Machine-Learned Force Field for Molecular Dynamics Simulations of Nonequilibrium Ammonia Synthesis on Iron Catalysts

Ammonia (NH₃) is one of the most important industrial chemicals. The conventional NH₃ synthesis method─the Haber-Bosch process─converts atmospheric nitrogen (N₂) into NH₃ using H₂ with an iron (Fe) catalyst. However, this process requires high pressures (100-200 atm) and temperatures (700-800 K) near thermal equilibrium. Recently, Fe-based nanocatalysts have been reported to produce promising NH₃ yields under atmospheric pressures and temperature-modulated nonequilibrium conditions. Understanding the mechanism of nonequilibrium catalysis with programmed temperature variation could help to optimize this fully electrified and less energy-intensive process. Although reactive molecular dynamics (RMD) simulations can be a useful tool to model nonequilibrium catalytic processes, they require the development of accurate force fields (i.e., interatomic potentials). Here, we present a machine-learned (ML) force field within the Deep Potential MD (DPMD) framework, trained using periodic density functional theory (DFT) calculations, to model NH₃ synthesis on Fe catalysts with various surface adsorbates such as *N, *H, *N₂, *H₂, *NH, *NH₂, and *NH₃. We generated the DFT data from static models of elementary reactions on the most stable (110) surface of body-centered cubic Fe, which then were augmented by data from constant number of particles-volume-temperature (NVT) DFT-MD trajectories at various temperatures. Finally, we utilized the fully optimized ML force field to investigate reaction dynamics at an Fe(110) surface at linearly increasing temperatures using NVT-DPMD simulations. Our simulations indicate that pulsed temperature ramping could prove favorable for NH₃ synthesis. For example, we conducted ramping under multiple sets of conditions: (i) from 900 to 1200 K over periods of 0.1-0.3 ns for Fe surfaces precovered with N or NH along with H; and (ii) from 300 to 600 K over 0.1-0.3 ns for Fe surfaces precovered with NH₃. While our simulations so far are limited to short time scales (very rapid heating), these observations shed light on the mechanism of the high NH₃ synthesis rate achieved in a novel temperature-modulated nonequilibrium catalytic reactor using pulsed heating and cooling.