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Methods for nitrogen activation by reduction and oxidation

Nature Reviews Methods Primers volume 1, Article number: 56 (2021) Cite this article

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Abstract

The industrial Haber–Bosch process to produce ammonia (NH3) from dinitrogen (N2) is crucial for modern society. However, N2 activation is inherently challenging and the Haber–Bosch process has significant drawbacks, as it is highly energy intensive, is not sustainable owing to substantial CO2 emissions primarily from the generation of H2 and requires large, centralized facilities. New strategies of sustainable N2 activation, such as low-temperature thermochemical catalysis and (photo)electrocatalysis, have been pursued, but progress has been hindered by the lack of rigour and reproducibility in the collection and analysis of results. In this Primer, we provide a holistic step by step protocol, applicable to all nitrogen-transformation reactions, focused on verifying genuine N2 activation by accounting for all contamination sources. We compare state-of-the-art results from different catalytic reactions following the protocol’s framework, and discuss necessary reporting metrics and ways to interpret both experimental and density functional theory results. This Primer covers various common pitfalls in the field, best practices to improve reproducibility and cost-efficient methods to carry out rigorous experimentation. The future of nitrogen catalysis will require an increase in rigorous experimentation and standardization to prevent false positives from appearing in the literature, which can enable advancing towards practical technologies for the activation of N2.

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Fig. 1: Historical development of N2 activation.
Fig. 2: Fundamental challenges of N2 activation.
Fig. 3: General flow chart of experimentation.
Fig. 4: Experimental set-up for electrochemical and thermochemical N2 activation.
Fig. 5: Isotopic and non-isotopic NH3 detection methods.
Fig. 6: State-of-the-art literature overview of electrochemical, photo(electro)chemical and thermochemical N2 activation.
Fig. 7: Example data of lithium-mediated NH3 synthesis in THF with LiClO4 salt and EtOH as the proton source.
Fig. 8: Recommended reports of catalytic performances for thermal NH3 synthesis.
Fig. 9: Density functional theory results.
Fig. 10: Homebuilt glass circulation pump and full gas recirculating set-up with homebuilt activated copper catalyst for gas cleaning.

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Acknowledgements

Y.S.-H acknowledges support by Toyota Research Institute through the Accelerated Materials Design and Discovery Program. H.I. acknowledges support from the Imperial–MIT Department of Materials Exchange Program. S.Z.A. and I.C. acknowledge funding by Villum Fonden, part of the Villum Center for the Science of Sustainable Fuels and Chemicals (V-SUSTAIN grant 9455) and Innovationsfonden (E-ammonia grant 9067-00010B). P.C. and X.Z. were supported by the National Natural Science Foundation of China (Grant Nos 21633011 and 21988101). The material based upon work by A.J.M. and B.M.C. was supported by the National Science Foundation under Grant No. 1943707. I.E.L.S. and J.B. acknowledge financial support from the Engineering and Physical Sciences Research Council (EP/M0138/1), the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 866402) and the National Research Council Canada through the Materials for Clean Fuels Challenge Program. The authors thank M. Hatzell and Z. J. Xu for insightful discussion regarding N2 oxidation, and V. Shadravan for helpful advice on thermochemical catalysis. The authors acknowledge J. Montoya for providing data on the vibrational frequencies and free energy calculations for N2 reduction on Ru(211) for Fig. 9a.

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  1. These authors contributed equally: Haldrian Iriawan, Suzanne Z. Andersen.

Authors and Affiliations

  1. Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

    Haldrian Iriawan & Yang Shao-Horn

  2. Department of Materials, Imperial College London, London, UK

    Haldrian Iriawan, Jesús Barrio & Ifan E. L. Stephens

  3. Department of Physics, Technical University of Denmark, Kongens Lyngby, Denmark

    Suzanne Z. Andersen & Ib Chorkendorff

  4. Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

    Xilun Zhang & Ping Chen

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

    Xilun Zhang

  6. School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA

    Benjamin M. Comer & Andrew J. Medford

  7. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

    Yang Shao-Horn

  8. Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA

    Yang Shao-Horn

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Contributions

Introduction (H.I., S.Z.A. and Y.S.-H.); Experimentation (S.Z.A., H.I., X.Z., J.B., P.C., I.E.L.S., I.C. and Y.S.-H.); Results (S.Z.A., H.I., X.Z., B.M.C., J.B., P.C., I.C., A.J.M. and Y.S.-H.); Applications (H.I. and S.Z.A.); Reproducibility and data deposition (H.I. and I.C.); Limitations and optimizations (S.Z.A.); Outlook (H.I., S.Z.A., J.B., X.Z. and I.E.L.S.); Overview of the Primer (H.I., S.Z.A. and Y.S.-H.). All authors discussed and edited the full manuscript.

Corresponding authors

Correspondence to Ping Chen, Andrew J. Medford, Ifan E. L. Stephens, Ib Chorkendorff or Yang Shao-Horn.

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Glossary

Faradaic efficiency

The efficiency at which charge, in the form of electrons, participates in a specific electrochemical reaction.

Activation barriers

The minimal amounts of energy required for reactants to undergo a chemical reaction. These are the energy difference between the reactant and the transition state.

Standard potentials

The potentials (V) of a reversible electrode at standard state with ions at an effective 1 M concentration at a pressure of 1 atm.

Electrochemical half-cell reactions

Either oxidation reactions on the anode electrode where an electron is lost or reduction reactions on the cathode electrode where an electron is gained.

Electric arc-generated hot plasma

A discharge of electric current across a spatial gap, sustained by the presence of a thermally ionized plasma, which allows for the flow of said current.

Density functional theory

(DFT). A computational quantum mechanical modelling method used to investigate the electronic structure of many-body systems.

Reaction orders

The power dependence of the rate on the concentration of each reactant, which is an experimentally determined parameter that can have fraction values.

Tafel analysis

A method used to determine an electrochemical systems transfer coefficient via voltammograms, thereby providing information about the electrochemical mechanism and catalytic activity.

Ohmic correction

Accounting for the ohmic resistance of the media to accurately determine the potential at the surface of the electrode.

Quantum yield

Determining the number of times a specific event occurs per absorbed photon by the system in question.

Zero-point energy

The lowest possible energy that a quantum mechanical system contains, which includes fluctuations in the lowest energy state from the Heisenberg uncertainty principle.

Pareto-optimal frontier

A curve that contains physically possible optimal trade-offs between activity and stability.

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Iriawan, H., Andersen, S.Z., Zhang, X. et al. Methods for nitrogen activation by reduction and oxidation. Nat Rev Methods Primers 1, 56 (2021). https://doi.org/10.1038/s43586-021-00053-y

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