Kinetic Modeling Analysis of Ar Addition to Atmospheric Pressure N2-H2 Plasma for Plasma-Assisted Catalytic Synthesis of NH3.

Abstract

Zero-dimensional kinetic modeling of atmospheric pressure Ar-N2-H2 nonthermal plasma was carried out to gain mechanistic insights into plasma-assisted catalytic synthesis of ammonia. Ar dilution is a common technique for tailoring plasma discharge properties and has been shown to enhance NH3 formation when added to N2-H2 plasma. The kinetic model was developed for a coaxial dielectric barrier discharge quartz wool-packed bed reactor operating at near room temperature using a kHz-frequency plasma source. With 30% Ar mixed in a 1:1 N2-H2 plasma at 760 Torr, we find that NH3 production is dominated by Eley-Rideal (E-R) surface reactions, which heavily involve surface NHx species derived from N and H radicals in the gas phase, while the influence of excited N2 molecules is negligible. This is contrary to the commonly proposed mechanism that excited N2 molecules created by Penning excitation of N2 by Ar(4s) and Ar(4p) play a significant role in assisting NH3 formation. Our model shows that the enhanced NH3 formation upon Ar dilution is unlikely due to the interactions between Ar and H species, as excited Ar atoms have a weak effect on H radical formation through H2 dissociation compared to electrons. We find that excited Ar atoms contribute to 28% of the N radical production in the gas phase via N2 dissociation, while the rest are dominated by electron-impact dissociation. Furthermore, Ar species play a negligible role in the product NH3 dissociation. N2 conversion sensitivity analyses were carried out for electron number density (ne) and reduced electric field (E/N), and contributions from Ar to gas-phase N radical production were quantified. The model can provide guidance on potential reasons for observing enhanced NH3 formation upon Ar dilution in N2-H2 plasma beyond changes in the discharge characteristics.