Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst

Abstract

Ammonia (NH3) is pivotal to the fertilizer industry and one of the most commonly produced chemicals1. The direct use of atmospheric nitrogen (N2) had been challenging, owing to its large bond energy (945 kilojoules per mole)2,3, until the development of the Haber–Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N≡N bond and make the process more efficient. These include using alkali and alkaline earth metal oxides as promoters to boost the performance of traditional iron- and ruthenium-based catalysts4,5,6 via electron transfer from the promoters to the antibonding bonds of N2 through transition metals7,8. An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals9,10. This strategy has facilitated ammonia synthesis from N2 dissociation11 and enabled catalytic operation under mild conditions; however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N2 (refs. 12,13,14,15). Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N2. In addition, the nickel metal loaded onto the nitride dissociates H2. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturally abundant elements.

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Fig. 1: Electronic and crystal structure of Ni/LaN.
Fig. 2: Catalytic activity of Ni/LaN.
Fig. 3: Isotope effect on Ni/LaN.
Fig. 4: DFT studies of the reaction path of Ni/LaN for ammonia synthesis.

Data availability

The data that support the findings of this study are available from the corresponding authors on reasonable request.

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Acknowledgements

This work was supported by a MEXT Element Strategy Initiative to Form Core Research Center (grant number JPMXP0112101001). Part of this work was supported by a PRESTO Grant (number JPMJPR18T6) from the Japan Science and Technology Agency (JST) and Kakenhi Grants-in-Aid (numbers 17H06153, JP19H05051 and JP19H02512) from the Japan Society for the Promotion of Science (JSPS). T.-N.Y. is supported by a JSPS fellowship for International Research Fellows (number P18361). Y.L. is supported by a JSPS fellowship for young scientists (number 18J00745). We thank Y. Sato (Tokyo Institute of Technology) for technical support in the AES measurements and J. Wu (Tokyo Institute of Technology) for discussions.

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Contributions

H.H. proposed the idea behind the research and supervised the project. T.-N.Y., Y.L., J.L. and M.K. performed the synthesis, characterization and catalytic measurements. S.-W.P. and T.T. conducted the model construction and DFT calculations. M.S. conducted STEM measurements. T.-N.Y., S.-W.P., Y.L., M.K. and H.H. co-wrote the paper. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Masaaki Kitano or Hideo Hosono.

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Extended data figures and tables

Extended Data Fig. 1 Schematics of reaction mechanisms.

ad, Comparison of the structural and reaction mechanism for ammonia synthesis over transition metal (TM)-loaded catalysts: a, TM/MgO; b, TM/C12A7:e; c, TM/LiH; d, TM/LaN.

Extended Data Fig. 2 DFT calculation results.

a, Calculated projected DOS of Ni8/La32N32, with detailed atomic configuration. The Ni[1], La[2], N[2] and N[1] atoms are emphasized using dashed lines. Substantial overlapping of the projected DOS of Ni (Ni[1]) and the nearest N (N[2]), observed at around −4 eV < E − EF < −2 eV, is much more stable than the interaction between Ni (Ni[1]) and the second-nearest N (N[1]). b, Proposed reaction barriers for N2 dissociation on VN sites of the Ni-loaded LaN surface. In scenario I, two N2 molecules are activated at two adjacent VN sites in the initial step (IS), and the two dissociated top N atoms combine to form a new N2 molecule with a barrier of 2.46 eV in the final step (FS). In scenario II, a N2 molecule is activated at the VN site (IS) and the dissociated top N atom is transferred to an adjacent Nlattice site with a hopping barrier of 3.34 eV (FS). c, Proposed reaction mechanism for N2 dissociation on Ni(111) surface. Owing to the extremely weak interaction between N2 and Ni, the adsorption energy is nearly 0.00 eV and the reaction barrier for the dissociation of N2 is calculated to be 1.55 eV. The structures of intermediates and transition states (TSs) for the key elementary steps are shown in the reaction paths. d, The Ni8/La32N31 model and ENV at different N sites.

