Green nitrogen fixation: Increased NOx production by catalysis using the effluent of a microwave air plasma

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 α -Al 2 O 3 catalyst support pellets were found to increase NO x yields and achieve higher NO 2 selectivity. Three critical parameters are assessed: the temperature, the amount of α -Al 2 O 3 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 α -Al 2 O 3 material. However, integrated over time the results clearly support a catalytic effect, producing additional NO x compared to plasma-only operation. Previous modelling efforts show negligible amounts of plasma species and no non-thermal behaviour at the start of the pellet bed. So, the downstream pellets effectively interact with a thermal gas at a relatively high temperature of 500–1000 °C . Despite the fact that the gas is thermalised, catalytic reactions can benefit from the relevant vibrational populations of N 2 and O 2 . Therefore, the additional NO x production is speculated to be the result of high-temperature catalysis. • First successful microwave-air plasma downstream catalysis achieved for NO x synthesis • Roughly 10% NO x increase from Al 2 O 3 pellets at no additional energy cost • Temperature, amount of catalyst, and catalyst proximity to plasma investigated • 500–1000 °C catalytically active, likely due to vibrational population of N 2 and O 2