Production of nitric/nitrous oxide by an atmospheric pressure plasma jet

Abstract

Absolute densities of nitrous species were studied in an atmospheric pressure RF plasma jet. The measurement of NO and N 2 O densities has been performed mainly by means of ex situ quantum-cascade laser absorption spectroscopy via a multi-pass cell in Herriot configuration. The dependence of the species’ production on individual parameters such as power, flow and oxygen, nitrogen and water admixture is shown. NO and N 2 O densities are found to increase with absorbed power, while an increase in the gas flow induces a decrease of these densities due to a reduction in residence time of the gas in the plasma. Actually, a change of these two parameters, absorbed power and gas flow, induces a variation of energy density. The higher energy density, the higher NO and N 2 O densities. The NO and N 2 O densities are strongly gas mixture dependent. A change of that parameter allows to choose between a NO-rich or a N 2 O-rich regime. NO and N 2 O densities increase as a function of the N 2 admixture, while increasing oxygen, above a minimum value, reduces the densities of both NO and N 2 O. When adding water instead of oxygen to the gas mixture the reduction in the NO density is much less. For maximal NO and N 2 O formation a ratio of about He/N 2 /O 2 = 99.5/0.36/0.07 is found to be the most efficient in the μ -APPJ. However, it was found that the absorbed power in the plasma always reduces with increasing admixtures. The validation of the results obtained with quantum-cascade absorption spectroscopy with mass spectrometry shows how the two measurement techniques can complement each other. Finally a comparison of our results and others works is presented.