[Paper Review] Study of vibrational kinetics of CO2 and CO in CO2-O2 plasmas under non-equilibrium conditions

C. Fromentin, Tiago Silva+5 more|arXiv (Cornell University)|Oct 14, 2022

Plasma Applications and Diagnostics4 citations

TL;DR

This study develops and validates a self-consistent plasma kinetic model for CO2-O2 mixtures under non-equilibrium conditions, incorporating detailed vibrational kinetics of CO2 and CO, including electron impact excitation, V-T, and V-V processes. The model explains that O2 addition reduces CO2 dissociation and lowers CO2 vibrational temperature due to enhanced V-T relaxation with O atoms, while increasing CO vibrational temperature via back reactions.

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, the back reactions with O2 contribute to the lower CO2 dissociation fraction with increased O2 content in the mixture.

Motivation & Objective

• To understand the influence of O2 addition on CO2 dissociation and vibrational energy distribution in non-equilibrium plasmas.
• To extend a validated CO2-only kinetic model to include CO vibrational states and their interactions with CO2, O2, and O atoms.
• To explain experimental observations of reduced CO2 dissociation and altered vibrational temperatures upon O2 doping.
• To quantify the role of V-T and V-V processes in energy transfer between CO2, CO, and O species.
• To validate the extended model against experimental data from a DC glow discharge in the 0.4–5 Torr pressure range.

Proposed method

• A self-consistent kinetic model is extended to include vibrational states of CO up to v = 10 and CO2 up to v1 = 2, v2 = v3 = 5 for symmetric stretch, bending, and asymmetric stretch modes.
• The model incorporates electron-induced vibrational excitation/de-excitation (e-V), vibration-to-translation (V-T) relaxation, and vibration-to-vibration (V-V) energy transfer processes.
• Collisional processes involving CO, CO2, O2, and O atoms are included in the V-T and V-V transfer networks.
• The model is validated by comparing simulated vibrational temperatures and CO2 dissociation fractions against experimental data from a DC glow discharge at 0.4–5 Torr.
• The kinetic scheme accounts for all possible V-V energy transfers between CO2 and CO molecules.
• Simulations are performed across a range of O2 concentrations to assess their impact on vibrational and dissociative behavior.

Experimental results

Research questions

• RQ1 How does O2 addition affect the vibrational temperature of CO2 modes in non-equilibrium CO2-O2 plasmas?
• RQ2 What is the role of O atoms in enhancing V-T relaxation and reducing CO2 vibrational energy?
• RQ3 Why does CO2 dissociation fraction decrease with increasing O2 content in the plasma?
• RQ4 How does O2 influence the vibrational excitation of CO, and what processes drive its increased vibrational temperature?
• RQ5 To what extent do back reactions with O2 suppress CO2 dissociation in CO2-O2 plasmas?

Key findings

• O2 addition leads to a measurable decrease in CO2 vibrational temperature across all modes due to enhanced V-T relaxation with O atoms.
• The increased V-T efficiency with O atoms is identified as the primary cause for the reduced vibrational energy in CO2.
• CO2 dissociation fraction decreases with increasing O2 content, primarily due to back reactions involving O2.
• The vibrational temperature of CO increases with O2 addition, indicating enhanced vibrational excitation through collisional processes.
• The model shows good quantitative agreement with experimental data for vibrational temperatures and dissociation fractions in the 0.4–5 Torr pressure range.
• The inclusion of CO vibrational kinetics significantly improves the model's ability to reproduce experimental trends in CO2-O2 mixtures.