
==== Front
Natl Sci Rev
Natl Sci Rev
nsr
National Science Review
2095-5138
2053-714X
Oxford University Press

10.1093/nsr/nwae251
nwae251
Research Article
Chemistry
Nsr/1
AcademicSubjects/MED00010
AcademicSubjects/SCI00010
Ru3@Mo2CO2 MXene single-cluster catalyst for highly efficient N2-to-NH3 conversion
Zhang Cong Formal analysis Investigation Software Writing - original draft School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China

Wang Ze-Hui Investigation Software Writing - original draft Shaanxi Key Laboratory of Catalysis, Institute of Theoretical and Computational Chemistry, School of Chemistry and Environment Science, Shaanxi University of Technology, Hanzhong 723000, China
Department of Chemistry and Guangdong Provincial Key Laboratory of Catalytic Chemistry, Southern University of Science and Technology, Shenzhen 518055, China

Wang Haiyan Investigation School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China

Liang Jin-Xia Project administration Supervision Writing - review & editing School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China

Zhu Chun Project administration Supervision Writing - review & editing School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China
Department of Chemistry and Guangdong Provincial Key Laboratory of Catalytic Chemistry, Southern University of Science and Technology, Shenzhen 518055, China

https://orcid.org/0000-0002-8456-3980
Li Jun Conceptualization Project administration Supervision Writing - review & editing Department of Chemistry and Guangdong Provincial Key Laboratory of Catalytic Chemistry, Southern University of Science and Technology, Shenzhen 518055, China
Department of Chemistry and Engineering Research Center of Advanced Rare-Earth Materials of Ministry of Education, Tsinghua University, Beijing 100084, China
Fundamental Science Center of Rare Earths, Ganjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou 341000, China

Corresponding author. E-mail: liangjx2009@163.com
Corresponding author. E-mail: czhu2014@163.com
Corresponding author. E-mail: junli@tsinghua.edu.cn
9 2024
26 7 2024
26 7 2024
11 9 nwae25119 3 2024
30 6 2024
03 7 2024
10 9 2024
© The Author(s) 2024. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.
2024
https://creativecommons.org/licenses/by/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Single-cluster catalysts (SCCs) representing structurally well-defined metal clusters anchored on support tend to exhibit tunable catalytic performance for complex redox reactions in heterogeneous catalysis. Here we report a theoretical study on an SCC of Ru3@Mo2CO2 MXene for N2-to-NH3 thermal conversion. Our results show that Ru3@Mo2CO2 can effectively activate N2 and promotes its conversion to NH3 through an association mechanism, in which the rate-determining step of NH2* + H* → NH3* has a low energy barrier of 1.29 eV. Notably, with the assistance of Mo2CO2 support, the positively charged Ru3 cluster active site can effectively adsorb and activate N2, leading to 0.74 |e| charge transfer from Ru3@Mo2CO2 to the adsorbed N2. The supported Ru3 also acts as an electron reservoir to regulate the charge transfer for various intermediate steps of ammonia synthesis. Microkinetic analysis shows that the turnover frequency of the N2-to-NH3 conversion on Ru3@Mo2CO2 is as high as 1.45 × 10−2 s−1 site−1 at a selected thermodynamic condition of 48 bar and 700 K, the performance of which even surpasses that of the Ru B5 site and Fe3/θ-Al2O3(010) reported before. Our work provides a theoretical understanding of the high stability and catalytic mechanism of Ru3@Mo2CO2 and guidance for further designing and fabricating MXene-based metal SCCs for ammonia synthesis under mild conditions.

This study thsoretically predicts that MXene-based Ru3@Mo2CO2 can effectively activate N2 and promote its conversion to NH3 via an association mechanism.

N2 reduction reaction
single-cluster catalyst
Ru3@Mo2CO2
DFT
ab initio molecular dynamics simulation
National Natural Science Foundation of China 10.13039/501100001809 22363001 21963005 21763006 22250710677 China National Key Research and Development Plan Project 10.13039/501100012166 2022YFA1503900 Guizhou University 10.13039/501100003459 202140
==== Body
pmcINTRODUCTION

Ammonia synthesis (N2 + 3H2 → 2NH3) is one of the most important processes for agriculture, industrial productions and energy resources [1–3]. Yet the large-scale production of ammonia relies on the Haber–Bosch process [4–6], which requires high temperature and pressure (typically ∼500°C and ∼200 bar) [7–9] to directly dissociate the chemisorbed N2 over the bulk surfaces of Fe- and Ru-based catalysts [10–15]. Due to its high bonding energy of 941 kJ/mol and high N2-ionization potential of 15.1 eV [16], industrial N2-to-NH3 conversion accounts for ∼2% of global energy consumption and a large amount of CO2 emissions [8,17]. Moreover, limited by the Brønsted–Evans–Polanyi (BEP) scaling relation [18], the activity of catalysts and the reaction rate are not compatible with effective N2-to-NH3 conversion at low temperature. Here the BEP linear relationship between the activation energy and the enthalpy change of an elementary reaction regulates the dissociation barrier of N2 and the desorption energies of NHx that scale linearly with the adsorption energy of an N atom. Therefore, it is ideal to develop highly active catalysts for N2 fixation and activation under mild conditions.

Similar to single-atom catalysts (SACs, such as Pt1/FeOx [19]), which represent atomically precise heterogeneous catalysts featuring a substrate-anchored, stable and reactive (metal or non-metal) single-atom-based active center toward selective catalytic conversion of chemical compounds [20], single-cluster catalysts (SCCs) [21–25] provide tunable and atomically precise catalytic active sites for handling complicated redox reactions in heterogeneous catalysis. The supported single clusters of SCCs can provide multiple-atom active sites that show a synergistic effect for efficiently catalyzing complex reactions [21,26,27]. It is well documented that transition-metal single clusters anchored on suitable substrates can enhance the catalytic activity for N2-to-NH3 conversion, or even break the constraint of BEP scaling relation through the association mechanism involving the gradual hydrogenation of N≡N followed by the N–N cleavage for N2-to-NH3 conversion [22,23,28–30]. Our recent theoretical studies also showed that the synergistic effect of small metal single clusters of SCCs, such as Fe3 on θ−Al2O3 (010) surface [23] and Rh1Co3 on CoO (011) surface [22], could effectively catalyze NH3 synthesis at multiple-atom sites under relatively low temperature.

Currently, noble metal Ru-based catalysts are the second generation for ammonia synthesis due to their high activity under low temperature and pressure conditions [3,31,32]. A number of studies have shown that the geometric and electronic structures of the Ru active sites of Ru-based catalysts are sensitive to N2 activation for NH3 synthesis [33–39]. Moreover, the synergistic effect between the surface species and the supported Ru clusters, and the size effect of Ru-based catalysts, can facilitate thermal N2-to-NH3 conversion under mild conditions. For example, the synergism of the surface Sm−H species and Ru clusters on Ru/Sm2O3 SCCs [28] and the synergistic effect between the surface TiCN species and the supported Ru cluster on Ru/ZrH2 [40] can significantly improve their catalytic activity for NH3 synthesis from N2 under mild conditions. In addition, the small Ru size in Rux/BaCeO3 is efficient with regard to NH3 synthesis as it enhances hydrogen spillover [36]. The size sensitivity of graphene-supported Ru catalysts from nanoparticles to subnanometric clusters and atomic clusters were also explored for ammonia synthesis experimentally and theoretically [41]. Therefore, it is crucial to explore the catalytic process and understand the mechanism of Ru-based SCCs for thermal ammonia synthesis at mild conditions.

