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Article

Tailoring of Three-Atom Metal Cluster Catalysts for Ammonia Synthesis

by
Shuo Wang
,
Tingting Zhao
and
Likai Yan
*
Institute of Functional Material Chemistry, Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(5), 869; https://doi.org/10.3390/catal13050869
Submission received: 16 March 2023 / Revised: 9 May 2023 / Accepted: 9 May 2023 / Published: 11 May 2023
(This article belongs to the Special Issue Theory-Guided Electrocatalysis and Photocatalysis)
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Abstract

:
Electrocatalytic nitrogen reduction reaction (NRR) can realize the green production of ammonia while developing electrocatalysts with high selectivity and ability is still an ongoing challenge. Two-dimensional (2D) graphitic carbon nitride (CN) frameworks can provide abundant hollow sites for stably anchoring several transition metal (TM) atoms to facilitate single-cluster catalysis, promising to overcome the problems of low activity and poor selectivity in the process of ammonia synthesis. Herein, extensive density functional theory (DFT) calculations were performed to investigate the feasibility of six bimetallic triatomic clusters FexMoy (x = 1, 2; x + y = 3) supported on C6N6, C2N, and N-doped porous graphene (NG) as NRR electrocatalysts. Through a systematic screening strategy, we found that the Fe2Mo–NG possesses the highest activity with a limiting potential of –0.36 V through the enzymatic mechanism and could be the promising catalyst for NH3 synthesis. The Fe2Mo moiety in Fe2Mo–NG moderately regulates the electron transfer between reaction intermediates and NG, which is ascribed to enhanced performance. This work accelerates the rational design of catalysts in the field of NRR and contributes to broadening the understanding of cluster catalysis.

