculations show that the Fe2 site remains the active center with N-doping varying from 1 to 4, and N-doping will not change the selectivity toward eNRR to HER. Most importantly, with the lowest
The origin of different eNRR performance of Fe3@NxC20-x (x = 0~4)
As an eNRR catalyst, the free energy change results show that the limiting potential of Fe3@N4C16 (0.45 eV) is smaller than that of Fe3@C20 (0.59 eV) and is even better than the reported UL values FeN4/G (FeN4 moiety embedded in graphene, 1.30 eV)[63] and FeN4/FeMoPPc (FeN4 moiety embedded with polyphthalocyanine, 0.66 eV)[64]. To figure out the origin of the significantly improved eNRR activity of Fe3@N4C16, we analyzed the geometric and electronic structures of Fe2 sites in Fe3@NxC20-x(x = 0~4). As illustrated in Supplementary Table 6, the Fe−Fe bond length of the Fe2 site shows slight variation for x = 0~3 (dFe−Fe = 2.22~2.24 Å) and an obvious drop to 2.16 Å for x = 4, and the Fe2-d orbitals exhibit a better hybridization with the adsorbed N2-p orbitals [Supplementary Figure 9], which further leads to more efficient activation of N−N as demonstrated by the lower ΔG of the first hydrogenating step. Besides, both the d-band center and the p-band center of Fe2-bonded C/N in Fe3@N4C16 have moderate values among Fe3@NxC20-x(x = 0~4), showing an inverse volcano relationship [Figure 5 and Supplementary Table 6].
Figure 5. The variation of ΔG for the first hydrogenating of side-on N2 against the d-band center (A) and p-band center of
CONCLUSIONS
In summary, we theoretically investigated the eNRR performance of Mo3@C20 and Fe3@C20 monolayers based on the experimentally synthesized 2D Ag3@C20 structure. Our calculations revealed that Fe3@C20 exhibits good activity and selectivity for eNRR with a predicted limiting potential of -0.59 V. By further investigation of the synergy between the active Fe2 site and its coordination, we found that the N-containing environment would tune the electronic properties and, thus, enhance the catalytic efficiency, where 2D
DECLARATIONS
Authors’ contributionsPerformed the computations and drafted the manuscript: Han B
Provided guidance and revised the manuscript: Li F
Availability of data and materialsSupplementary materials are available from the Journal of Materials Informatics or from the authors.
Financial support and sponsorshipThis work was supported by the National Natural Science Foundation of China (11964024), the “Grassland Talents” project of the Inner Mongolia autonomous region (12000-12102613), and the young science and technology talents cultivation project of Inner Mongolia University (21200-5223708). We thank the computational support from Beijing PARATERA.
Conflicts of interestAll authors declared that there are no conflicts of interest.
Ethical approval and consent to participateNot applicable.
Consent for publicationNot applicable.
Copyright© The Author(s) 2023.
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Han, B.; Li, F. Regulating the electrocatalytic performance for nitrogen reduction reaction by tuning the N contents in Fe3@NxC20-x (x = 0~4): a DFT exploration. J. Mater. Inf. 2023, 3, 24. http://dx.doi.org/10.20517/jmi.2023.32
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