Catalytic properties of SCCs are determined by both the metal cluster and support. MXene, as a new class of 2D carbide, nitride and carbonitride of transition-metal nanomaterials, is a robust new type of support that can bind Ru clusters tightly. MXenes are derived from the MAX-phase precursors by etching the ‘A’ element, in which ‘M’ stands for early d-block transition metal element, ‘A’ for the main group's sp-block element, and ‘X’ for C and/or N atom [42–44]. Due to its high stability, extremely easily tunable atomic surface with various single-atom adsorption sites, outstanding electronic conductivity and excellent catalytic properties [45–47], MXene is a robust substrate for SACs. Furthermore, MXene can evolve into thermally stable Mn+1XnTx by bonding Mn+1Xn with terminal atoms T (T = O, F, S, OH, etc.) [48–51], thus further expanding adsorption and doping sites for single atoms (SAs) on its surface. Based on its structural features, a large number of MXene-based metal and non-metal [52] SACs have been studied theoretically and/or experimentally [53,54]. Tao et al. reported that by adjusting the annealing temperature, transition metal Ru clusters of different sizes can be obtained on MXene Ti3C2Tx catalysts, indicating that it is feasible to load clusters of specific sizes on MXene [55]. Additionally, a large number of studies have shown that MXenes can effectively adsorb, activate or dissociate N2 [56–58]. Recently, a Mo2CO2 MXene-supported catalyst with highly dispersed Ru clusters was successfully synthesized [59]. The interplay of MXenes and Ru nanoparticles can further promote catalytic efficiency in ammonia synthesis [60]. Tan et al. reported that the single-atom Ru-modified Mo2CO2 MXene exhibits excellent electrocatalytic performance for nitrogen fixation [61].

We have therefore chosen 2D O-functional Mo2C (Mo2CO2) MXene as the support to anchor the small Ru3 cluster to build the Ru-based Ru3@Mo2CO2 SCC, taking advantage of its chemical stability [62,63] and controllable sites [64] to stably anchor SAs or single clusters [42]. The triatomic Ru3 cluster is particularly stable due to effective metal–metal bonding when compared with Ru2 and Ru4 clusters, as shown in the case of an Mx cluster supported on graphydiyne [65]. Ab-initio molecular dynamics (AIMD) simulations revealed that an Ru3 cluster could be stably anchored on the surface of Mo2CO2 MXene by bonding with three surface O atoms. Based on the stable structure of Ru3@Mo2CO2, the optimal association mechanism for thermal N2-to-NH3 conversion at low temperature was explored. A series of theoretical analysis methods were performed to explore the catalytic properties of Ru3@Mo2CO2 for NH3 synthesis. Finally, the microkinetic simulations predicted the turn-over frequencies (TOFs) through different reaction paths for ammonia synthesis catalyzed by Ru3@Mo2CO2.

COMPUTATIONAL DETAILS

The geometries optimization and electronic structure calculations were performed with spin-polarized density functional theory (DFT) using the Vienna ab initio simulation package (VASP) [66,67]. A generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was used to describe the electronic exchange-correlation potential [68]. A cut-off energy of 450 eV was used for the plane-wave basis sets. A conjugated gradient method with a converging tolerance of 0.02 eV/Å for the force on each atom was used for the full geometry optimization without any restrictions. A vacuum distance of 15 Å was set to avoid artificial interlay interaction between the adjacent units of the periodic 2D materials. The van der Waals interaction was taken into account using the dispersion correction of Grimme's method (DFT-D3) [69,70]. A 3 × 3 × 1 Monkhorst-Pack grid was adopted for geometry optimization, while a 11 × 11 × 1 Monkhorst-Pack grid was used for the density-of-states (DOS) calculations. An integrated crystal orbital Hamilton population (ICOHP) was used to analyze the bonding/antibonding population between the adsorbates and the substrate Mo2CO2 [71,72]. The transition states were searched using the dimer method [73] and further confirmed by vibrational frequency analysis. Moreover, the stability and the adsorption behavior of Ru3 cluster anchored on the surface of Mo2CO2 were estimated by using AIMD simulations in a canonical ensemble (NVT) at 473 and 673 K for 20 ps with a time step of 1 fs.

Microkinetic simulations were performed to elucidate the TOFs and the conversions of ammonia synthesis on Ru3@Mo2CO2 using CatMAP software [74]. For surface reactions, the rate constants for the forward and backward elementary reaction were determined by the Eyring equation [75]. The harmonic approximation that treats all degrees of freedom as vibrational modes can be used to estimate the free energies of adsorbates at different temperatures. Detailed descriptions of microkinetic simulation methods are provided in the literature [76–78].

The quantum theory of atom-in-molecule (QTAIM) [79] was used to calculate the atomic Bader charges. This has been shown to provide intriguing bonding and electronic properties of SACs [80]. Following the convention of thermodynamics, the adsorption energy (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{E}_{{\mathrm{ads}}}}$\end{document}) of an adsorbate (X) on the adsorbent Ru3@Mo2CO2 was defined by equation (1)

(1) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{eqnarray*} {{E}_{{\mathrm{ads}}}}{\mathrm{\ = \ }}{{E}_{{\mathrm{X \ldots R}}{{{\mathrm{u}}}_{\mathrm{3}}}{\mathrm{@M}}{{{\mathrm{o}}}_{\mathrm{2}}}{\mathrm{C}}{{{\mathrm{O}}}_{\mathrm{2}}}}}{\mathrm{\ - \ }}{{E}_{{\mathrm{R}}{{{\mathrm{u}}}_{\mathrm{3}}}{\mathrm{@M}}{{{\mathrm{o}}}_{\mathrm{2}}}{\mathrm{C}}{{{\mathrm{O}}}_{\mathrm{2}}}}}{\mathrm{\ - \ }}{{E}_{\mathrm{X}}} \end{eqnarray*}\end{document}

where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{E}_{{\mathrm{X \ldots R}}{{{\mathrm{u}}}_{\mathrm{3}}}{\mathrm{@M}}{{{\mathrm{o}}}_{\mathrm{2}}}{\mathrm{C}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$\end{document}, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{E}_{{\mathrm{R}}{{{\mathrm{u}}}_{\mathrm{3}}}{\mathrm{@M}}{{{\mathrm{o}}}_{\mathrm{2}}}{\mathrm{C}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{E}_{\mathrm{X}}}$\end{document} represent the total energies of the X···Ru3@Mo2CO2 adsorption system, Ru3@Mo2CO2 and free X species, respectively. In addition, the electron density difference of the Ru3@Mo2CO2 or X···Ru3@Mo2CO2 adsorption system was calculated by equation (2)

(2) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{eqnarray*} \Delta \rho \ = {{\rho }_{{\mathrm{AB}}}} - {{\rho }_{\mathrm{A}}} - {{\rho }_{\mathrm{B}}} \end{eqnarray*}\end{document}

where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{\rho }_{{\mathrm{AB}}}}$\end{document} is the electron density of the entire Ru3@Mo2CO2 (or X…Ru3@Mo2CO2 system), \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{\rho }_{\mathrm{A}}}$\end{document} is the electron density of the Ru3 cluster (or X), and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{\rho }_{\mathrm{B}}}{\mathrm{\ }}$\end{document}is the electron density of Mo2CO2 (or Ru3@Mo2CO2), respectively.

