NH₃ Synthesis in a Dielectric Barrier Discharge Reactor : A Study from Atmospheric to Low Pressure

Abstract

N2-H2 discharges are systematically studied using a coaxial dielectric barrier discharge reactor for ammonia synthesis by means of mass spectrometry, electrical characterization and high-resolution emission spectroscopy. The influence of packing is investigated by accommodating chemically inert SiO2 beads in the discharge volume from 920 to 13 mbar. Above 275 mbar, the discharge is dominated by filaments associated with intense microdischarges, whereas at lower pressures, the plasma becomes diffuse and occupies a large volume. In presence of packing, the intensity of the microdischarges at 920 mbar are strongly suppressed, while the electrical and emission properties of the diffuse plasma remain largely unaffected. The absence of intense microdischarges in the diffuse mode at low pressures eliminates important NH3 dissociation channels. Decreasing the pressure below 100 mbar leads to a significant increase in [NH3] with SiO2 beads. This is attributed to both an increase of E/n, which favours H2 and N2 dissociation, and consequently to an increase in plasma-surface reactions involving H and N towards ammonia formation. Investigations at 50 mbar reveal that introducing SiO2 beads in contact with the plasma has a more limited impact on [NH3] than at 920 mbar. The emission spectra are dominated by the second positive system of N2, first negative system of N2+, and Hα, with no evidence of excited NH*. The rotational temperature of N2(C) is mostly affected by [N2] in [H2] in the empty reactor at 920 mbar, reaching about 808 K at 75 vol.% N2. With packing or at 50 mbar the rotational temperature remains at ≈ 400 K. For all tested conditions, the vibrational temperatures of N2(C) lie in the range of 3500 - 3900 K.