论文中英文摘要格式作者姓名:袁荃论文题目:稀土/锆基和稀土/铝基有序介孔结构的可控合成及性质研究作者简介:袁荃,女,1982年2月出生,2004年9月师从于北京大学严纯华教授,于2009年7月获博士学位。
中文摘要本文选择稀土/锆基复合氧化物和氧化铝体系作为主要研究体系。
前者具有丰富的催化和电学性质,而氧化铝是极为重要的催化剂载体材料。
利用高分子嵌段共聚物为模板辅助的溶胶凝胶方法,我们对这两大类化合物的有序介孔结构进行了可控合成,在合成制备的基础上,借助多种表征手段和测试方法,我们系统研究了材料的介孔结构、组成和性能的关系,探索了材料的实际与潜在应用价值。
(1) 稀土/锆基复合氧化物体系通过溶胶-凝胶法结合溶剂挥发诱导自组装的方法,在嵌段共聚高分子为软模板的条件下,制备得到了有序介孔结构的铈锆固溶体,合成过程中无外加酸碱,通过前驱体水解过程中的自我调节获得有序组装结构。
所得介孔铈锆固溶体具有长程有序的二维六方(p6mm)的介孔结构,铈锆比例可调,孔壁高度晶化,对于Ce0.2Zr0.8O2 (x = 0.8),确认具有稳定的四方相t,而对于Ce1-x Zr x O2 (x = 0.4-0.7),其相结构被指认为主要是亚稳的t"相结构。
氮气吸脱附分析表明产物具有较大的比表面积和较窄的孔径分布,介孔结构热稳定性能够达到600 ︒C。
以合成得到的有序介孔铈锆固溶体为载体,担载Pt催化剂纳米颗粒,获得了250 ︒C下CO100%氧化的催化活性。
将催化剂用于环己烯的加氢反应,发现很好的抑制了副反应环己烯脱氢反应的发生,加氢转化率高达91%。
在有序介孔铈锆固溶体可控合成研究的基础上,将这种一步加入多种金属盐前驱体且不引入任何外加酸碱的合成方法进一步扩展,成功得到了有序介孔结构的氧化钇稳定的氧化锆(meso-Y2O3-ZrO2)和氧化钪稳定的氧化锆(meso-Sc2O3-ZrO2)。
发现锆的前驱体ZrOCl2⋅8H2O的运用是获得有序介孔结构的关键,引入的Cl-能与有机-无机界面处的无机物种强烈的相互作用,在自组装过程中扮演着重要的角色。
合成得到的meso-Y2O3-ZrO2和meso-Sc2O3-ZrO2具有二维六方孔道结构,孔壁高度晶化,Y和Sc的含量可以在较大范围内进行调节。
稀土元素的掺杂使得获得的介孔材料稳定在四方相,并且介孔结构的热稳定性比纯的介孔氧化锆高出很多,700 ︒C处理后仍然保持有序介孔结构。
材料具有较高的比表面积和均一的孔道结构。
通过苯的加氢反应催化测试,证明系列稀土锆基复合氧化物有序介孔材料在催化领域有潜在的应用价值。
以Ti的醇盐为前驱体,制备合成了有序介孔TiO2-ZrO2,不同Ti/Zr比例,获得了丰富的相态结构。
分析发现样品具有大的比表面积和均一的孔径。
研究了得到的系列介孔TiO2-ZrO2作为光催化剂催化罗丹明(RhB)的降解活性,发现能获得比商用氧化钛粉末P25更好的降解效率,归结为介孔结构带来的大的比表面积提供了更多的接触面积和活性位点。
研究发现对于RhB的降解反应,介孔TiO2-ZrO2复合氧化物的晶化程度和比表面积起到双重作用。
考察了材料在光解水制氢反应中的催化活性,发现不同处理温度获得的材料均有制氢活性,分析认为制氢催化活性受到比表面积、孔隙率和晶化度三个因素的影响。
(2) 氧化铝体系利用简单的软模板法合成得到了系列有序介孔氧化铝,仔细考察了实验条件如前驱体、表面活性剂种类和比例、酸度调节剂的种类、反应温度、湿度、时间和脱除模板剂的方法等,得到了合成有序介孔氧化铝的最优化条件。
发现了具有配位能力的羧酸根离子对介孔结构起着至关重要的调控作用,一方面,羧酸根离子与有机-无机界面处的铝离子配位,阻止了溶液中Cl-对铝离子的配位干扰;另一方面,羟基酸还可以与嵌段共聚物的聚氧乙烯基团通过氢键和范德华力发生相互作用,将另一个配位对象铝离子固定在界面处。
通过体系中加入种类不同但具有类似结构的羟基羧酸,均合成得到了有序介孔氧化铝,验证了这种作用在合成中的优势。
合成得到的系列介孔氧化铝均具有高度有序的二维六方介孔结构,800 ︒C以上处理就可以得到孔壁晶化的有序介孔γ-Al2O3,有序孔道结构的热稳定性高达1000 ︒C。
系列介孔氧化铝均具有很高的比表面积和可调的孔道尺寸,表面具有大量的酸性位点。
考察对不同尺寸分子的加氢催化,证明了介孔孔道对反应物分子尺寸的选择性作用。
采用一步合成方法,对上述有序介孔氧化铝体系引入稀土元素,成功得到了系列稀土(Ce,La,Y)修饰的有序介孔氧化铝,这种新型有序介孔材料具有高的比表面积、高的热稳定性和规整的孔道结构。
考察了引入的稀土离子的种类、含量对介孔结构的影响,观察到了稀土元素在孔壁中的均匀分布状态。
考察了少量稀土修饰(8 mol% Ce)的有序介孔氧化铝作为载体担载贵金属对一氧化碳催化氧化的反应活性,发现催化活性与单纯的有序介孔氧化铝相比,有显著提高,归结为介孔结构里均匀分布的氧化铈对催化反应的促进作用。
关键词:稀土、锆基氧化物、氧化铝、介孔材料、溶胶-凝胶合成、溶剂蒸发诱导自组装、配位作用、催化、光催化Controlled Synthesis and Properties Studies of Rare Earth/Zr-based and Rare Earth/Al-based Oxides with Ordered MesostructuresQuan Y uanABSTRACTThis dissertation is devoted to the controlled synthesis and properties studies of rare earth functionalized Zr-based composites and alumina with ordered mesostructures. Using sol-gel synthetic method with block copolymers as templates, a series of novel mesoporous materials including ceria-ziconia solid solutions, yttria-stabilized zirconia, scandia-stabilized zirconia, titania-zirconia composites, alumina, and rare earth modified alumina have been achieved by elaborately controlling the synthetic parameters. The as-prepared mesostructures have been characterized by various measurements with respect to mesoporous structure, phase, texture, and surface state, so as to obtain