Behavior of N2 and nitrogen oxides in nonthermal plasma chemical processing of hazardous air pollutants

Nonthermal plasma chemical behavior of N₂-O₂ mixed gases and nitrogen oxides such as N₂O, NO, and NO₂ was investigated to obtain baseline information on the generation of active oxygen species and the formation of inorganic byproducts in the nonthermal plasma chemical processing of hazardous air pollutants (HAPs) with a ferroelectric packed-bed reactor. Ozone concentrations were too low, even in air, to oxidatively decompose 300-1000 ppm of HAPs. The O₂ concentration in N₂-O₂ was the determining factor in the formation of all the nitrogen oxides. N₂O formation was enhanced with increases in O₂ concentration and in specific energy density, while a threshold value was observed at around 5% of O₂ concentration in the formation of NO and NO₂. Rate-suppressing effect by O₂, detailed byproduct analyses, and thermochemical data suggest that NO_x decomposes in its reactions with nitrogen atoms derived from N₂ dissociation, and that the unimolecular N-O cleavage predominantly occurs for N₂O. The behavior of nitrogen oxides and their precursors was not affected by hydrogen atoms evolved from hydrogen-rich HAPs such as ethylene and benzene. Halogenated HAPs enhanced NO_x formation and NO₂ selectivity. Different additive effects of chlorinated and brominated HAPs were observed in the formation of NO_x and N₂O, indicating the involvement of different active oxygen species.