Graphical Abstract

1. Introduction

The chemical conversion process of abundant N2 (78% of air content) into ammonia (NH3) has important implications for promoting a sustainable low-carbon society and sustaining all life forms. Ammonia is not only an essential chemical raw material but also an excellent hydrogen storage material [1,2]. The predominant commercialization method for large-scale production of NH3 in industry is the Haber–Bosch (HB) process, which is plagued by severe operating conditions of high temperature (300–500 °C) and pressure (150–300 atm) [3]. In order to alleviate energy consumption and contribute to sustainable low-carbon development, the electrocatalytic nitrogen reduction reaction (NRR) has been envisaged as an alternative method that can activate N2 under mild operating conditions [4,5]. Tremendous efforts have been made to solve the stability, selectivity, and efficiency of electrocatalysts in the NRR process [6,7,8,9].
To date, single-atom catalysts (SACs) containing transition metal (TM) atoms have triggered great interest and have been widely applied in the field of electrocatalytic nitrogen reduction due to their efficient utilization of the active species [10,11,12,13]. Two-dimensional (2D) materials can serve as a versatile platform to form expedient metal–N coordination sites as promising catalytic centers due to their huge specific surface area, and excellent physical and chemical properties. g–C3N4 [14], C2N [15], C6N6 [16], boron nitride [17], and nitrogen-doped porous graphene [10] have extensively been studied as substrates for catalyzing NRR SACs. Despite the attractive promises, SACs have two demerits, namely low amount of metal atom loading [18,19,20] and lack of synergistic active sites to be capable of cooperatively functioning for NRR [12,21]. One promising strategy to address these problems is to introduce metallic or nonmetallic atoms to form dual and few atoms catalysts [7,22,23,24], resulting in better dispersed single atoms at a higher amount and playing a synergistic role in optimizing the interaction between the active site and the reactant.
Many experimental and theoretical works have demonstrated that metal cluster catalysts have exhibited great potential to activate small molecules such as CO2, NO3, CH4, etc. [25,26,27,28,29,30,31]. Metal clusters anchored on carbon materials are commonly stabilized by strong metal-support bonding. Due to the highly dispersed atoms, adjacent active sites can also act synergistically, playing a synergistic role in optimizing the interaction between the active site and the reactant [32]. In 2019, Yang et al. synthesized Ru nanoparticles anchored on N-doped carbon which was applied as an efficient catalyst for the hydrogenation of nitrobenzene to aromatic amines [33]. The CO oxidative coupling reaction can be catalyzed by PdxCuy/GDY (x = 1, 2, 3, 4; x + y ≤ 4) in which the introduction of Cu not only improved the formation activity of dimethyl oxalate (DMO) but also controlled the ratio of Cu:Pd to effectively adjust the selectivity of DMO [34]. Confining multiple Fe and Cu atoms in the surface cavities in graphitic carbon nitride can capture N2 molecules/intermediates with optimized strength, significantly increasing the ammonia yield and the Faradaic efficiency of NRR [35]. There are also theoretical works on triatomic cluster catalysts for understanding the underlying intrinsic factors and screening candidates for efficient NRR [36,37,38,39,40]. These works demonstrate that the size of the metal particles reduced to the ultra-small cluster obviously changes the electronic properties of catalysts, so the number and type of metal atoms play a key role in the enhancement of the electron transfer process, which will further influence the catalytic performance.
Inspired by natural nitrogenase Fe–Mo cofactors for N2 fixation under ambient conditions, researchers devote themselves to developing Fe- and Mo-based catalysts for NRR [41,42,43,44]. In the 1960s, the enzyme FeMo was used to fix N2 under mild conditions and without the consumption of H2. However, the biological N2 fixation process is slow and sensitive to the environment [45]. These works manifest the vital role of Fe- and Mo-based catalysts towards N2 fixation. It is highly desired to develop an efficient catalyst containing Fe and Mo active sites for mild N2 hydrogenation to NH3. Accordingly, it is pivotal to find suitable substrates anchoring Fe and Mo clusters to evaluate the performance of electrocatalysts.
In the present work, DFT calculations were performed to focus on how Fe2Mo and FeMo2 clusters affect the activity and selectivity of NRR. Three planar 2D materials (CNs) including C6N6, C2N, and nitrogen-doped graphene (NG) were selected as potential FexMoy supports. It is found that heterogeneous triatomic clusters are embedded in the pore of the substrate. The computational results show that the Fe2Mo cluster anchored on CNs shows a higher activity of NRR. The N2 prefers a “side-on” adsorption mode for further hydrogenation process. The Fe2Mo–NG favors the enzymatic mechanism with a rather low limiting potential of −0.36 V. Based on the analyses of electronic property, the Fe2Mo–NG exhibits high performance for NRR owing to its more electrons near the Fermi level that results in activating the triple N≡N bond. Furthermore, we suggest that designing heterogeneous triatomic clusters anchored on 2D materials is a potential application of three-atom catalysts for N2 fixation.

2. Results and Discussion

2.1. Structure and Stability

Triple metal atoms of dual types (i.e., both Fe and Mo) deposited on C6N6, C2N, and N doped graphene monolayer (NG) to form six different structural models (Fe2Mo–C6N6, Fe2Mo–C2N, Fe2Mo–NG, FeMo2–C6N6, FeMo2–C2N, and FeMo2–NG) as shown in Figure 1. To estimate the possibility and stability of Fe2Mo and FeMo2 clusters deposited on the three mentioned 2D materials, we calculated the binding energy (Eb) by the following formula: Eb = EFexMoy@sub − Esub − xEFe − yEMo, where EFexMoy@sub, EFe, EMo, and Esub represent the electronic energies of total catalysts, Fe atom, Mo atom and the pristine supports, (x = 1, 2; x + y = 3). All these three substrates can bind FexMoy clusters with binding energies in a range from −12.04 to −13.43 eV (Figure S1), which suggests FexMoy clusters can strongly bind with six N atoms and are highly stable [24]. In order to check the thermal stability of candidates, AIMD simulations were performed. As shown in Figure S2, the geometric structures are well preserved without significant distortion when the temperature reaches 500 K for 10 ps with a time step of 1 fs, suggesting the high stability of FexMoy–CNs.
To gain deep insight into the strong interaction between TM clusters and substrates, we computed the corresponding charge transfer based on Bader charge analysis. As shown in Table 1, there is significant charge transfer from TM atoms to the substrate in the range of 0.35–0.88 e−. More strikingly, Mo atoms transfer more electrons to the substrate than Fe atoms. Bader charge analysis indicates a strong interaction between FexMoy clusters and substrates. Charge density differences (Figure S3) also illuminate that significant electrons transfer from the FexMoy cluster to the substrate, which is beneficial to facilitate N2 activation.
Electrical conductivity is important for efficient electrocatalytic performance during the NRR. The electron transport properties were evaluated by the projected density of states (PDOS) of FexMoy–CNs, as shown in Figure S4. All FexMoy–CNs exhibit metallic properties, indicating their excellent electron transport properties. For Fe2Mo–CNs, the PDOS near the Fermi level is mainly contributed by the Fe d-orbital, while the PDOS of FeMo2–CN is mainly contributed by the Mo atom. This confirms that TM atoms can contribute to the domination of d-electrons near the Fermi level and consequently show the conductivity and catalytic activity of FexMoy–CNs which is in favor of electrochemical NRR. Compared with Fe and Mo atoms, the N atom has the inert behavior of N2 adsorption.

