【作者】 刘鸿飞；

【导师】 季生福；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2013， 博士

【摘要】 磁性纳米催化剂作为一种集高催化活性、可磁性回收和重复使用于一身的新型纳米催化剂,在光催化、有机合成催化、生物催化等液相催化领域表现出传统催化剂无法比拟的优越性能。其中,以Fe3O4纳米颗粒为核,通过表面包覆不同的材料制备的磁性核/壳纳米复合材料因其优异的化学稳定性和表面性能、均匀的颗粒尺寸以及良好的液相分散性能成为一种优异的催化剂载体,被用来制备磁性纳米催化剂。本论文对以Fe3O4纳米颗粒为基础的磁性纳米催化剂进行了系统的研究。Fe3O4纳米微球的制备主要采用溶剂热法及其表面活性剂改性方法,然后,制备了介孔SiO2包覆和介孔铝氧化物包覆的磁性核/壳纳米微球，通过负载Pd纳米颗粒制备了Pd/γ-AlOOH@Fe3O4、Pb/γ-AlOOH@Fe3O4和Pd/γ-Al2O3@Fe3O4催化剂,通过包覆TiO2制备了TiO2/SiO2@Fe3O4光催化剂。采用X射线衍射仪(XRD)、N2吸脱附分析仪、透射电镜(TEM)、振动样品磁强计(VSM)和扫描电镜(SEM)等手段对磁性载体和催化剂的物化性能和结构进行了表征。分别考察了三种磁性Pd纳米催化剂对Heck反应的催化性能和重复使用性能,并考察了TiO2/SiO2@Fe3O4催化剂对有机染料废水的光催化性能和重复使用性能。主要研究工作如下：1、首次采用十六烷基三甲基溴化铵(CTAB)对溶剂热法合成Fe3O4进行改性。CTAB充分发挥了密封剂、定向团聚导向剂和分散剂的作用。当CTAB浓度为0.102mol/L,晶化时间为12h时,合成的Fe3O4微球具有优异的超顺磁性和单分散性,微球粒径分布约为230nm～250nm。同时,制备的Fe304微球不需要任何表面处理和预包覆,便可进行介孔Si02和γ-AlOOH的包覆。制备得到的SiO2@Fe3O4和γ-AlOOH@Fe3O4均具有清晰的核/壳结构和超顺磁性。其中,Si02壳层具有丰富的蠕虫状短程有序介孔,平均孔径约为2.1nm;γ-AlOOH壳层由片状γ-AlOOH构成,孔径分布约为2nm～5nm。在两种介孔材料的包覆过程中,CTAB分子为介孔壳层的沉积提供了异相成核中心,同时为介孔结构的形成提供了模板剂。2、通过超声波辅助法超快速(30min)制备了均匀单分散的meso-SiO2@Fe3O4微球，微球具有规整的核/壳结构和清晰的介孔SiO2壳层。制备过程中,CTAB浓度(CCTAB)、超声波强度(P)和超声时间(乃对微球的核/壳结构、分散性和介孔结构具有显著影响。在CCTAB=6.86mmol/L,P=150W和r=30min的最佳制备条件下,微球的BET比表面积和BJH孔体积分别高达468.6m2/g和0.35cm3/g。我们认为超声波的加速作用主要体现在三个方面：一方面加速了TEOS的水解-凝聚过程；另一方面加速了CTAB分子与水解前驱体之间的自组装过程；最后还可以加速介孔Si02在磁核表面的沉积。3、对于Pd/meso-SiO2@Fe3O4(E)催化剂,当Pd负载量为5.32wt%时,Pd纳米颗粒高分散地分布于载体的表面和壳层介孔孔道中,粒径分别为4nm～6nm和2nm～3nm。在溴苯与丙烯酸丁酯的Heck反应中,反应温度为120℃,Pdmol%为0.01mo1%时,反应12h溴苯转化率即可达到约100%,比文献报道的其他负载型Pd基Heck反应催化剂具有更高的催化活性。5.32%Pd/meso-SiO2@Fe3O4(E)催化剂具有优异的磁性回收和重复使用性能,重复使用8次后,溴苯的转化率依然高达92%以上,催化剂损失小于5%,催化剂中Pd含量达到5.15wt%。而且催化剂的核/壳结构保存完整,Pd纳米颗粒依然高分散地分布于载体上,粒径约为2nm～4nm。4、对于Pd/γ-AlOOH@Fe3O4和Pd/γ-Al2O3@Fe3O4两种催化剂。当Pd负载量为1.5wt%～6.0wt%时,前者中Pd纳米颗粒高分散地分布于片状γ-AlOOH上,平均粒径约为6nm～8nm,后者的Pd纳米颗粒存在明显的团聚。在溴苯与苯乙烯的Heck反应中,两种催化剂均表现出比其他负载型Pd基催化剂更高的催化活性。其中,4.5%Pd/γ-AlOOH@Fe3O4在120℃时完全催化转化溴苯仅为12h,同时,催化剂可重复使用8次,溴苯转化率保持在90%以上,催化剂回收率达到93%以上,并且催化剂结构和Pd负载量没有明显变化。5、对于核/壳结构TiO2/SiO2@Fe3O4磁性纳米光催化剂。当TiO2包覆量为50%,SiO2包覆量为6%时,TiO2层厚度约为8nm～10nm, BET比表面积和BJH孔体积分别达到495.3m2/g和0.54cm3/g。在光催化降解罗丹明B(RhB)时,50%TiO2/6%SiO2@Fe3O4催化剂在60min的光催化反应后,RhB的降解率可达到98.1%,重复使用8次后,光催化活性没有明显降低,而且催化剂的回收率可达90%以上。通过对原有催化剂的TiO2壳层进行优化,使其具有可见光响应。9%TiO2/6%SiO2@Fe3O4催化剂在60min的高压汞灯光照条件下,可完全降解亚甲基蓝和甲基橙,光催化活性远远高于P25,且重复使用8次后,催化剂的光催化活性无明显下降。更多还原

【Abstract】 Magnetic nano-catalysts, a kind of new nano-catalyst with favorable magnetic recovery and high catalytic activity, have wide application prospects.in liquid phase catalysis, such as photocatalysis, organic synthesis catalysis, and biocatalysis. Particularly, the magnetic core/shell nano-composites composed by a Fe3O4core and a specific shell have been regarded as one kind of desirable catalysts carrier to prepare magnetic nano-catalysts because of their good chemical stability and surface properties, uniform