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Spontaneous N2 formation by a diruthenium complex enables electrocatalytic and aerobic oxidation of ammonia

Nature Chemistry volume 13, pages 1221–1227 (2021)Cite this article

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Abstract

The electrochemical conversion of ammonia to dinitrogen in a direct ammonia fuel cell (DAFC) is a necessary technology for the realization of a nitrogen economy. Previous efforts to catalyse this reaction with molecular complexes required the addition of exogenous oxidizing reagents or application of potentials greater than the thermodynamic potential for the oxygen reduction reaction—the cathodic process of a DAFC. We report a stable metal–metal bonded diruthenium complex that spontaneously produces dinitrogen from ammonia under ambient conditions. The resulting reduced diruthenium material can be reoxidized with oxygen for subsequent reactions with ammonia, demonstrating its ability to spontaneously promote both half-reactions necessary for a DAFC. The diruthenium complex also acts as a redox mediator for the electrocatalytic oxidation of ammonia to dinitrogen at potentials as low as −255 mV versus Fc0/+ and operates below the oxygen reduction reaction potential in alkaline conditions, thus achieving a thermodynamic viability relevant for the future development of DAFCs.

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Fig. 1: Overview of a nitrogen/ammonia fuel economy and progress towards catalysed DAFCs.
Fig. 2: Synthesis and structural characterization of key Ru2 compounds.
Fig. 3: Electrochemical and spectroscopic investigations of Ru2-catalysed ammonia oxidation.
Fig. 4: Equipment used and data collected in Faradic efficiency experiments.
Fig. 5: Ru2 electronic structural features and proposed mechanism of ammonia reactivity.

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Data availability

Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 1945089 (7), 1945090 (2), 1945091 (4), 1945092 (3) and 2006690 (5). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. Source data for Figs. 3 and 4 and Extended Data Fig. 6 are provided with the article. Source data for Extended Data Fig. 1 can be found at https://figshare.com/articles/dataset/Berry_ED_Fig1_mnova/15060798. All other data supporting the findings of this study are available within the article and its Supplementary Information, or from the corresponding author upon reasonable request.

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Acknowledgements

We thank the Department of Energy for funding (DE-SC0021021). We also thank S.-C. Wang in the Hermans group as well as E. Canales and K. Rivera-Dones in the Huber group at UW-Madison for help with the mass spectrometry measurements of labelled nitrogen. We thank M. Aristov and A. Wheaton for assistance with crystallographic data. Crystallographic data were collected on a Cu Kα instrument that was funded by the NSF (CHE-1919350) and on a Mo Kα instrument funded by a generous gift from Paul and Margaret Bender.

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Author notes

  1. These authors contributed equally: Michael J. Trenerry, Christian M. Wallen.

Authors and Affiliations

  1. Department of Chemistry, University of Wisconsin-Madison, Madison, WI, USA

    Michael J. Trenerry, Christian M. Wallen, Tristan R. Brown, Sungho V. Park & John F. Berry

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  1. Michael J. Trenerry
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Contributions

M.J.T. performed all electrochemical and spectroelectrochemical experiments and conducted the computational modelling. C.M.W. performed all synthesis and non-crystallographic characterization of diruthenium complexes, preparation of ammonia solutions, ammonia reactivity experiments and other spectroscopic experiments. T.R.B. performed preliminary synthesis and characterization on diruthenium ammine complexes that directly informed this work. S.V.P. performed crystallographic characterization of the diruthenium compounds. M.J.T., C.M.W. and J.F.B. wrote the manuscript. All authors provided feedback during the manuscript preparation and approved the final manuscript.

Corresponding author

Correspondence to John F. Berry.

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Competing interests

M.J.T., C.M.W., T.R.B., S.V.P. and J.F.B. have submitted a provisional patent application based on the work described here (US patent application number P200013US01).

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Peer review information Nature Chemistry thanks Dai Oyama 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

Extended Data Fig. 1 1H-NMR spectra monitoring the reaction of 2 with ammonia in CH3CN-d3.

Left: Portion of spectra focused on the Ar-H signals in the diruthenium complex. Right: Portion of spectra focused on the NH3/NH4+ signal shifting over the course of the experiment. Spectra are collected at 1-hour intervals.

Extended Data Fig. 2 Spectroelectrochemical cell used in controlled current Coulometry experiments.

