Hydrogenation of plasma-excited nitrogen over an alumina catalyst for ammonia synthesis

Ammonia (NH₃) is a potential hydrogen carrier as alternative fuel and feedstock for hydrogen production. In this study, plasma synthesis of NH₃ was conducted in a packed-bed dielectric barrier discharge (DBD) reactor using Al₂O₃ as the catalyst. In order to explore the mechanism of hydrogenation of plasma-excited nitrogen for NH₃ synthesis, the whole NH₃ synthesis process was divided into three steps including N₂ activation, hydrogenation of plasma-excited N(a), and desorption of NH₃(a) from catalyst. The effects of reaction conditions on the three steps and corresponding NH₃ production were examined. Results showed that more plasma-excited nitrogen species were formed through N₂ activation at higher N₂ flow rate, discharge time and discharge power for N₂ activation. Hydrogenation of plasma-excited N(a) to form NH₃(a) was improved by more discharge time at the second step. Higher discharge temperature for N(a) hydrogenation favored NH₃(a) desorption from catalyst and increased NH₃ production at the second step, with the total NH₃ yield slightly changed. In addition, one-step NH₃ synthesis in plasma was investigated and compared with the three-step process. The results will provide reference for catalyst and reactor design in plasma synthesis of ammonia.