First-Principles Insights into the Thermocatalytic Cracking of Ammonia-Hydrogen Blends on Fe(110): 1. Thermodynamics
John Mark P. Martirez, Emily A. Carter
Abstract
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 NH3 to 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 NH3 decomposition catalysts. Here, we explore the equilibrium composition(s) of the Fe(110) surface when exposed to NH3–H2 mixtures. We predict that both N and NH partially cover the Fe(110) surface at 300–400 °C (far above the NH3 decomposition-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 H2 production 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 NH3 decomposition. Thus, catalysts that remain active toward N–H bond breaking but with weakened N binding relative to Fe are needed to enable high-turnover catalytic NH3 decomposition to release H2.