2.2. N2 Adsorption

From a theoretical assessment perspective, N2 spontaneously adsorbed on the catalyst surface (ΔG*N2 < 0 eV) is an important prerequisite for efficient NRR electrocatalysts. Thus, all possible initial adsorption configurations were considered by adjusting N2 to connect with single metal (Fe/Mo), dual metal (Fe–Fe, Mo–Mo, Fe–Mo), and triple metal atoms in end-on and side-on modes as shown in Figure S5. By comparing the adsorption energies, the most stable configuration and the bond length of the activated N2 are shown in Figure S6. For Fe2Mo–CNs, N2 tends to be adsorbed on Fe and Mo bimetals via the more stable side-on pattern. For the FeMo2 anchored on CN substrates, N2 tends to be adsorbed on two Mo atoms with a side-on configuration. The activation of N≡N is directly associated with its bond elongation. Compared with the free N2 molecule (dN-N = 1.11 Å), the length of the N–N bond is significantly elongated after absorbing on the FexMoy–CNs, and the value ranges from 1.15 Å to 1.22 Å. As shown in Figure 2a, the N2 adsorption-free energies (ΔG*N2) on FexMoy–CNs are all negative. According to ΔG*N2, the N2 molecule preferably adsorbs on FexMoy–CNs with side-on configuration. The charge transfers of N2 adsorption with optimal configuration were analyzed and the results are shown in Table 1. It can be seen that electrons transfer from the FexMoy cluster to the adsorbed N2 are around 0.70–0.86 e−. After N2 adsorption, the two metals with N2 adsorption give more electrons.

