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Article

Density Functional Theory Study of Triple Transition Metal Cluster Anchored on the C2N Monolayer for Nitrogen Reduction Reactions

by
Shifa Xiao
1,
Daoqing Zhang
1,
Guangzhao Wang
2,
Tianhang Zhou
3,4,* and
Ning Wang
5,*
1
College of Physics Science and Technology, Lingnan Normal University, Zhanjiang 524048, China
2
Key Laboratory of Extraordinary Bond Engineering and Advanced Materials Technology of Chongqing, School of Electronic Information Engineering, Yangtze Normal University, Chongqing 408100, China
3
College of Carbon Neutrality Future Technology, China University of Petroleum (Beijing), Beijing 102249, China
4
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China
5
School of Science, Key Laboratory of High Performance Scientific Computation, Xihua University, Chengdu 610039, China
*
Authors to whom correspondence should be addressed.
Molecules 2024, 29(14), 3314; https://doi.org/10.3390/molecules29143314
Submission received: 26 June 2024 / Revised: 8 July 2024 / Accepted: 11 July 2024 / Published: 13 July 2024
(This article belongs to the Special Issue Development of Nanomaterials for Energy and Environmental Applications)
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Abstract

:
The electrochemical nitrogen reduction reaction (NRR) is an attractive pathway for producing ammonia under ambient conditions. The development of efficient catalysts for nitrogen fixation in electrochemical NRRs has become increasingly important, but it remains challenging due to the need to address the issues of activity and selectivity. Herein, using density functional theory (DFT), we explore ten kinds of triple transition metal atoms (M3 = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) anchored on the C2N monolayer (M3-C2N) as NRR electrocatalysts. The negative binding energies of M3 clusters on C2N mean that the triple transition metal clusters can be stably anchored on the N6 cavity of the C2N structure. As the first step of the NRR, the adsorption configurations of N2 show that the N2 on M3-C2N catalysts can be stably adsorbed in a side-on mode, except for Zn3-C2N. Moreover, the extended N-N bond length and electronic structure indicate that the N2 molecule has been fully activated on the M3-C2N surface. The results of limiting potential screen out the four M3-C2N catalysts (Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N) that have a superior electrochemical NRR performance, and the corresponding values are −0.61 V, −0.67 V, −0.63 V, and −0.66 V, respectively, which are smaller than those on Ru(0001). In addition, the detailed NRR mechanism studied shows that the alternating and enzymatic mechanisms of association pathways on Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N are more energetically favorable. In the end, the catalytic selectivity for NRR on M3-C2N is investigated through the performance of a hydrogen evolution reaction (HER) on them. We find that Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N catalysts possess a high catalytic activity for NRR and exhibit a strong capability of suppressing the competitive HER. Our findings provide a new strategy for designing NRR catalysts with high catalytic activity and selectivity.

