Ambient Carbon-Neutral Ammonia Generation via a Cyclic Microwave Plasma Process

A novel reactor methodology was developed for chemical looping ammonia synthesis processes using microwave plasma for pre-activation of the stable dinitrogen molecule before reaching the catalyst surface. Microwave plasma-enhanced reactions benefit from higher production of activated species, modularity, quick startup, and lower voltage input than competing plasma-catalysis technologies. Simple, economical, and environmentally benign metallic iron catalysts were used in a cyclical atmospheric pressure synthesis of ammonia. Rates of up to 420.9 μmol min^-1 g^-1 were observed under mild nitriding conditions. Reaction studies showed that both surface-mediated and bulk-mediated reaction domains were found to exist depending on the time under plasma treatment. The associated density functional theory (DFT) calculations indicated that a higher temperature promoted more nitrogen species in the bulk of iron catalysts but the equilibrium limited the nitrogen converion to ammonia, and vice versa. Generation of vibrationally active N₂ and, N₂⁺ ions is associated with lower bulk nitridation temperatures and increased nitrogen contents versus thermal-only systems. Additionally, the kinetics of other transition metal chemical looping ammonia synthesis catalysts (Mn and CoMo) were evaluated by high-resolution time-on-stream kinetic analysis and optical plasma characterization. This study sheds new light on phenomena arising in transient nitrogen storage, kinetics, effect of plasma treatment, apparent activation energies, and rate-limiting reaction steps.