2.3. NRR Mechanism

Based on previous studies [42,46,47], the formation of *NNH and the last hydrogenation step to form *NH3 via the protonation process are usually considered potential-determining steps (PDS) due to high energy barriers. The values of ΔGN2-N2H and ΔGNH2-NH3 for studied FexMoy–CNs with N2 side-on adsorption were calculated and shown in Figure 2b. The calculated ΔGN2-N2H ranges from 0.21 to 0.69 eV and the ΔGNH2-NH3 are in the range of 0.36 to 1.11 eV, showing that ΔGNH2-NH3 are significantly higher than ΔGN2-N2H. The ΔGN2-N2H and ΔGNH2-NH3 of FexMoy–CNs except for FeMo2–C6N6 are significantly smaller than the ΔGmax(0.98 eV) of best bulk TM catalyst Ru (0001) (purple dotted line). In order to confirm the selected catalysts are active and efficient, the systems with ΔGN2-N2H and ΔGNH2-NH3 below 0.65 eV (pink dotted line) [42,48,49] are considered for further study. Concerning this standard, only Fe2Mo–CNs can be promising candidates for further hydrogenation to unveil the NRR mechanism.
Based on the previous analysis, the N2 molecule is preferably adsorbed on Fe2Mo–CNs with a side-on configuration. Therefore, only enzymatic and consecutive mechanisms are considered (Figure 3a). Along with the enzymatic pathway, the N2 molecule is initially adsorbed on Fe2Mo–CNs with both two N atoms binding to Fe and Mo atom, followed by six H+/e− pairs alternately attack on both N atoms which result in the formation of two NH3 molecules. For consecutive mode, the protons (H+) continuously attack one N atom until the two NH3 are released in succession. Next, we investigate the detailed NRR process by adding H atoms (H+/e− pair) one by one on three Fe2Mo–CNs as presented in Figure 3b,c. The *NHNH intermediate is more favorable than the *NNH2 intermediate due to the more negative ΔG value for the hydrogenation of *NNH intermediate (−0.21 vs. 0.19 eV for Fe2Mo–C6N6, −0.01 vs. 0.23 eV for Fe2Mo–C2N, −0.06 vs. 0.18 eV for Fe2Mo–NG), suggesting that the enzymatic pathway is more likely to occur. For –, the *NNH formation is endothermic and demands energy input with a ΔG of 0.48 eV. The intermediate *NNH is further hydrogenated to generate the *NHNH, NHNH2, and NH2NH2 with the ΔG of −0.21, −0.20, and −0.21 eV, respectively. The next step (*NH2NH2 → NH3 + *NH2) could proceed spontaneously with ΔG = −1.35 eV, meanwhile releasing the first NH3. The remaining intermediate *NH2 further reacts with one proton–electron pair (H+ + e−) to form *NH3 with ∆G of 0.53 eV. For Fe2Mo–C2N, the first H+/e– pair attacks the *N2 species to form *NNH with ∆G of 0.21 eV. Subsequently, the H+/e– pair alternately attacks the N atom of reaction intermediates until the first NH3 molecule is released with ∆G values of –0.01 eV for *NNH → *NHNH, −0.37 eV for *NHNH → *NHNH2, −0.26 eV for *NHNH2 → *NH2NH2 and −1.25 eV for *NH2NH2 → *NH2, respectively. The last hydrogenation step (*NH2 → *NH3) requires a ∆G of 0.43 eV. For Fe2Mo–NG, the free energy of the first hydrogenation is 0.20 eV. Afterward, the *NNH species is continuously attacked by H+/e– pair to form *NHNH, *NHNH2, *NH2NH2, and *NH2 along the enzymatic pathway, which is carried out spontaneously with ∆G = −0.07 eV, −0.15 eV, −0.23 eV, and −1.40 eV, respectively. The formation of the second NH3 is endothermic with an energy input of 0.36 eV.
According to the above analysis, it can be found that the elementary process with the lowest required potential is the last hydrogenation step for the Fe2Mo–C6N6, Fe2Mo–C2N, and Fe2Mo–NG with a limiting potential of −0.53, −0.43 and −0.36 V, respectively. The optimized adsorption configurations of intermediates along the enzymatic pathway on three Fe2Mo–CNs are illustrated in Figure S7. In a previous study, ΔGmax values of homonuclear metal clusters Fe3 and Mo3 supported on N-doped porous graphene are 0.70 and 0.66 eV [50], which are larger than those of Fe2Mo–CNs in the present work. Furthermore, Fe2Mo–NG with a limiting potential of –0.36 V is to be predicted as a more promising candidate as an NRR electrocatalyst.
Since the NRR process takes place in an aqueous solution, we used an implicit solvation model to evaluate the effect on eNRR. Take Fe2Mo–NG for example, Fe2Mo–NG has a stronger N2 adsorption (ΔGN2 = −1.25 eV). The *NH2 → *NH3 is the PDS with a ΔG of 0.41 eV. When the implicit solvation model is used, the results are consistent with those obtained in a vacuum, but the numerical values are slightly different (Figure S8). So the solvent molecules have little effect on the NRR process. The adsorption of H2O on the FeMo site of Fe2Mo–NG was also evaluated to predict the stability in practical applications. By calculation, the Gibbs free energy of H2O at the FeMo site is −0.78 eV, which is less than the value of N2 at Fe2Mo–NG through the end or side configuration (−1.05 eV and −1.14 eV), indicating H2O does not compete during NRR. Therefore, the solvent molecules have a small effect on eNRR.
The desorption of ammonia from the catalyst surface can continuously reexpose active sites which is thus important for the entire catalytic cycle. Herein, NH3 desorption from the Fe2Mo–CNs needs to overcome the thermodynamic barrier of 1 eV. In previous theoretical and experimental work, NH3 can be desorbed when the desorption energy exceeds 1 eV [1,46,51]. Moreover, from the experimental point of view, the produced ammonia will not be released in a gaseous form but is protonated to NH4+ ions in strongly acidic solutions. Therefore, the desorption of NH3 from the Fe2Mo–CNs is not a big obstacle.

