Computational design of spatially confined triatomic catalysts for nitrogen reduction reaction

Wei Pei; Wenya Zhang; Xueke Yu; Lei Hou; Weizhi Xia; Zi Wang; Yongfeng Liu; Si Zhou; Yusong Tu; Jijun Zhao

doi:10.20517/jmi.2023.35

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Research Article | Open Access | 20 Dec 2023

Computational design of spatially confined triatomic catalysts for nitrogen reduction reaction

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Wei Pei^1,#,*

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Wenya Zhang^1,#

, ...

Jijun Zhao²

J Mater Inf 2023;3:26.

10.20517/jmi.2023.35 | © The Author(s) 2023.

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Abstract

The electrocatalytic process of nitrogen reduction reactions (NRR) offers a promising approach towards achieving sustainable ammonia production, acting as an environmentally friendly replacement for the conventional Haber-Bosch method. Density functional theory calculations have been utilized to design and investigate a set of catalysts known as triple-atom catalysts (TACs) for electrochemical NRR, which are supported on graphite-C₃N₃ nanosheets. Herein, we have systematically evaluated these TACs using stringent screening to assess their catalytic performance. Among the candidates, supported Pt₃, Re₃, and Ru₃ trimers emerged as highly active with decent selectivity, involving a limiting potential range of -0.35~-0.11 V. According to analysis of electronic properties, we determined that high NRR activity stems from the d-π* electron-accepting and -donating mechanism. Significantly, the correlation between chemical activity of TACs and electronic structure was established as a pivotal physical parameter, which has led to the conclusion that we can precisely control the catalytic behavior of transition metal trimer clusters by selecting appropriate metal elements and designing moderate cluster-substrates interactions. In summary, these theoretical studies not only enhance our understanding of how catalytic properties are governed by metal-support interactions, regulating stability, activity, and selectivity, but also offer a useful method for screening and designing novel TACs for NRR.

Keywords

Density theory calculation, triple-atom catalysts, nitrogen reduction reaction, metal-support interactions

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INTRODUCTION

Ammonia (NH₃) plays a pivotal role as a primary precursor for the production of chemical fertilizers, nitric acid, biofuel energy, plastic, synthetic fiber, etc.^[1-4]. Industrially, NH₃ is primarily synthesized through the Haber-Bosch process, involving the reaction of N₂ and H₂ at high temperatures (T > 700 K) and pressure conditions (P > 200 atm)^[5-8]. However, this method consumes a substantial amount of global energy and contributes to significant greenhouse gas emissions^[9,10]. Therefore, it is an urgent need to develop sustainable and clean methods for yielding NH₃ products. Electrochemical nitrogen reduction reactions (NRR) utilizing renewable energy to convert N₂ to NH₃ have recently emerged as a promising alternative^[11-18]. For instance, Geng et al. reported the excellent NRR activity of Ru on N-doped carbon, achieving a 29.6% Faradaic efficiency (FE) and an NH₃ production rate of 120.9 μg·mg^-1·h^-1^[19]. Additionally, the achieved NH₃ yield rate reached 3.665 mg·h^-1·mg_Ru^-1 at a potential of -0.21 V, where the researchers successfully developed a novel approach using N-doped porous carbon (PC) to encapsulate single Ru sites for efficient NRR while maintaining the FE below 9%^[20]. A 5,7-membered carbon ring-involved PC was developed for the electrocatalytic NRR of Ru-embedded PCs by Han et al.^[21]. These materials, with an impressive NH₃ yield rate as high as 67.8 ± 4.9 μg·h^-1·mg_cat^-1 and a high FE of 19.5% ± 0.6%, exhibit remarkably favorable catalytic NRR properties as electrocatalysts, surpassing the majority of documented single-atom NRR catalysts. Moreover, theoretical research and calculation models play a key role in predicting highly active and selective catalytic materials^[22], providing important reference values for catalyst preparation. According to density functional theory (DFT) calculations, Azofra et al. reported that V₃C₂ possesses the best NRR activity with a 0.64 eV activation barrier among d²-d⁴ M₃C₂ Mxenes^[23]. Some theoretical work has studied the NRR catalyzed by TM@N₄-G, in which the central transition metal (TM) atom is coordinated by four pyridinic nitrogen atoms. The results show that the limiting potentials of Ti@N₄ (0.69 eV) and V@N₄ (0.87 eV)^[24] are shown to exhibit lower free energy for NRR than that of the Ru(0001) stepped surface (0.98 eV)^[25]. Despite significant progress in this field, many obstacles remain, including high overpotentials (> 0.6 V) and low FEs (9%~29.6%) for NRR. Hence, the development of highly efficient and selective NRR electrocatalysts to facilitate mild condition synthesis of ammonia is crucial.