RESULTS AND DISCUSSION

Stability of Ru3 cluster on Mo2CO2

The optimized lattice parameters a = b = 2.86 Å for the primitive cell of Mo2CO2 are in agreement with the previous results (2.88 Å) [81]. Herein, the 3 × 3 supercell of Mo2CO2 support is used to anchor the Ru3 cluster, as shown in Fig. S1 of the supplementary data. The optimized stable structure of Ru3@Mo2CO2 is displayed in Fig. 1a, in which three Ru–O bonds were obviously formed after anchoring the Ru3 cluster onto Mo2CO2 of Ru3@Mo2CO2, with the three Ru–O bond lengths all at 2.14 Å, which is close to the Ru–O single bond distance [82]. The electron density difference and Bader charge analysis indicate that electron densities are enriched between Ru3 and the three surface O atoms of Mo2CO2, and the charge of 1.34 |e| is transferred from the Ru3 cluster to Mo2CO2, as shown in Fig. 1b. Furthermore, the calculated partial density-of-states (PDOS) of Ru3@Mo2CO2 (Fig. 1c) shows that there is a relatively large overlap between Ru 4d orbitals of the Ru3 cluster and O 2p orbitals of the three surface O atoms at the occupying states near the Fermi level, and the integrated crystal orbital Hamiltonian population (−ICOHP) value (−0.26) of Ru–O in Ru3@Mo2CO2 (Fig. 1d) demonstrates Ru and O interaction states lie in the bonding region below the Fermi level, indicating that the Ru3 cluster is strongly anchored on Mo2CO2 MXene for forming a Ru3@Mo2CO2 SCC. The Ru3-MXene interaction is therefore a result of the Ru–O covalent bonding, and the metal cluster–support interaction dictates the charge transfer, the oxidation state of Ru and the catalytic properties of Ru3@Mo2CO2 for N2-to-NH3 conversion.

Figure 1. The stability and electronic properties of Ru3@Mo2CO2. (a) Optimized geometry and (b) calculated electron density difference of Ru3@Mo2CO2. (c) PDOS and (d) −COHP of Ru–O in Ru3@Mo2CO2. (e) Fluctuations of the total energy and average bond lengths of Ru–Ru in the Ru3 cluster and Ru–O on the Ru3@Mo2CO2 during AIMD simulations at a temperature of 473 K. (f) The dissociation of Ru3 to Ru2 and Ru1.

To further explore the stability of Ru3 clusters on Mo2CO2 in a Ru3@Mo2CO2 SCC, AIMD simulations were performed at 473 and 673 K for 20 ps, as shown in Fig. 1e and Fig. S2, respectively. Starting from the optimized stable structure of the Ru3@Mo2CO2 SCC, the geometric configuration of the anchored Ru3 cluster on the surface of Mo2CO2 largely maintains a regular triangular structure, and the statistical average bond lengths of Ru–Ru (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\bar{d}$\end{document}Ru–Ru) and Ru–O (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\bar{d}$\end{document}Ru–O) are 2.39 and 2.17 Å, respectively, showing that the Ru3 cluster binds strongly on the surface of Mo2CO2. Moreover, with regard to the stability of the Ru3 cluster, Fig. 1f shows that the dissociation of one Ru atom from the Ru3 cluster in Ru3@Mo2CO2 reactant (R), yielding the metastable Ru single atom and Ru2 dimer as products (P), requires a high energy barrier of 2.72 eV (TS) and an endothermic reaction energy of 0.79 eV. These results demonstrate the stability of Ru3 clusters on Mo2CO2 support in Ru3@Mo2CO2 SCCs.

Adsorption of N2 and H2

It is well known that the degree of N2 activation plays a crucial role in thermal N2-to-NH3 conversion, especially for the first associative reaction step of *N2 + *H → *NNH, which is often the rate-determining step (RDS) for NH3 synthesis [37,83]. Here we firstly investigate the possible configurations of N2 adsorption on the surface of Ru3@Mo2CO2 (Fig. 2a). Due to the strong N≡N triple bond, the N2 prefers molecular adsorption in the end-on and side-on modes rather than dissociative adsorption on the positively charged Ru3 cluster of Ru3@Mo2CO2. As shown in Fig. 2a and Table S1, the most stable adsorption mode of N2 is the end-on adsorption (I) at one Ru atom of the Ru3 cluster with an adsorption energy of −1.37 eV, in which the bond length of *N2 is 1.16 Å and the Bader charge of *N2 is only −0.26 |e|, suggesting that the adsorbed N2 in the end-on adsorption mode is weakly activated. However, the metastable N2 adsorbed in side-on adsorption mode can be well activated by three Ru atoms of the Ru3 cluster in IV (rather than by one or two Ru atoms in II and III), as shown in Fig. 2a. This strong adsorption arises from a significant charge transfer of 0.74 |e| from Ru3@Mo2CO2 to the adsorbed N2 with an N–N bond length of 1.25 Å in IV, which is close to the N–N double bond length of 1.20 Å [82]. In comparison, there is only 0.34 |e| charge transfer from isolated Ru3 clusters to the adsorbed N2 with an N–N bond length of 1.20 Å, indicating that the multiple-atom site of the SCC is advantageous for N2 activation [84,85].

Figure 2. The stability and electronic properties of N2/H2 adsorbed on Ru3@Mo2CO2. (a) The correlation of the N−N bond lengths and Bader charges of *N2. (b) Electron density difference of *N2 and Ru3@Mo2CO2. (c) PDOS and −COHP of Ru−N. (d) The correlation of the H–H distance and the adsorption energies of *H2 with the co-adsorption of N2 + H2 on Ru3@Mo2CO2.

Consistent with previous studies [86–88], the two adsorption modes of N2 can be transformed into each other at room temperature. Here, N2 on the Ru3 cluster of Ru3@Mo2CO2 is easily converted from end-on adsorption (I) to side-on adsorption (IV), with an energy barrier of only 0.55 eV, as shown in Fig. S3. Moreover, the calculated charge density difference (Fig. 2b) of structure IV shows that N2 is activated by Ru3@Mo2CO2 with strong Ru–N bonds formed, leading to a decrease in charge density on N–N and the enrichment of charge density on Ru–N. Furthermore, PDOS and −ICOHP electronic properties analysis of structure IV shows that the (N2)3σ→(Ru)4d donation and (Ru)4d→(N2)1π* back-donation interactions result in the side-on adsorbed N2 on Ru3@Mo2CO2, which further facilitates N2 activation, as shown in Fig. 2c.

The dissociative adsorption of H2 on the surface of Ru3@Mo2CO2 may also play an important role in the catalytic processes of NH3 synthesis, as shown in a previous study [89]. The adsorption of H2 and co-adsorption of H2 + N2 on Ru3@Mo2CO2 are explored. As shown in Fig. 2d, Fig. S4a and Table S2, H2 can easily undergo dissociative adsorption on the Ru3 cluster of Ru3@Mo2CO2 and the calculated dissociative adsorption energies of H2 in structures Ⅴ and Ⅵ are −1.01 and −1.03 eV, respectively, which are less than that (−1.31 eV) of N2 adsorption on the Ru3 cluster of Ru3@Mo2CO2 in structure IV, suggesting that N2 is preferentially adsorbed on the Ru3 cluster. Therefore, based on structure IV, the dissociatively adsorbed H2 can co-adsorb with the adsorbed N2 on the Ru3 cluster of Ru3@Mo2CO2, as shown in Fig. 2d. The most stable co-adsorption configuration of H2 + N2 is structure IX, for which the distance (2.09 Å) of H and H atoms is longer than that in structures VII and VIII. Moreover, it is evident that a Ru–H bond is formed in structure IX due to the obvious electron density accumulation between the two adsorbed H atoms and the Ru3 cluster (Fig. S4b). The calculated PDOS and −ICOHP of structure IX, where Ru–H interacting states lie in the bonding region below the Fermi level (Fig. S4c), indicate that H2 is also strongly dissociatively adsorbed on Ru3@Mo2CO2.

