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 (rH2) depend exponentially on temperature. The catalyst achieves a steady-state TOF of 36.4 s–1 and an rH2 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 rH2 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 NHx species should enhance its overall NH3 decomposition activity.