Preparation And Catalytic Properties Of Supported Bimetallic Nitrides

Transitional metallic nitrides have attracted much attention as potential catalysts for hydrogenation reaction due to their similar properties as the transitional group VIE metals, however there are issues concerning with the preparation and catalytic performances of the metallic nitrides, which still are needed to explore. Comparison with the reactions of HDS, HDN and F-T synthesis, the study on the catalytic performances of transitional metallic nitrides for ammonia decomposition and synthesis has not been carried out, especially for the supported bimetallic nitrides. A great restriction for commercial catalyst NiO/MgO for the decomposition of ammonia has not been got through because of the drawbacks of short-life, poor tolerance to H2S and severe reaction conditions. In this thesis, the preparation of supported transitional bimetallic nitrides and the catalytic activity for ammonia decomposition were studied. Also, the catalytic activity of the supported bimetallic nitrides for ammonia synthesis was preliminary investigated.1) The effects of Co, Ni and Fe additives, synergic action between Fe, Co, Ni and Mo on the ammonia decomposition activity of transitional metallic nitrides were conducted, and the comparison study on the catalytic performances of nitrides and their corresponding precursor oxides were also investigated. The additions of Co, Ni and Fe on the catalytic activity of MoN/γ-Al2O3 have a significant influence. The ammonia decomposition activities reached the highest values when the atomic ratios were 0.8 for Co/Mo and Ni/ (Ni+Mo), 1.0 for Fe/Mo. These results might be interpreted with the decrease of nitrogen adsorption energy on Mo in the presence of Co, Ni and Fe. Also, Co (or Ni, Fe)-Mo binary sites might have high activity than Mo-Mo sites for ammonia decomposition. The results of XRD, TPR-MS and TG show that the oxides precursors have been nitrated in our experimental conditions. The nitration processes for CoxMOyN/γ-Al2O3 and NixMOyN/γ-Al2O3 are slower than that for FexMoyN/y-Al2O3. And the results drawn from the TG profiles show the passivation treatment has not resulted in the complete oxidation, but only a flimsy oxide layer is formed.2) The supports on the catalytic activity of bimetallic nitrides for ammonia decomposition have remarked influence. MgAl2O4 is the best support in all of supports for the ammonia decomposition of bimetallic nitrides.3) Preparation conditions of supported bimetallic nitrides have a significant influence on the activities of ammonia decomposition. The ammonia decomposition activity of bimetallic nitrides does not increase successively with the increase of the GHSV of NH3 during the nitration process, and there is a best value of the GHSV of ammonia. Furthermore, the catalytic activity reached the highest value with the heating rate of 2℃/min during nitration process. Based on the effect of the GHSV of NH3 in the nitration process or the heating rate on the specific surface area and the particle size, it can be suggested that ammonia decomposition on the supported bimetallic nitrides is a structure-sensitive reaction.4) Calcination temperature and time have an marked influence on the ammonia decomposition activity over supported bimetallic nitrides. The highest activity for ammonia decomposition are observed when the oxide were calcined at 600# for 5h.5) Ammonia synthesis over supported bimetallic nitrides was preliminary investigated. The results indicate that supported bimetallic nitrides are more active than monometallic nitrides and unsupported bimetallic nitrides for ammonia synthesis.