The Preparing And Performances Of Transition Metal Mixed Oxides Derived From Layered Double Hydroxide Compounds For Catalytic Combustion Of Methane

Methane, as a typical greenhouse effect gas, is estimated to have a GWP (global warming potential) value around25times higher compared to carbon dioxide. Low concentration methane could be hardly utilized as energy due to its lower calorific value, while its emissions into the atmosphere would cause greenhouse effect. As for the abatement of methane emissions, the catalytic total oxidation attracts lots of attentions recently due to its lower operating temperature and lower energy cost. The improvements of the catalytic activity, thermal stability and economical efficiency are the key points of its real application. Layered double hydroxides (LDHs) derived catalysts, for its well performance in surface area, thermal stability and phase dispersion, also became a promising material in the area of catalytic oxidation materials.In this thesis, several transition metals like Mn, Co etc. were incorporated into LDHs to prepare a series of catalysts for methane catalytic oxidation combustion, and then their catalytic behaviors have been discussed via several characterization processes.First, Mg6MnxAl2-xLDO (x=0.5,1.0,1.5,2.0) catalysts have been prepared via the calcination of Mg-Al-LDHs with replacing Al3+by Mn3+through co-precipitation deposition method at a constant pH value. Then, their catalytic activities of methane combustion were investigated. It was observed that the catalytic activity increased with the rising of manganese content. When all alumina was substituted with manganese, the optimal activity was obtained that T90approached to530℃. After a series of characterizations, it was obtained that the main phase of that catalyst was spinel phase and the surface area would decrease with the increase in manganese content. However, the redox capacity of catalysts increased greatly with the addition of Mn, which could be the main reason of the improvement of activity. Furthermore, Co and Zn ions were introduced into LDHs by replacing Mg ion, the resulting LDO catalysts were evaluated regarding their activities of methane catalytic oxidation. Co substitution was proved to be the optimal one. After that, based on the above-mentioned findings, we prepared a series of CoxMg6-xMn2LDO (x=1.5,3.0,4.5,6.0) catalyst with both Co and Mn substitution (Co replaced Mg and Mn replaced Al). The best activity was obtained for the Co4.5Mg1.5Mn2LDO with methane’s90%conversion at ca.485℃. Though the addition of Co would increase the redox ability of the catalyst, a certain amount of magnesium was necessary for high activity. Magnesium oxide would be beneficial to the dispersion of the cobalt and manganese mixed oxides. Meanwhile, the existence of Mg could reduce the surface area lose after calcination and increase the surface active oxygen content. Moreover, the magnesium might also accelerate the C-H break of the methane molecule on catalysts’surface.