First-Principles Insights into the Thermocatalytic Cracking of Ammonia-Hydrogen Blends on Fe(110): 1. Thermodynamics

Ammonia (NH₃) is being considered as a practical means to transport hydrogen (H₂) because of its higher volumetric energy density for the same temperature and pressure. Thermodynamics suggest high temperature is needed to decompose NH₃to nitrogen (N₂) and H₂. Furthermore, overcoming decomposition kinetic barriers requires a catalyst. Via density functional theory, we study this reaction on a model catalyst: the close-packed (110) facet of α-Fe. Specifically, we predict detailed in-operando temperature- and pressure-dependent surface phase diagrams on this benchmark catalyst that offer insights for the design of optimal NH₃decomposition catalysts. Here, we explore the equilibrium composition(s) of the Fe(110) surface when exposed to NH₃-H₂mixtures. We predict that both N and NH partially cover the Fe(110) surface at 300-400 °C (far above the NH₃decomposition-formation coexistence temperature at standard partial pressures of 1 bar: ∼180 °C) and 2-4 bar of total reactor pressure. At the equilibrium N/NH coverage, these species inhibit coadsorption of H, indicating that direct H₂production may occur. However, from thermodynamics alone, removal of N/NH as N₂(g) is extremely unfavorable even at these elevated temperatures-effectively deactivating the surface toward further NH₃decomposition.