Extended Data Fig. 3 Powder XRD patterns.

ae, Powder XRD pattern of various catalysts: a, fresh LaH3 (blue), LaN (red) and Ni/LaN (green) bulk; b, fresh (green) and used (purple) Ni/LaN bulk; c, fresh LaN nanoparticles (NPs; red) and used Ni/LaN NPs (blue); d, Ni/ScN bulk; e, Ni/YN bulk.

Extended Data Fig. 4 TEM, SEM and EDX studies.

ad, Atom-resolved HR-TEM image of LaN regions in fresh (a, b) and N-deficient (c, d) Ni/LaNV along the (111) (a, c, d) and (001) (b) directions. The inset of a shows the corresponding crystal structure of LaN along the (111) direction. The inset of b shows the (001) direction of LaN. La and N atoms are represented as grey and blue balls, respectively. e, HR-TEM images and EDX mapping results for Ni(12.5 wt%)/LaN nanoparticles after 100 h of reaction. fj, SEM and corresponding EDX analysis of fresh (f) and used (g–j) Ni(5 wt%)/LaN bulk after 100 h of reaction. Reaction conditions: catalyst, 0.1 g; WHSV, 36,000 ml gcat−1 h−1; 400 °C, 0.1 MPa.

Extended Data Fig. 5 AES observations.

af, AES spectra for N (a, d), La (b, e) and Ni (c, f), for fresh Ni/LaN, H2-pretreated Ni/LaNV and used Ni/LaN. Panels a–c (left) compare the spectra between fresh Ni/LaN and H2-pretreated Ni/LaNV; d–f compare the spectra between fresh and used Ni/LaN. To determine the location of VN, Ar plasma was used to etch the sample surface during the AES measurement (ac, right). After Ar spattering, the N content became almost the same as that in the LaN bulk, showing that the VN were generated only on the surface of LaN. By contrast, the signals of La remained largely unchanged between the surface and the bulk. The change in the Ni concentration was obvious because it was deposited only on the surface of LaN (c). The depth composition variation of used Ni/LaN bulk shows that the N peak (about 387 eV) also remains largely unchanged after reaction (d), demonstrating that the surface VN generated in situ should be occupied by N2 molecules during ammonia synthesis. La and Ni peaks also remains largely unchanged after reaction (e, f).

Extended Data Fig. 6 Catalytic performance and related characterizations of ammonia synthesis.

ad, Ammonia synthesis activity over Ni/LaN bulk (a, b) and Ni/LaN NPs (c, d), with various amounts of Ni loading. e, Time course of ammonia synthesis over different batches of Ni/LaN NPs and Ni/LaN bulk catalysts. f, Pressure dependence of the ammonia synthesis activity over Ni/LaN NPs. g, h, TPD profile (g) and Raman spectrum (h) for used Ni/LaN bulk after 100 h of reaction. i, Ammonia synthesis activity over various metals supported on LaN bulk catalysts. Ni(C5H5)2, Fe2(CO)9, Co2(CO)8 and Ru3(CO)12 were used as Ni, Fe, Co and Ru precursors, respectively. Error bars in ad, f and i represent the standard deviation from three independent measurements. Reaction conditions: catalyst, 0.1 g; WHSV, 36,000 ml gcat−1 h−1; 340−400 °C, 0.1−0.9 MPa.

Extended Data Fig. 7 Comparison of activities and H2 temperature-programmed reduction with different nitride-supported Ni.

a, Ammonia synthesis activity and VN formation energies over various nitride-supported Ni catalysts. Error bars represent the standard deviation from three independent measurements. Reaction conditions: catalyst, 0.1 g; WHSV, 36,000 ml gcat−1 h−1; 400 °C, 0.1 MPa. b, Cumulative amount of NH3 generated over various nitride-supported Ni catalysts under pure H2 as a function of time. c, H2 temperature-programmed reduction (TPR) profiles for various nitride-supported Ni catalysts.

Extended Data Table 1 Summary of various transition-metal-based catalysts in ammonia synthesis
Extended Data Table 2 Kinetic parameters of selected catalysts
Extended Data Table 3 DFT-calculated energy changes for elementary reaction steps over Ni/LaN

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Ye, T., Park, S., Lu, Y. et al. Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst. Nature 583, 391–395 (2020). https://doi.org/10.1038/s41586-020-2464-9

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