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 alpha-Al2O3, anatase TiO2 and basic MgO, CaO), surface area and porosity (alpha-Al2O3 and gamma-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 gamma-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 mu m yielded a 64% higher concentration of NH3 than 1300 mu m 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 N-2/H-2 feed flow ratio, total flow rate and argon dilution were also investigated. In contradiction to catalytic ammonia synthesis, plasma-assisted synthesis favors a N-2/H-2 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 gamma-Al2O3.