2.4. Origin of NRR Activity on Fe2Mo-CNs

Revealing the origin of NRR activity of the electrocatalysts holds important guiding significance for designing and developing highly active catalysts. Compared with the PDOSs of six FexMoy–CNs before nitrogen adsorption, Fe2Mo–CNs have more electronic states near the Fermi level and show stronger adsorption capacity toward nitrogen than FeMo2–CNs. The PDOS of the N2 adsorption on the Fe2Mo–CNs was also investigated. As shown in Figure 4a, there are obvious orbital hybridizations between the d orbitals of Fe2Mo clusters and the 2p orbitals of adsorbed N2, indicating the multiple “pull–push effect”. In addition, the integrated-crystal orbital Hamilton population (ICOHP) analysis of the N–N bond was performed to reveal the N2 activation degree (Figure 4b). Generally, the more negative ICOHP indicates the stronger bonding of N–N and thus the less activation of N2 molecule [42,52]. The ICOHP value for N–N bond in the N2 molecule is −22.99, and was computed to be −15.65 for Fe2Mo–NG, and is less negative than that of the N–N bond in the Fe2Mo–C6N6 (−15.80) and Fe2Mo–C2N (−15.80), indicating that the Fe2Mo triatomic active centers on NG can effectively adsorb and activate N2. In addition, the charge density difference further confirms the electron transfer between the FexMoy cluster and the adsorbed N2 as shown in Figure 4c. The positive and negative charges mainly distribute on the trimeric cluster and N2, implying that the triatomic cluster acts as both electron donor and acceptor, which is well consistent with the above PDOS analysis. The combination of empty and occupied d-orbitals can accept electrons from the σ-orbital of N2 and donate electrons back to antibonding π orbitals of N2. Particularly, the π back-donation weakens the N–N triple bond which is crucial for effective N2 activation [24]. As mentioned above, two metal atoms play a direct role in the pulling and pushing process to greatly activate inert N≡N triple bonds. Compared with Fe2@NG (UL = −0.62 V) [53], Mo2@NG (UL = −0.39 V) [54], or FeMo@NG (UL = −0.51 V) [55], the comparable or lower limiting potential of FexMoy–CNs exhibit higher NRR catalytic activities than these predicted double-atom catalysts. We speculate that the third metal atom could be considered as the reservoir for storing or supplying d electrons when needed.
Calculating the charge variation in elementary protonation steps can further reveal the origin of activity for NRR [56,57]. As shown in Figure 5a, the Fe2Mo–CN was divided into three moieties: the substrate without three TM atoms (moiety 1), Fe2Mo unit (moiety 2), and the adsorbed NxHy species (moiety 3). When the N2 molecule adsorbed on Fe2Mo–CN, *N2 gained 0.73~0.86 e−, which is mainly contributed by the triatomic metal cluster. In the following hydrogenation steps, except *N2H4 in Fe2Mo–C2N and Fe2Mo–NG, and *NH3, interestingly, the adsorbed NxHy species always gain electrons. Intense charge variations occur between the adsorbed NxHy species and trimeric metal cluster, which not only accept electrons but also donate electrons from each other. It indicates that the exposed Fe2Mo cluster is vital for the electrochemical NRR. Therefore, moiety 1 can be considered an electron reservoir, while moiety 2 acts as an electron transmitter to moiety 3.