Over the last few years, the design and synthesis of catalysts have been revolutionized by the advent of atom dispersions^[26,27]. Single-atom catalysts (SACs) have attracted attention for their high-specific activity and maximum metal utilization efficiency^[28-32]. For example, He et al. found that a graphdiyne monolayer supported with 11 TM atoms (TM@GDY) exhibits exceptional stability as an electrocatalyst for hydrogen evolution reactions (HER) and oxygen evolution reactions (OER), involving an overpotential range of 0.01~0.46 V^[33]. Moreover, Liu et al. conducted research on the electrocatalytic generation of NH₃ from N₂ at room temperature and atmospheric pressure, using nitrogen-doped PC embedded in cobalt, achieving a high ammonia generation rate of 0.86 μmol·cm^-2·h^-1^[34]. Alternatively, other single metal atoms anchored on N-modified carbon-based materials, such as graphitic carbon nitride (g-C₃N₄) and defective graphene, are promising electrocatalysts for NRR with 0.34 V potential^[35]. Despite the potential benefits of SACs, a significant challenge persists in balancing the reaction rate and FE for NH₃ synthesis, mainly due to the involvement of multiple reactive species in the NRR. This challenge remains a major obstacle in the practical application of SACs for the controlled and efficient electrocatalytic generation of NH₃.

Atomic clusters^[36-38], on the other hand, possess unique geometric and electronic properties owing to their strong quantum size effects, rendering them highly promising for catalytic applications. The catalytic activities and product selectivity of supported metal nanoclusters (NCs) were governed with atomic precision by tuning the size and composition of metal NCs. Experimentally, the use of well-dispersed Pt₂ dimers on graphene resulted in a unique catalytic system that significantly increased the aqueous hydrogenation rate of ammonia-borane in comparison to isolated individual Pt-ions. Specifically, the specific rate of the reaction was found to be approximately 17 times higher when using the Pt₂ dimers^[39]. Furthermore, a Mo₃ trimer on graphdiyne nanosheets was observed to exhibit the highest activity, selectivity, and stability towards NRR based on the designed screening criteria, involving a calculated initial voltage of -0.32 V^[40]. More importantly, the presence of several active sites in a catalyst is essential to broaden the range of adsorbate binding capabilities and to facilitate the catalysis of a broader spectrum of complex reactions to yield diverse products. Tian et al. fabricated diatomic Fe₂ NCs anchored on mesoporous carbon nitride, which demonstrated remarkable catalytic activity in the conversion of substilbene to oxide, achieving a high selection of 93% while maintaining a transformation rate of 91%^[41]. Ru₃ stabilized on nitrogen-doped carbon nanosheets was reported to efficiently catalyze the selective oxidation of alcohols^[42]. Moreover, theoretical studies have suggested that supported metal NCs exhibit remarkable abilities in catalyzing reactions involving the activation of inert reactant molecules or requiring multiple reaction centers^[43-47]. For example, a tetramer immobilized in nitrogen-doped graphene monolayers shows optimal surface activities for N₂ activation and subsequent reaction to yielding NH₃ products, involving the calculational on-set potential of -0.45 V, significantly lower than that of supported single atoms and dimers^[47]. Li et al. discovered that Rh₃-C₂N exhibited superior catalytic performance in the enzymatic pathway with a limiting voltage of only -0.45 V compared to Rh₁-C₂N and Rh₂-C₂N^[48]. Furthermore, Zheng et al. have designed a set of electrocatalysts known as triple-atom catalysts (TACs). The theoretical insights suggest that the Co₃-N₄ system has a superior catalytic activity, attaining a boundary potential of -0.41 V via the enzymatic mechanism for NRR^[46].