Graphical Abstract

1. Introduction

Ammonia (NH3) is an important raw chemical material that plays an essential role in industry, agricultural production, energy storage and conversion, and other fields [1,2,3]. At present, industrial ammonia synthesis mainly relies on the traditional Haber–Bosch process [4,5]. Since this technology requires high temperatures and pressure, it not only consumes vast energy, but also emits a large amount of greenhouse gasses. Therefore, against the backdrop of the energy crisis and increasing environmental concerns, developing new processes for efficiently synthesizing ammonia under mild conditions is urgent [6].
Compared with the Haber–Bosch method, the electrocatalytic approach for achieving nitrogen reduction reactions (NRRs) can theoretically be carried out at room temperature and pressure [7,8]. Meanwhile, the sources of raw water and nitrogen are extensive, which provides an opportunity for achieving the green synthesis of ammonia under mild conditions [9]. In recent years, electrocatalytic nitrogen reduction for ammonia production has attracted significant attention, and related research has shown a rapid growth trend [10,11,12,13]. However, the current study shows that although electrocatalytic technology can achieve the green synthesis of ammonia, the thermodynamic and kinetic obstacles to the production of ammonia via electrocatalytic nitrogen reduction at room temperature and pressure are enormous due to the high stability of the N≡N triple bond and the slow adsorption of nitrogen [14,15]. Moreover, the selectivity of the nitrogen reduction reaction and the ammonia production rate are greatly reduced due to the hydrogen precipitation competition reaction [16]. Therefore, how to improve the ammonia production rate and the catalyst selectivity at the same time is the biggest challenge in the study of electrocatalytic nitrogen reduction at ambient temperature and pressure.
The two-dimensional material known as C2N has recently emerged as a subject of interest among researchers, thanks to its remarkable stability, cavity structure, high specific surface area, and other distinctive attributes [17,18,19,20]. Its spacious cavities make C2N a suitable support for anchoring metal atoms to catalyze various chemical reactions [21]. Metal clusters featuring exposed atomic interfaces and distinct electronic configurations have garnered significant attention in multiphase catalysis [22]. Transition metals like Fe-, Ru-, and Co-based complexes, with their d-orbitals capable of donating electrons to the empty π*-orbitals of N2 and accepting electrons from its σ-orbitals, enhance N2 adsorption, making them suitable for nitrogen reduction reaction (NRR) catalysis [23,24,25,26]. In response to the rising interest in single-atom catalysts for efficient NRR electrocatalysts [27,28,29], dual-atom and triple-atom catalysts have also been experimentally and theoretically explored for their catalytic performance in NRRs [30,31,32]. For instance, the work proposed by Chen et al. indicates that due to the unique characteristics of M3 (M = Mn, Fe, Co, and Ni) active sites, the triple-atom catalysts exhibit better catalytic activity towards NRRs than single-atom and double-atom catalysts [33]. Liu et al. studied Fe3 clusters anchored on the surface of Al2O3 as multiphase catalysts for NRRs [34]. They discovered that their comparable activity to Ru catalysts, which is attributed to the large spin polarization, low iron oxidation state, and multi-step oxidation–reduction ability of Fe3 clusters.
Motivated by the above studies, a series of triple transition metal atoms (M3 = 3d transition metal, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) anchored on the C2N monolayer (M3-C2N) are designed as electrocatalysts for NRRs, and the electronic structures and NRR catalytic mechanisms are systematically investigated using density functional theory (DFT). Firstly, the binding energy of triple transition metal atoms on C2N is calculated to evaluate the stability of M3-C2N catalysts to prescreen the promising candidates for NRR catalysts. The negative binding energies manifest that the triple transition metal clusters can be stably anchored on the N6 cavity of the C2N monolayer. Next, as the first step of the NRR, the adsorption and activation of N2 molecules on the surface of M3-C2N are studied via the adsorption structure, adsorption energy, and electronic structures. Moreover, the catalytic activity and mechanisms are systematically investigated based on the Gibbs free energy of the whole NRR process. According to the limiting potential, we screen out four highly active NRR catalysts, including Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N, while they can suppress the competitive hydrogen evolution reaction. Among them, Co3-C2N exhibits the highest NRR activity with a limiting potential of −0.61 V. This study provides a comprehensive understanding of the stability, activity, and selectivity of M3-C2N as NRR catalysts, which can guide the further experimental exploration of M3-C2N or other related reactions.

