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 CO2 conversion using warm plasma is combined with chemical kinetic modeling. CH4 (0-30%) and C2H4 (0-15%) are added to CO2 in 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% CH4 or C2H4 effectively eliminates all O-2 from the product gas at the cost of a dramatic decrease in the conversion of CO2. Increasing the fraction of hydrocarbons, the CO2 conversion, the CO yield, and the energy efficiency increase, and for more than 5% CH4 or C2H4, they surpass that of pure CO2. The gas mixture notably does not contain O-2, opening a potential new avenue for CO2 dissociation using warm plasmas, where the only byproducts are water and H-2. 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 O-2 in the case of small CH4 additions, and for larger CH4 additions, 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.