Ammonia Decomposition on Lithium Imide Surfaces: A New Paradigm in Heterogeneous Catalysis

The industrial production of commodity chemicals often requires extreme conditions of temperature and pressure. 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 Li2NH catalyzed decomposition of ammonia at T = 750 K. This process has been intensively studied for its possible use in a hydrogen-based economy. 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. 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 N2 and H2 molecules become possible.