Controllable Synthesis Of Ceria-based Micro/Nanocomposites And Their Catalytic Property

As a main component in environmental pollutants, the efficient removal of carbon monoxide (CO) has attracted more and more attentions due to their compliance with the principles of green chemistry. In recent decades, ceria (CeO2) based micro-and nanomaterials are highly desirable as the promosing catalysts in CO oxidation because of its better oxygen storage capacity, which is associated with the facile redox cycle between Ce3+and Ce4+. The rational design of CeO2-based catalysts, underpinned by the theories of structure-function relationships, has the potential to provide more active sites even enhanced performance. The research in this thesis focus on the modification of inner structures in order to develop the novel CeO2 based materials with controllable morphologies and superior catalytic activities, including hierarchical porous materials, multicomponent composites with the guest transition metal ions and noble metal species. The achievements are as follows:(1) The use of materials with larger surface area and uniform pore size is advanced to promote CO diffusion. The hierarchical mesoporous CeO2 structures with flowerlike morphology have been synthesized by a mild hydrothermal method. The flowerlike cerium oxalate precursor was first obtained at room temperature, followed by the hydrothermal formation of CeO2 structures by using the H2O2 as the oxidation agent. By means of various characterization techniques, the formation mechanism of final products and the influence of reaction parameters were investigated. It demonstrated that the morphology of cerium oxalate precursor was tuned by varying the amount of oxalic acid. In alkaline hydrothermal solutions, H2O2 with defined amount is believed to play a vital role in the further conversion from Ce2(C2O4)3 to CeO2 with original morphology. The as prepared flowerlike CeO2 products present larger surface area compared with that of CeO2 obtained by the calcination of Ce2(C2O4)3 (147.6 and 94.8 m2/g, respectively). Furthermore, it shows enhanced redox behavior and CO catalytic activity with T100 of 293℃, while the CeO2 obtained by calcination of Ce2(C2O4)3 only presents 40% conversion at the same temperature.(2) By moderating the Cu2+doping concenration, a series of Cu2+doped CeO2 nanospheres with different hollowness and size were fabricated in the fixed glycol-water solvothermal system. The solid nanospheres could be converted to core-shell, then to hollow structures with the increase of Cu/Ce molar ratio while their size becomes smaller. The sample (Cu/Ce molar ratio of 4.1%) could achieve the complete CO conversion at 215℃ which is two times higher than that of pure CeO2. The detailed investigation confirmed that the Cu2+ions not only increase the active sites in CO oxidation, but also act as the directing agent in the morphology formation which accelerate the nucleation rate and induce the following Ostwald ripening process.(3) A series of nickel-ceria nanospheres were prepared in the glycol-acetic acid solvothermal system through adjusting the volume of introduced Ni(NO3)2 solution. With the increase of Ni/Ce molar ratio, the small loose nanospheres were replaced by larger solid structures while their porous structures were also different. By comparison with the cobalt-or copper-ceria products, it can be demonstrated that the synergistic effect among various reaction parameters (e.g., the glycol, acetic acid, water and guest metal species) is very important to the formation of controllable morphologies. Furthermore, the obtained nickel-ceria structures show lower temperature with 100% CO conversion and superior thermal stability. The samples with Ni/Ce molar ratio of 6.83% could finish CO converison completely at 200℃. The XPS and H2-TPR results are sufficient to proof that the catalytic active species are the lattice defects, well dispersed NiO and Ni-[O]-Ce species at the interface.(4) Multicomponent composites were fabricated via the Cu2O assisted templates with controlling the shape, chemical composition and size. First, a novel synthesis of Cu2O cubes have been reported by the decomposition of Cu(OAc)2 in solvothermal system. Moreover, hollow CeO2-Cu2O (1), core-shell NiO@Cu2O (2) and hollow CeO2-NiO-Cu2O (3) structures were obtained when Cu2O cubes were subjected to ion exchange reactions with Ce (IV) and Ni (II) ions. The catalytic results show the composites 1-3 possess superior performance and lower activation energies barrier towards CO conversion compared with parent Cu2O cubes. The introduction of CeO2 species also guaranteed that the composites 1 and 3 exhibit good thermal stability against sintering.(5) The functionalization of Au nanoparticles on CeO2 surface was utilized to prepare Au/CeO2 composites. The hollow CeO2 nanospheres were obtained in glycol-HCl mixed system. Different from the conventional reports about acid etching, it has been demonstrated that both H+ and Cl" is vital in our synthesis of hollow CeO2 nanospheres as the structure directing agent. Moreover, The Au/CeO2 composites with modification of ～5nm Au nanoparticles could have a complete CO conversion at 170℃ which is lower than that in pure CeO2 of 310℃. The XPS and H2-TPR results displayed the occurrence of Au0、Auδ+and Ce3+species at the interface between Au nanoparticles and CeO2 which may act as active centers in CO oxidation.