2. Results and Discussion

2.1. Structure and Stability of M3-C2N

The pristine C2N monolayer containing 48 C and 24 N atoms is optimized, and the optimized lattice constant is a × b = 16.83 Å × 16.77 Å, which is consistent with the previous literature [35,36]. As shown in Figure 1a, the N6 cavity of the C2N structure is 5.56 Å, which is large enough to anchor triple metal atoms. Moreover, the N atoms surrounding the N6 cavity exhibit electron-rich properties, and due to the electron loss of the C atom, the N atom is negatively charged. Therefore, N atoms endow the N6 cavity environment with high electron enrichment properties, making the cavity an ideal place to contain positively charged TM ions. Figure 1b exhibits the optimized structure of Co triple metal clusters anchored on the N6 cavity of C2N, and other optimized structures of M3-C2N are displayed in the Supplementary Materials (Figure S1). The average bond length of the Co-N bond is 1.89 Å, indicating a strong interaction between Cu and N atoms. In addition, the difference in charge density clearly shows the accumulation and depletion of charges around the Co and adjacent N atoms. It is worth noting that the high charge density around the Co atoms facilitates the subsequent adsorption of N2 molecules. To evaluate the stability of the M3-C2N catalysts, the binding energy of triple transition metal atoms on the C2N monolayer is calculated, as displayed in Figure 1c. The ∆Eb values of all catalysts are less than 0 eV, confirming the stability of the M3-C2N system and the triple transition metal clusters stably anchored on the N6 cavity of the C2N monolayer.

2.2. N2 Adsorption on M3-C2N

For the whole NRR process, the adsorption and activation of N2 molecules on the catalyst surface is a crucial step. Therefore, the adsorption performance of N2 on the surface of M3-C2N catalysts is investigated, and the adsorption configurations are shown in Figure 2. It can be seen that except for on Zn3-C2N, N2 is more energetically favorable when adsorbed in the side-on mode on all M3-C2N, which is consistent with the research findings in the literature. Compared with the N-N bond length (1.12 Å) of free N2 molecule, the N-N bond length of N2 after adsorption has been elongated to varying degrees, indicating that N2 has been activated on M3-C2N. On Zn3-C2N, the bond length of N-N is still 1.12 Å, and N2 is far from the catalyst surface, indicating that the N2 molecule cannot be adsorbed on it, so the NRR activity of Zn3-C2N will not be discussed later. In addition, the charge density differences plot also suggests that the adsorbed N2 interacts with M3-C2N, activating the N≡N triple bond. The adsorption energies of N2 on the surface of M3-C2N catalysts are summarized in Figure 3a. It can be seen that the adsorption of N2 on Sc3-C2N, Ti3-C2N, and V3-C2N is exceedingly strong, and the Eads-N2 values are −3.78 eV, −3.96 eV, and −3.23 eV, respectively. The adsorption strength of N2 on Cu3-C2N is the most weak with only −0.17 eV values of Eads-N2. The range of adsorption energy values on the other six catalysts is from −0.84 eV to −1.60 eV. To further elucidate the adsorption and activation of N2, the scaling relationship between the charge transfer from M3-C2N to the adsorbed N2 and the adsorption energy Eads-N2 is studied and displayed in Figure 3b. It can be seen that the most charge transfer is from M3-C2N to N2 on Sc3-C2N, Ti3-C2N, and V3-C2N, corresponding to 1.84e, 1.62e, and 1.33e, respectively, which is consistent with the adsorption strength of them and the N-N bond length of adsorbed N2. There is a significant positive correlation between charge transfer and Eads-N2. That is to say, the more charge transfer, the stronger the N2 adsorption, and the more negative the value of Eads-N2. In order to better understand the adsorption of N2, the project density of states of the M3-C2N after adsorption of N2 is shown in Figure 3c,d, taking the strongest adsorption on Ti3-C2N and the weakest adsorption on Zn3-C2N as examples. It can be seen that for Ti3-C2N, there is a significant orbital hybridization between the Ti-d and N-p orbitals. For Zn3-C2N, due to weak adsorption, the N2 molecule still maintains a high DOS without orbital hybridization with Zn-d.

