Elevated Pressure Effects on Plasma-Driven Ammonia Synthesis: Insights from Experiments and Kinetic Modeling

Abstract

Nonthermal plasma (NTP) presents a promising pathway for sustainable ammonia synthesis under mild conditions, enabling activation of nitrogen without the need for high thermal input. While most studies to date have focused on plasma ammonia synthesis under ambient pressure, the potential benefits of elevated pressure, such as improved thermodynamic favorability and enhanced compatibility with downstream ammonia separation technologies, remain underexplored. In this work, we investigate plasma-driven ammonia synthesis under elevated pressure by combining experimental measurements with detailed plasma chemical kinetics modeling. A dielectric barrier discharge plasma reactor was employed, with the system pressure controlled up to 3 bar using a high-pressure regulator. Contrary to thermodynamic expectations, the experimental results reveal that increasing pressure suppresses ammonia yield in the plasma environment, primarily due to a reduction in the reduced electric field (E/N), which diminishes the energy of electrons available for molecule activation. The underlying reaction mechanism was elucidated using in situ optical diagnostics and chemical kinetics simulations. Path flux analysis confirms that N₂ is dissociated by energetic electrons into N and excited N(²D) species, which are subsequently hydrogenated to form NH and NH₂ radicals. These intermediates recombine via NH₂ + H(+M) → NH₃(+M) and NH + H₂ + M → NH₃ + M to form ammonia. Notably, elevated pressure does not alter the dominant reaction pathways but significantly influences the reaction rates and plasma characteristics. Sensitivity analysis highlights that the electron-impact dissociation of N₂ [e + N₂ → e + N + N(²D)] is the rate-limiting step and has the greatest promoting effect on ammonia formation. These insights offer guidance for optimizing plasma operating conditions and advancing the practical application of plasma-assisted ammonia synthesis under pressurized conditions.

Nonthermal plasma presents a promising pathway for sustainable ammonia synthesis under mild conditions, enabling activation of nitrogen without the need for high thermal input. While most studies to date have focused on plasma ammonia synthesis at ambient pressure conditions, the potential benefits of elevated pressure, such as improved thermodynamic favorability and enhanced compatibility with downstream ammonia separation technologies, remain underexplored. In this work, we investigate plasma-driven ammonia synthesis under elevated pressure conditions by combining experimental measurements with detailed plasma chemical kinetics modeling.