Nitrogen fixation: Synergy between air plasma and heterogenous catalyst for NO_x production

Using compressed air as feedgas, nitrogen fixation is achieved through the formation of NO_x, directly via a 2.45 GHz microwave plasma and through interactions of microwave plasma activated species with a catalyst surface. α-Al₂O₃ pellets and 5 wt% WO₃ on Al₂O₃ pellet support were investigated as catalysts in a post-plasma packed-bed configuration. The produced NO_x densities were quantitatively measured using gas-phase Fourier Transform Infrared (FTIR) spectroscopy. Plasma parameters were fixed to a flowrate of 22.4 slm, a pressure of 965 mbar, and an input power of 600 W. The catalyst temperature increased passively (heated over time by the hot exhaust of the plasma). The energy cost of the reactor was significantly reduced (up to 36%) by α-Al2O3 pellets, from 3.76 MJ/mol N-fixed to 2.42 MJ/mol N-fixed. Measurements show that the NO₂/NO ratio correlates with the total NO_x production, with higher ratios resulting in higher efficiencies. This is attributed to an oxygen absorbing and donating effect of the α-Al₂O₃ surface, which enhances NO oxidation to NO₂. This takes NO out of the equilibrium, allowing enhanced total NO_x production. The beneficial effect of the catalyst is found to be very temperature sensitive. Within the temperature window of 480-530 K the catalyst improved performance, outside this range it reduced performance. Overall, it is demonstrated that heterogeneous post-plasma catalyst can significantly enhance microwave plasma’s energy efficiencies for NO_x production.