【作者】 张钺伟；

【导师】 沈美庆；

【作者基本信息】 天津大学 ， 化学工艺， 2011， 博士

【摘要】 铈锆固溶体储氧材料是三效催化剂的重要组成部分,对拓宽催化剂的空燃比窗口,稳定催化性能有着重要的作用。纳米催化材料由于尺寸效应表现出有别于传统材料的物化性能及催化活性,受到广泛关注。本文采用两相法制备纳米铈锆固溶体,并通过与传统铈锆进行储放氧性能比较,明确了纳米材料特有的尺寸效应在铈锆固溶体上的体现;制备了介孔纳米铈锆球体和具有壳核结构的铈锆/氧化铝复合氧化物,为制备具有抗高温烧结性能的催化剂材料做了基础研究。系统研究了两相法制备纳米铈锆固溶体的成核、生长机理,发现铈前驱体的水解速率、晶体的成核及生长速率更快,在反应初始阶段首先得到富铈相纳米团簇,而后锆逐渐进入纳米晶体中,并最终形成均一的铈锆固溶体;与常规铈锆固溶体比较,纳米级Ce0.6Zr0.4O₂样品的[Ce³⁺]/[Ce⁴⁺]比值由0.233提高至0.403,400℃时样品OSC由0.237 mmol[O]/g提高至0.393 mmol[O]/g,反应速率由0.016μmol CO₂·s^-1提高至0.021μmol CO₂·s^-1,反应活化能由61.2 KJ/mol降低至46.1 KJ/mol。采用气溶胶辅助自组装方法制备了介孔纳米铈锆固溶体,考察了酸、表面活性剂等参数对铈锆固溶体结构及性能的影响。结果表明:随着有机酸酸性变弱,晶体的成核、生长速率减慢,可以得到具有中空结构和层状有序排列结构的产物;采用亲水端更长的F127为表面活性剂有利于提高材料的比表面积和孔体积,分别为104 m2/g和0.29 cm3/g,且样品具有更高的储放氧能力,新鲜样品500℃时的OSC值为0.551 mmol[O]/g,老化样品700℃时的OSC为0.851 mmol[O]/g;HCl用量提高后,材料形状的完整性下降,老化处理后结构容易被破坏,导致样品储放氧性能大幅度下降。以形成乳液或微乳液为实验基础,制备具有壳核模型的铈锆/氧化铝复合氧化物,并对样品进行了储放氧性能测试。结果表明:甲苯/水反应体系工艺较为繁琐,得到的样品氧化铝层较厚,储放氧性能较差;采用四氢呋喃/水反应体系得到的样品氧化铝厚度为1～2nm,氧化铝层厚度最薄,制备工艺最为简单,新鲜样品500℃时的OSC为0.569 mmol[O]/g,老化样品700℃的OSC为0.390 mmol[O]/g,在同系列样品中具有最高的储放氧性能;采用气溶胶辅助自组装方法可以快速、连贯地制备具有壳核结构的铈锆/氧化铝复合氧化物,但表面活性剂减少后,氧化铝层的厚度不均。更多还原

【Abstract】 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.更多还原