Physico-chemical mechanism of surface dielectric barrier discharge product change based on spectral diagnosis

Abstract

To gain an insight into the interaction mechanism among the gaseous products of atmospheric pressure air plasma, a surface dielectric barrier discharge is used as a study object. The dynamic processes of characteristic products (nitric oxide NO and ozone O<sub>3</sub>) are measured by in-situ Fourier infrared spectroscopy and UV absorption spectroscopy. The real energy density of the plasma is calculated by Lissajous figure and ICCD optical image. The gas temperature is obtained by fitting the emission spectrum of the second positive band of the nitrogen molecule. The results show that the real energy density and gas temperature are highly positively correlated with the applied voltage and frequency. Higher applied voltages and frequencies can lead to lower peak absorbance of O<sub>3</sub> and higher absorbance of NO, and accelerate the conversion of the products from O<sub>3</sub>-containing state into O<sub>3</sub>-free state. The microscopic mechanism of the product change is revealed by analyzing the effects of the real energy density and gas temperature on the major generation and quenching chemical reactions of the characteristic products. The analysis points out that there are two major reasons for the disappearance of O<sub>3</sub>, i.e. the quenching effect of O and O/O<sub>2</sub> excited state particles on O<sub>3</sub> and the quenching effect of NO on O<sub>3</sub>. And the mechanism that the disappearance of O<sub>3</sub> accelerates with the increase of energy density and gas temperature, is as follows. The increase of real energy density means that the energy injected into the discharge region is enhanced, which intensifies the collision reaction, thereby producing more energetic electrons and reactive oxygen and nitrogen particles. Since the discharge cavity is gas-tight, the rapid generation of O leads to a rapid increase in the ratio of O to O<sub>2</sub>, which accelerates the decomposition of O<sub>3</sub>; besides, the gas temperature is raised due to the intensification of the collision reaction. Whereas the gas temperature can change the rate coefficients of the chemical reactions involving the excited state particles of nitrogen and oxygen to regulate the production and quenching of the products. The increase of gas temperature has a negative effect on O<sub>3</sub>. The higher the gas temperature, the lower the rate of O<sub>3</sub> generation reaction is but the higher the rate of dissociation, which is thought to be the endogenous cause of the rapid disappearance of O<sub>3</sub>. In contrast, the gas temperature rising can significantly elevate the reaction rate of NO production and reduces its dissociation rate. This contributes to the faster production of massive NO, resulting in an accelerated quenching process of NO to O<sub>3</sub>, which can be considered as the exogenous cause of the rapid disappearance of O<sub>3</sub>. In a word, the present study contributes to a better understanding of the physico-chemical process in atmospheric pressure low-temperature plasma.