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–1g–1were
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 N2and,
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.