2.5. NRR Selectivity of the FexMoy–CNs

An ideal NRR catalyst should be able to suppress the main competing reaction HER effectively. Therefore, the selectivity of FexMoy–CNs is evaluated in this section. The calculated Gibbs free energy change of N2 adsorption (ΔG*N2) and single H atom adsorption energy (ΔG*H) are shown in Figure 6a. The blue (upper-left side) and white (lower-right side) regions of FexMoy–CNs show the selectivity toward NRR and HER (TM site), respectively. It is observed that except FeMo2–C6N6, FexMoy–CNs favorably interact with the N2 molecules, indicating that most FexMoy–CNs can be applied as NRR catalysts since the N2 adsorption on active site takes priority over that of a hydrogen atom. The difference in the limiting potentials between NRR and HER can be used to estimate the selectivity. The UL(NRR) − UL(HER) versus UL(NRR) relationship for these six NRR catalysts is depicted in Figure 6b. The larger the UL(NRR) − UL(HER) is, the higher the selectivity toward NRR is. The values of UL(NRR) − UL(HER) on Fe2Mo–CNs are positive. Such evidence indicates that Fe2Mo–CNs are highly selective toward the NRR [58,59].

3. Computational Details

All calculations were performed using the Vienna ab initio simulation package (VASP) [60]. The ionic cores were described by the projector-augmented wave (PAW) method. The Perdew, Burke, and Ernzerhof (PBE) functional in the generalized gradient approximation (GGA) was used to describe electron exchange and correction [61,62]. The wave functions of valence electrons were expanded using a plane wave basis set and 450 eV cut-off energy was selected. Van der Waals (vdW) interactions were taken into account using the DFT–D3 method [63]. A k-point grid of 3 × 3 × 1 was used to optimize each intermediate along the NRR process, while denser k-points of 5 × 5 × 1 were carried out for electronic property calculations. All atoms were fully relaxed until the maximum residual force and energy reached the threshold values of 0.01 eV Å−1 and 10−4 eV, respectively. A vacuum space with 15 Å thickness was imposed to avoid interlayer interactions. The ab initio molecular dynamics (AIMD) simulation was performed at 500 K to assess the thermal stability of the catalyst.
Based on the computational hydrogen electrode model CHE model [64], the Gibbs free energy change (ΔG) of each elementary step is defined as ΔG = ΔE + ΔEZPE − TΔS + ΔGU + ΔGpH. ΔE is the electronic energy change directly obtained from DFT calculations. ΔEZPE and ΔS are changes in the zero-point energy and entropy, which are obtained from frequency calculations at 298.15 K. ΔGU = −n × e × U, where n is the number of electrons transferred. ΔGpH = −kB × T × ln[H+] = −kB × T × pH × ln10, where kB is the Boltzmann constant. In this work, T was set to 298.15 K and pH = 0. UL is the limiting potential, which is determined by the most positive ΔG (ΔGmax) of each elementary step at zero electrode potential and computed by UL = −ΔGmax/e. For the most promising catalyst (Fe2Mo–NG) identified by the vacuum calculations, we used an implicit solvation model (with a dielectric constant of 78.4 F/m) implemented in VASPsol [65,66] to examine the solvent effect on its NRR process.

