A surface mechanism for O3 production with N2 addition in dielectric barrier discharges

Abstract

Ozone, O3, is a strong oxidizing agent often used for water purification. O3 is typically produced in dielectric barrier discharges (DBDs) by electron-impact dissociation of O2, followed by three-body association reactions between O and O2. Previous studies on O3 formation in low-temperature plasma DBDs have shown that O3 concentrations can drop to nearly zero after continued operation, termed the ozone-zero phenomenon (OZP). Including small (<4%) admixtures of N2 can suppress this phenomenon and increase the O3 production relative to using pure O2 in spite of power deposition being diverted from O2 to N2 and the production of nitrogen oxides, N x O y . The OZP is hypothesized to occur because O3 is destroyed on the surfaces in contact with the plasma. Including N2 in the gas mixture enables N atoms to occupy surface sites that would otherwise participate in O3 destruction. The effect of N2 in ozone-producing DBDs was computationally investigated using a global plasma chemistry model. A general surface reaction mechanism is proposed to explain the increase in O3 production with N2 admixtures. The mechanism includes O3 formation and destruction on the surfaces, adsorption and recombination of O and N, desorption of O2 and N2, and NO x reactions. Without these reactions on the surface, the density of O3 monotonically decreases with increasing N2 admixture due to power absorption by N2 leading to the formation of nitrogen oxides. With N-based surface chemistry, the concentrations of O3 are maximum with a few tenths of percent of N2 depending on the O3 destruction probability on the surface. The consequences of the surface chemistry on ozone production are less than the effect of gas temperature without surface processes. An increase in the O3 density with N-based surface chemistry occurs when the surface destruction probability of O3 or the surface roughness was decreased.