Interplay among ozone and nitrogen oxides in air plasmas: Rapid change in plasma chemistry

Air is obligatorily used as a supply gas in most of the plasma-aided processing (e.g., plasma-catalysis) systems because it is the most cost-effective approach, but the simultaneous production of gaseous ozone and nitrogen oxides is an unavoidable issue. Consequently, facilitating the separate production of ozone and nitrogen oxides is the primary requirement for achieving the desired plasma performance for specific purposes, yet it is poorly studied. Here, we report the chemical interplay among ozone and nitrogen oxides (i.e., NO, NO2, NO3, N2O4, N2O5) in air-based plasmas using surface dielectric barrier discharge (DBD) actuators as a reference plasma. A gas-tight chamber containing a DBD apparatus was designed for in-situ optical absorption spectroscopy to measure the concentrations of the chemicals. The temporal evolution of each chemical was successfully observed, and a rapid change in the plasma-chemistry mode (i.e., O3- to NO2-dominant mode) was clearly demonstrated. As a control parameter, the gas temperature, which ranged from 25 °C to 250 °C, was set in separate experiments. As the gas temperature was increased, ozone decomposed faster, while NO2 became a dominant chemical species in the reactor earlier. All experimental results were compared with zero-dimensional modeling results, and the interplay among ozone and nitrogen oxides in air plasma was qualitatively analyzed. The gas temperature, which can be influenced by ohmic heating of the plasma itself, convective cooling from external or internal gas flow, or external settings, should be carefully considered for future study and commercialization.