Thermal N2-to-NH3 conversion mechanisms

To study the thermal conversion of N2 to NH3 catalyzed by Ru3@Mo2CO2, the possible associative and dissociative reaction mechanisms are explored. The schematic diagram in Fig. 3 shows four possible associative and one dissociative reaction pathway for N2-to-NH3 conversion. For the associative mechanism, after the first hydrogenation of the activated N2 to form *NNH, it is either further hydrogenated to form *NHNH (alternative I) or *NNH2 (distal I), or dissociated into *N + *NH (alternative II), respectively. Moreover, *NHNH can further decompose into *NH + *NH in the alternative Ⅲ pathway. However, for the dissociative mechanism, the activated N2 is firstly dissociated into two *N on Ru3 cluster, and then the two *N are gradually hydrogenated to form NH3. Detailed schematic descriptions of the five reaction pathways for the thermal conversion of N2 to NH3 catalyzed by Ru3@Mo2CO2, and the calculated energy profiles for these reaction pathways with the corresponding optimized structures, are shown in Fig. 3 and Figs S5–S7, respectively.

Figure 3. Schematic diagram of the possible reaction pathways of N2 to NH3 on Ru3@Mo2CO2 (* denotes surface-adsorbed species).

Because of the strong triple bond of N2, for most iron and ruthenium metal-based catalysts [11,23,28,90–93], the dissociative step of *N2 → *N + *N is often identified as the RDS for NH3 thermal synthesis. On the B5 site of the bulk Ru metal surface, the N2 dissociation barrier is expected to be much lower than on supported clusters because of the Ru(0) oxidation state. However, as shown in Fig. 4a and Fig. S5, for Ru3@Mo2CO2 the N–N bond cleavage barrier energy in the dissociative pathway is only 1.22 eV for the transition state of TS(a2–a3) (a2 to a3), because of the synergistic effect [22,23] on the multiple-Ru sites of the Ru3 cluster in favor of N2 activation with an N–N bond length of 1.25 Å. With the hydrogenation of *N on the Ru3 cluster of Ru3@Mo2CO2, 1.34 eV barrier energy is required for TS(a5–a6) to form *NH + *NH. Then a higher barrier energy of 1.41 eV (TS(a8–a9)) is needed to form *NH2 + *NH2 species in a9. Next, the two generated *NH2 are successively hydrogenated to form two NH3 from a10 to a11 with a barrier of 1.36 eV, and from a13 to a14 with a barrier of 1.28 eV. Therefore, it is obvious that the step from a8 to a9 with barrier energy of 1.41 eV is the RDS in the dissociative pathway of N2-to-NH3 conversion on Ru3@Mo2CO2 in Fig. 4a.

Figure 4. (a) Predicted possible reaction pathways of NH3 synthesis on Ru3@Mo2CO2. (b) Energy profile of the optimal associative alternative pathway I.

Compared to the direct breaking of the N–N bond of *N2 in the aforementioned dissociative mechanism, it is more favorable for *N2 hydrogenation to form *NNH in b4 with a barrier energy of 0.47 eV (TS(b3–b4)) through the associative pathways [22,23], and the bond length of N–N is stretched to 1.35 Å in b4, which is close to the N–N single bond [82]. In the alternative pathway I, the generated *NNH can be further hydrogenated into *HNNH through TS(b6–b7) with a slightly high barrier energy of 1.03 eV, and the N–N bond length is further lengthened to 1.39 Å. Subsequently, the second H2 is dissociatively adsorbed on the Ru3 cluster in b8, and then one of the *H atoms attracts *HNNH species to form *HNNH2 with a barrier of 0.91 eV (TS(b8–b9)), followed by the N–N bond breaking to form *NH and *NH2 (0.29 eV, TS(b9–b10)). Then the remaining *H attacks *NH2 to yield the first NH3, with a barrier energy of 1.16 eV from b10 to b11. Finally, the remaining *NH is hydrogenated twice to yield the second NH3 from b15 to b16, in which the step of *NH2 + *H → *NH3 is the RDS in the alternative pathway I for NH3 synthesis, with a high barrier energy of 1.29 eV, as shown in Fig. 4a and b, and Fig. S6.

Associative distal pathway I is different from alternative pathway I starting from b4, where the *NNH species is further hydrogenated into *NNH2 with a slightly high barrier of 1.37 eV (TS(b4–c1)), and the N≡N triple bond is approximately activated to single bond (1.41 Å) in *NNH2 species by the Ru3 cluster. Thus, the NNH2 is easily broken into *N and *NH2 on the Ru3 cluster (0.40 eV, TS(c1–c2)). Furthermore, the subsequent hydrogenation reactions of *NH2 + *H → NH3 (c3 to c4) and *N + *H → *NH (c5 to b12) are more easily compared to the formation of *NNH2, owing to the two lower barrier energies of 1.29 and 1.31 eV, as shown in Fig. 4a and Fig. S7. Accordingly, the formation of *NNH2 is the RDS in associative distal pathway I for the conversion of N2 to NH3.

For the associative alternative pathways in Fig. 4a and Fig. S7, based on the structures of b4 and b7, the N–N bonds in *NNH and *HNNH species are easy to break directly in alternative pathway II from b4 to d1 (0.71 eV, TS(b4–d1)) and in alternative pathway III from b7 to a6 (0.46 eV, TS(b7–e1)), respectively. Then, the resulting *N and *NH species undergo hydrogenation along the dissociation pathway to form NH3.

Overall, the N–N bond cleavage energy barrier (0.71 eV, TS(b4–b5)) in *NNH is obviously lower than that (1.22 eV, TS(a2–a3)) in *N2, resulting in NH3 synthesis preferentially following the associative pathway on Ru3@Mo2CO2 under moderate thermodynamic conditions. The RDSs of the above five reaction pathways all involve the hydrogenation steps for the formation of *NHx species, and the reaction barrier energies are 1.29 eV (b15 to b16) in alternative pathway I, 1.37 eV (b4 to c1) in distal pathway I and 1.41 eV (a8 to a9) in alternative pathway II, Ⅲ and dissociation mechanisms (Fig. 4a and Fig. S5), respectively, showing that efficient thermal N2-to-NH3 conversion on the Ru3 cluster supported by Mo2CO2 MXene is dominated by associative alternative pathway I at mild conditions.

To further explore the effect of thermodynamic conditions, the Gibbs free energy profile is calculated at 48 bar and 700 K. As shown in Fig. S8, although the free energy changes slightly, the optimal reaction pathway and the RDS remain unchanged, where the energy barrier only increases by 0.02 eV.

Origin of N2-to-NH3 conversion on Ru3@Mo2CO2

The configurations of the active center, support and the intermediate species, especially the oxidation states of the active center, are the key [94,95] to understanding the essence of NH3 synthesis through the multi-step redox reactions catalyzed by Ru3@Mo2CO2. Therefore, we further analyzed the Bader charges of Mo2CO2 support, Ru3 cluster, NxHy species and N2, and the changes of N–N bond length along the optimal associative alternative pathway I for the N2-to-NH3 conversion, as shown in Fig. 5 and Tables S3 and S4. As seen from Fig. 5 and Table S3, the Bader charge of Mo2CO2 support is about −1.40 |e| and remains almost unchanged throughout the whole process of NH3 synthesis, indicating that Mo2CO2 support here serves as the electron reservoir for the anchored Ru3 cluster, which tends to effectively adsorb and activate N2. This originates from its ability to effectively accept the electron lone pair of N2, thereby decreasing the bonding of N2. Furthermore, the oxidation state of the anchored Ru3 cluster significantly increases after the adsorption of N2, with its Bader charge increasing from +1.34 |e| in a1 to +2.13 |e| in a2. This charge transfer arises from the back-donation of Ru α-spin d-electrons to the antibonding orbital of N2 through d-π* orbital interaction, which involves the energy-level-matched d orbitals of the Ru3 cluster and the antibonding π* orbital of N2, as shown in Fig. 5. Thus, the N–N bond is significantly weakened so that the hydrogenation of the activated N2 becomes easier than that on the isolated Ru3 cluster [92].

