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Direct electroconversion of air to nitric acid under mild conditions

Nature Synthesis volume 3, pages 76–84 (2024)Cite this article

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Abstract

Nitric acid (HNO3) synthesis is an important industrial process but typically requires multiple energy-intensive steps, including the synthesis and subsequent oxidation of ammonia. Here we report the direct conversion of air to HNO3 under ambient conditions through a hydroxyl radical (OH­·)-mediated hetero-homogeneous electrochemical route. The conversion proceeds at a reductive potential of 0 V versus the reversible hydrogen electrode at the cathode and can achieve a Faradaic efficiency of 25.37% with over 99% HNO3 selectivity. Experimental and theoretical investigations reveal that OH· produced from Fe2+-induced dissociation of in situ generated H2O2 can efficiently activate N2. This delivers a high HNO3 productivity of \(141.83\,\upmu{\mathrm{mol}}\,{\mathrm{h}}^{-1}\,{\mathrm{g}}_{\mathrm{Fe}}^{-1}\), which is 225 times that of directly using H2O2 as the oxidant. This process opens an energy-efficient and green route for the direct conversion of air to HNO3 under mild conditions.

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Fig. 1: Comparison of the reaction conditions of different routes for the conversion of N2 to HNO3.
Fig. 2: Catalytic performance testing for the electroconversion of air to HNO3.
Fig. 3: Investigation of the reaction intermediates in the electroconversion of air to HNO3.
Fig. 4: Reaction mechanism from DFT calculations.

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Data availability

The data supporting the finding of the study are available in the paper and its Supplementary Information. Source data are provided with this paper and in the Figshare repository (https://doi.org/10.6084/m9.figshare.23652639).

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Acknowledgements

Financial support was provided by the National Natural Science Foundation of China (numbers 21988101, 22225204, 21890753 to D.D., and number 22272170 to L.Y.), the Ministry of Science and Technology of China (number 2022YFA1504500 to D.D.), and the Strategic Priority Research Program of the Chinese Academy of Science (number XDB36030200 to D.D.). We thank Z. Peng and Z. Zhao from the Dalian Institute of Chemical Physics for help with in situ infrared and in situ Raman measurements.

Author information

Author notes

  1. These authors contributed equally: Shiming Chen, Suxia Liang.

Authors and Affiliations

  1. State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

    Shiming Chen, Suxia Liang, Rui Huang, Mo Zhang, Yao Song, Yunlong Zhang, Shuo Tao, Liang Yu & Dehui Deng

  2. School of Chemistry, Dalian University of Technology, Dalian, China

    Rui Huang

  3. University of Chinese Academy of Sciences, Beijing, China

    Yao Song, Yunlong Zhang, Shuo Tao, Liang Yu & Dehui Deng

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  1. Shiming Chen
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Contributions

D.D. conceived the project. S.C. carried out the experiments and the manuscript preparation. L.Y. supervised the DFT calculations and manuscript revision. S.L. performed DFT calculations. R.H., M.Z., Y.S., Y.Z. and S.T. assisted with experiments and data analysis. All authors contributed to scientific discussion of the manuscript. S.C., S.L., L.Y. and D.D. wrote the paper.

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Correspondence to Liang Yu or Dehui Deng.

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Nature Synthesis thanks Guofeng Wang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary handling editor: Alexandra Groves, in collaboration with the Nature Synthesis team.

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Extended data

Extended Data Table 1 Comparison between the Faradaic Efficiency (FE) of the cathodic electroconversion of air to HNO3 and previously reported anodic electro-oxidation of N2 to HNO3 over metal-oxide catalysts at current densities higher than 1 mA·cm−2
Full size table
Extended Data Table 2 The standard molar Gibbs free energy of formation (ΔfGmΘ) for the molecules and ions used in this work
Full size table

Extended Data Fig. 1

H-type electrolytic cell separated with Nafion membrane.

Extended Data Fig. 2 Investigation of the reaction performances by using directly added H2O2 and in-situ generated H2O2.

a, Schematic overview of N2 oxidation using in-situ generated H2O2 from oxygen reduction reaction (ORR). b, The histogram of HNO3 formation rates and Faradaic Efficiency with different concentration of FeSO4 (Typical reaction conditions: 0.25 mol·L−1 Li2SO4 with pH adjusted to 3 by H2SO4, PO2/PN2 =1/1, 0 V vs. RHE). c, The schematic overview that N2 was oxidated to HNO3 by ·OH from Fenton reaction. d, The formation rate of HNO3 at different concentrations of FeSO4 (8 mol·L−1 of H2O2 and 100 mL·min−1 of N2). The produced trace amount of HNO3 is due to the use of high concentration H2O2 (8 mol·L−1), which can still produce certain amount of OH radicals for N2 oxidation even in the absence of FeSO4.

Source data

Extended Data Fig. 3 Catalytic performance testing for the electroconversion of air to HNO3 with different electrodes.

Catalytic performance of graphite rod, Pt Foil, Cu Foil and Ag Foil. Typical reaction conditions: 0.25 mol·L−1 Li2SO4 with pH adjusted to 3 by H2SO4, PO2/PN2 = 1/1, 0 V vs. RHE, 1 mmol·L−1 FeSO4. Data are presented as mean values +/− standard deviation. The errors were obtained by repeating the reaction for three times (n = 3).

Source data

Extended Data Fig. 4 Investigation of the reaction intermediates in the electroconversion of air to HNO3 with UV-vis absorbance spectra.

The UV-vis spectra of H2N2O2 and NO3− standards and experiment sample were performed in the operando conditions. Typical reaction conditions: 0.25 mol·L−1 Li2SO4 with pH adjusted to 3 by H2SO4, PO2/PN2 =1/1, 0 V vs. RHE, 1 mmol·L−1 FeSO4.

Source data

Extended Data Fig. 5 Investigation of the reaction intermediates in the electroconversion of air to HNO3 with in-situ surface-enhanced infrared absorption spectroscopy (SEIRAS) 2D spectra.

Typical reaction conditions: 0.25 mol·L−1 Li2SO4 with pH adjusted to 3 by H2SO4, PO2/PN2 =1/1, 0 V vs. RHE, 1 mmol·L−1 FeSO4.

Source data

Extended Data Fig. 6

Potential energy surface for searching the transition state of O radical (·O) formation from hydroxyl radical (·OH).

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Supplementary information

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Chen, S., Liang, S., Huang, R. et al. Direct electroconversion of air to nitric acid under mild conditions. Nat. Synth 3, 76–84 (2024). https://doi.org/10.1038/s44160-023-00399-z

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