Motivated by the substantial progress in both experimental and theoretical studies, we have systematically investigated the stability of 3d-5d TM trimers embedded on C₃N₃ nanosheets (marked as TM₃@C₃N₃) using DFT calculations and evaluated their electrocatalytic performances for NRR. The theoretical results indicate that TM₃@C₃N₃ (TM = Re, Ru, Pt) holds significant promise electrocatalysts for the NRR, demonstrating excellent performance and feasibility. Significantly, the Re₃@C₃N₃ system demonstrates exceptional catalytic activity, surpassing other catalysts, with a limiting potential of -0.11 V using the consecutive mechanism. Furthermore, we established a correlation between the inherent electronic characteristics of TM₃@C₃N₃ and its catalytic performance, identifying significant physical parameters. Additionally, we advocate the concept of designing TACs as a strategic approach to drive the development of advanced electrocatalysts within the framework of green hydrogen economics.

MATERIALS AND METHODS

The Vienna ab initio simulation package (VASP) was utilized to conduct spin-polarized DFT computations, employing the plane wave basis set^[49,50]. We employed projector augmented wave (PAW) potentials along with an energy cutoff of 500 eV^[51,52]. The Perdew-Burke-Ernzerhof (PBE) functional of generalized gradient approximation (GGA) was employed as the exchange-correlation functional^[53]. It has been extensively tested and successfully applied to an array of solid-state and molecular systems^[22-26,54]. The van der Waals (vdW) interaction was investigated using Grimme’s semiempirical DFT-D3 scheme with dispersion correction^[55,56]. The energy and force convergence thresholds were established at 10^-4 eV and 0.02 eV·Å^-1, correspondingly. To avoid any interaction between two periodic units, a (2 × 2) supercell of C₃N₃ monolayer with a 16 Å vacuum space along the z-direction was employed. Moreover, the Brillouin zone was sampled at the k-point using a 2 × 2 × 1 grid^[57]. Ab initio molecular dynamics (AIMD) simulations were performed at a temperature of 300 K in an NVT ensemble for a duration of 10 ps in order to estimate the thermodynamic stability of the TM₃@C₃N₃ systems.

The binding energies (E_b) were calculated to judge the thermodynamic stabilities of the designed TM₃@C₃N₃ systems, as follows:

(1)

$$ E_{b} = E_{\rm TM_{3}@C_{3}N_{3}} - E_{\rm C_{3}N_{3}} - E_{\rm TM_{3}} $$

where $$ E_{\rm TM_{3}@C_{3}N_{3}} $$, $$ E_{\rm C_{3}N_{3}} $$, and $$ E_{\rm TM_{3}} $$ denote the total energy of C₃N₃ substrate anchored with transition-metal trimeric clusters, the optimized pristine C₃N₃, and the isolated transition-metal trimeric clusters, respectively. The electronic adsorption energy (E_ads) of reaction intermediates on TM₃@C₃N₃ substrates can be computed by the following formula:

(2)

$$ E_{\rm ads} = E_{\rm tot} - E_{\rm cat} - E_{\rm adsorbate} $$

where E_tot is the total energy of TM₃@C₃N₃ substrates adsorbed by the intermediate, and E_cat and E_adsorbate are the energy of TM₃@C₃N₃ and the adsorbed intermediate, respectively. According to the proposal by Nørskov et al.^[58-60], the calculated hydrogen electrode (CHE) model can be employed to determine the Gibbs free energy change (∆G) for each individual step in the electrochemical hydrogenation process, and the ∆G value is calculated by employing the formula as follows:

(3)

$$ \Delta G = \Delta E + \Delta E_{\rm ZPE} - T\Delta S + \Delta G_{U} + \Delta G_{\rm pH} $$

where ∆E is the total reaction energy gained from the DFT calculations. ∆E_ZPE and ∆S are the changes in zero-point energy and entropy, respectively. The zero-point energy and entropy were determined by calculating the vibrational frequencies. The temperature (T) was set to 298.15 K in this research. ∆G_U denotes the impact of the applied potential (U) and is equal to -neU, where n corresponds to the number of electrons transferred. The correction of pH, ∆G_pH, represents the free energy and can be calculated according to the formula: ∆G_pH = -K_BT × pH × ln10. The pH value is assumed to be 0, and K_B refers to the Boltzmann constant. The quantities of transferred electrons and the potential of the applied electrode are represented by e and U, respectively. The potential determination step (PDS), which has the maximum ∆G value, can be used to determine the limiting potential (U_limiting) of the entire reduction process in an acid solution using the following formula:

