| Cerium oxide, as one of the most important rare earth oxides, has great potential in catalysis, sensor technology and biomedical applications. Transition metal oxide catalytic oxidation usually involves oxygen exchange between the gas reactants and the lattice oxygen. In Mars van Krevelan reaction mechanism of catalytic reaction, oxygen vacancy, act as the reaction center, produced and eliminated with Ce4+(?)Ce3+ redox cycle. Generally, the oxidation process of Ce3+is much faster, but the reduction process of Ce4+is relatively slow. Therefore, enhance the reducibility of the Ce4+ plays an important role in catalytic reaction. It has been indicated that the reduction of ceria proposed to control the nature of the oxygen vacancy, since the rate-controlling steps, oxygen diffusion, depended on the type, size, and concentration of oxygen vacancies. In the photocatalytic reaction, oxygen vacancy can be used as trapping center of electrons or holes to capture the excited electrons or holes generated from the motivated with ultraviolet or visible light. Oxygen vacancy could also effectively inhibit the restructuring of the electronic-hole, thus consequently improve its catalytic activity. Therefore, the studies on oxygen vacancies in oxide materials played an important role in the preparation of catalysts with high catalytic activity.This dissertation includes the following two parts:1. Ascorbic acid-treatment effect of ceria nanorods on Au nanoparticle deposition and CO oxidation.In this part, deposition behavior of Au on CeO2 support was studied by mediating the content of oxygen vacancies using ascorbic acid (VC) treatment. Oxygen vacancies were introduced on CeO2 nanorods (NRs) by VC reduction, and Au nanoparticles (NPs) were loaded by deposition-precipitation (DP) method. The formation of oxygen vacancies and its effects on the Au NPs deposition were evaluated by catalytic CO oxidation. The reaction mechanism of Au anchoring was closely linked to the concentration of oxygen vacancies. VC treatment induced large number of oxygen vacancies on CeO2 NRs, resulting in highly increased reducibility of CeO2 and strong interaction between Au and CeO2. Consequently, Au3+cations were reduced directly with fast reduction rate instead of hydrolysis into hydroxychloro gold (III) complex [Au(OH)xCl4-x]- that was generally generated during the DP procedure. Such strong charge transfer interaction between the oxygen vacancies and Au3+leads to sintering of the reduced Au species to form Au NPs with bigger size and uneven distribution, and to decreased Ce3+/Ce4+ ratio with a decrement of surface oxygen atoms, as well as the reduction of Au3+ species to Au+, which are altogether connected to the catalysis activity loss.2. Preparation of yttrium (Y) doped CeO2 nanotubes implementation the increase of oxygen vacancy on catalysts.In this part, controlling the oxygen vacancy concentration in the ceria is achieved by Y-doping. With the Kirkendall effect and a sacrificial template method, nanohollow structure was obtained by HNO3 treatment from the Y-doped Ce(OH)C03, namely YxCe1-x(OH)CO3 (x=0,0.05,0.10,0.30,0.50). With x increasing from 0 to 0.50, nanohollow structure from nanotube to nanohollow spheres are obtained. All the as-obtained Y-doped CeO2 nanohollow structures demonstrate the typical cubic fluorite-type structure of CeO2 in the XRD pattern. However, the diffraction peaks shifted to higher 2θ region along with increasing the Y ions concentration, on account of the substitutional effects of Y3+. Meanwhile, the specific surface area increases with increasing the Y3+ content. The actual proportions of the different Y doping concentration were detected by HAADF. The highest doping content of Y was about 30%. Raman and XPS analysis demonstrate that the relative proportions of Ce3+/Ce4+ as well as the oxygen species, mostly the surface absorbed-OH, increased with increasing Y doping content, which may expected to have vital effects on catalysis. |
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