【作者】 李霞章；

【导师】 陈志刚；

【作者基本信息】 江苏大学 ， 材料学， 2009， 博士

【摘要】 CeO₂是一类重要的稀土氧化物,在现代高科技领域有着巨大的发展潜力。纳米化后的CeO₂具有特殊的性质及应用,相应地在多孔结构的实现,及微观形貌的要求也越来越高。因此,纳米CeO₂的孔结构与形貌控制及其应用研究成为迫切需要解决的课题。本文研究了不同介孔结构、微观形貌的纳米CeO₂合成方法和过程机理,并探索了其在CO转化、染料废水降解的催化氧化性能。主要研究内容和成果概括如下:1.以柠檬酸为整合剂,CTAB为模版剂,利用改进的溶剂挥发诱导自组装（EISA）方法合成了高比表面积的介孔CeO₂。考察了Ce³⁺与CTAB的摩尔比以及煅烧温度对介孔CeO₂粉体比表面与孔径的影响,结果表明柠檬酸、CTAB与硝酸铈的摩尔比为1:1:1,煅烧温度为300℃下得到的介孔CeO₂的比表面积最大(205 m²·g^-1)、孔道相对规则,其催化活性最高。在介孔CeO₂的合成过程中发现随着煅烧温度升高,比表面积减小,介孔逐渐被破坏,相应地催化活性降低。2.以乙酸铈为无机源,P123（或F127）嵌段共聚物为模版剂,利用溶剂挥发诱导自组装的方法首次合成得到了孔壁晶粒高度定向排列的介孔介晶CeO₂。In-situTEM原位观测表明,在600℃至800℃高温加热状态下,孔壁完整性仍然得到了很好的维持,在于晶粒取向高度一致的条件下晶粒的无序长大受到了很好的抑制,从而避免了孔道的塌陷。当ZrO₂掺杂摩尔份数x小于等于0.3时,Ce_1-xZr_xO₂仍为固溶的单一立方相,且孔壁的定向排列未受影响,暴露晶面仍以{200}为主,当x超过0.3后,四方相开始析出,使得孔壁晶粒逐渐向无序取向转化,并最终成为立方相CeO₂与四方相ZrO₂的复合态。CO氧化探针反应表明,由于介孔介晶Ce_0.7Zr_0.3O₂维持了高活性面{200}.的暴露,且比表面积达到最大,显示了最低的T₅₀值,当掺杂量进一步升高后,T₅₀值也相应升高,归因于晶粒取向杂乱后的活性位减少与比表面积的降低。3.在水热条件下以硝酸铈和六次甲基四胺（HMT）为原料首次合成出了棱柱状形貌的介晶CeO₂,研究了反应浓度比、反应温度、溶剂类型对CeO₂形貌结构的影响。提出棱柱状形貌的介晶CeO₂的形成机制遵循相临晶界面一致原则,纳米晶粒沿着轴向定向聚集形成。相对于普通块状CeO₂粉体,介晶CeO₂粉体的紫外吸收特征峰发生了红移,其光子带隙E_g=3.02eV,小于块状CeO₂粉体光子带隙（E_g=3.19eV）,推测跟棱柱状形貌的介晶CeO₂存在的孪晶等缺陷有关。4.以自制碳球为模板,HMT为沉淀剂,通过层层自组装的方法制得了CeO₂/碳球复合材料,经煅烧处理后得到了CeO₂空心球,其中空直径约为250 nm,外壳厚度约为20 nm。研究了硝酸铈与碳球的质量比与煅烧温度对空心结构的影响。CeO₂空心球和无孔CeO₂标样对CO催化氧化比较后,发现CeO₂空心球由于具有良好的气体通透性与吸附性,催化活性得到明显提升。5.以嵌段共聚物P123为表面活性剂,水热合成了管径约为20nm左右,长度为500nm-1μm的CeO₂纳米管。研究了水热反应时间、反应温度、表面活性剂浓度对形貌演化的影响,研究表明,以六方晶系的Ce（OH）₃为晶种,纳米管的形成遵循“溶解-异向生长-自卷曲”机制。探索了不同形貌CeO₂催化氧化处理亚甲基蓝染料废水的降解效果,研究发现CeO₂纳米管相对于CeO₂纳米颗粒和CeO₂纳米棒,对染料的脱色率有明显提高,归因于其良好的中空通透结构对有机大分子的吸附,及其高活性面{220}的暴露引起的高氧化还原活性。6.首次以改性的天然凹凸棒石为模板成功合成了管径为20-40 nm的CeO₂纳米管。由于ATP可以通过热酸反应完全转化为外表面富含有羟基的活性SiO₂纳米棒,容易吸附Ce³⁺并与其发生置换,在加入HMT加热发生Ce³⁺/OH^-/Ce³⁺/OH^-…交替沉积的层层自组装反应。研究表明硝酸铈的加入量是合成可用前驱体的关键,硝酸铈与活性SiO₂纳米棒的质量比为3:1时包覆效果最好。脱除硬模版后,产物的煅烧温度也是一个关键因素,在500℃热处理下,晶面之间的晶面转角错位最小,晶粒熔接性最好。更多还原