Figure 5. (a) The Bader charge variation of the adsorbates of the Ru3 cluster, NxHy species, dinitrogen and Mo2CO2 support, and the changes of N–N bond lengths at every step of the catalytic cycle along the optimal associative alternative pathway I. (b) PDOS and schematic illustrations of the 4d orbitals of the Ru3 cluster on Mo2CO2, 2p-orbitals of the N2 gas molecule, and their interaction within Ru3N2/Mo2CO2. (c) PDOS and schematic illustrations for Ru3NNH/Mo2CO2.

After the first hydrogenation of N2, the Ru3 cluster further back-donates its β-spin d-electrons to the antibonding orbital of HNN, resulting in its lower hydrogenation (0.47 eV) and dissociation energy barriers (0.71 eV) than those of N2. Moreover, the change of the Bader charges of NxHy species and the Ru3 cluster are complementary in the whole of associative alternative pathway I, indicating that Ru3 serves as a reservoir to regulate the charge transfer for N2 synthesis, in which the calculated Bader charges of Ru3 range from +1.82 to +2.13 |e|. After the *NH3 desorbs from the surface of Ru3@Mo2CO2, the oxidation state of the Ru3 cluster returns to its initial low valence state from b15 to b16. It appears that the change of the Bader charges of NxHy species and the Ru3 cluster are complementary in the whole process of NH3 synthesis (Fig. 5a), showing that charges transfer from the anchored Ru3 cluster to the adsorbed N2 and NxHy species to further facilitate N2 hydrogenation during the process of N2-to-NH3 conversion.

In addition, the complete cleavage of the N≡N triple bond undergoes five stepwise stages in optimal associative alternative pathway I (Fig. 5a and Table S4): N2 (g) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\mathop \to \limits^{\bigcirc{1}} $\end{document} *N2  \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\mathop \to \limits^{\bigcirc{2}} $\end{document} *NNH \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\mathop \to \limits^{\bigcirc{3}} $\end{document} *NHNH \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\mathop \to \limits^{\bigcirc{4}} $\end{document} *NH····NH2  \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\mathop \to \limits^{\bigcirc{5}} $\end{document} *NH2 + *NH. The bond length of N–N changes from 1.08 Å in gas-phase N2 to an N–N double bond of 1.25 Å in *N2, to an approximate N–N single bond of 1.35 Å in *NNH, to an N–N single bond of 1.41 Å in *NHNH, to an N–N distance of 1.48 Å with very weak interaction in *NH and *NH2, until the N–N bond complete cleavage (3.31 Å) for *NH2 and *NH.

Microkinetic simulations of N2-to-NH3 conversion on Ru3@Mo2CO2

To directly examine the catalytic activity of N2-to-NH3 conversion under realistic conditions, we further performed microkinetic simulations to estimate the TOF. The TOF is calculated under a pressure range of 1–100 bar and a temperature range of 300–1050 K. As shown in Fig. 6 and Tables S5 and S6, the TOF of the N2-to-NH3 conversion on the Ru3@Mo2CO2 SCC is <10−10 s−1 site−1 below 400 K, due to its RDS energy barrier of 1.29 eV. With an increase in temperature, the TOF rapidly increases. However, at a pressure of 1 bar, when the temperature exceeds 640 K, the TOF decreases with the increase in temperature, which is due to the influence of the entropy effect and the fact that N2 cannot stably adsorb on the metal for reaction. With the increased pressure, the TOF continues to increase. When the temperature reaches 610 K and the pressure reaches 100 bar, the reaction easily overcomes the energy barrier of the RDS and the TOF of N2-to-NH3 conversion of Ru3@Mo2CO2 is 1.02 × 10−3 s−1 site−1. Moreover, the catalytic performance of Ru3@Mo2CO2 for ammonia synthesis even surpasses that of the well-known Ru B5 site on Ru-metal and Fe3/θ-Al2O3(010) [23,96] due to its TOF reaching 1.45 × 10−2 s−1 site−1 at 48 bar and 700 K. Furthermore, we further compare the TOF of the N2-to-NH3 conversion on Ru3@Mo2CO2 through the dissociation pathway and the association alternative pathway I. As shown in Fig. 6, in the range of the temperature and pressure considered, the TOF in alternative pathway I is significantly larger than that in the dissociation pathway, which further confirms that the ammonia synthesis on Ru3@Mo2CO2 will prefer alternative pathway I, which can bypass the restriction of BEP relation.

Figure 6. Microkinetic simulations. (a) TOF per site of ammonia synthesis on Ru3@Mo2CO2 mapped with pressure (1–100 bar) and temperature (300–1050 K). H2: N2 = 3 : 1, and the conversion ratio of NH3 is 10%. (b) TOF contributions from the alternative pathway-I mechanism and the dissociative mechanism at constant pressures of 100 and 48 bar and constant temperatures of 700 and 610 K, respectively.

Moreover, we further explored the evolution of surface coverage at 48 bar and 700 K using MKMCXX code [77]. As shown in Fig. S9, at this thermodynamic condition, as the reaction progresses, the coverage degree of H and N2 first increases and then decreases, while the coverage degree of intermediate NH2* increases continuously, indicating that the synthesis of ammonia can proceed smoothly at 48 bar and 700 K conditions.

CONCLUSIONS

The first-principles calculations have been performed on the stability and electron structure of Ru3@Mo2CO2, and its catalytic performance for ammonia synthesis from nitrogen has been investigated. It is shown that Ru3@Mo2CO2 has high stability, originating from the strong bonding interaction between the anchored Ru3 cluster and its surrounding O atoms. N2 is found to stably adsorb on Ru3@Mo2CO2 by both end-on and side-on adsorption modes, and gets obviously activated due to the d-π interaction between N2 and the Ru3 cluster.

Four possible mechanisms for ammonia synthesis, including one dissociation mechanism and three association mechanisms, are studied in detail. The results show that association alternative pathway I is the most feasible for N2-to-NH3 conversion on Ru3@Mo2CO2, in which the RDS is NH2* + H* → NH3* with a low energy barrier of 1.29 eV. The high catalytic performance of Ru3@Mo2CO2 for ammonia synthesis originates from the anchored Ru3 cluster, which can serve as an electron reservoir to effectively regulate the charge transfer during the ammonia synthesis process, with the assistance of the support Mo2CO2. Moreover, microkinetic analysis shows that the TOF of N2-to-NH3 conversion on Ru3@Mo2CO2 via association alternative pathway I is as high as 1.45 × 10−2 s−1 site−1 at 48 bar and 700 K. The theoretical results show that Ru3@Mo2CO2 is a promising SCC for ammonia synthesis directly from nitrogen and hydrogen. This work demonstrates the potential and capacity of SCCs featuring atomically precise active sites in the rational design of high-performance heterogeneous catalysts.

Supplementary Material

nwae251_Supplemental_File

ACKNOWLEDGEMENTS

Calculations were performed using supercomputers at Shanghai Supercomputing Center and the CHEM high-performance computing cluster (CHEM-HPC) located at the Department of Chemistry, Southern University of Science and Technology (SUSTech). Computational resources were also supported by the Center for Computational Science and Engineering at SUSTech.

FUNDING

This research was supported by the National Natural Science Foundation of China (NSFC, 22363001, 21963005, 21763006 and 22250710677), the NSFC Center for Single-Atom Catalysis (22388102), the National Key R&D Project (2022YFA1503900) and the Natural Science Special Foundation of Guizhou University (202140).

