Synthesis And Catalytic Oxidation Performance Of CeO₂-based Catalysts

With the development of world’s economy, the environmental deterioration and the shortage of energy has become more and more serious and drawn considerable attention. As a clean fuel, methane has many advantages, such as its abundant reserves, low cost, etc. The catalytic combustion of methane is an efficient strategy, which exhibits high energy efficiency and releases more heat and less pollutant. And the key is the preparation of the high active and stabile catalysts.In this thesis, the abundant and low cost CeO2was employed as the catalyst materials for the catalytic combustion of methane. The methane combustion over CeO2-basedcatalysts was systematically investigated. By loading the transition metal oxides and adding alkaline earth metalson CeO2, the catalytic activity and stability of CeO2were remarkably improved for the catalytic combustion. Furthermore, a novel hydrothermal synthesis method was developed to prepare CeO2with high performance for CO oxidation and other rare earth oxide nano-materials. The main research results are achieved as follows.1. The influence of preparation methods and calcined temperatures on the catalytic activities of CeO2for methane combustion was investigated. The results show that CeO2(D) prepared by direct thermal decomposition exhibits the higher performance due to its smaller crystal size and better reduction property. The activity of CeO2(D) is increased greatly after supporting transition metals oxides MOx (M=Cu, Zn, Fe, Co and Ni). It has been found that the catalytic activities of MOx/CeO2catalysts with the optimum loading are decreased for methane combustion in the following order of10wt.%NiO/CeO2>10wt.%Co3O4/CeO3=7wt.%CuO/CeO2-5wt.%Fe2O3/CeO2>10wt.%ZnO/CeO2. T10, T50and T90of10wt.%NiO/CeO2catalyst is333℃,415℃and467℃, respectively. It is because those activities of surface oxygen and bulk oxygen in NiO/CeO2catalyst are higher than that of other MOx/Ce02catalysts.2. The interaction between NiO and CeO2in NiO/CeO2catalyst was studied in detail. The results show that, in the NiO/CeO2catalyst, Ni2+can incorporate into the lattice of CeO2, enhance the oxygen vacancy concentration, the activity and the amount of surface oxygen in CeO2and promote the mobility of bulk oxygen in CeO2and decrease the activation energy of methane combustion. By means of support CeO2NiO can be well dispersed on the surface to prevent the aggregation of NiO nanoparticles, and support CeO2can improve the reduction property of NiO because of the interaction between CeO2and NiO and provide much more oxygen species for methane combustion. After being used in the reaction stream at600℃for150h, the10wt.%NiO/CeO2catalyst can yet keep high activity, for instance,100%of methane can reach. After10wt.%NiO/CeO2catalyst was used in the reaction including3.1%water vapor content in stream, its catalytic performance is slightly descended, indicating that the catalyst has good water-resistant property.3. To improve the thermal stability of NiO/CeO2catalyst, the NiO/CeO2catalyst was modified with Ba (NiO-Ba/CeO2). When the Ba content is l wt.%, the NiO-Ba/CeO2-650(calcined at650℃) catalyst possesses the relatively high activity and stability, its T10, T50and T90is as low as346℃,431℃and505℃, respectively, which is25℃,21℃and26℃lower than those of NiO/CeO2-650catalyst. The characterization results show that, the addition of Ba can improve the activity of surface oxygen in NiO/CeO2catalyst to a certain extent, and then enhance the reduction property of the catalyst.4. A novel and facile hydrothermal approach has been developed to synthesize hierarchical3D flowerlike CeO2microspheres, in which two commercial surfactants were used and the synthesis condition was very mild. It is interesting that the prepared CeO2possesses multilevel pore structure and low bandgap energy. Prepared flowerlike CeO2microspheres show high catalytic performance for the photocatalytic degradation of methyl orange and CO oxidation. Other rare earth oxides (Tb4O7, Nd2O3and La2O3) and ZnO with hierarchical structure can be prepared by this mild strategy. The prepared La2O3and interesting luminescent property, and ZnO exhibits good photocatalytic activity.