Influence of oxygen on the ro-vibrational kinetics of a non-equilibrium discharge in CO₂-O₂ mixtures

Storing excess renewable energy in hydrocarbons produced from CO2 potentially solves the intermittency issue of renewable energy sources in a green manner. The required reduction of CO2 to CO can be efficiently accomplished with non-equilibrium plasma conversion. On an industrial scale, effects of impurities on the reduction must be taken into account. During this study, the effects of oxygen are considered, as the impurity O2 is both a product of the reduction reaction and abundant in air. In this paper, the influence of O2 addition on the ro-vibrational kinetics of a pulsed DC CO2 glow discharge at 2.5 to 6.0Torr — serving as a model non-equilibrium system — is studied in situ with quantum cascade laser infrared absorption spectroscopy. The temporal evolution of the ro-vibrational temperatures is measured, as well as the conversion of CO2 to CO. Trends in the temperature evolutions when increasing the flow rate from 7.4 to 30.0 sccm, varying the pressure, and increasing the O2 admixture up to 90% in increments of 10% are utilised to determine the underlying kinetic processes. Our results show that any decrease in the conversion of CO2 to CO caused by increasing O2 addition cannot be attributed to an induced change in the vibrational kinetics, since the asymmetric stretch mode of CO2 — which is associated with dissociation via vibrational excitation — is not quenched. Measured changes in the temporal temperature trends are explained by species-dependent intra- and intermolecular collisional energy transfer processes.