How dynamics changes ammonia cracking on iron surfaces

Ammonia is a promising hydrogen carrier, being rich in hydrogen and ease of transport. However, a microscopic characterization of the ammonia cracking reaction is still lacking, hindered by extreme operando conditions. Leveraging state-of-the-art molecular dynamics, machine learning potentials, and enhanced sampling methods, we offer an atomistic view of the adsorption, diffusion, and dehydrogenation processes of NHx (x=1, 3) on two representative surfaces at operando temperature of 700 K. Dynamics pervasively affects all steps of decomposition, including on the stable (110) surface where the high mobility of reaction intermediates affects the reactivity. The role is even more dramatic on the (111) surface, where the mobility of Fe surface atoms introduces new adsorption sites and alters the dehydrogenation barriers. In both cases, a detailed analysis of reactive events shows that there is never a single transition state, but it is always an ensemble composed of at least two pathways. Notwithstanding, a unified mechanism can be identified by following the charge transfer along the different reaction pathways.