Left: A photograph of the spectroelectrochemical cell, loaded with a solution of 5 with excess ammonia in the working electrode chamber and a solution of FcPF6 sacrificial oxidant in the counter electrode chamber. Right: A schematic of the spectroelectrochemical cell detailing its assembly.

Extended Data Fig. 3 Electrode and chemical processes occurring during controlled current Coulometry.

Diruthenium species in the [Ru2]4+ oxidation state are electrochemically oxidized to the [Ru2]5+ oxidation state at the surface of the reticulated vitreous carbon (RVC) working electrode (WE). The redox potential for this transformation is −255 mV vs Fc0/+ in CH3CN. Applied potential at WE is adjusted throughout the experiment to maintain a constant oxidizing current of +0.5 mA. Exogenous ammonia in solution spontaneously reacts with [Ru2]5+ species to produce dinitrogen and ammonium while regenerating [Ru2]4+ species. The ferrocenium cation in FcPF6 is electrochemically reduced to neutral ferrocene at the surface of the platinum counter electrode (CE). The working electrode and counter electrode chambers are divided by a fine glass frit allowing for ion exchange.

Extended Data Fig. 4 Electronic structural features and proposed mechanism of ammonia reactivity with an imido intermediate.

A: DFT orbital calculation showing the Ru-Ru-NH π* LUMO in 9. B,C: DFT-calculated geometry (B) and drawn representation (C) of transition state TS2 showing the formation of an N-N bond via nucleophilic attack of NH3 on 9.

Extended Data Fig. 5 Electronic structural features and proposed mechanism of ammonia reactivity with a nitrido intermediate.

A: DFT orbital calculation showing the Ru-Ru-N π* LUMO in 10. B,C: DFT-calculated geometry (B) and drawn representation (C) of transition state TS3 showing the formation of an N-N bond via nucleophilic attack of NH3 on 10.

Extended Data Fig. 6 Titration of 2 with N2H4 in CH3CN monitored by electronic absorption spectroscopy.

The trace of absorbance at 533 nm versus equivalents of N2H4 shows a distinct end to the reaction after the addition 1.25 equivalents.

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–20, Discussion and Tables 1–18. Supplementary tables providing DFT-optimized coordinates are included in a separate Excel spreadsheet in the additional supplementary files.

Supplementary Tables

DFT-optimized cartesian coordinates for compounds 6, 6+, 8, 9, 10, TS1, TS2 and TS3.

Supplementary Data 1

Crystallographic data for compound 2. CCDC reference 1945090.

Supplementary Data 2

Crystallographic data for compound 3. CCDC reference 1945092.

Supplementary Data 3

Crystallographic data for compound 4. CCDC reference 1945091.

Supplementary Data 4

Crystallographic data for compound 5. CCDC reference 2006690.

Supplementary Data 5

Crystallographic data for compound 7. CCDC reference 1945089.

Supplementary Data 6

mnova file with raw NMR data for compounds 2, 5 and 6.

Supplementary Data 7

Statistical source data for Supplementary Fig. 8.

Supplementary Data 8

Statistical source data for Supplementary Fig. 10.

Supplementary Data 9

Statistical source data for Supplementary Fig. 11.

Supplementary Data 10

Statistical source data for Supplementary Fig. 12.

Supplementary Data 11

Statistical source data for Supplementary Fig. 13.

Supplementary Data 12

Statistical source data for Supplementary Fig. 14.

Supplementary Data 13

Statistical source data for Supplementary Fig. 15.

Supplementary Data 14

Statistical source data for Supplementary Fig. 16.

Supplementary Data 15

Statistical source data for Supplementary Fig. 17.

Supplementary Data 16

Statistical source data for Supplementary Fig. 18.

Supplementary Data 17

Statistical source data for Supplementary Fig. 19.

Supplementary Data 18

Statistical source data for Supplementary Fig. 20.

Source data

Source Data Fig. 3

Statistical source data for Fig. 3.

Source Data Fig. 4

Statistical source data for Fig. 4.

Source Data Extended Data Fig. 6

Statistical source data for Extended Data Fig. 6.

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Trenerry, M.J., Wallen, C.M., Brown, T.R. et al. Spontaneous N2 formation by a diruthenium complex enables electrocatalytic and aerobic oxidation of ammonia. Nat. Chem. 13, 1221–1227 (2021). https://doi.org/10.1038/s41557-021-00797-w

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