Characterization of mesoporous silica packed bed reactors for plasma synthesis of ammonia*

Plasma-catalytic ammonia production at atmospheric pressure has the advantage of small scalable design, easy intermittent operation, and use of renewable energy, but to meet the ammonia production it needs intelligent design of catalytic materials with different properties than traditional catalysts. One of our main hypotheses is that since plasma aides in the dissociation of N₂ trough vibrational excitations and produces H* radicals, the catalysts need to adsorb H* to allow it to react with plasma activated nitrogen, and bond to nitrogen weakly. Mesoporous oxides such as silica (SBA-15) have these properties and also produce a stable plasma due to their low conductivity, are chemically stable, and have a porous structure that facilitates diffusion and surface reactions. We use optical emission spectroscopy (OES), Fourier transform infrared absorption spectroscopy (FTIR-AS), and electrical measurements to characterize two plasma-calytic packed bed dielectric barrier reactors, one packed with SBA-15 and another with SBA-15 with 10% Ag, operating in a 1:1 N₂ and H₂ gas mixture. OES analysis shows similar parameters for two reactors with a 10% higher vibrational excitation temperature and lower rotational temperature in SBA-15 reactor as compared to SBA-15-Ag. At the same applied voltage SBA-15-Ag produces <20% higher concentration of NH₃, but the highest 11,000 ppm of NH₃ was for SBA-15 reactor.  Plasma characterization can help choose the catalysts for ammonia production.