a systematical understanding of the self-assembly process, catalytic, and photocatalytic properties from the viewpoint of both fundamental and technological interests. The revealment of the intrinsic correlation between structure, composition, and property for these mesoporous materials has demonstrated that they could serve as good candidates for such applications in catalysis and photocatalysis.The contents of this dissertation are described as follows:(1) Rare earth functionalized Zr-based composites with ordered mesostructuresHighly ordered mesoporous ceria-zirconia solid solutions (Ce1-x Zr x O2) are prepared via a novel direct method, which based on a sol-gel process combined with evaporation induced self-assembly in ethanol using block copolymer Pluronic P123 as template, ceric nitrate and zirconium oxide chloride as precursors. In the whole synthesis procedure, the hydrolysis-condensation behaviour and assembly process are self-adjusted with no additional acid or base. Under the optimized temperature and humidity conditions, a series of mesoporous ceria-zirconia with different Ce/Zr ratios can be obtained. The obtained solid solutions have long range ordered mesostructures with 2D hexagonal (p6mm) symmetry, crystalline walls, high surface areas, and thermal stability up to 600 ︒C. The mesoporous Ce1-x Zr x O2solid solutions have proved to be ideal catalyst supports, for 100% conversion occurs at 250 °C in CO oxidation employing Pt/meso-Ce0.5Zr0.5O2as catalyst. This catalyst is also employed for cyclohexene hydrogenation and exhibits 85 - 92% conversion over a wide temperature range (20 °C to 350 °C) combined with a few benzene (hydrogenation product) generated above 150 °C. These results suggest that such ordered mesoporous supports havepotential applications in many catalytic fields.Based on the successful synthesis of ordered mesoporous Ce1-x Zr x O2 solid solutions, yttria- and scandia-stabilized zirconia (meso-Y2O3-ZrO2and meso-Sc2O3-ZrO2) are prepared by the similar simple general method. The choice of ZrOCl2⋅8H2O as precursor is the key for getting highly ordered mesostructures. Cl-introduced strongly interacts with the organic species at the organic-inorganic interface, as a result, playing an important role in self-assembly process. The as-synthesized meso-Y2O3-ZrO2and meso-Sc2O3-ZrO2possess 2D hexagonal pore structures, highly crystalline walls, and tunable compositions. Not only does the doping of rare earth make the materials possess stable t phase, but it improves the mesostructures’ thermal stability up to 700 ︒C as well, which is much higher than pure ziconia mesostructures’. With the privi leges of high surface areas and uniform pore size distributions, these rare earth functionalized Zr-based mesoporous composites prove having potentials in catalytic field.With tetrabutyl titanate and zirconium oxide chloride as titanium and zirconium sources, a controlled synthesis of highly ordered mesoporous