2.3. NRR Mechanism and Activity on M3-C2N

The dissociation and association pathways are the two common mechanisms for electrocatalytic NRRs. For the dissociation pathway, it is difficult for the catalyst to break the N≡N bond of the adsorbed N2 molecule. Therefore, only the association pathway on M3-C2N is investigated in this work. For the association pathway, before the formation of the first NH3 molecule, the two N atoms of N2 remain bound to each other. It can be further divided into the distal and consecutive mechanisms (the protons continuously attack a N atom until the first NH3 molecule is produced), as well as the alternating and enzymatic mechanisms (the protons alternately bind the two N atoms). Herein, the two NRR mechanisms on Co3-C2N are shown in Figure 4a, and the adsorbed N2 molecule is gradually hydrogenated to produce NH3 gas. As we all know, the activity of electrocatalysts can be estimated by the limit potential (UL). Therefore, the UL values of the M3-C2N catalysts are calculated and displayed in Figure 4b, and the Ru(0001) (UL = 0.98 eV) catalyst is chosen as a benchmark to evaluate the electrocatalytic NRR activity of M3-C2N due to it having the highest theoretical activity on the surface of the bulk metal. It can be seen that the UL values of Sc3-C2N, Ti3-C2N, and V3-C2N are larger than that of Ru(0001), indicating their poor catalytic activity for NRR. The other six M3-C2N catalysts with UL values less than 0.98 eV exhibit a better catalytic activity than Ru(0001). It is worth noting that the limit potential of Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N are relatively small, and the corresponding values are −0.61 V, −0.67 V, −0.63 V, and −0.66 V, so their free energy diagrams for NRRs are detailed in Figure 5. It can be seen that for Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N, the step of *NNH → *NHNH is more significant downhill than the *NNH → *NNH2 step, indicating that the alternating and enzymatic mechanisms on them are more energetically advantageous. In addition, the first step of the NRR shows goes downhill for all four M3-C2N catalysts, precisely because of the strong N2 adsorption, which also suggests that the adsorption and activation of N2 molecules can occur at room temperature. The potential-limiting step (PDS) on Co3-C2N and Ni3-C2N is the *NHNH2 → *NH2NH2 step, and the corresponding ΔG values are 0.61 eV and 0.66 eV. For Cr3-C2N and Fe3-C2N, the final step of hydrogenation *NH2 → *NH3 is the PDS; and their ΔG values are 0.67 eV and 0.63 eV. Therefore, Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N are the candidates for NRR catalysts, and Co3-C2N possesses the highest NRR activity.

2.4. Selectivity Evaluation for NRR on M3-C2N

Furthermore, an ideal electrocatalyst for NRR should possess a high stability and activity and effectively suppress the hydrogen evolution reaction (HER) to achieve high production for NH3. The HER is the most problematic yet dominant side reaction in the NRR. Therefore, the adsorption free energy of H (ΔG*H) on M3-C2N catalysts is calculated and summarized in Figure 6a. If the ΔG*H values are close to 0 eV, it means that H* cannot easily cover the metal surface and will not block the active sites for NRRs. Although, the ΔG*H values on all M3-C2N are lower than those on Ru(0001) (ΔG*H = −0.35 eV), some M3-C2N exhibit a relatively high HER activity, indicating that the HER process that occurred on some M3-C2N surfaces could be hindered effectively. In addition, the difference in limiting potential between the NRR and HER (UL(NRR)—UL(HER)) is calculated to estimate the catalytic selectivity for NRR on M3-C2N; the scaling relationship between the UL(NRR)—UL(HER) and UL(NRR) is plotted in Figure 6b. Notably, Mn3-C2N, Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N have a relatively high NRR selectivity. Therefore, Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N not only possess a high NRR activity, but also exhibit the highest selectivity for NRRs.