AUTHOR CONTRIBUTIONS

Li J. directed the research. Liang J.X., Zhu C. and Zhang C. conducted the DFT calculations. Zhang C., Wang Z.H., Wang H.Y., Liang J.X., Zhu C. and Li J. analyzed the data. All the authors discussed the results and co-wrote the manuscript.

Conflict of interest statement. None declared.
==== Refs
REFERENCES

1. Ghoreishian  SM, Shariati  K, Huh  YS  et al.  Recent advances in ammonia synthesis over ruthenium single-atom-embedded catalysts: a focused review. Chem Eng J  2023; 467 : 143533.10.1016/j.cej.2023.143533
2. Wang  Q, Guo  J, Chen  P. Recent progress towards mild-condition ammonia synthesis. J Energy Chem  2019; 36 : 25–36.10.1016/j.jechem.2019.01.027
3. Fang  H, Liu  D, Luo  Y  et al.  Challenges and opportunities of Ru-based catalysts toward the synthesis and utilization of ammonia. ACS Catal  2022; 12 : 3938–54.10.1021/acscatal.2c00090
4. Haber  F, van Oordt  G. Über die Bildung von Ammoniak den Elementen. Z Anorg Allg Chem  1905; 44 : 341–78.10.1002/zaac.19050440122
5. Ertl  G . Reactions at surfaces: from atoms to complexity (Nobel lecture). Angew Chem Int Ed  2008; 47 : 3524–35.10.1002/anie.200800480
6. Guo  J, Chen  P. Ammonia history in the making. Nat Catal  2021; 4 : 734–5.10.1038/s41929-021-00676-0
7. Kandemir  T, Schuster  ME, Senyshyn  A  et al.  The Haber–Bosch process revisited: on the real structure and stability of “ammonia iron” under working conditions. Angew Chem Int Ed  2013; 52 : 12723–6.10.1002/anie.201305812
8. Chen  JG, Crooks  RM, Seefeldt  LC  et al.  Beyond fossil fuel–driven nitrogen transformations. Science  2018; 360 : eaar6611.10.1126/science.aar6611 29798857
9. Wang  L, Xia  M, Wang  H  et al.  Greening ammonia toward the solar ammonia refinery. Joule  2018; 2 : 1055–74.10.1016/j.joule.2018.04.017
10. Gong  Y, Li  H, Wu  J  et al.  Unique catalytic mechanism for Ru-loaded ternary intermetallic electrides for ammonia synthesis. J Am Chem Soc  2022; 144 : 8683–92.10.1021/jacs.2c01899 35507518
11. Logadóttir  Á, Nørskov  JK. Ammonia synthesis over a Ru(0001) surface studied by density functional calculations. J Catal  2003; 220 : 273–9.10.1016/S0021-9517(03)00156-8
12. Ertl  G, Lee  SB, Weiss  M. Kinetics of nitrogen adsorption on Fe(111). Surf Sci  1982; 114 : 515–26.10.1016/0039-6028(82)90702-6
13. Bozso  F, Ertl  G, Grunze  M  et al.  Interaction of nitrogen with iron surfaces: I. Fe(100) and Fe(111). J Catal  1977; 49 : 18–41.10.1016/0021-9517(77)90237-8
14. Strongin  DR, Carrazza  J, Bare  SR  et al.  The importance of C7 sites and surface roughness in the ammonia synthesis reaction over iron. J Catal  1987; 103 : 213–5.10.1016/0021-9517(87)90109-6
15. Reuter  K, Plaisance  CP, Oberhofer  H  et al.  Perspective: on the active site model in computational catalyst screening. J Chem Phys  2017; 146 : 040901.10.1063/1.4974931 28147553
16. Belt  ST, Scaiano  JC, Whittlesey  MK. Determination of metal-hydride and metal-ligand (L = CO, N2) bond energies using photoacoustic calorimetry. J Am Chem Soc  1993; 115 : 1921–5.10.1021/ja00058a043
17. Foster  SL, Bakovic  SIP, Duda  RD  et al.  Catalysts for nitrogen reduction to ammonia. Nat Catal  2018; 1 : 490–500.10.1038/s41929-018-0092-7
18. Bligaard  T, Nørskov  JK, Dahl  S  et al.  The Brønsted–Evans–Polanyi relation and the volcano curve in heterogeneous catalysis. J Catal  2004; 224 : 206–17.10.1016/j.jcat.2004.02.034
19. Qiao  B, Wang  A, Yang  X  et al.  Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat Chem  2011; 3 : 634–41.10.1038/nchem.1095 21778984
20. Zhuo  H-Y, Zhang  X, Liang  J-X  et al.  Theoretical understandings of graphene-based metal single-atom catalysts: stability and catalytic performance. Chem Rev  2020; 120 : 12315–41.10.1021/acs.chemrev.0c00818 33112608
21. Xing  D-H, Xu  C-Q, Wang  Y-G  et al.  Heterogeneous single-cluster catalysts for selective semihydrogenation of acetylene with graphdiyne-supported triatomic clusters. J Phys Chem C  2019; 123 : 10494–500.10.1021/acs.jpcc.9b02029
22. Ma  X-L, Liu  J-C, Xiao  H  et al.  Surface single-cluster catalyst for N2-to-NH3 thermal conversion. J Am Chem Soc  2017; 140 : 46–9.10.1021/jacs.7b10354 29244491
23. Liu  J-C, Ma  X-L, Li  Y  et al.  Heterogeneous Fe3 single-cluster catalyst for ammonia synthesis via an associative mechanism. Nat Commun  2018; 9 : 1610.10.1038/s41467-018-03795-8 29686395
24. Liu  J-C, Xiao  H, Li  J. Constructing high-loading single-atom/cluster catalysts via an electrochemical potential window strategy. J Am Chem Soc  2020; 142 : 3375–83.10.1021/jacs.9b06808 31994381
25. Liu  S, Wang  M, Ji  H  et al.  Altering the rate-determining step over cobalt single clusters leading to highly efficient ammonia synthesis. Natl Sci Rev  2021; 8 : nwaa136.10.1093/nsr/nwaa136 34691629
26. Wang  G, Jiang  X-L, Jiang  Y-F  et al.  Screened Fe3 and Ru3 single-cluster catalysts anchored on MoS2 supports for selective hydrogenation of CO2. ACS Catal  2023; 13 : 8413–22.10.1021/acscatal.3c00617
27. Ji  S, Chen  Y, Fu  Q  et al.  Confined pyrolysis within metal–organic frameworks to form uniform Ru3 clusters for efficient oxidation of alcohols. J Am Chem Soc  2017; 139 : 9795–8.10.1021/jacs.7b05018 28696113
28. Zhang  X, Liu  L, Wu  A  et al.  Synergizing surface hydride species and Ru clusters on Sm2O3 for efficient ammonia synthesis. ACS Catal  2022; 12 : 2178–90.10.1021/acscatal.1c05985
29. Shu  P, Qi  X, Peng  Q  et al.  Heterogeneous metal trimer catalysts on Mo2TiC2O2 MXene for highly active N2 conversion to NH3. Mol Catal  2023; 539 : 113036.10.1016/j.mcat.2023.113036
30. Ma  X-L, Yang  Y, Xu  L-M  et al.  Theoretical investigation on hydrogenation of dinitrogen triggered by singly dispersed bimetallic sites. J Mater Chem A  2022; 10 : 6146–52.10.1039/D1TA08350C
31. Lin  B, Fang  B, Wu  Y  et al.  Enhanced ammonia synthesis activity of ceria-supported ruthenium catalysts induced by CO activation. ACS Catal  2021; 11 : 1331–9.10.1021/acscatal.0c05074
32. Zhou  Y, Xu  C-Q, Tan  Z  et al.  Integrating dissociative and associative routes for efficient ammonia synthesis over a TiCN-promoted Ru-based catalyst. ACS Catal  2022; 12 : 2651–60.10.1021/acscatal.1c05613
