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

Abstract

The industrial production of commodity chemicals often requires extreme conditions of temperature and pressure [1]. Yet, in industrial reactors the catalyst remains active for a long time notwithstanding the harsh operating conditions. This challenges a static picture of the catalytic process. To explain the long-term stability of the industrial catalysts we invoke instead a highly dynamical scenario. We illustrate this concept with an ab initio quality simulation of the Li 2 NH catalyzed decomposition of ammonia [2, 3] at T = 750 K. This process has been intensively studied for its possible use in a hydrogen-based economy [4,5,6,7,8,9]. However, a full understanding of the way it works is still lacking. We show that when exposed to the reactants, the surface structure of the catalyst changes and a dynamic fluctuating steady state is activated allowing the reaction to proceed until the flow of reagents is discontinued. This interfacial reactive fluctuating state is induced by the reaction of ammonia with the surface imides NH 2− + NH 3 → 2NH − 2 . This discommensuration induces large fluctuations in the top surface layers, which now behave almost like an ionic liquid. In this activated environment, a series of reactions that eventually lead to the release of N 2 and H 2 molecules become possible.

Electrochemical Degradation of Pt3Co Nanoparticles Investigated by Off-Lattice Kinetic Monte Carlo Simulations with Machine-Learned Potentials

“…The ab initio computation of the activation barriers in electrochemical reactions is a formidable task due to the intricate nature of atomic environments and the hurdles faced when incorporating electrode potentials in the first-principles calculations (i.e., grand-canonical calculations). We note that considerable efforts are being made to tackle each of these challenges. − Additionally, advancements are also being made in directly simulating solvent environments and electrochemical reactions using MLPs. − …”

How Does Structural Disorder Impact Heterogeneous Catalysts? The Case of Ammonia Decomposition on Non-stoichiometric Lithium Imide

“…The electrons released during this reaction can be accommodated either in a diffuse surface state or in a localized state, similar to what happens in pure lithium imide (Figure S8 in the Supporting Information). 34 Once formed, the diazanediide is stabilized by a cloud of Li + cations and it can eventually lead to a chain of reactions resulting in N 2 and H 2 formation. Given the increased number of amides, there is also another pathway to form the N−N bond, between an imide and an amide, with a subsequent abstraction of the excess hydrogen from [ ] HN NH 2 .…”

“…The data illustrate that an increase in the occupation number of the adlayer is concomitant with a slight decrease in the occupation of the two uppermost layers as the amide concentration increases. Furthermore, the presence of NH 2 – anions on the surface also favors imide–amide proton transfer reactions, ,,,− and interlayer exchanges (see Figures S2–S4 in the Supporting Information for statistical analyses), which were not possible in the pure Li 2 NH. This effect is clearly visible from the analysis of the spatial distribution of the N and Li atoms described by the probability P α ( z ) (α = N, Li) of finding an atom of species α at a given value of the z coordinate (see Figures b) and d)).…”

“…The distributions P N ( z ) in the first top layers broaden as x increases, corresponding to a wider fluctuation of the amides in the top layer. Concurrently, Li atoms move to balance the negative charge of amide groups, following a mechanism analogous to the one described in ref .…”

“…11 Progress in the construction of DFT-quality machinelearning-based reactive potentials 16−22 and the use of enhanced sampling techniques 23−27 to accelerate the occurrence of reactive events 28−33 have recently enabled the simulation of the ammonia decomposition process on pristine lithium imide. 34 This has allowed the catalytic mechanism of superionic Li 2 NH to be unraveled and the key factors determining its activity to be unveiled. In particular, our simulations 29 have shown that surface imides quickly react with the incoming ammonia molecules, inducing the formation of a few diffusive almost liquid-like interfacial layers that act as a catalytic medium, promoting the complex steps that lead to the cracking of ammonia and the eventual release of N 2 and H 2 molecules.…”