Study of vibrational kinetics of CO2 and CO in CO2–O2 plasmas under non-equilibrium conditions

Abstract

This work explores the effect of O2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent model, previously validated for pure CO2 discharges, is further extended by adding the vibrational kinetics of CO, including electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The vibrational kinetics considered include levels up to v = 10 for CO and up to v 1 = 2 and v 2 = v 3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and accounts for e-V, V-T in collisions between CO, CO2 and O2 molecules and O atoms and V-V processes involving all possible transfers involving CO2 and CO molecules. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge ignited in pure CO2 and CO2–O2, operating at pressures in the range 0.4–5 Torr (53.33–666.66 Pa). The experimental results show a lower vibrational temperature of the different modes of CO2 and a decreased dissociation fraction of CO2 when O2 is added to the plasma but an increase of the vibrational temperature of CO. On the one hand, the simulations suggest that the former effect is the result of the stronger V-T energy-transfer collisions with O atoms which leads to an increase of the relaxation of the CO2 vibrational modes. On the other hand, two main mechanisms contribute to the lower CO2 dissociation fraction with increased O2 content in the mixture: the back reaction, CO(a3Πr) + O2 → CO2 + O and the recombinative detachment O− + CO → e + CO2.