(4)

$$ U_{\text {limiting }}=-\Delta G_{\max } / e $$

RESULTS AND DISCUSSION

Geometry and stability of TM₃@C₃N₃

Since the NRR is a complex process including various reactive species, the use of effective screening descriptors is crucial. As illustrated in Figure 1, we propose a standard strategy for screening candidate catalysts for N₂ reduction reaction. (1) Thermodynamic stability: TACs should have thermodynamics (∆E_b < 0 eV, where ∆E_b is the binding energy of TM₃ atoms on C₃N₃). Additionally, their dynamic stability should be confirmed through AIMD simulations at 300 K, ensuring that the structure remains stable without deformation; (2) Surficial activity: N₂ should undergo complete activation ($$ \Delta G_{\rm \ast N_{2}} $$ < 0.50 eV, indicating chemical adsorption of N₂, where the asterisk * denotes the adsorption site)^[61]; (3) Energy cost: the potential range of NRR reaction on Ru-based catalysts with high catalytic performance under mild conditions is -0.50~-0.56 V. Therefore, in this paper, catalytic properties of NRR: the limit potential, U_limiting, is used to evaluate NRR activity. To ensure low energy costs, we use a standard of 0.55 eV. For the first and last hydrogenation steps, which are typically the most likely limiting steps in a reaction, it is desirable to have ∆G values that are as low as possible ($$ \Delta G_{\ast \rm{N_{2}}\rightarrow \text{NNH}} $$ ≤ 0.55 eV and $$ \Delta G_{\rm{NH_{2}}\rightarrow \rm{NH_{3}}} $$ ≤ 0.55 eV)^[61,62]. The calculation of ∆G is performed using Equation (3)^[61]; (4) NRR vs. HER selectivity: to ensure high NRR selectivity, the Gibbs free energy value of hydrogen adsorption should exceed that of nitrogen. These criteria collectively form a robust filtration strategy for identifying potential NRR catalysts.

Computational design of spatially confined triatomic catalysts for nitrogen reduction reaction

Figure 1. The schematic diagram for screening candidate TACs for NRR. HER: Hydrogen evolution reactions; NRR: nitrogen reduction reactions; TACs: triple-atom catalysts.

Next, we first consider the stability of the metal trimer on C₃N₃ before delving into the electroreduction process. Through extensive geometric optimization and configuration search, we obtained the structures of TM₃@C₃N₃. As summarized in Figure 2A, these metal trimer clusters show the strong binding strength on the C₃N₃ monolayer, involving E_b of -9.57~-4.22 eV, which indicates decent thermodynamic stabilities. The E_b is calculated by Equation (1) with the detailed data shown in Table 1. Furthermore, according to the above calculation, it is worth noting that Pd₃@C₃N₃ possesses the weakest binding strength among the 21 selected systems. In light of this, we conducted AIMD at 300 K for 10 ps to evaluate the stability of TM₃@C₃N₃, with Pd₃@C₃N₃ considered as a representative. Figure 2B and C illustrates the oscillations of the DFT total energies (E) relative to the initial conditions and temperature (T), along with the d_Pd-N and d_Pd-Pd, indicating dynamic fluctuations near the initial condition. According to the captured structures of Pd₃@C₃N₃ under different times [Figure 2D], it is preserved well and maintains structural stability, in which the vertical buckling exhibits minimal fluctuation, measuring less than 0.10 Å. Therefore, combined with the above calculations, we can confidently assert that these TM₃@C₃N₃ systems have robust structural stabilities.

Figure 2. (A) Computed the binding energies (ΔE_bind) of triple-atoms anchored on C₃N₃. The grey, blue, and light green are C, N, and metal atoms, respectively; (B) Variations of the temperature (T) and total energies (E); and (C) the distances of Pd-N (d_Pd-N) and Pd-Pd (d_Pd-Pd) vs. time for AIMD simulations of Pd₃@C₃N₃; (D) Pd₃@C₃N₃ structures are captured within 10 ps, with the Pd shown in green. AIMD: Ab initio molecular dynamics.

