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Production of ammonia via a chemical looping process based on metal imides as nitrogen carriers

Abstract

Ammonia is a promising carbon-free energy carrier, but is currently synthesized industrially under harsh conditions. Synthesizing ammonia using lower temperatures and pressures could therefore improve its prospects as a chemical means to store and transport energy. Here we report that alkali and alkaline earth metal imides function as nitrogen carriers that mediate ammonia production via a two-step chemical looping process operating under mild conditions. Nitrogen is first fixed through the reduction of N2 by alkali or alkaline earth metal hydrides to form imides and, subsequently, the imides are hydrogenated to produce NH3 and regenerate the metal hydrides. The oxidation state of hydrogen therefore switches between −1 (hydride), 0 (H2) and +1 (imide and NH3). Late 3d metals accelerate the reaction rates of both steps. The chemical loop mediated by BaNH and catalysed by Ni produces NH3 at 100 °C and atmospheric pressure.

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Fig. 1: Chemical looping processes for ammonia synthesis.
Fig. 2: Thermodynamic analyses of steps I and II in AH-CL.
Fig. 3: Nitridation of hydride samples and hydrogenation of post-nitridized samples.
Fig. 4: Nitridation and hydrogenation of TM–AH samples.
Fig. 5: Kinetic measurements of N2 fixation and hydrogenation of BaH2-CL.
Fig. 6: Performances of Ni–BaH2 (LiH) for ammonia production.
Fig. 7: Conceptual set-up for sustainable ammonia synthesis via AH-CL.

Data availability

The data that support the plots within this paper and other finding of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors are grateful for the financial support from National Natural Science Foundation of China (Grant Nos. 21633011 and 21603220), Sino-Japanese Research Cooperative Program of Ministry of Science and Technology (2016YFE0118300), DICP (DICP DMTO201504), Youth Innovation Promotion Association CAS (No. 2018213) and Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM).

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P.C. conceived the research and wrote the paper. J.G. coordinated the experimental work. W.G. performed the synthesis, characterization and ammonia production rate measurements of the materials. All authors discussed the results and commented on the manuscript.

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Correspondence to Jianping Guo or Ping Chen.

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Supplementary methods, Supplementary Figures 1–10, Supplementary Tables 1–4, Supplementary references

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Gao, W., Guo, J., Wang, P. et al. Production of ammonia via a chemical looping process based on metal imides as nitrogen carriers. Nat Energy 3, 1067–1075 (2018). https://doi.org/10.1038/s41560-018-0268-z

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