Plasma-enabled sustainable ammonia synthesis at atmospheric pressure: The role of catalysts on synergistic effect

Abstract

Ammonia (NH3) has received remarkable interest for the synthesis of many nitrogenous product and as storage hydrogen media. However, the synthesis of NH3 remains challenging due to the high pressure and temperature from the dissociation of the stable N≡N triple bond. Here, we report an environmentally friendly plasma-thermal catalytic process for the sustainable synthesis of NH3 over M/CeO2 and M/MgO catalysts (M = Ru, Fe) with dielectric barrier discharge (DBD) reactor at atmospheric pressure, which showed excellent activity and high temperature stability. The synergistic effect strongly depended on the support properties, that CeO2-supported catalysts were more obvious below 300 °C compared with the MgO-supported catalysts regardless of the active metals (Ru or Fe). The highest NH3 synthesis rate of 6.8 mmol g−1 h−1 was achieved over Ru/CeO2 at 400 °C under a ratio of N2:H2 = 1:3. This value was much higher compared with that using DBD reactor reported in the literature. The structure-activity relation of catalysts was characterized through X-ray photoelectron spectroscopy, temperature programmed reduction by hydrogen, Raman spectra and the in-situ diffuse reflectance infrared Fourier transform spectroscopy (Iin-situ DRIFTS). In addition to the gas-phase reaction mediated by plasma and the catalyst surface reaction, the increased NH3 concentration under plasma-catalysis condition can be attributed the synergistic effect between plasma and catalyst, where plasma irradiation promoted the desorption of NH3 and NHx species. These results are helpful for the design of highly efficient catalysts and the understanding for the role of plasma in plasma-enabled green chemistry.