4. Conclusions

In summary, triatomic transition metals (Fe2Mo and FeMo2) coordinated with six nitrogen atoms anchored carbon nitrogen materials were designed and the N2 fixation and reduction mechanism were extensively explored based on spin-polarized density functional theory calculations. Fe2Mo–CNs could not only effectively capture and convert the N2 molecule with limiting potentials of −0.53, −0.43, and −0.36 V, respectively, but also suppress the competitive HER. With the considerations of activity, selectivity, and stability, it is believed that designed Fe2Mo–NG is proposed as an ideal candidate for catalytic nitrogen reduction to ammonia. The origin of high NRR activity is attributed to the interaction between metal clusters and CN substrate, the exposed metal clusters moiety can provide the electrons required for the process of hydrogenation. Our findings open up numerous possibilities to provide FexMoy-based materials with promising NRR performance and might motivate a new strategy for developing novel cluster catalysts with high efficiency.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13050869/s1, Figure S1: Binding energies of FexMoy@CN (x = 1, 2; x + y = 3); Figure S2: Variation of the total energy and temperature against time for the AIMD simulation on FexMoy–CNs; Figure S3: Charge density differences of FexMoy@CN (x = 1, 2; x + y = 3); Figure S4. Computed projected density of states (PDOSs) of FexMoy–CNs; Figure S5: All possible initial N2 adsorption configurations; Figure S6: The configurations and bond length of N2 adsorption on FexMoy–CNs via end-on and side-on patterns; Figure S7: Corresponding geometric structures of intermediates along the enzymatic pathway on (a) Fe2Mo–C6N6, (b) Fe2Mo–C2N, and (c) Fe2Mo–NG; Figure S8: Free energy diagrams of NRR by considering solvation effect for enzymatic pathway on Fe2Mo–NG.

Author Contributions

Conceptualization, S.W. and L.Y.; methodology, S.W. and T.Z.; software, S.W.; validation, S.W., T.Z. and L.Y.; formal analysis, S.W.; investigation, S.W.; resources, S.W.; data curation, S.W.; writing—original draft preparation, S.W.; writing—review and editing, L.Y.; visualization, S.W.; supervision, L.Y.; funding acquisition, L.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Natural Science Foundation of Science and Technology Department of Jilin Province (20210101131JC).

Data Availability Statement

Not available.