particle size, and favorable liquid dispersion performance.In this dissertation, the magnetic nano-catalysts based on Fe3O4nanoparticles were systematically researched. Fe3O4nanoparticles were prepared by solvothermal method and surfactant modified solvothermal method. Then, Magnetic core/shell microspheres were prepared by coating mesoporous SiO2and mesoporous aluminum oxides on Fe3O4nanoparticles. Finally, Pd/mmso-SiO2@Fe3O4, Pd/y-AlOOH@Fe3O4and Pd/y-Al2O3@Fe3O4catalysts were synthesized by supporting Pd nanoparticles, and TiO2/SiO2@Fe3O4photocatalysts were prepared by coating TiO2. All magnetic supporters and catalysts were characterized by XRD, N2adsorption-desorption, TEM, VSM, and SEM. The Heck reaction catalytic acivity and reusability of Pd/meso-SiO2@Fe3O4, Pd/γ-AlOOH@Fe3O4and Pd/γ-Al2O3@Fe3O4magnetic core/shell nano-catalysts were studied. Meanwhile, the catalytic performances of TiO2/SiO2@Fe3O4catalysts for photocatalytic degradation dyestuffs (RhB, methyl orange, and methylene blue) were evaluated. The main work of the dissertation was as follows:1. Cetyltrimethyl ammonium bromide (CTAB) was used, for the first time, as modifier to improve the solvothermal synthesis of Fe3O4microspheres. CTAB molecules played the roles of capping agent, crystal growth oriented agent, and dispersant. When the CTAB concentration was0.102mol/L and the solvothermal time was12h, obtained Fe3O4microspheres exhibited favorable superparamagnetism and monodispersion, and the particle size distribution was about230nm-250nm. Meanwhile, the Fe3O4microspheres without any other treatment or pre-coated could be coated by mesoporous SiO2and y-AlOOH. The obtained SiO2@Fe3O4and y-AlOOH@Fe3O4microspheres presented obvious core/shell structure and superparamagnetism. Thereinto, the SiO2shell showed abundant of worlike pores with2.1nm of average pore size, and the y-AlOOH shell with2:0nm-5.0nm of mesopores was composed of many near nanosheets. The CTAB molecules could serve as nucleation seeds for precipitation of mesoporous shells and act templates for the formation of mesoporous structure during the coating process.2. Monodispersed meso-SiO2@Fe3O4microspheres were rapidly synthesized (30min) by ultrasonic-assisted method, and the microspheres exhibited uniform core/shell structure with obvious mesoporous shell. During the preparation process, the morphology and dispersion of microspheres and mesoporous structure of silica shell were significantly influenced by initial concentration of CTAB (CCTAB),ultrasonic irradiation power (P) and ultrasonic irradiation time (T). Under the best preparation condition (CCTAB=6.86mmol/L, P=150W and T=30min), the BET surface area and BJH pore volume of microspheres were468.6m/g and0.35cm/g, respectively. The acceleration effects of