Microwave Plasma-Assisted CO₂ Dissociation with Methane and Ethylene: Improved Conversion and Selectivity Controlled by Afterglow Chemistry

An experimental investigation into the addition of small hydrocarbons to gas discharges for CO₂conversion using warm plasma is combined with chemical kinetic modeling. CH₄(0–30%) and C₂H₄(0–15%) are added to CO₂in microwave plasmas operated at 150 to 900 mbar, 1 kW, and 9.4 SLM, in an attempt to remove oxygen from the effluent. Introduction of only 1% CH₄or C₂H₄effectively eliminates all O₂from the product gas at the cost of a dramatic decrease in the conversion of CO₂. Increasing the fraction of hydrocarbons, the CO₂conversion, the CO yield, and the energy efficiency increase, and for more than 5% CH₄or C₂H₄, they surpass that of pure CO₂. The gas mixture notably does not contain O₂, opening a potential new avenue for CO₂dissociation using warm plasmas, where the only byproducts are water and H₂. Measurements show that the effluent composition and especially the selectivity are more or less independent of the plasma size and the gas temperature. Using a quasi-1D chemical kinetic model for the afterglow, it is shown that H and OH radicals have a determining effect on the selectivity and the effluent composition: The radicals catalyze the recombination of CO and O₂in the case of small CH₄additions, and for larger CH₄additions, they are found to drive the water gas shift reaction. The model accurately reproduces the experimental selectivities and shows that processes in the afterglow primarily control the composition of the effluent.