Table 1

DFT-calculated the average distances of TM-N (d_TM-N) and TM-TM (d_TM-TM), vertical buckling between TM₃ and C₃N₃ monolayer (d_TM-C), combining energy (ΔE_bind), d orbital centroid (ε_d), magnetic moment (Mag), and the number of CT between TM₃ and C₃N₃ monolayers

System	d_TM-N (Å)	d_TM-TM (Å)	h_TM-C (Å)	ΔE_bind (eV)	ε_d (eV)	Mag (μB)	CT (e)
Ti	2.01	2.52	1.27	-9.14	0.72	3.23	1.60
V	2.03	2.38	1.53	-9.01	0.46	0.00	0.58
Cr	1.94	1.93	0.89	-8.63	-0.56	0.00	0.24
Mn	1.97	2.33	0.97	-7.44	-1.49	4.96	0.60
Fe	1.92	2.27	1.47	-6.98	-1.93	6.16	0.33
Co	1.88	2.22	1.23	-7.35	-1.53	3.00	0.47
Ni	1.86	2.21	1.17	-7.47	-1.58	0.00	0.46
Y	2.18	3.53	2.01	-9.48	1.87	0.99	1.27
Zr	2.09	3.00	1.37	-9.46	0.77	1.92	1.97
Nb	2.07	2.59	1.31	-7.95	0.77	0.77	1.63
Mo	2.09	2.19	1.23	-7.51	-0.92	0.61	0.38
Ru	2.03	2.51	1.73	-7.38	-1.59	0.00	0.30
Rh	2.07	2.55	1.50	-6.46	-1.75	0.71	0.35
Pd	2.11	2.59	1.37	-4.22	-2.09	0.00	0.24
Hf	2.89	2.10	1.41	-9.57	1.01	1.24	1.85
Ta	2.55	2.10	1.28	-9.07	0.39	0.07	1.00
W	2.45	2.08	1.17	-6.59	-0.35	0.11	0.88
Re	2.35	1.99	1.06	-9.13	-1.01	0.96	0.65
Os	2.43	2.06	1.20	-8.18	-1.41	0.00	0.47
Ir	2.42	2.02	0.68	-7.70	-2.02	0.00	0.40
Pt	2.51	2.21	1.77	-5.25	-2.53	0.02	0.26

CT: Charge transfers; DFT: density functional theory; TM: transition metal.

Activation of N₂ on TM₃@C₃N₃

In the overall electrochemical NRR process, the primary and critical step involves the adsorption and activation of N₂ molecules, a process of utmost importance as it is responsible for activating the N≡N triple bond, laying the foundation for the smooth protonation to follow. Figure 3A shows that the activation of N₂ is facilitated via an electron transfer mechanism, where the partially filled d orbitals of TM atoms accept electrons from N₂ molecules while simultaneously donating d electrons to the anti-bonding orbitals (π*) of *N₂ in the reverse direction. Therefore, this interaction strengthens the TM–N bond while attenuating N≡N. Notably, the metal double or triple-atom centers are critical to maximizing the activation effect by donating a greater number of electrons to N₂ in comparison to monatomic active sites^[63-66]. Using dual-metal or triple-atom centers proves to be a more effective approach in promoting N₂ activation. Employing this approach provides an advantage in surmounting the barrier encountered during the initial step in the hydrogenation of NRR. By incorporating multiple metal atoms in the catalytic centers, the activation of N₂ becomes more efficient, enhancing the overall performance of the NRR process.

Figure 3. (A) Simplified schematic illustration of N₂ binding to single- and double-atom sites; (B) Computed molecular orbitals showing the electronic structure of free N₂ and N₂ absorbed on Pt₃/g-C₃N₃. The red dashed line is the d-band center; (C) The adsorption free energies ($$ \Delta G_{\ast \rm{N_{2}}} $$) of *N₂ on TM₃@C₃N₃. The number of CT of *N₂ with (D) side-on and (E) end-on configurations vs. its adsorption energies ($$ \Delta E_{\ast \rm{N_{2}}} $$); (F) The d-orbital center (ε_d) of TM₃@C₃N₃vs.$$ \Delta E_{\ast \rm{N_{2}}} $$. The inset is the differential charge density of N₂ on TM₃@C₃N₃ and TM₃@C₃N₃. CT: Charge transfers; DOS: density of states; TM: transition metal.