ultrasonic were mainly manifested in three aspects: accelerating the hydrolysis-condensation process of TEOS, accelerating the coassembly of hydrolyzed precursors and templates, and accelerating the deposition of silica oligomers on the magnetic particles.3. For the Pd/meso-SiO2@Fe3O4(E) catalysts, Pd nanoparticles were loaded on the surface of meso-SiO2@Fe3O4(E) supporters and into the mesopores of SiO2shell. The sizes of Pd nanoparticles were4nm-6nm and2nm～3nm, respectively. In the Heck reaction of bromobenzene and butyl acry late, the conversion of bromobenzene over5.32%Pd/meso-SiO2@Fe3O4(E) catalyst reached about100%under condition of120℃of reaction temperature,0.01mol%of Pd mole content, and12h of reaction time. Compared with the other supported Pd catalysts reported in literatures, Pd/meso-SiO2@Fe3O4(E) catalysts exhibited higher catalytic performance. Furthermore,5.32%Pd/meso-SiO2@Fe3O4(E) catalyst presented excellent magnetic recycling and reusability. After recycled for8times, the conversion of bromobenzene was above92%, the loss of catalyst was below5%, and the Pd loading was about5.15wt%. The catalyst kept uniform core/shell structure and the Pd nanoparticles with2nm～4nm of particle size kept highly dispersed state.4. For the Pd/γ-AlOOH@Fe3O4and Pd/γ-Al2O3@Fe3O4catalysts, the monodispersed Pd nanoparticles with6nm-8nm of particle size were loaded on the γ-AlOOH sheets of Pd/γ-AlOOH@Fe3O4, but the Pd nanoparticles on the latter exhibited obvious aggregation. In the Heck reaction of bromobenzene and styrene, both kinds of catalysts showed higher catalytic activity than the other supported Pd catalysts reported in literatures. Thereinto, the conversion of bromobenzene over4.5%Pd/γ-AlOOH@Fe3O4catalyst reached about100%under condition of120℃of reaction temperature and12h of reaction time. After recycled for8times, the conversion of bromobenzene was above90%, the recovery of catalyst was above93%, and morphology and Pd loading of catalyst were kept well.5. For the50%TiO2/6%SiO2@Fe3O4magnetic core/shell photocatalysts, the thickness of the homogeneous anatase TiO2layer was about8nm-10nm, and the BET surface area and the BJH pore volume were495.3m2/g and0.54cm3/g, respectively. The50%TiO2/6%SiO2@Fe3O4microspheres exhibited the best photocatalytic performance. The conversion of RhB achieved up to98.1%after60min UV irradiation. After recycled for8times, it also maintained high degradation rate and catalyst recovery. In addition, the photocatalysts received visible-light reponse after structure optimization. Two kinds of dyes (methyl orange and methylene blue) were completely degradated by9%TiO2/6%SiO2@Fe3O4catalyst after60min irradiation of high-pressure mercury lamp. Meanwhile, the catalyst exhibited favorable recovery and reusability.更多还原