Study on the physico-chemical mechanism of SDBD product change based on spectral diagnosis

To gain insight into the interaction mechanism among the gaseous products of atmospheric pressure air plasma, using a surface dielectric barrier discharge as the study object, the dynamic processes of characteristic products (nitric oxide NO and ozone O3) concentrations were measured by in-situ Fourier infrared spectroscopy and UV absorption spectroscopy, respectively; the real energy density of the plasma was calculated by Lissajous figure and ICCD optical image; and the gas temperature was obtained by fitting the emission spectrum of the second positive band of the nitrogen molecule. The results showed that the real energy density and gas temperature were highly positively correlated with the applied voltage and frequency. Higher applied voltages and frequencies could lead to lower peak absorbance of O3 and higher absorbance of NO, and accelerate the conversion of the products from O3-containing state to O3-free state. The microscopic mechanism of the product changes was 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 pointed out that there are two major reasons for the disappearance of O3, the quenching effect of O and O/O2 excited state particles on O3 as well as the quenching effect of NO on O3. And the mechanism of O3 disappearance accelerates with the increase of energy density and gas temperature are following: as the real energy density increases, it means that the energy injected into the discharge region is boosted, which intensifies the collision reaction to produce more energetic electrons as well as 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 O2, which accelerates the decomposition of O3; besides, the gas temperature was raised due to the intensification of the collision reaction. Whereas the gas temperature could 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 O3. The higher the gas temperature is, the lower the rate of O3 generation reaction is, but the higher the rate of dissociation is, which was thought to be the endogenous cause of the rapid disappearance of O3. In contrast, the rise in gas temperature could significantly elevate the reaction rate of NO production and decrease its dissociation rate. This contributes to the faster production of massive NO, resulting in an accelerated quenching process of NO to O3, which could be considered as the exogenous cause of the rapid disappearance of O3. In a word, based on the analysis above, it contributes to a better understanding of the physico-chemical processes in atmospheric pressure low-temperature plasma.