Revealing the source of NRR activity in electrocatalysts provides guidance for designing and developing highly active catalysts. To uncover the fundamental mechanism behind the activation of N₂, we employed the PBE functional to calculate the partial density of states (PDOS), enabling us to gain deeper insights into the nature of the interactions and the extent of N₂ activation facilitated by the metal trimers^[67,68]. The enhanced ability of Pt₃/g-C₃N₃ to adsorb and activate N₂, as compared to the free N₂, can be attributed primarily to the presence of vacant and filled d orbitals. As displayed in Figure 3B, the empty 5d orbitals of Pt₃ receive electrons from N₂, resulting in the formation of bonding states. Simultaneously, a robust d-2π* orbital coupling takes place, causing the 2π* orbital from N₂ to be fractionally filled in the vicinity of the Fermi level. The partial filling is accomplished by the electron feedback from the occupied d orbitals of Pt₃ to the 2π* orbital of N₂. The observed outcome provides additional evidence to support the presence of strong d-π* orbital coupling in close proximity to the Fermi level. Additionally, it affirms the significant overlap between the d orbitals of trimeric clusters and the filled orbitals of absorbed N₂. This finding strongly aligns with the earlier analysis and supports the proposed mechanism of N₂ activation.

To further understand the interacting nature of N₂ and TM₃@C₃N₃ catalysts, we conducted calculations to determine the $$ \Delta G_{\rm{N_{2}}\ast} $$ value. The corresponding adsorption free energy for N₂ adsorption on TM₃@C₃N₃ is calculated by Equation (3), with the detailed data presented in Figure 3C and Supplementary Table 1, respectively. It is not difficult to see that for most TM₃@C₃N₃ catalysts, the $$ \Delta G_{\rm{N_{2}}\ast} $$ value is close to the zero, ranging from 0.35 to -5.12 eV, implying that the N₂ molecule exhibits favorable adsorption characteristics on these catalysts. In comparison to the original N≡N triple bond length in N₂ molecules [Table 1], regardless of whether the N₂ molecule is adsorbed in an end-on or side-on configuration, a noticeable elongation is observed in the N≡N triple bond, indicating that the adsorption of N₂ on the selected 21 catalysts can attenuate the N≡N, facilitating the activation of the subsequent NRR process. According to Bader charge analysis, Figure 3D and E shows that the TM atoms and N atoms are indicated to undergo a significant charge transfer, and the metal trimer cluster loses an electron ranging from 0.24 to 1.97 e [Table 1]. As shown in Figure 3F, the adsorption energy of N₂ is linearly related to the d-orbital center (ε_d) of the metal clusters. The higher the d orbital center of the TM₃@C₃N₃ system, the more favorable its interaction with the π* orbital of N₂ and provides more charge transfer to the N₂ molecule, thus showing a stronger binding capacity for N₂ adsorption^[69-72]. Guided by these essential parameters, the catalytic response of TM trimer clusters can be finely regulated and manipulated by designing moderate cluster-carrier interactions through the selection of suitable metal elements.