3. Computational Methods

All computational studies were executed utilizing the Perdew−Burke−Ernzerhof (PBE) functional [37] implemented in the Vienna Ab Initio Simulation Package (VASP 5.4.4) [38,39]. Spin polarization was incorporated in all calculations, and the electron–ion interactions were described through the projector-augmented wave method with a 450 eV cutoff energy. Atomic structures were fully relaxed until the force on each atom was smaller than 0.02 eV/Å, while the energy convergence was set to 10−5 eV. To account for van der Waals (vdW) interactions, Grimme’s DFT-D3 approach was implemented [40]. For geometry relaxation, a 3 × 3 × 1 k-point grid centered at the gamma point was employed, and a 20 Å vacuum space along the z-direction was introduced to prevent periodic image interactions. To simulate the electrolyte solution and address solvation effects, the VASPsol code with an implicit solvation model was utilized [41,42].
The binding energy of triple transition metal atoms on a C2N monolayer is calculated to evaluate the stability of M3 on the C2N monolayer:
∆Eb = (EM3-C2N − EC2N − 3EM)/3
where EM3-C2N, EC2N, and EM are the total energies of M3-C2N, C2N, and metal atoms, respectively.
The adsorption energies of reaction species on the M3-C2N catalyst are determined by:
Eads = Etot − Especies − EM3-C2N
where Etot and Especies are the total energies of the M3-C2N with adsorbed reaction species and the isolated reaction species.
Based on the computational hydrogen electrode (CHE) model given by Nørskov and coworkers, the Gibbs free energy change (∆G) for each fundamental step of the NRR is obtained by using the following equation:
ΔG = ΔE + ΔZPE − TΔS
where ΔE is the reaction energy difference in each hydrogenation step in the NRR pathways. ΔZPE and TΔS are the changes in zero-point energy and entropy (T = 298.15 K), respectively.
Limiting potential (UL) was obtained from the maximum free energy change (ΔGmax) among all elementary steps along the lowest-energy pathway by:
UL = −ΔGmax/e

4. Conclusions

In summary, the triple transition metal atoms (M3 = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) anchored on the C2N monolayer (M3-C2N) as electrocatalysts for NRRs are systematically investigated. Based on the negative binding energies of triple transition metal clusters on C2N, the conclusion can be drawn that these ten metal clusters can be stably anchored on the N6 cavity of the C2N structure. The N2 adsorption results indicate that except for Zn3-C2N, the N2 molecule can be stably adsorbed in the side-on mode on all the M3-C2N catalysts. In addition, the results combining adsorption configurations and electronic structure demonstrate that the charges transfer from the M3-C2N to N2, activating the N2 molecule. The positive correlation of the scaling relationship between the charge transfer and the adsorption energy illustrates that the more charge transfer, the stronger the N2 adsorption. The analysis of Gibbs free energy changes suggests that the alternating and enzymatic mechanisms of the association pathway on Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N, which have the relatively low limiting potentials of −0.61 V, −0.67 V, −0.63 V, and −0.66 V, are more energetically advantageous. Moreover, the potential-limiting step (PDS) on both Co3-C2N and Ni3-C2N is the *NHNH2 → *NH2NH2 step, while that on Cr3-C2N and Fe3-C2N is the step of *NH2 → *NH3. Finally, we investigate the competitive reaction of HER on M3-C2N, and it can be concluded that five catalysts (including Mn3-C2N, Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N) have a relatively high NRR selectivity. Overall, Co3-C2N, Cr3-C2N, Fe3-C2N, and Ni3-C2N catalysts can effectively inhibit the competitive HER with a favorable limiting potential. We hope this work can provide a valuable clue for the experimental explorations of the triple transition metal clusters anchored on C2N catalysts and can promote more experimental and theoretical methods to design new high-performance NRR catalysts for efficient NH3 production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29143314/s1, Figure S1. Optimized structures of M3-C2N.

