Plasma-catalytic ammonia synthesis in a dielectric barrier discharge reactor: A combined experimental study and kinetic modeling

Plasma-catalytic ammonia synthesis in a dielectric barrier discharge reactor has emerged as a possible route for electrification of nitrogen fixation. In this study, we use a combination of experiments and a plasma kinetic model to investigate the ammonia synthesis from N₂ and H₂, both with and without a solid packing material in the plasma zone. The effect of plasma power, feed flow rate, N₂:H₂ feed ratio, gas residence time, temperature, and packing material (MgAl₂O₄ alone or impregnated with Co or Ru) on the ammonia synthesis rate were examined in the experiments. The kinetic model was employed to improve our understanding of the ammonia formation pathways and identify possible changes in these pathways when altering the N₂:H₂ feed ratio. A higher NH₃ synthesis rate was achieved when increasing the feed flow rate, as well as when increasing the gas temperature from 100 to 200 °C when a packing material was present in the plasma. At the elevated temperature of 200 °C, an optimum in the NH₃ synthesis rate was observed at an equimolar feed ratio (N₂:H₂ = 1:1) for the plasma alone and MgAl₂O₄, while a N₂-rich feed was favored for Ru/MgAl₂O₄ and Co/MgAl₂O₄. The optimum in the synthesis rate with the N₂-rich feed, where high energy electrons are more likely to collide with N₂, suggests that the rate-limiting step is the dissociation of N₂ in the gas phase. This is supported by the kinetic model when packing material was used. However, for the plasma alone, the model found that the N₂ dissociation is only rate limiting in H₂-rich feeds, whereas the limited access to H in N₂-rich feeds makes the hydrogenation of N species limiting.