Ammonia Synthesis Using Atmospheric Dbd Plasma Coupled With Catalysis

Ammonia(NH3) synthesis technology plays a central role in the development of the basic chemical industries and has an extremely important position in national economy. It is not only used as a nitrogenous fertilizer and a raw material for industry but also a potential alternative fuel for energy field. Ammonia fuel conference conducted in Kansas City on October of 2009 addressed that liquid ammonia was one of the realistic solution that made sense. Due to the high stability of nitrogen(N2), NH3 is industrially synthesized from the mixture of N2 and H2 through the Haber-Bosch(HB) process under high temperature and pressure with huge energy consumption. Therefore, scientists are engaged in pursuing the target of ammonia synthesis at normal temperature and pressure.Non-thermal plasma technology(NTP) with the strong ability of bond breaking and molecule reorganization has been widely used in ammonia synthesis. N2 and H2 dissociated and into active N, H and NHx radicals to form NH3 at atmospheric or low pressure plasma coupled with catalysis, which changes the rule of traditional ammonia synthesis completely. Among the discharge modes, packed bed dielectric barrier discharge(DBD) plasma is regarded as an excellent source of ideal energetic electrons. In this paper, the technology of atmospheric packed bed DBD plasma coupled with catalysis was first used for the ammonia synthesis. The research mainly focuses on three aspects: packed catalyst screening, optimization of process parameters and mechanism of ammonia synthesis in packed bed DBD reactor coupled catalysis. The main achieved conclusions are as follows:(1) The Ru-based catalysts were prepared with Al2O3 and Mg O as supports and Ru(NO)(NO3)3 as precursor. Their activities of ammonia synthesis were evaluated in packed bed DBD reactor. Meantime, the Ru-based catalysts were characterized before and after discharging. Results showed that that the activities of catalysts were ordered by Ru/L-Mg O>Ru/Al2O3 > L-Mg O > Al2O3 > H-Mg O. The characterization results of SEM-EDS, BET showed that the mechanical intension of Al2O3 with high surface area and small bore was higher than L-Mg O with small surface area and big bore in DBD plasma system. The pore structure of L-Mg O happened to collapse after discharge for 30 hours, leading to the agglomeration of Ru loaded on L-Mg O;(2) The process parameters were optimized in DBD reactor with N2 and H2 in certain ratio and different catalysts as dielectric material(one-step ammonia synthesis). The parameters included volume ratio of N2 and H2(VN2/H2), gas flow rate, discharge power and discharge temperature. Results showed that the best VN2/H2 were 2:1 for the above five catalysts. Higher flow rate could eliminate the influence of external diffusion and advantageous for ammonia synthesis. The mass yield of NH3(g NH3/s) increased with the increase of discharge power. The discharge temperature had a vital influence on the mass yield of NH3(g NH3/s) which increased with the increase of the discharge temperature. It is more difficult for the NH3(a) to desorb from Al2O3 catalyst below 300 ℃. The results of NH3-TPD on Al2O3 catalyst also demonstrated that it was a weak acidic catalyst;(3) To further understand of reaction mechanism of ammonia synthesis during the plasma-catalytic process. The ammonia synthesis process was separated into three successive steps in DBD reactor(three-step ammonia synthesis), including(1) N2 activation on the catalysts by N2 plasma;(2) hydrogenation of plasma-excited N(a) by H2 plasma;(3) desorption of NH3(a) from Al2O3 catalyst after H2 plasma. The effects of process parameters and catalysts on the hydrogenation performance of plasma-excited N(a) was investigated. Results showed that higher the discharge power, more plasma-excited N(a) were produced to form NH3. When the plasma-excited N(a) chemisorbed on the catalysts were saturated when discharge power was more than 26 W, the total NH3 amount produced on Al2O3 started to level off. More NH3(a) were desorbed from Al2O3 at higher discharge temperature. In term of H2 discharge step, the amount of NH3 produced by hydrogenation of plasma-excited N(a) chemisorbed on different catalysts was also ordered by Ru/L-Mg O>Ru/Al2O3>L-Mg O>Al2O3 > H-Mg O, which was consistent with catalytic evaluation. It illustrated that the process of hydrogenation of plasma-excited N(a) was the critical process on the ammonia synthesis in DBD plasma coupled with catalysis.