Frequency Effects on the Vibrational States and Conversion of CO2 in Radio Frequency Discharges Under Martian Pressure

Abstract

In recent years, with the progress of Mars exploration, the in situ utilization of CO2 resources in the Martian atmosphere has become a widely discussed subject. Nonequilibrium plasma technology as an efficient molecular activation approach has great potential for CO2 conversion. In this study, a fluid model is established to discuss driving frequency effects on the vibrational states and conversion of CO2 in radio frequency (RF) discharges at a given power density under Martian pressure. The calculated CO2 conversion and energy efficiency depending on the specific energy input are in good agreement with the experimental measurements. The simulation results found that a considerable fraction of electron energy is transferred to the asymmetric stretching modes of CO2, and the characteristic times for electron impact vibrational excitation and vibration–vibration (VV) relaxation are small, which results in a larger density of higher vibrational levels. The density of vibrational levels of CO2 increases with the driving frequency at a constant power density, which can be explained by the enhancement of vibrational excitation and VV relaxation. The computational data also show that more CO, O, and O2 are produced as the driving frequency increases at a constant power density, and the vibration-induced dissociation with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textrm {CO}_{2}\textrm {v}_{8}$ </tex-math></inline-formula> gradually replaces the direct dissociation to be the main reaction for the production of CO and O. In addition, the increase in driving frequency causes more electron energy to be transferred to the asymmetrical stretch modes, and the vibrational energy of these vibrational states of CO2 can reduce the activation barrier of the reaction, improving the CO2 conversion and energy efficiency. This study contributes to a deeper insight into the frequency effects on plasma chemistry in RF CO2 discharges under Martian pressure and shows the optimized ways for the decomposition of CO2 on Mars.