Reaction mechanism of electrochemical NRR

As shown in Figure 4A, one reaction mechanism for the electrocatalytic synthesis of ammonia, namely the enzymatic mechanism, is depicted, and an additional consecutive mechanism is presented in Supplementary Figure 1A. The distal mechanism and the alternative mechanism are also detailed in Supplementary Figure 1B. A common feature among them is that the N atom at one end of N₂ is adsorbed on the catalyst, while the N element at the other end is not adsorbed. However, the N element at the far end preferentially reacts with the H proton. In the distal mechanism, N atoms leaving the surface of the catalyst preferentially react with H protons and release as ammonia, leaving *N adsorbed on the catalyst, which will start hydrogenation and form ammonia. An alternative mechanism involves hydrogenating two N atoms using six proton-electron pairs to form two NH₃ molecules. Contrarily, in the enzymatic pathway, the N₂ molecule is first decomposed into adsorbed *N atoms, and then, *N is gradually hydrogenated into ammonia. The hydrogenation process follows the same pathway as the alternative mechanism. In the consecutive mechanism, one of the N atoms undergoes hydrogenation first and subsequently reacts with the remaining N atoms, resulting in the formation of the second NH₃ molecule. Owing to the distinct structures and performances of catalysts, the NRR reaction mechanism often varies. Nevertheless, numerous studies have shown that the initial or final step can be considered the potential determinant independent of NRR mechanisms^[73-75]. The proton step with the maximal positive free energy change (∆G_max) is defined as the PDS. It is well known that the ideal catalyst for an electrochemical NRR should meet the following criteria: (1) The ∆G_max of the two key steps is below 0.55 eV, that is, $$ \Delta G_{\ast \rm{N_{2}}\rightarrow \ast \text{NNH}} $$ ≤ 0.55 eV and $$ \Delta G_{\ast \rm{NH_{2}}\rightarrow \ast \rm{NH_{3}}} $$ ≤ 0.55 eV, so the performance of the catalyst in question is superior to that of the best pure metal or nanometal cluster catalyst; (2) $$ E_{\rm ads(\ast N_{2})} - E_{\rm ads(\ast H)} $$ < 0, which proves that N₂ has good selectivity in catalysts.

Figure 4. (A) Schematic illustration of Enzymatic mechanism towards NH₃ formation on TM₃@C₃N₃. Diagrams for N₂ electroreduction via enzymatic mechanism on (B) Pt₃@C₃N₃ and (C) Ru₃@C₃N₃; (D) The optimized geometry of various intermediates on the Pt₃@C₃N₃ structure along the enzymatic pathway of NRR. NRR: Nitrogen reduction reactions.

For a prompt evaluation of TM₃@C₃N₃ catalysts regarding their NRR performance, we calculated the ∆G for the initial and final stages of NRR under open-circuit conditions (U = 0). The resulting values, along with corresponding diagrams, are presented in Table 2 and Supplementary Figure 2. Four classifications are considered to evaluate the critical steps, with the critical point set at 0.55 eV. Consequently, six systems met the above criteria, demonstrating decent catalytic activity in electrochemical NRR. In the case of Pt₃@C₃N₃, the associated free energy diagram is presented in Figure 4B. The Pt₃@C₃N₃ exhibits a propensity for NRR via enzymatic mechanisms, wherein N₂ adopts a side-on configuration. In the enzymatic mechanism, the initial step of hydrogenation involves the formation of a chemical bond between a hydrogen atom and one of the N atoms, followed by alternating bonding to the two N atoms until a second NH₃ molecule is produced. The ∆G_max change is 0.24 eV, establishing the final hydrogenation step as the PDS of the entire electrochemical NRR. Similarly, we investigated the NRR pathway on Ru₃@C₃N₃, as depicted in Figure 4C, revealing a ∆G_max of 0.35 eV.

Table 2

DFT-calculated potential-determining steps (including reaction mechanisms and corresponding reactions) and corresponding limiting potentials (U_L) for TM₃@C₃N₃ (TM = Re, Pt, Ru, Rh, Ta, Ir)

TM₃@C₃N₃	Potential-determining step		U_L (V)
TM₃@C₃N₃	Mechanism	Reaction	U_L (V)
Re	Consecutive	HNNH + H⁺ + e^- → HNNH₂	-0.11
Pt	Enzymatic	NH₂ + H⁺ + e^- → NH₃	-0.24
Ru	Enzymatic Consecutive	N₂ + H⁺ + e^- → NNH NNH+ H⁺ + e^- → NNH₂	-0.35 -0.40
Rh	Enzymatic Consecutive	N₂ + H⁺ + e^- → NNH	-0.40
Ta	Consecutive	HNNH + H⁺ + e^- → HNNH₂	-0.42
Ir	Enzymatic Consecutive	NH₂ + H⁺ + e^- → NH₃	-0.54

DFT: Density functional theory; TM: transition metal.

