Nanosecond pulsed discharge for CO ₂ conversion: kinetic modeling to elucidate the chemistry and improve the performance

We study the mechanisms of CO ₂ conversion in a nanosecond repetitively pulsed (NRP) discharge, by means of a chemical kinetics model. The calculated conversions and energy efficiencies are in reasonable agreement with experimental results over a wide range of specific energy input values, and the same applies to the evolution of gas temperature and CO ₂ conversion as a function of time in the afterglow, indicating that our model provides a realistic picture of the underlying mechanisms in the NRP discharge and can be used to identify its limitations and thus to suggest further improvements. Our model predicts that vibrational excitation is very important in the NRP discharge, explaining why this type of plasma yields energy-efficient CO ₂ conversion. A significant part of the CO ₂ dissociation occurs by electronic excitation from the lower vibrational levels toward repulsive electronic states, thus resulting in dissociation. However, vibration-translation (VT) relaxation (depopulating the higher vibrational levels) and CO + O recombination (CO + O + M → CO ₂ + M), as well as mixing of the converted gas with fresh gas entering the plasma in between the pulses, are limiting factors for the conversion and energy efficiency. Our model predicts that extra cooling, slowing down the rate of VT relaxation and of the above recombination reaction, thus enhancing the contribution of the highest vibrational levels to the overall CO ₂ dissociation, can further improve the performance of the NRP discharge for energy-efficient CO ₂ conversion.