Acknowledgments

We acknowledge the Institute of Theoretical Chemistry, Jilin University for providing the computational resources for this work. We also thank LvLiang Cloud Computing Center of China, and the calculations were performed on TianHe-2.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The top and side views for the optimized structures of Fe2Mo and FeMo2 clusters supported by (a,d) C6N6, (b,e) C2N, and (c,f) NG. The number labeled were corresponding to M1, M2, and M3 in Table 1.
Figure 1. The top and side views for the optimized structures of Fe2Mo and FeMo2 clusters supported by (a,d) C6N6, (b,e) C2N, and (c,f) NG. The number labeled were corresponding to M1, M2, and M3 in Table 1.
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Figure 2. (a) N2 adsorption free energies (ΔG*N2) on FexMoy–CNs via end-on and side-on patterns. (b) Calculated free energy change of the first (*N2 → *N2H) and the last PCET steps (*NH2 → *NH3) on various FexMoy–CNs for NRR via side-on adsorption configuration.
Figure 2. (a) N2 adsorption free energies (ΔG*N2) on FexMoy–CNs via end-on and side-on patterns. (b) Calculated free energy change of the first (*N2 → *N2H) and the last PCET steps (*NH2 → *NH3) on various FexMoy–CNs for NRR via side-on adsorption configuration.
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Figure 3. (a) Schematic depiction of the enzymatic and consecutive pathways for NRR on Fe2Mo–CNs (* denoted Fe2Mo–CN). Free-energy diagram along the minimal energy path at zero and limiting potential for the eNRR on (b) Fe2Mo–C6N6, (c) Fe2Mo–C2N, and (d) Fe2Mo–NG.
Figure 3. (a) Schematic depiction of the enzymatic and consecutive pathways for NRR on Fe2Mo–CNs (* denoted Fe2Mo–CN). Free-energy diagram along the minimal energy path at zero and limiting potential for the eNRR on (b) Fe2Mo–C6N6, (c) Fe2Mo–C2N, and (d) Fe2Mo–NG.
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Figure 4. (a) PDOS of N2 2p and TM d orbital after N2 adsorption of Fe2Mo–CNs; (b) The crystal Hamilton population (-COHP) of N–N bond of N2 on the Fe2Mo–CNs; (c) The charge density difference after N2 adsorption on Fe2Mo–C6N6, Fe2Mo–C2N and Fe2Mo–NG. The Fermi level is set to 0 eV. The isosurface value is set to 0.003 e/Å3 (Cyan and yellow colors represent the depletion and accumulation of charge, respectively).
Figure 4. (a) PDOS of N2 2p and TM d orbital after N2 adsorption of Fe2Mo–CNs; (b) The crystal Hamilton population (-COHP) of N–N bond of N2 on the Fe2Mo–CNs; (c) The charge density difference after N2 adsorption on Fe2Mo–C6N6, Fe2Mo–C2N and Fe2Mo–NG. The Fermi level is set to 0 eV. The isosurface value is set to 0.003 e/Å3 (Cyan and yellow colors represent the depletion and accumulation of charge, respectively).
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Figure 5. (a) schematic diagram of three moieties on Fe2Mo–CN. Moieties 1, 2, and 3 represent the CN substrate, the Fe2Mo unit, and the adsorbed NxHy, respectively. Bader charge variations of the three moieties for (b) Fe2Mo–NG, (c) Fe2Mo–C2N, and (d) Fe2Mo–C6N6 along the enzymatic pathway (* denoted Fe2Mo–CN).
Figure 5. (a) schematic diagram of three moieties on Fe2Mo–CN. Moieties 1, 2, and 3 represent the CN substrate, the Fe2Mo unit, and the adsorbed NxHy, respectively. Bader charge variations of the three moieties for (b) Fe2Mo–NG, (c) Fe2Mo–C2N, and (d) Fe2Mo–C6N6 along the enzymatic pathway (* denoted Fe2Mo–CN).
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Figure 6. (a) ΔG*H vs. ΔG*N2 for N2 adsorbed FexMoy–CNs with the most stable adsorption configurations. Dashed line indicates ΔG*H = ΔG*N2. (b) UL(NRR) − UL(HER) versus UL(NRR), where the colored area shows better performance compared to that of the metallic catalysts benchmark.
Figure 6. (a) ΔG*H vs. ΔG*N2 for N2 adsorbed FexMoy–CNs with the most stable adsorption configurations. Dashed line indicates ΔG*H = ΔG*N2. (b) UL(NRR) − UL(HER) versus UL(NRR), where the colored area shows better performance compared to that of the metallic catalysts benchmark.
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Table
Table 1. Computed Bader charge (e−) of FexMoy cluster anchored on supports before and after adsorbing N2 (− represents losing e−, + represents gaining e−). The number of Fe and Mo is defined in Figure 1.
Table 1. Computed Bader charge (e−) of FexMoy cluster anchored on supports before and after adsorbing N2 (− represents losing e−, + represents gaining e−). The number of Fe and Mo is defined in Figure 1.
SystemBader Charge (e−)Bader Charge (e−) * N2
M1M2M3M1M2M3N2
Fe2Mo–C6N6−0.35−0.50−0.88−0.52−0.68−1.23+0.75
FeMo2–C6N6−0.47−0.80−0.65−0.62−1.03−1.15+0.85
Fe2Mo–C2N−0.45−0.50−0.78−0.64−0.52−1.18+0.70
FeMo2–C2N−0.61−0.70−0.74−0.52−1.12−1.04+0.83
Fe2Mo–NG−0.80−0.50−0.51−1.24−0.54−0.62+0.73
FeMo2–NG−0.64−0.78−0.61−1.08−1.10−0.53+0.86
The * denoted FexMoy–CN.
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Wang, S.; Zhao, T.; Yan, L. Tailoring of Three-Atom Metal Cluster Catalysts for Ammonia Synthesis. Catalysts 2023, 13, 869. https://doi.org/10.3390/catal13050869

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Wang S, Zhao T, Yan L. Tailoring of Three-Atom Metal Cluster Catalysts for Ammonia Synthesis. Catalysts. 2023; 13(5):869. https://doi.org/10.3390/catal13050869

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Wang, Shuo, Tingting Zhao, and Likai Yan. 2023. "Tailoring of Three-Atom Metal Cluster Catalysts for Ammonia Synthesis" Catalysts 13, no. 5: 869. https://doi.org/10.3390/catal13050869

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