Deciphering the synergy between plasma and catalyst support for ammonia synthesis in a packed dielectric barrier discharge reactor

Plasma-assisted ammonia synthesis in a packed dielectric barrier discharge (DBD) reactor at atmospheric pressure is presented in this work. A broad range of materials (commonly used as catalyst supports) with various chemical properties (acidic α-Al₂O₃, anatase TiO₂ and basic MgO, CaO), surface area and porosity (α-Al₂O₃ and γ-Al₂O₃), dielectric properties (quartz wool, TiO₂, and BaTiO₃), have been investigated for synergetic effects by packing them in the discharge zone of the DBD reactor. All the materials showed a substantial effect on ammonia production, which can be explained solely as a result of the effect of packing on plasma formation and not by a synergy between plasma and surface catalysis. Size and shape of packing material are found to be the key parameters in enhancing the performance. Quartz wool, closely followed by γ-Al₂O₃, produces the highest concentration of ammonia at 2900 and 2700 ppm, respectively, due to their ability to generate dense filamentary microdischarges. Particles with a diameter of 200 µm yielded a 64% higher concentration of NH₃ than 1300 µm particles - because of amplified electric field strength from increased particle-particle contact points. The specific energy input per unit volume also displayed a significant impact on ammonia production. The process parameters such as N₂/H₂ feed flow ratio, total flow rate and argon dilution were also investigated. In contradiction to catalytic ammonia synthesis, plasma-assisted synthesis favors a N₂/H₂ feed ratio 2 instead of the stoichiometric feed ratio of 0.33. At 0.4 l min^-1, 3500 ppm of ammonia was produced with an energy efficiency of 1.23 g NH₃ kWh^-1. Dilution with 2-5 vol% of argon yielded a 2% improvement in the concentration and energy efficiency, which seems insignificant considering the added practical challenges posed by gas separation. To achieve even higher ammonia concentration and energy efficiencies, it is recommended to support transition metal on γ-Al₂O₃.