Ⅰ.Catalytic Combustion Of Methane On Mixed Oxides-Containing MgO Catalysts Ⅱ.Ab Initio Study On The Reaction Mechanism Of Methane Combustion

The main problem in catalytic combustion processes is to find suitable catalysts which have high stability and high activity. Magnesium oxide has a melting point of 2850℃. As a result of this, magnesium oxide can maintain a large surface area at high temperature compared to most other oxides and can thus be used as a catalyst support for high temperature applications. In this thesis, NiO/M_xO_y-MgO catalysts(M= Ce, Cu, Cr, La, Y, Mn, Ti) were systemically investigated for the first time. The main results of the research are as follows:1. A sol-gel method was employed to prepare NiO/M_xO_y-MgO (M= Ce, Cu, Cr, La, Y, Mn, Ti) catalysts for methane combustion. The effect of M on CH4 combustion activity was investigated. M improved the activity of NiO/MgO catalyst. NiO/M_xO_y-MgO (M= Cu, Mn) catalysts were found to have the highest catalytic activity, but their thermal stability was the worst. NiO/M_xO_y-MgO (M= La, Ce, Y) catalysts exhibited the intermediate activity, but their thermal stability was high. In the same time, BET, XRD and TEM results indicated that NiO/M_xO_y-MgO (M= Cu, Mn) catalysts were sintered by second evaluation, and NiO/M_xO_y-MgO (M= La, Ce, Y) catalysts possessed better ability to resist sintering than other catalysts. NiO/Y₂O₃-MgO catalyst had the best thermal stability.2. A series of NiO/(Y₂O₃)_x-(MgO)_1-2x catalysts for methane combustion were studied. There was optimum concentration of Y₂O₃ (x=0.075) in Y-modified NiO/MgO, at which the catalyst showed maximum methane combustion activity. NiO/(Y₂O₃)_0.075-(MgO)_0.85 catalyst calcined at 1000℃ for 5h exhibited high activity and good thermal stability.3. A series of M(=Ni, Co, Cu)/(CeO₂)_x-(MgO)_1-x catalysts for methane combustion were investigated. There was optimum concentration of CeO₂ (x=0.1) in Ce-modified NiO/MgO, at which the catalyst showed the maximum methane combustion activity and the best thermal stability. The different active component has a great effect on the activity and the thermal stability of catalyst. (CeO₂)_0.1-(MgO)_0.9-supported Cu catalyst has the highest activity, but it was sintered by second evaluation, due to a strong interaction between copper and the support. The activity of NiO/(CeO₂)_0.1-(MgO)_0.9 catalyst was similar to that of CuO/(CeO₂)_0.1 -(MgO)_0.9 catalyst and NiO/(CeO₂)_0.1-(MgO)_0.9 catalyst had the best thermal stability. Through second evaluation, the structure of NiO/(CeO₂)_0.1-(MgO)_0.9 catalyst calcined at 100℃ has no any change as compared with that of fresh catalyst, attaining stabilization. This catalyst has a specific surface area of 14.32 m²Â·g^-1 and high activity as well as good thermal stability. *...