First-Principles Insights into the Thermocatalytic Cracking of Ammonia-Hydrogen Blends on Fe(110). 2. Kinetics

Ammonia (NH₃) is an energy-rich molecule that is routinely synthesized from nitrogen (N₂) and hydrogen (H₂). NH₃’s more favorable physical properties compared to H₂ suggests it may offer a way to more conveniently store, transport, and, when needed, extract H₂ via thermal decomposition. However, the high kinetic barrier and endoergicity to decompose to H₂ and N₂ require high temperatures. The standard reaction free energy indicates nearly 100% thermodynamic conversion to the diatomic molecules only at ∼673 K and higher. However, even at these temperatures, a catalyst, e.g., iron (Fe), is needed for favorable kinetic conversion. Here, we explore via density functional theory the kinetics of NH₃ decomposition on the most stable facet of body-centered cubic Fe, namely, (110), under typical high-temperature and finite-pressure operando conditions. We predict coverage-dependent energetics of elementary surface reactions, often neglected in atomic-scale modeling. From these models, we find the recombinative desorption of adsorbed N as N₂ is rate-determining at 573.15-773.15 K and even at an extreme case of 1173.15 K. From microkinetic modeling, we find that the steady-state turnover frequencies (TOFs) for N₂ and H₂ generation rates (r_H2) depend exponentially on temperature. The catalyst achieves a steady-state TOF of 36.4 s^-1 and an r_H2 of 0.107 μmol cm^-2 s^-1 for a feed of 1.8 bar NH₃ with 0.2 bar H₂ at 1173.15 K. However, at 773.15 K, with the same feed composition and velocity, the steady-state TOF and r_H2 decrease to 0.14 s^-1 and 4.10 × 10^-4 μmol cm^-2 s^-1, respectively, as the process is significantly hindered by slow N₂ desorption. Although at first glance counterintuitive, our simulations suggest that surface modifications that reduce Fe’s reactivity toward NH_x species should enhance its overall NH₃ decomposition activity.