No 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 v1 = 2 and v2 = v3 = 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, 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.

Vibrational excitation represents an efficient channel to drive the dissociation of CO2 in a non-thermal plasma. Its viability is investigated in low-pressure pulsed discharges, with the intention of selectively exciting the asymmetric stretching mode, leading to stepwise excitation up to the dissociation limit of the molecule. Gas heating is crucial for the attainability of this process, since the efficiency of vibration-translation relaxation strongly depends on temperature, creating a feedback mechanism that can ultimately thermalize the discharge. Indeed, recent experiments demonstrated that the timeframe of vibration-translation non-equilibrium is limited to a few milliseconds at ca. 6 mbar, and shrinks to the μs-scale at 100 mbar. With the aim of backtracking the origin of gas heating in pure CO2 plasma, we perform a kinetic study to describe the energy transfers under typical non-thermal plasma conditions. The validation of our kinetic scheme with pulsed glow discharge experiments enables to depict the gas heating dynamics. In particular, we pinpoint the role of vibration-vibration-translation relaxation in redistributing the energy from asymmetric to symmetric levels of CO2, and the importance of collisional quenching of CO2 electronic states in triggering the heating feedback mechanism in the sub-millisecond scale. This latter finding represents a novelty for the modelling of low-pressure pulsed discharges and we suggest that more attention should be paid to it in future studies. Additionally, O atoms convert vibrational energy into heat, speeding up the feedback loop. The efficiency of these heating pathways, even at relatively low gas temperature and pressure, underpins the lifetime of vibration-translation non-equilibrium and suggests a redefinition of the optimal conditions to exploit the “ladder-climbing” mechanism in CO2 discharges.

No abstract