Plasma-catalytic assisted ammonia synthesis: Reactive molecular dynamics study

Abstract

Plasma catalytic assisted ammonia synthesis is currently considered a promising approach that enables ammonia synthesis under low temperature and low pressure. In this paper, ReaxFF molecular dynamics (MD) method was first used to study the impact of varying electric fields and different plasma-generated active species on ammonia synthesis, aiming to uncover the formation mechanisms and synthetic pathways of NH₃ from the molecular-level under plasma assistance. The results indicate that the electric field has the optimal range. With the increase of electric field strength, the collision frequency between N₂ and H₂ molecules does not increase linearly due to the polarization phenomenon, but increases first and then decreases, which affects the production of ammonia. As the electric field is greater than −0.01 V/Å, the ammonia production begins to decline due to the decreased molecular collision frequency as well as the electric-field induced ammonia dissociation. The results also show that the plasma-generated active species significantly promote NH₃ formation. Compared to the plasma-generated excited state and the ion, plasma-generated radicals such as N, H, NH and NH₂ have a more significant promoting effect on ammonia synthesis due to the acceleration of ammonia synthesis elementary reactions and the reduction of starting time significantly. In the aspect of molecular level, a new ammonia synthesis reaction pathway is discovered: N₂→N₂H→N₂H₂→N₂H₃→NH₃, which was never reported in previous studies. In addition, by decoupling the gas-phase reactions and dissociative adsorption reactions on the catalyst, it verified a new adsorption reaction path: N(s)→NH(s)→N₂H(s)→N₂H₂(s) at molecular level. This study provides valuable insights into the complete dynamic mechanism of plasma catalyst assisted NH₃ synthesis in the molecular level.