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

TL;DR

This study uses kinetic modeling to understand ammonia synthesis in atmospheric pressure Ar-N2-H2 plasma, revealing that surface reactions dominate NH3 formation and challenging previous assumptions about excited N2 molecules' roles.

Contribution

The paper presents a detailed kinetic model showing that NH3 formation mainly occurs via surface reactions, with minimal influence from excited N2 molecules, providing new mechanistic insights.

Findings

NH3 formation is dominated by surface Eley-Rideal reactions.

Excited N2 molecules have negligible impact on NH3 synthesis.

Ar's role in N radical production is primarily through N2 dissociation by electrons.

Abstract

Zero-dimensional kinetic modeling of atmospheric pressure Ar-N2-H2 nonthermal plasma was carried out to gain mechanistic insights into ammonia formation during plasma-assisted catalysis of ammonia synthesis. The kinetic model was developed for a coaxial dielectric barrier discharge (DBD) 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) plays 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 is 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 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 plasmas beyond changes to the discharge characteristics.