Reforming Of Methane With CO₂ And O₂ To Syngas Over Ni/SiO₂ Based Catalysts In A Fluidized Bed Reactor

The production of syngas from methane is one of the most promising utilization approaches of natural gas. Reforming of CH₄ with CO₂ and O₂ to produce synthesis gas is a green process and it has many advantages compared to partial oxidation of CH₄ and CO₂ reforming of CH₄. Firstly, by coupling the exothermic partial oxidation reaction with the endothermic reforming reaction, the methane-to-syngas conversion can be operated in a safer manner than the partial oxidation process. Secondly, by changing the feed composition, one can control the product ratio of H₂/CO according to the need of the postprocess. Thirdly, the combined reaction can decrease the carbon deposition significantly than the CO₂ reforming of CH₄ reaction. In a word, the combined reaction has the advantages of both partial oxidation of methane and CO₂ reforming of CH₄, and it overcomes the shortcomings of the both reactions. So it is expected that this process can be applied in industry in recent years. The Ni/SiO₂ based catalysts for the combined reaction in a fluidized bed reactor were focused on in this thesis. And some interesting results were achieved. The principal results are summarized as follows.The catalysts of Ni/SiO₂ prepared from nickel citrate as precursors (designated as ANiSC, the numbers of A% is represented as the nickel loading) performed excellent activity and high stability in the combined reaction at 700℃. It was found that the NiO species and nickel metals on ANiSC catalysts were much smaller than those on the catalysts of Ni/SiO₂ prepared from nickel nitrate as precursors (designated as ANiSN, the numbers of A% is represented as the nickel loading). It was learned by H₂-TPR that there were strong interactions between NiO and SiO₂ and form nickel silicate like species which were difficult to reduce on the ANiSC catalysts. The strong interaction between NiO and SiO₂ may be the main reason that results in high dispersion of Ni over ANiSC catalysts. Propably, the high catalytic activity and high stability of ANiSC catalysts were attributed to the small particles of nickel and the strong interaction between metal and support.The Ni/SiO₂ catalysts modified by Al₂O₃ (Ni-Al₂O₃/SiO₂) were prepared using citrate salts and nitrate salts as precursors, respectively. For the catalysts using nitrate salts as precursors (ANi_xAl_ySN, The numbers of A%, x/y are represented as the nickel loading and molar ratio of Ni/Al, respectively), the addition of Al₂O₃ can increase the catalysis activity and stability; For the catalysts using citrate salts as precursors (ANi_xAl_ySC, The numbers of A%, x/y are represented as the nickel loading and molar ratio of Ni/Al, respectively), however, the addition of Al₂O₃ decreased the catalytic activity and stability. Samples of the catalysts were investigated using H₂-TPR, XRD and UV-Raman techniques. From the characterization results, some conclusions could be deduced: For the catalysts of ANi_xAl_ySN, the addition of Al₂O₃ enhanced the interaction between the NiO and Al₂O₃ on the catalysts, so the reduction temperature of NiO shifted to high temperature. The reduction temperature of NiO increased and the crystallite size of Ni metal on catalysts after reduction became much smaller, while increasing the content of Al₂O₃. For the catalysts of ANi_xAl_ySC, the addition of Al₂O₃ weakened the interaction between the NiO and SiO2 on the catalysts, so the reduction temperature of NiO shifted to low temperature. The reduction temperature of NiO decreased and the crystallite size of Ni metal on catalysts after reduction became much bigger, while increasing the content of Al₂O₃. These may be the reasons why the addition of Al₂O₃ increased the catalysis activity and stability of ANi_xAl_ySN catalysts but decreased the catalysis activity and stability on ANi_xAl_ySC catalysts.