The Structure-Activity Performance Relation Of CuO_x-CeO₂Oxide Composite

Recent developments in both nanoscience and material characterization techniques have extended the fundamental studies of heterogeneous catalysis from the traditional UHV studies of model catalysts based on single ctystals and single crystal thin films. The structure-catalytic performance relation of catalysts is one of the main scientific issues in heterogeneous catalysis. Many structural factors of a catalyst nanoparticle, including size, morphology, defect, and phase, influence its catalytic performance, making the unambiguous correlation between the structure and the catalytic perfoamance difficult. In this thesis, we have systematically studied the structure-catalytic performance relation of CuOx-CeO2catalysts. We have controlled their structures by doping and morphology-controlling strategies, and examined their catalytic performance in CO oxidation and CH4oxidation. The main findings are summarized as follow:1. Structure-catalytic performance relation of metal ion doped CeO2composites. Ce-Fe, Ce-Zr and Ce-Co oxide composites were synthesized by co-precipitation method. The cubic fluoride CeO2-liked solid solution could be formed, but the solubility of metal ions in CeO2lattice differs much:Ce-Zr solid solution can form regardless of the Zr concentration; Ce-Fe solid solution forms with Fe molar percentages not larger than30%; Ce-Co solid solution only forms with Co molar percentages not higher than5%. Metal ion doping enhances the catalytic performance of Cu2O. For the Ce-Fe solid solutions, the proportional relation between the catalytic activity in CO oxidation reaction and the density of oxygen vacancies proves that the formation of oxygen vacancies is one of the key elementary steps. The doping manner and location of Fe3+in solid solutions have been successfully identified with X-ray Absorption Fine Structure and the selectively chemisorption of methyl orange molecule. With the increasing of the Fe3+concentration, the doping manner of Fe3+varies from the substituting-site doping to the interstitial-site doping and from bulk to surface. For Ce-Zr solid solutions, the oxide lattice gets most seriously disordered when the Ce/Zr molar ratio is around1, giving the highest catalytic activity.2. Structure-catalytic performance relation of CuOx/CexFe1-xO2catalysts. CuOx/CexFe1-xO2have been synthesized using Ce-Fe solid solutions as supports. For CuOx/CexFe1-xO2catalysts with the same support but different Cu loadings, CuO is the main copper species; with the increase of the Cu loading, CuO species changes from the highly dispersed species to the bulk CuO nanoparticles. The highly dispersed CuO species is the active species in CO oxidation. For CuOx/CexFe1-xO2catalysts with the same Cu loading but different supports, the dispersion of CuO decreases with the increase of Fe3+concentration in CexFe1-xO2oxide solutions, therefore, the catalyst activity in CO oxidation decreases with the increase of Fe3+concentration.3. Morphology effect of CeO2support on CuO/CeO2catalysts. Three kinds of well-defined shaped CeO2nanocrystals have been synthesize by hydrothermal method:nanocube exposing (100) facet, nanorod exposing (110) and (100) facet, and naonpolyhedron exposing (111) and (100) facets, and employed as the supports to prepare CuO/CeO2catalysts by impregnation method. The oxygen vacancy density in CeO2nanocrystals varies with their morphology, in which CeO2nanorods are with the highest density of oxygen vacancies. The loading of CuO increases the oxygen vacancy density of the CeO2support. The morphology of CeO2support influences the dispersion of CuO, in which and CeO2nanocubes are the worst support. In the preferential CO oxidation reaction, the CO conversion seems to be determined by the amount of highly dispersed Cu species for catalysts with low Cu loadings, but by the oxygen vacancy density for catalysts with high Cu loadings.4. Morphology-dependent surface restructuring and catalytic performance of Cu2O nanocrystals. Cu2O naoncubes exposing (100) facet and nanooctahedra exposing (111) facet have been synthesized. In CO oxidation reaction, surface restructuring of both types of Cu2O nanocrystals occurrs, forming CuO thin film on their surfaces. The surface structure and catalytic performance of restructured CuO film are controlled by the surface structure of underlying Cu2O nanocrystals. CuO thin film on Cu2O nanooctahedra is much more active than that on Cu2O nanocubes, and CO oxidation on these two types of CuO thin films follows different reaction mechanisms. By optimizing the restructuring conditions, the catalytic performance of CuO thin film on Cu2O nanocubes can be optimized, demonstrating that controlled surface restructuring of catalysts nanoparticles can be utilized as a method to optimize their catalytic performance.5. Synthesis of novel CeO2-Cu2O nanostructures. Two methods have been developed to synthesize novel CeO2-Cu2O nanostructures starting from Cu2O nanocrystals. One method is template sacrifice method, in which cubic Cu2O nanocrystals and Ce4+was used as template and precursor, respectively. Ce(OH)x shell forms on the Cu2O surface and the Cu2O core shrinks simultaneously via the redox reaction. The cubic Ce(OH)x/Cu2O shell-core nanostructure can be eventually formed. When the Ce4+concentration is high, double-wall Ce(OH)x/Cu2O shell-core nanostructure can be obtained. Calcination of Ce(OH)x/Cu2O shell-core nanostructures can form corresponding CeO2/Cu2O nanostructures. The other method is deposition-precipitation method, in which Ce3+was used as precursor. Ce(OH)x was deposited on the surface of Cu2O nanooctahedron by controlling the pH value of solution. Whisker-liked Ce(OH)x nanostructure on Cu2O grows with increasing concentration of Ce3+and OH-. CeO2/Cu2O could be obtained by calcining the Ce(OH)x/Cu2O nanostructures.Above experimental results systematically demonstrate the influence of doping and morphology of CuOx/CeO2catalysts on their catalytic performance. Doping can enhance the lattice disorder of CeO2, enhancing the catalytic performance of CeO2either as the catalyst or as the support. The morphology of oxide nanocrystals determines the exposed crystal planes, i.e. the atomic surface composition and surface structure, thus strongly affecting their catalytic performance and their surface restructuring behaviors during heterogeneous catalytic reactions when employed as the catalyst, and their interaction with the supported components and the catalytic performance of supported catalysts when employed as the support.