Controlled Synthesis, Characterization And Catalytic Properties Of Gold, Ceria And Their Composite Materials

Owing to their unique quantum size effects, macroscopical quantum tunneling effects, nanocatalysts demonstrate many specific properties. Noble metal, rare earth oxide and their composites are widely used in fuel cells, advanced ceramics, three-way catalyst, oxygen sensors, petrochemical industry and other important fields. The key scientific and technological issue of these catalysts is to further improve their activity, stability and utilization efficiency. In this thesis, Au, CeO2and their composites were selected and several novel structures and morphologies of them have been prepared via wet chemical method (hydrothermal, solvothermal) by selectively controlling the preparation conditions. Uniform CeO2hierarchical microstructures, Ce1-xZnxO2-δ solid solution nanodisks and five-fold-twinned Au nanoparticles have been successfully prepared. The as-obtained products were characterized by different measurement technologies. The formation mechanism and catalytical properties of the products were briefly discussed and summarized, which may provide synthetic approaches and theoretical foundation for the synthesis of other noble metal and rare earth oxide micro/nanomaterials.The main content of this thesis is as following:(1) Preparation and catalytic activity of highly uniform CeO2hierarchical microstructuresHighly uniform pancake-like CeOHCO3hierarchical microstructures have been successfully prepared by a simple gelatin-assisted mixed-solvothermal route. Ceria hierarchical microstructures with similar morphology were obtained after thermal treatment of the CeOHCO3hierarchical microstructures at700℃for4h. The CeOHCO3microstructures can be selectively obtained by varying the composition of solvent, concentration of gelatin and alkali source. The as-prepared products were characterized by X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electronic microscope (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and selected area electron diffraction (SAED). The possible formation process of the CeOHCO3microstructures was briefly discussed. Gold coated ceria nanostructures were also prepared and show excellent catalytic activity in the conversion of carbon monoxide, the T50and T90are at240℃and300℃, respectively. Compared with the conventional catalysts, their catalytic has been improved to some extent.(2) Uniform and porous Ce1-xZnxO2-δ solid solution nanodisks:preparation and their CO oxidation activityA series of Ce1-xZnxO2-δ solid solution catalysts were prepared by a surfactant-free coprecipitation method. The catalytic activity of ceria nanostructures was enhanced by increasing density of oxygen vacancy defects in these catalysts by doping. The morphology of the nanodisks can be facilely controlled by changing reaction parameters such as dosage of reagent, reaction time and temperature. In addition, Ce1-xZnxO2-δ catalyst supported with Au was also prepared, and its activity for CO oxidation was greatly enhanced, due to the acceleration of gas-phase oxygen activation and transferring via spillover. It turns out that the synthesized Ce1-xZnxO2-δ materials exhibit efficient activity for removing CO by catalytic oxidation, which may find promising applications in many fields.(3) Facile synthesis and characterization of concave decahedra enclosed by high-index facets and truncated decahedra with a large sizeTwo kinds of five-fold-twinned Au decahedra and dodecagonal plates have been successfully synthesized by changing the reaction conditions and their growth mechanism was discussed.40high-index{221} facets were observed in the concave decahedron. The truncated decahedral gold nanocrystals with a large size of250-350nm were obtained for the first time, which breaks the assumption that the truncated five-fold-twinned Au nanoparticles can only be obtained with the size below5nm. This work provides a facile way to synthesize concave decahedra, truncated decahedra and dodecagonal plates with controlled nanostructures.(4) Highly Branched Concave AuPd Bimetallic Nanocrystals with Excellent Electrocatalytic Activity and Highly Efficient SERSNanocrystals (NCs) of face-centered cubic (fcc) metals have no intrinsic driving force for growing into highly anisotropic nanostructures, making the solution-phase synthesis of metal branched multipods extremely challenging. In this paper, we report the synthesis of highly branched Au-Pd bimetallic nanostructures through seed-mediated co-reduction of Au and Pd precursors in an aqueous solution at room temperature. Interestingly, we observe the branches with concave structures, which have received much attention for generating high-index facets on their surface. A mechanism for the evolution of twinned, sharp-tipped surface protrusions has been proposed based on time-dependent experiments and transmission electron microscopic studies. The obtained nanostructures show relatively large surface areas and excellent electrocatalytic activity for the oxidation of ethanol. Meanwhile, we study the near-field enhancement properties of Au-Pd nanocrystals experimentally by surface-enhanced Raman scattering (SERS) using crystal violet as the probe molecules, exhibiting a highly sensitive SERS response.