Author Contributions

Conceptualization, D.Z. and G.W.; Data curation, S.X.; Formal analysis, D.Z.; Funding acquisition, T.Z.; Resources, S.X.; Software, G.W.; Validation, D.Z.; Visualization, N.W.; Writing—original draft, S.X.; Writing—review and editing, T.Z. and N.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Doctor Scientific Research of Lingnan Normal University No. ZL2006 and No. ZL22001, the Young Innovative Talents Program in Guangdong Province Colleges and Universities No. 2022KQNCX042, the National Natural Science Foundation of China (22308376 and 20220242), the Outstanding Young Scholars Foundation of China University of Petroleum (Beijing) (ZX20230080), and the Foundation of United Institute for Carbon Neutrality (CNIF20230209).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 29 03314 g001
Figure 1. (a) Optimized structure of C2N. (b) The charge density difference of Co3-C2N. (Isosurface = 0.003 e/Bohr3. Yellow represents the charge increase, and cyan represents the charge decrease.) (c) The binding energy of triple transition metal atoms on the C2N monolayer.
Figure 1. (a) Optimized structure of C2N. (b) The charge density difference of Co3-C2N. (Isosurface = 0.003 e/Bohr3. Yellow represents the charge increase, and cyan represents the charge decrease.) (c) The binding energy of triple transition metal atoms on the C2N monolayer.
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Figure 2. Optimized adsorption configurations and charge density differences of N2 adsorbed on M3-C2N. The isosurface value is 0.003 e/Bohr3.
Figure 2. Optimized adsorption configurations and charge density differences of N2 adsorbed on M3-C2N. The isosurface value is 0.003 e/Bohr3.
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Figure 3. (a) Adsorption energies of N2 on the M3-C2N catalysts. (b) Charge transfer from M3-C2N to the adsorbed N2 as a function of Eads-N2. The partial density of states (PDOS) of N2 on (c) Ti3-C2N and (d) Zn3-C2N.
Figure 3. (a) Adsorption energies of N2 on the M3-C2N catalysts. (b) Charge transfer from M3-C2N to the adsorbed N2 as a function of Eads-N2. The partial density of states (PDOS) of N2 on (c) Ti3-C2N and (d) Zn3-C2N.
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Figure 4. (a) Schematic diagram of the NRR catalyzed by Co3-C2N and the corresponding adsorption intermediate. (b) Theoretical limiting potential UL of M3-C2N.
Figure 4. (a) Schematic diagram of the NRR catalyzed by Co3-C2N and the corresponding adsorption intermediate. (b) Theoretical limiting potential UL of M3-C2N.
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Figure 5. Gibbs free energy diagrams for NRR on (a) Co3-C2N, (b) Cr3-C2N, (c) Fe3-C2N, and (d) Ni3-C2N, respectively.
Figure 5. Gibbs free energy diagrams for NRR on (a) Co3-C2N, (b) Cr3-C2N, (c) Fe3-C2N, and (d) Ni3-C2N, respectively.
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Figure 6. (a) Free energy diagrams for the HER on M3-C2N. (b) Liming potential (UL) versus UL(NRR)—UL(HER) on M3-C2N.
Figure 6. (a) Free energy diagrams for the HER on M3-C2N. (b) Liming potential (UL) versus UL(NRR)—UL(HER) on M3-C2N.
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Xiao, S.; Zhang, D.; Wang, G.; Zhou, T.; Wang, N. Density Functional Theory Study of Triple Transition Metal Cluster Anchored on the C2N Monolayer for Nitrogen Reduction Reactions. Molecules 2024, 29, 3314. https://doi.org/10.3390/molecules29143314

AMA Style

Xiao S, Zhang D, Wang G, Zhou T, Wang N. Density Functional Theory Study of Triple Transition Metal Cluster Anchored on the C2N Monolayer for Nitrogen Reduction Reactions. Molecules. 2024; 29(14):3314. https://doi.org/10.3390/molecules29143314

Chicago/Turabian Style

Xiao, Shifa, Daoqing Zhang, Guangzhao Wang, Tianhang Zhou, and Ning Wang. 2024. "Density Functional Theory Study of Triple Transition Metal Cluster Anchored on the C2N Monolayer for Nitrogen Reduction Reactions" Molecules 29, no. 14: 3314. https://doi.org/10.3390/molecules29143314

APA Style

Xiao, S., Zhang, D., Wang, G., Zhou, T., & Wang, N. (2024). Density Functional Theory Study of Triple Transition Metal Cluster Anchored on the C2N Monolayer for Nitrogen Reduction Reactions. Molecules, 29(14), 3314. https://doi.org/10.3390/molecules29143314

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