33. Sun  K, Zou  X, Sun  X  et al.  Structure and reaction condition dependent mechanism for ammonia synthesis on Ru-based catalyst. Appl Surf Sci  2023; 613 : 156060.10.1016/j.apsusc.2022.156060
34. Raróg-Pilecka  W, Miśkiewicz  E, Szmigiel  D  et al.  Structure sensitivity of ammonia synthesis over promoted ruthenium catalysts supported on graphitised carbon. J Catal  2005; 231 : 11–9.10.1016/j.jcat.2004.12.005
35. Song  Z, Cai  T, Hanson  JC  et al.  Structure and reactivity of Ru nanoparticles supported on modified graphite surfaces: a study of the model catalysts for ammonia synthesis. J Am Chem Soc  2004; 126 : 8576–84.10.1021/ja031718s 15238017
36. Zhou  Y, Wang  J, Liang  L  et al.  Unraveling the size-dependent effect of Ru-based catalysts on ammonia synthesis at mild conditions. J Catal  2021; 404 : 501–11.10.1016/j.jcat.2021.10.024
37. Jacobsen  CJH, Dahl  S, Hansen  PL  et al.  Structure sensitivity of supported ruthenium catalysts for ammonia synthesis. J Mol Catal A Chem  2000; 163 : 19–26.10.1016/S1381-1169(00)00396-4
38. Dahl  S, Logadottir  A, Egeberg  RC  et al.  Role of steps in N2 activation on Ru(0001). Phys Rev Lett  1999; 83 : 1814–17.10.1103/PhysRevLett.83.1814
39. Zhou  Y, Sai  Q, Tan  Z  et al.  Highly efficient subnanometer Ru-based catalyst for ammonia synthesis via an associative mechanism. Chin J Chem Eng  2022; 43 : 177–84.10.1016/j.cjche.2022.01.015
40. Li  L, Zhang  T, Cai  J  et al.  Operando spectroscopic and isotopic-label-directed observation of LaN-promoted Ru/ZrH2 catalyst for ammonia synthesis via associative and chemical looping route. J Catal  2020; 389 : 218–28.10.1016/j.jcat.2020.05.039
41. Li  L, Jiang  Y-F, Zhang  T  et al.  Size sensitivity of supported Ru catalysts for ammonia synthesis: from nanoparticles to subnanometric clusters and atomic clusters. Chem  2022; 8 : 749–68.10.1016/j.chempr.2021.11.008
42. Naguib  M, Kurtoglu  M, Presser  V  et al.  Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv Mater  2011; 23 : 4248–53.10.1002/adma.201102306 21861270
43. Naguib  M, Come  J, Dyatkin  B  et al.  MXene: a promising transition metal carbide anode for lithium-ion batteries. Electrochem Commun  2012; 16 : 61–4.10.1016/j.elecom.2012.01.002
44. Wei  Y, Zhang  P, Soomro  RA  et al.  Advances in the synthesis of 2D MXenes. Adv Mater  2021; 33 : 2103148.10.1002/adma.202103148
45. Zhao  D, Chen  Z, Yang  W  et al.  MXene (Ti3C2) vacancy-confined single-atom catalyst for efficient functionalization of CO2. J Am Chem Soc  2019; 141 : 4086–93.10.1021/jacs.8b13579 30699294
46. Zhu  J, Ha  E, Zhao  G  et al.  Recent advance in MXenes: a promising 2D material for catalysis, sensor and chemical adsorption. Coord Chem Rev  2017; 352 : 306–27.10.1016/j.ccr.2017.09.012
47. Papadopoulou  KA, Chroneos  A, Parfitt  D  et al.  A perspective on MXenes: their synthesis, properties, and recent applications. J Appl Phys  2020; 128 : 170902.10.1063/5.0021485
48. Kamysbayev  V, Filatov  AS, Hu  H  et al.  Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes. Science  2020; 369 : 979–83.10.1126/science.aba8311 32616671
49. Xie  Y, Naguib  M, Mochalin  VN  et al.  Role of surface structure on Li-ion energy storage capacity of two-dimensional transition-metal carbides. J Am Chem Soc  2014; 136 : 6385–94.10.1021/ja501520b 24678996
50. Wang  J, Dong  S, Ding  B  et al.  Pseudocapacitive materials for electrochemical capacitors: from rational synthesis to capacitance optimization. Natl Sci Rev  2017; 4 : 71–90.10.1093/nsr/nww072
51. Talib  SH, Lu  Z, Bashir  B  et al.  CO oxidation on MXene (Mo2CS2) supported single-atom catalyst: a termolecular Eley-Rideal mechanism. Chin Chem Lett  2023; 34 : 107412.10.1016/j.cclet.2022.04.010
52. Wang  X, Su  Y, Song  M  et al.  Design single nonmetal atom doped 2D Ti2CO2 electrocatalyst for hydrogen evolution reaction by coupling electronic descriptor. Appl Surf Sci  2021; 556 : 149778.10.1016/j.apsusc.2021.149778
53. Zhao  R, Chen  Y, Xiang  H  et al.  Two-dimensional ordered double-transition metal carbides for the electrochemical nitrogen reduction reaction. ACS Appl Mater Interfaces  2023; 15 : 6797–806.10.1021/acsami.2c19911 36705631
54. Gao  Y, Zhang  S, Sun  X  et al.  Computational screening of O-functional MXenes for electrocatalytic ammonia synthesis. Chin J Catal  2022; 43 : 1860–9.10.1016/S1872-2067(21)64011-1
55. Liu  T, Zhang  W, Chen  T  et al.  Regulating the coordination environment of ruthenium cluster catalysts for the alkaline hydrogen evolution reaction. J Phys Chem Lett  2021; 12 : 8016–23.10.1021/acs.jpclett.1c01936 34433277
56. Gouveia  JD, Rocha  H, Gomes  JRB. MXene-supported transition metal single-atom catalysts for nitrogen dissociation. Mol Catal  2023; 547 : 113373.10.1016/j.mcat.2023.113373
57. Zhang  X, Wang  S, Yan  L  et al.  Mo3C2: active electrocatalysts for spontaneous nitrogen reduction reaction. J Phys Chem C  2023; 127 : 22530–6.10.1021/acs.jpcc.3c04000
58. Amrillah  T, Hermawan  A, Alviani  VN  et al.  MXenes and their derivatives as nitrogen reduction reaction catalysts: recent progress and perspectives. Mater Today Energy  2021; 22 : 100864.10.1016/j.mtener.2021.100864
59. Wu  Y, Wang  L, Bo  T  et al.  Boosting hydrogen evolution in neutral medium by accelerating water dissociation with Ru clusters loaded on Mo2CTx MXene. Adv Funct Mater  2023; 33 : 2214375.10.1002/adfm.202214375
60. Gouveia  JD, Morales-García  Á, Viñes  F  et al.  Facile heterogeneously catalyzed nitrogen fixation by MXenes. ACS Catal  2020; 10 : 5049–56.10.1021/acscatal.0c00935
61. Peng  W, Luo  M, Xu  X  et al.  Spontaneous atomic ruthenium doping in Mo2CTX MXene defects enhances electrocatalytic activity for the nitrogen reduction reaction. Adv Energy Mater  2020; 10 : 2001364.10.1002/aenm.202001364
62. Cheng  C, Zhang  X, Yang  Z  et al.  Cu3-cluster-doped monolayer Mo2CO2 (MXene) as an electron reservoir for catalyzing a CO oxidation reaction. ACS Appl Mater Interfaces  2018; 10 : 32903–12.10.1021/acsami.8b12318 30157637