The relevant intermediate geometries of Pt₃@C₃N₃ for each step are presented clearly in Figure 4D. It is evident that throughout the entire NRR process, the initial step (*N₂ → *NNH) is identified as the PDS. Briefly speaking, for Pt₃@C₃N₃ catalysts, the corresponding limiting potential (U_L) value is -0.24 V, while for Ru₃@C₃N₃ catalysts, the U_L value is -0.35 V. Consequently, by applying the U_L to the surfaces of Pt₃@C₃N₃ and Ru₃@C₃N₃ catalysts, it is ensured that all electron transfer steps occur spontaneously without any uphill energy barriers, which is beneficial to the production of NH₃, where the reaction process of Ru₃@C₃N₃ also follows an enzymatic mechanism.

It is worth emphasizing that the development of efficient NRR catalysts remains challenging due to the competition with HER^[66,76,77]. An ideal NRR catalyst would demonstrate significantly higher NRR activity and considerably lower HER activity. To assess selectivity, we calculated the N₂ adsorption energy and hydrogen adsorption energy on the designed TM₃@C₃N₃ catalysts using Equation (2). A more negative difference between N₂ adsorption energy and hydrogen adsorption energy indicates higher selectivity for NRR. The results are presented in Supplementary Figure 3. It is evident that both Pt₃@C₃N₃ and Ru₃@C₃N₃, which were previously identified as having the highest NRR activity, exhibit markedly higher selectivity for NRR over HER. These findings imply that these catalysts can ensure a high Faraday efficiency in catalytic electrochemical NRR.

CONCLUSIONS

In summary, this study provides a systematic investigation into the potential of C₃N₃-loaded TACs in electrocatalytic NRR using DFT calculations. Employing a stringent “five-step” filtering strategy, we have identified TM₃@C₃N₃ (TM = Pt, Ru, Re) as highly promising candidates with the attributes of low energy cost, high selectivity, remarkable stability (both thermodynamic and kinetic), and remarkably low limiting potentials (-0.35~-0.11 V). Our analysis of electronic properties highlights that the exceptional NRR activity can be ascribed to the electron acceptance and donation mechanism involving d-π* interactions. This mechanism, combined with charge density differences and PDOS, underscores the qualifications of TM₃@C₃N₃ as electrocatalysts. Furthermore, we have explored the linkage between chemical activity and the electronic structure of TM₃@C₃N₃ surfaces, revealing a pivotal physical parameter that allows precise control over the catalytic performance of TM trimer clusters. In conclusion, this work significantly improves our overall comprehension regarding the stability, activity, and selectivity of TM₃@C₃N₃ electrocatalysts. Moreover, it provides a valuable methodology for efficiently screening and designing innovative TACs specifically tailored for NRR. We anticipate that these findings will ignite further exploration and experimentation, both in theory and practice, aiming to unlock the vast potential of TACs in NRR and its associated electrochemical reactions.

DECLARATIONS

Authors’ contributions

Made substantial contributions to the conception and design of this article, writing, and editing: Pei W, Zhang W, Yu X

Made substantial contributions to the collation of literature and writing: Liu Y, Zhou S, Tu Y, Zhao J

Performed data analysis and discussion: Hou L, Xia W, Wang Z

Availability of data and materials

Supplementary Materials are available from the Journal of Materials Informatics or from the authors.

Financial support and sponsorship

This work was supported by the National Natural Science Foundation of China (Nos. 12304300, 11974068, 91961204, and 12075201), the Natural Science Foundation of Jiangsu Province (Nos. BK20230555 and BK20230563), the Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 23KJB140019), the Fundamental Research Funds for the Central Universities of China (Nos. DUT20LAB110 and DUT22ZD103), Outstanding Doctor Program of Yangzhou City “Lv Yang Jin Feng” (No. YZLYJFJH2022YXBSO84), and Open Research Fund of CNMGE Platform & NSCC-TJ (CNMGE2023007). The authors acknowledge the computer resources provided by the Shanghai Supercomputer Center.

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

Supplementary Materials

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Cite This Article

Research Article

Open Access

Computational design of spatially confined triatomic catalysts for nitrogen reduction reaction

Wei Pei

, ... Jijun Zhao

How to Cite

Pei, W.; Zhang, W.; Yu, X.; Hou, L.; Xia, W.; Wang, Z.; Liu, Y.; Zhou, S.; Tu, Y.; Zhao, J. Computational design of spatially confined triatomic catalysts for nitrogen reduction reaction. J. Mater. Inf. 2023, 3, 26. http://dx.doi.org/10.20517/jmi.2023.35

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