Synergy between microwave air plasma and Al₂O₃ as catalyst for NO_xproduction

This work experimentally investigated a 2.45 GHz microwave air plasma and a catalyst bed placed in its downstream region. The plasma condition was fixed at 965 mbar,

22.4 slm, and 600 W, while the catalyst bed temperature was the result of the heating from the plasma effluent over time. Downstream Al₂O₃ catalyst support pellets were found to increase NO_x yields and achieve higher NO₂ selectivity. Thus, three critical parameters are assessed that influence the surface process: the temperature, the amount of Al₂O₃ pellets, and the distance between the pellet bed and the plasma. Higher temperatures, greater amounts of pellets, and closer proximity of the pellets to the plasma all enhance the additional NO_x formed. The findings are convoluted by NO_x adsorption and desorption on the Al₂O₃ material. However, integrated over time the results clearly support a catalytic effect, producing additional NO_x compared to plasma-only operation. Modelling shows negligible amounts of plasma species at the start of the pellet bed. So, the downstream pellets effectively interact with a thermal gas at a relatively high temperature of 500-900^◦C. Therefore, the additional NO_x production is likely the result of high-temperature catalysis.