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Reactant-induced dynamics of lithium imide surfaces during the ammonia decomposition process

Nature Catalysis volume 6, pages 829–836 (2023)Cite this article

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Abstract

Ammonia decomposition on lithium imide surfaces has been intensively investigated owing to its potential role in a sustainable hydrogen-based economy. Here, through advanced molecular dynamics simulations of ab initio accuracy, we show that the surface structure of the catalyst changes on exposure to the reactants and a dynamic state is activated. It is this highly fluctuating state that is responsible for catalysis and not a well-defined static catalytic centre. In this activated environment, a series of reactions that eventually leads to the release of N2 and H2 molecules becomes possible. Once the flow of reagent is terminated, the imide surface returns to its pristine state. We suggest that by properly engineering this dynamic interfacial state one can design improved catalytic systems.

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Fig. 1: Bulk and surface structure of Li2NH.
Fig. 2: Schematic representation of the initial reaction steps.
Fig. 3: Free energy surface for diazanediide formation.
Fig. 4: Schematic of one representative set of decomposition reactions.
Fig. 5: Simplified scheme outlining the catalytic process.

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Data availability

All the inputs and instructions to reproduce the results presented in this article can be found in the PLUMED-NEST repository (plumID 23.028).

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Acknowledgements

We acknowledge the help of V. Rizzi in the initial stages of the simulation and useful discussions with E. Trizio and M. Bernasconi. We are grateful to N. Ansari for the help with the graphic. We thank V. Glezakou and R. Schlögl for reading a preliminary version of the paper and G. Ertl for his encouraging remarks. M.P. thanks R. Schlögl for sharing his insight into catalysis. This work closely reflects his vision. However, any error or misinterpretation is our own responsibility. Funding: This work was supported by funds from the AmmoRef project in the framework of the agreement between the Max Planck Institute and the Italian Institute of Technology. Computational resources were also provided by the Swiss National Supercomputing Centre (CSCS) under project ID nos. S1134 and S1183.

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Authors and Affiliations

  1. Italian Institute of Technology, Genoa, Italy

    Manyi Yang, Umberto Raucci & Michele Parrinello

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  1. Manyi Yang
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Contributions

M.Y., U.R. and M.P. made substantial contributions to the design and implementation of the work and wrote the paper.

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Correspondence to Michele Parrinello.

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Nature Catalysis thanks Josh Makepeace, Johannes Margraf and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–20, Table 1, Note, Discussion and Methods.

Supplementary Video 1

Diffusion mechanism of Li+ cations, This video shows the diffusion of a Li+ cation (represented as a red ball) from one tetrahedral site to another through an octahedral interstitial site.

41929_2023_1006_MOESM3_ESM.mp4

Supplementary Video 2. Superionic behaviour of the (111) surface. This video shows NH2− anions oscillating around the equilibrium positions while Li+ cations diffuse. We tagged for clarity one imide group (shown in the ball and stick representation) and Li+ cation (represented as a red ball).

Supplementary Video 3

Ammonia adsorption on the (111) surface. This video shows two NH3 reacting with two NH2− resulting in four NH2−. This process occurs spontaneously on the nanosecond timescale.

Supplementary Video 4

Amide diffusion on the surface after the reaction with ammonia. This video shows that NH2− moves frequently between the top layer and the adlayer. It can also move to the second layer. This process occurs spontaneously on the nanosecond timescale.

Supplementary Video 5

Imide diffusion on the surface after the reaction with ammonia. This video shows one of the NH2− moving from the top layer to the second layer. This process occurs spontaneously on the nanosecond timescale.

Supplementary Video 6

This video shows the Grotthus-like proton exchange between NH2− and NH2−. This process occurs spontaneously on the nanosecond timescale.

Supplementary Video 7

This video shows the diazanediide formation according to: \({{{{\rm{NH}}}}}^{2-}+{{{{\rm{NH}}}}}^{2-}\to {[{{{\rm{HN}}}}-{{{\rm{NH}}}}]}^{2-}+2{{{{\rm{e}}}}}^{-}\). The position of one NH2− before the reaction is marked with a grey transparent sphere. To simulate this step we used \({S}_{\mathrm{NN}}^{\mathrm{max}}\) as CV with ΔE = 180 kJ mol−1.

Supplementary Video 8

This video shows the reaction: \({{{{\rm{NH}}}}}_{{2}}^{-}\to [{{{\rm{NH}}}}]^{*} +{{{{\rm{H}}}}}^{-}\). The abstracted H atom in NH2− is marked with a grey transparent sphere. Lithium atoms within 2.5 Å from the abstracted H are displayed. The NH distance between the reactive atoms is reported in ångströms. To simulate this step we used \({S}_{\mathrm{HN}}^{\mathrm{min}}\) as CV with ΔE = 100 kJ mol−1.

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Yang, M., Raucci, U. & Parrinello, M. Reactant-induced dynamics of lithium imide surfaces during the ammonia decomposition process. Nat Catal 6, 829–836 (2023). https://doi.org/10.1038/s41929-023-01006-2

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