titania-zirconia nanocomposites with crystalline frameworks is demonstrated. To reveal the formation mechanism of ordered mesostructures, the Ti/Zr ratio, quantity of block copolymers, relative humidity of the surrounding atmosphere, and evaporation temperature have been investigated in detail. The nanocomposites possess ordered 2D hexagonal mesostructure, high surface area (up to 200 m2⋅g-1), large pore volume (0.1-0.3 cm3⋅g-1), and uniform pore size (3.5-4.1 nm). The titania-zirconia nanocomposites show good photocatalytic activities for photodegradation of Rhodamine B and hydrogen evolution in an aqueous suspension, which can be ascribed to the cooperative effect of the crystallinity and porosity of these materials.(2) O rdered mesoporous aluminasHighly ordered mesoporous aluminas with amorphous and/or crystalline γ-phase framework walls are synthesized by an easily accessible, reproducible, and high-throughput method via sol-gel route with block copolymers as the soft templates.Various experimental conditions, including precursor, surfactant, acid adjustor, temperature, humidity, reaction time, and calcinations temperature were investigated. A controlled coordination effect of hydro--carboxylic acid in mesostructure formation is discovered. The carboxylic ions provided by hydro-carboxylic acid serve as inhibitors for hydrolysis-condensation process of aluminum species through coordinating with one or two aluminum sites on very small clusters of AlOOH, becoming competitors against chloride ions, both of which coordinate with aluminum ions through the whole evaporation process. Meanwhile, hydro--carboxylic acids can interact with the block copolymers, and then protect the aluminum ions at the organic-inorganic interface from being affected by chloride ions. With this strategy, a series of ordered mesoporous aluminas with 2D hexagonal structure are readily obtained possessing a high thermal stability up to 1000 °C, high surface areas, tunable pore sizes, and large amount of surface Lewis acid sites. Acting as catalyst support, the as-synthesized ordered mesoporous alumina showsshape selectivity in the hydrogenation of the acetone, D-glucose, and D-(+)-cellobiose, indicating that the reaction rates are apparently related to the effects of steric hindrance, imposed by the molecular sizes of the reactants relative to the pore sizes of the alumina supports.Introduced by rare earth elements such as Ce, La, and Y, rare earth modified alumina with ordered mesoporous structures are fabricated. The effects of rare earth on mesostructure, thermal stability, and surface properties are investigated. In addition, excellent activities using catalysts based on these materials have primarily been explored and ascribe to the existence of rare earth in mesostructures.Key words: Rare earth, Zr-based oxide, Alumina, Mesoporous material, Sol-gelsynthesis, Evaporation induced self-assembly, Coordination effect, Catalysis,Photocatalysis。