Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier; however, its production is highly energy intensive. Conventional heterogeneous catalysts based on metallic iron or ruthenium mediate dinitrogen dissociation and hydrogenation through a relatively energy-expensive pathway. Here we report the ternary ruthenium complex hydrides Li4RuH6 and Ba2RuH6 as an alternative class of catalysts, composed of electron- and hydrogen-rich [RuH6] anionic centres, for non-dissociative dinitrogen reduction, where hydridic hydrogen transports electrons and protons between the centres, and the Li/Ba cations stabilize NxHy (x = 0–2, y = 0–3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates an associative reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multistep process, leading to ammonia production from N2 + H2 with superior kinetics under mild conditions.
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The data supporting the findings of this study are available within the article and its Supplementary Information or from the corresponding authors upon reasonable request. The atomic coordinates of the optimized electronic structures are available in the data repository https://data.dtu.dk/, and can be accessed using the link https://doi.org/10.11583/DTU.16621918.v1. Source data are provided with this paper.
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We acknowledge the beam time of the Shanghai Synchrotron Radiation Facility and Deutsches Elektronen-Synchrotron and the help of J.-C. Tseng and J. Bednarcik for in situ SR-PXD. P.C. and J.G. thank the National Natural Science Foundation of China (grant numbers 21988101, 21633011 and 21922205), the Dalian Institute of Chemical Physics (DCLS201701) and the K.C. Wong Education Foundation (GJTD-2018-06) for financial support; J.P., H.A.H. and T.V. thank the Villum Foundation for financial support through the research centre V-Sustain (#9455).
The authors declare no competing interests.
Peer review information Nature Catalysis thanks Bingyu Lin, Jingxiang Zhao and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended Data Fig. 1 The stability of bulk Li4RuH6 and Ba2RuH6 under different conditions.
a, TPR profile of Li4RuH6 in a mixture gas of NH3-Ar (NH3:Ar=0.5:99.5) and a mixture gas of NH3-H2-Ar (NH3:H2:Ar=0.5:75:24.5). b, In situ SR-PXD characterization of Li4RuH6 sample under atmospheric N2 and elevated temperatures (from 473 K to 773 K). c, In situ SR-PXD characterization of Ba2RuH6 sample under atmospheric H2-N2 (H2:N2 = 3:1) mixture and elevated temperatures (from 637 K to 923 K). d, TPR profile of Ba2RuH6 in a mixture gas of NH3-Ar (NH3:Ar=0.5:99.5) and a mixture gas of NH3-H2-Ar (NH3:H2:Ar=0.5:75:24.5). The above results show that the Li4RuH6 phase is stable up to ca. 651 K under N2 atmosphere. While upon co-feeding H2, it survives until 738 K (Fig. 1e). Li4RuH6 is also resistant to diluted NH3 especially in the presence of H2. Ba2RuH6 phase is stable up to 923 K under atmospheric H2-N2 mixture. Ba2RuH6 is also resistant to diluted NH3 especially in the presence of H2.
Extended Data Fig. 2 Measurements of kinetic parameters.
a and b are the Arrhenius plots of the Li4RuH6, ball-milled Li4RuH6 (Li4RuH6 (BM)), ball-milled mixture of Li4RuH6 and BN (Li4RuH6-BN-(BM)), ball-milled mixture of Li4RuH6 and MgO (Li4RuH6-MgO-(BM)), Li4RuH6/MgO, Ba2RuH6/MgO, Ru/MgO, and Cs-Ru/MgO catalysts. c to e are the dependences of ammonia synthesis rates on the partial pressures of NH3, N2, and H2, respectively, under a total pressure of 1 bar at 573 K over Li4RuH6/MgO, Ba2RuH6/MgO, Ru/MgO, and Cs-Ru/MgO catalysts.
Extended Data Fig. 3 Measurements of low-temperature NH3 synthesis rates using 1H NMR spectroscopy.
a, 1H NMR spectra of 14NH4Cl and 15NH4Cl solutions in the concentration range of 30 to 1000 μM (with equimolar concentrations of 14NH4+ and 15NH4+). b, Linear calibration curves for both 14NH4+ and 15NH4+ derived from a. c and d are the 1H NMR spectra of 14NH4+ and 15NH4+ signals of the sulfuric acid solutions which absorbed the outlet gas at 373 K, 423 K and 473 K, respectively, e and f are the corresponding 14NH3 and 15NH3 synthesis rates derived from the 1H NMR. Measurement conditions: Ba2RuH6/MgO catalyst 30 mg, H2:14N2(15N2) = 2:3, flow rate 10 ml min−1, and 1 bar. Error bars shown in e and f represent the standard deviation from three independent 1H NMR measurements.
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Wang, Q., Pan, J., Guo, J. et al. Ternary ruthenium complex hydrides for ammonia synthesis via the associative mechanism. Nat Catal 4, 959–967 (2021). https://doi.org/10.1038/s41929-021-00698-8