Plasma-assisted ammonia synthesis in a packed-bed dielectric barrier discharge reactor: roles of dielectric constant and thermal conductivity of packing materials

Abstract In this article, plasma-assisted NH 3 synthesis directly from N 2 and H 2 over packing materials with different dielectric constants (BaTiO 3 , TiO 2 and SiO 2 ) and thermal conductivities (BeO, AlN and Al 2 O 3 ) at room temperature and atmospheric pressure is reported. The higher dielectric constant and thermal conductivity of packing material are found to be the key parameters in enhancing the NH 3 synthesis performance. The NH 3 concentration of 1344 ppm is achieved in the presence of BaTiO 3 , which is 106% higher than that of SiO 2 , at the specific input energy (SIE) of 5.4 kJ·l −1 . The presence of materials with higher dielectric constant, i.e. BaTiO 3 and TiO 2 in this work, would contribute to the increase of electron energy and energy injected to plasma, which is conductive to the generation of chemically active species by electron-impact reactions. Therefore, the employment of packing materials with higher dielectric constant has proved to be beneficial for NH 3 synthesis. Compared to that of Al 2 O 3 , the presence of BeO and AlN yields 31.0% and 16.9% improvement in NH 3 concentration, respectively, at the SIE of 5.4 kJ·l −1 . The results of IR imaging show that the addition of BeO decreases the surface temperature of the packed region by 20.5% to 70.3°C and results in an extension of entropy increment compared to that of Al 2 O 3 , at the SIE of 5.4 kJ·l −1 . The results indicate that the presence of materials with higher thermal conductivity is beneficial for NH 3 synthesis, which has been confirmed by the lower surface temperature and higher entropy increment of the packed region. In addition, when SIE is higher than the optimal value, further increasing SIE would lead to the decrease of energy efficiency, which would be related to the exacerbation in reverse reaction of NH 3 formation reactions.