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Light-driven ammonia synthesis under mild conditions using lithium hydride

Abstract

Photon-driven chemical processes are usually mediated by oxides, nitrides and sulfides whose photo-conversion efficiency is limited by charge carrier recombination. Here we show that lithium hydride undergoes photolysis upon ultraviolet illumination to yield long-lived photon-generated electrons residing in hydrogen vacancies, known as F centres. We demonstrate that photon-driven dehydrogenation and dark rehydrogenation over lithium hydride can be fulfilled reversibly at room temperature, which is about 600 K lower than the corresponding thermal process. As light-driven F centre generation could provide an alternative approach to charge carrier separation to favour chemical transformations that are kinetically or thermodynamically challenging, we show that light-activated lithium hydride cleaves the N≡N triple bond to form a N–H bond under mild conditions. Co-feeding a N2/H2 mixture with low H2 partial pressure leads to photocatalytic ammonia formation at near ambient conditions. This work provides insights into the development of advanced materials and processes for light harvesting and conversion.

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Fig. 1: Photon-assisted dehydrogenation of LiH.
Fig. 2: The photon-driven reversible dehydrogenation and dark rehydrogenation of LiH.
Fig. 3: Photon-assisted nitrogen fixation mediated by LiH.
Fig. 4: Photocatalytic ammonia synthesis over LiH.

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Data availability

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 Supplementary Information. Source data are provided with this paper.

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Acknowledgements

We thank F. X. Zhang for beneficial discussions. P.C. and J.G. are grateful for financial support from the National Key R&D Program of China (2021YFB4000400), National Natural Science Foundation of China (grant nos 21988101 and 21922205), Youth Innovation Promotion Association of the Chinese Academy of Sciences (nos Y2022060 and 2022180) and Liaoning Revitalization Talents Program (nos XLYC2007173 and XLYC2002076).

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Contributions

P.C. and J.G. conceived the project. P.C. and J.G. co-supervised the research and wrote the paper. Y.G. conducted most of the experimental work and prepared the original draught. H.W. conducted the DFT calculations and co-prepared the draught. K.C. and Q.W. assisted with materials synthesis. Z.L. and T.H. reviewed and edited the paper. All authors participated in the discussion and data analyses.

Corresponding authors

Correspondence to Jianping Guo or Ping Chen.

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Nature Chemistry thanks Jing Ma and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Schematics for charge carrier separation processes.

a, The electron-hole separation and recombination in conventional metal oxide or nitride photocatalysts. b, The electron-hole separation during the photolysis of LiH to form F center and H2.

Extended Data Fig. 2 FT-IR spectra of LiH samples after photo-driven N2 fixation.

LiH samples were treated under different conditions (LiH in 5 bar 14N2 pressure after illumination for 0.5 h-orange, LiH in 5 bar 15N2 pressure after illumination for 0.5 h-blue).

Source data

Extended Data Fig. 3 Measurements of NH3 synthesis rates using 1H-NMR spectroscopy.

a, 1H-NMR spectra of 14NH4Cl and 15NH4Cl solutions in the concentration range of 0.1 to 1.0 mM (with equimolar concentrations of 14NH4+ and 15NH4+). b, Linear calibration curves for both 14NH4+ and 15NH4+ derived from 1H-NMR. c and d are the 1H-NMR spectra of 14NH4+ and 15NH4+ signals of the sulfuric acid solutions which absorbed the outlet gas with different sample loadings and different illumination time. e, The corresponding fixed 14N and 15N amounts derived from the 1H-NMR and conductivity meter measurement. f, The corresponding 14N and 15N fixation rates derived from the 1H-NMR. Measurement conditions: LiH dispersed in cyclohexane, N2 pressure, 5 bar; light intensity, 398.2 mW/cm2.

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–22, Tables 1 and 2, refs. 1–3 and the atomic coordinates of the states in Fig. 3g.

Source data

Source Data Fig. 1

Statistical source data.

Source Data Fig. 2

Statistical source data.

Source Data Fig. 3

Statistical source data.

Source Data Fig. 4

Statistical source data.

Source Data Extended Data Fig. 2

Statistical source data.

Source Data Extended Data Fig. 3

Statistical source data.

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Guan, Y., Wen, H., Cui, K. et al. Light-driven ammonia synthesis under mild conditions using lithium hydride. Nat. Chem. 16, 373–379 (2024). https://doi.org/10.1038/s41557-023-01395-8

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