63. Wang  C, Shou  H, Chen  S  et al.  Mxene synthesis: HCl-based hydrothermal etching strategy toward fluoride-free MXenes. Adv Mater  2021; 33 : 2170209.10.1002/adma.202170209
64. Sosa  LF, de Souza  PM, Rafael  RA  et al.  Study of the performance of SiO2-supported Mo2C and metal-promoted Mo2C catalysts for the hydrodeoxygenation of m-cresol. Appl Catal B Environ  2023; 331 : 122720.10.1016/j.apcatb.2023.122720
65. Liu  J-C, Xiao  H, Zhao  X-K  et al.  Computational prediction of graphdiyne-supported three-atom single-cluster catalysts. CCS Chem  2022; 5 : 152–63.10.31635/ccschem.022.202201796
66. Kresse  G, Joubert  D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys Rev B  1999; 59 : 1758–75.10.1103/PhysRevB.59.1758
67. Kresse  G, Hafner  J. Ab initio molecular dynamics for liquid metals. Phys Rev B  1993; 47 : 558–61.10.1103/PhysRevB.47.558
68. Perdew  JP, Burke  K, Ernzerhof  M. Generalized gradient approximation made simple. Phys Rev B  1997; 77 : 3865–8.
69. Tkatchenko  A, Scheffler  M. Accurate molecular van der Waals interactions from ground-state electron density and free-atom reference data. Phys Rev Lett  2009; 102 : 073005.10.1103/PhysRevLett.102.073005 19257665
70. Grimme  S, Antony  J, Ehrlich  S  et al.  A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys  2010; 132 : 154104.10.1063/1.3382344 20423165
71. Nelson  R, Ertural  C, George  J  et al.  LOBSTER: local orbital projections, atomic charges, and chemical-bonding analysis from projector-augmented-wave-based density-functional theory. J Comput Chem  2020; 41 : 1931–40.10.1002/jcc.26353 32531113
72. Liu  X, Jiao  Y, Zheng  Y  et al.  Building up a picture of the electrocatalytic nitrogen reduction activity of transition metal single-atom catalysts. J Am Chem Soc  2019; 141 : 9664–72.10.1021/jacs.9b03811 31145607
73. Henkelman  G, Jónsson  H. A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives. J Chem Phys  1999; 111 : 7010–22.10.1063/1.480097
74. Medford  AJ, Shi  C, Hoffmann  MJ  et al.  CatMAP: a software package for descriptor-based microkinetic mapping of catalytic trends. Catal Lett  2015; 145 : 794–807.10.1007/s10562-015-1495-6
75. Eyring  H . The activated complex in chemical reactions. J Chem Phys  2004; 3 : 107–15.10.1063/1.1749604
76. Su  Y-Q, Xia  G-J, Qin  Y  et al.  Lattice oxygen self-spillover on reducible oxide supported metal cluster: the water–gas shift reaction on Cu/CeO2 catalyst. Chem Sci  2021; 12 : 8260–7.10.1039/D1SC01201K 34194718
77. Filot  IAW, van Santen  RA, Hensen  EJM. The optimally performing Fischer–Tropsch catalyst. Angew Chem Int Ed  2014; 53 : 12746–50.10.1002/anie.201406521
78. Filot  IAW, Broos  RJP, van Rijn  JPM  et al.  First-principles-based microkinetics simulations of synthesis gas conversion on a stepped rhodium surface. ACS Catal  2015; 5 : 5453–67.10.1021/acscatal.5b01391
79. Bader  RFW . Atoms in molecules. Acc Chem Res  1985; 18 : 9–15.10.1021/ar00109a003
80. Yu  Q . Theoretical studies of non-noble metal single-atom catalyst Ni1/MoS2: electronic structure and electrocatalytic CO2 reduction. Sci China Mater  2022; 66 : 1079–88.10.1007/s40843-022-2222-6
81. Björk  J, Rosen  J. Functionalizing MXenes by tailoring surface terminations in different chemical environments. Chem Mater  2021; 33 : 9108–18.10.1021/acs.chemmater.1c01264
82. Pyykkö  P, Atsumi  M. Molecular single-bond covalent radii for elements 1–118. Chem Eur J  2009; 15 : 186–97.10.1002/chem.200800987 19058281
83. Emmett  PH, Brunauer  S. The adsorption of nitrogen by iron synthetic ammonia catalysts. J Am Chem Soc  1934; 56 : 35–41.10.1021/ja01316a011
84. Rong  H, Ji  S, Zhang  J  et al.  Synthetic strategies of supported atomic clusters for heterogeneous catalysis. Nat Commun  2020; 11 : 5884.10.1038/s41467-020-19571-6 33208740
85. Liu  J, Cao  D, Xu  H  et al.  From double-atom catalysts to single-cluster catalysts: a new frontier in heterogeneous catalysis. Nano Select  2021; 2 : 251–70.10.1002/nano.202000155
86. Hu  J-N, Tian  L-C, Wang  H  et al.  Theoretical screening of single-atom electrocatalysts of MXene-supported 3d-metals for efficient nitrogen reduction. Chin J Catal  2023; 52 : 252–62.10.1016/S1872-2067(23)64501-2
87. Liu  G, Zhou  J, Zhao  W  et al.  Single atom catalytic oxidation mechanism of formaldehyde on Al doped graphene at room temperature. Chin Chem Lett  2020; 31 : 1966–9.10.1016/j.cclet.2019.12.023
88. Young  DC . Computational chemistry: a practical guide for applying techniques to real world problems. J Am Chem Soc  2001; 123 : 10142–3.
89. Gu  K, Lin  S. Sustained hydrogen spillover on Pt/Cu(111) single-atom alloy: dynamic insights into gas-induced chemical processes. Angew Chem Int Ed  2023; 62 : e202312796.10.1002/anie.202312796
90. Peng  X, Su  K, Fang  H  et al.  Colloid carbonization-stabilized Ru nanoparticle catalyst for efficient ammonia synthesis at mild conditions. Chem Eng Sci  2023; 278 : 118926.10.1016/j.ces.2023.118926
91. Bielawa  H, Hinrichsen  O, Birkner  A  et al.  The ammonia-synthesis catalyst of the next generation: barium-promoted oxide-supported ruthenium. Angew Chem Int Ed  2001; 40 : 1061–3.10.1002/1521-3773(20010316)40:6<1061::AID-ANIE10610>3.0.CO;2-B
92. Cui  C, Zhang  H, Cheng  R  et al.  On the nature of three-atom metal cluster catalysis for N2 reduction to ammonia. ACS Catal  2022; 12 : 14964–75.10.1021/acscatal.2c04146
93. Casey-Stevens  CA, Lambie  SG, Ruffman  C  et al.  Geometric and electronic effects contributing to N2 dissociation barriers on a range of active sites on Ru nanoparticles. J Phys Chem C  2019; 123 : 30458–66.10.1021/acs.jpcc.9b09563
94. Liang  J, Yu  Q, Yang  X  et al.  A systematic theoretical study on FeOx-supported single-atom catalysts: M1/FeOx for CO oxidation. Nano Res  2018; 11 : 1599–611.10.1007/s12274-017-1775-0
95. Liang  J-X, Lin  J, Liu  J  et al.  Dual metal active sites in an Ir1/FeOx single-atom catalyst: a redox mechanism for the water-gas shift reaction. Angew Chem Int Ed  2020; 59 : 12868–75.10.1002/anie.201914867
96. Vojvodic  A, Medford  AJ, Studt  F  et al.  Exploring the limits: a low-pressure, low-temperature Haber–Bosch process. Chem Phys Lett  2014; 598 : 108–12.10.1016/j.cplett.2014.03.003
