Synthesis And Performance Invesitigation Of Nanoscaled Ce-Zr-O Oxygen Storage Materials

As an oxygen storage materials and a key part of three-way catalyst, ceria-zirconia solid solution has important effect on broadening the window of air/fuel ratio and stabilizing the performance of catalyst. Owing to the size effect, nanocrystal presents unique physical-chemical and catalytic characteristic, which is much different from the bulk one. The nanocrystal ceria-zirconia solid solution was prepared through two-phase method. The size-effect of nanocrystal CeO₂-ZrO₂ was investigated by comparing the OSC performance of nanocrystal CeO₂-ZrO₂ and the bulk one. The mesoporous spherical CeO₂-ZrO₂ nanocrystal and CeO₂-ZrO₂/Al2O3 complex compounds with core/shell structure was synthesized, which provides elementary research about preparation of anti-aggregation catalyst.The CeO₂-ZrO₂ nanocrystal was prepared by two-phase method. The effect of reaction parameter, such as amount of capping-agents, on shape of products and further the characteristic of two-phase method were investigated. The process of nucleation was longer than normal and always overlapped with the crystallization process in the two-phase approach. The formation of the Ce-rich clusters was faster than that of Zr-rich clusters, leading to the formation of Ce-rich NCs at a short reaction time. Prolonging the reaction time allows further incorporation of the Zr component into the NCs, forming homogeneous NCs with the composition controlled by the ratios of the precursors used. The weaker interaction between capping agents and precursor accelerates the hydrolysis of precursor, nucleation and growth of crystal, and increase the crystalline of product.The different OSC performance between nanocrystal CeO₂-ZrO₂ prepared by two-phase method and bulk one prepared by sol-gel method was systemically investigated. The emergence of tetragonal phase moves towards higher zirconia concentrations with decreasing particle size mainly due to increasing zirconia solubility in cubic ceria. The reduced crystal size affords the NCs with higher concentrations of Ce³⁺ ([Ce³⁺]/[Ce⁴⁺]=0.403) and oxygen vacancy, shorter diffusion length, and lower CO-CO₂-conversion activation energy （46.1 KJ/mol）. These unique size-enhancing properties lead to NC-derivate CZ with significantly higher OSC （0.393 mmol[O]/g at 400 oC）, faster oxygen storage/release kinetics, and faster CO conversion (0.021μmol CO₂·s^-1 at 400 oC). The mesoporous CeO₂-ZrO₂ nanocrystal was prepared by aerosol-assisted self-assembly process. The effect of acid and surfactant on structure and OSC performance of produced CeO₂-ZrO₂ nanocrystal was investigated. The weaker acid nature of organic acid slows down the nucleation and growth process, leads to formation of CeO₂-ZrO₂ with hollow structure or showing a vesicular mesophase. Using block copolymer F127 with longer hydrophilic PEO is in favor of increasing the BET surface area （104 m2/g） and pore volume （0.29 cm3/g） and further enhancing the OSC performance （0.551 mmol[O]/g at 500 oC）. The double amount of HCl results in poor shape integrity and OSC performance of the mesoporous CeO₂-ZrO₂.Base on the formation of emulsion and micro-emulsion, the CeO₂-ZrO₂/Al₂O₃ complex compounds with core/shell structure was synthesized and their OSC performance was further tested. The procedure of toluene/water system is much more complicated and the layer of Al2O3 is thicker （¹0 nm） than others. The CeO₂-ZrO₂/Al₂O₃ with thinnest layer （1-2 nm） of Al2O3 and best OSC performance （0.569 mmol[O]/g at 500 oC）was synthesized by THF/water system. It is convenient and fast to prepare CeO₂-ZrO₂/Al₂O₃ when the aerosol-assisted self-assembly was adopted. The thickness of Al2O3 layer is not homogenous according to the decreased amount of P123.