【Abstract】 CeO₂ is an important type of rare earth material, and has widely applications in modern high-tech areas. The application of nanosized CeO₂ is largely depended on its micro-structure and morphologies. In this dissertation, we report the synthesis and characterization of mesostructured and morphology controlled ceria based nanomaterials, as well as their catalytic properties in CO oxidation and degradation of organic pollutants.1. Mesoporous CeO₂ particles with high surface area were synthesized via a modified evaporation-induced self assembly（EISA） method using citric acid as complexing agent, CTAB as surfactant respectively. The effects of Ce³⁺/CTAB molar ratio as well as calcination temperature on the surface area of mesoporous CeO₂ were investigated. It was found that the cerium oxide/surfactant mixture gave rise to pure and fluorite-structured CeO₂ after calcination at 300℃. Moreover, the textural analysis revealed high specific surface area (205 m²·g^-1) and mesoporous structure of the sample. The catalytic performances of mesoporous CeO₂ heated at various temperatures for CO oxidation were examined. The catalytic tests exhibited that the product had enhanced catalytic efficiency compared with the decomposed ceria.2. Novel mesoporous mesocrystal CeO₂ were synthesized using acetate salt as inorganic species and P123 as surfactant. Transmission electron microscopy revealed that the wall framework consists of a single phase based on the face-centered cubic CeO₂ and the polycrystalline crystals were highly oriented with the crystal axis [001] parallel to the pore channel if the Zr⁴⁺ molar fraction x was 0.3 or less, In-situ TEM demonstrated that the integrity of the framework could be maintained as high as 800℃on account that the random growth of the grains was suppressed. However, when the Zr⁴⁺ molar fraction was larger than 0.3, a mixture of cubic and tetragonal phases formed and the preferential crystal orientation disappeared as revealed by XRD and Raman measurements. It was proposed that the hydrophilic segments of P123 and the acetate group acted cooperatively and lead to the preferred crystals attachment facilitated by the strong dipole-dipole interactions following the manner of coherent interface. The N₂ sorption measurement suggested that the single phase solid solution at Zr⁴⁺ molar fraction 0.3 had the largest BET surface area and it consequently demonstrated the best catalytic performance for CO conversion due to the unique mesoporous mesocrystal structure with dominant exposure of highly active {200} planes and an enhanced redox property caused by adequate Zr⁴⁺ incorporation. 3.Prism-like mesocrystal CeO₂ was synthesized for the first time via hydrothermal method without any surfactant. The effect of the molar ratios of HMT to Ce（NO₃）₃·6H₂O , reaction temperature and solvent type on the morphology of the product was investigated. A plausible formation mechanism was put forward that the nanocrystals aggregated along with the epitaxial orientation following the manner of coherent interface .The UV-visible adsorption spectrum exhibited the red-shift phenomenon （E_g=3.02eV） compared with bulk CeO₂ particles （E_g=3.19eV） , presumably due to the existence of considerable defects in particular twin boundaries in this unique structure. 4. Hollow spherical CeO₂ were prepared via a layer-by-layer （LBL） method using self-made carbon spheres as sacrificial template, HMT as precipitant respectively. CO conversion was used as a catalytic test reaction. The obtained products exhibit well-defined hollow spherical structure with a diameter of ca. 250 nm as well as the thin shell about ca. 20 nm composed of various-oriented polycrystals. Catalytic test revealed that the activity of the hollow spherical products in CO oxidation were substantially higher than for a non-hollow sample, the hollow structure was believed to provide substantial space for the adsorption and desorption of gas molecules5. CeO₂ nanotubes were synthesized by a hydrothermal reaction assisted by P123. The effects of the reaction time, reaction temperature, and the surfactants concentration on the morphology evolution of the products were investigated. The formation of CeO₂ nanotube can be rationalized by the dissolution-anisotropic growth-rolling up mechanism. The experimental results showed that CeO₂ nanotube had enhanced efficient catalytic activity on degradation of methylene blue compared with nanoparticles and nanorods, which was ascribed to the tubular structure owning excellent adsorption capability with dye moleculars , as well as the dominant exposure of high-energy surface of {220}.6. CeO₂ nanotubes were synthesized facilely with a layer-by-layer deposition route templated by modified attapulgite for the first time. As a natural clay, attapulgite can be completely converted into amorphous SiO₂ nanorods with surface functionalized by-OH groups and acted as promising template. The obtained CeO₂ nanotubes had a uniform diameter ranging from 20-40 nm with a bundle-like structure. Under slow hydrolyzing of HMT, the oppositely-charged OH^- was released in the solution, and consequently Ce³⁺/OH^-/Ce³⁺/OH^-…deposited onto the carbon sphere following a layer-by-layer assembly. Appropriate calcination at 500℃was found to be crucial to solidify the framework on account of the well-fused neighboring crystals caused by the reduction of interfacial energy. This synthesis strategy by taking advantage of natural clay as hard template implies a simple and inexpensive way to prepare oxide nanotubes on a large scale for modern chemical synthesis.更多还原