【作者】 许士洪；

【导师】 上官文峰；

【作者基本信息】 上海交通大学 ， 环境工程， 2007， 博士

【摘要】 作为一种应用广泛的光催化剂,TiO₂以其无毒、催化活性高、氧化能力强、稳定性好而最为常用。在水处理领域,悬浆型光催化反应器因其高比表面积和良好的分散性而受到普遍的关注。然而,由于受到从水中回收纳米二氧化钛微粒的困扰,悬浆型光催化反应器仍然受到很大的限制。为了克服催化剂分离的困难,人们研究报道了将二氧化钛负载在玻璃珠、玻璃纤维、沸石等载体上的负载型光催化剂。然而,由于这些光催化剂载体较小的比表面积,大大降低了二氧化钛的负载量和光催化活性。另外,虽然TiO₂表现出了优异的光催化去除环境中污染物的能力,但是它是一种宽禁带的半导体材料,只能吸收太阳光中不足5%的紫外光,这大大限制了其在实际工程中的应用。因此,为了解决纳米二氧化钛微粒的分离回收难题和目前光催化过程中太阳能利用率低的问题,研究开发容易分离回收以及能够响应可见光提高太阳能利用率的光催化剂就成为当前光催化研究中的关键课题。纳米磁性颗粒因其具有巨大的比表面积和良好的分离回收特性,将其作为光催化剂的载体,有希望利用其优点来解决水中纳米二氧化钛微粒难以分离回收的困扰,使制备的复合TiO₂光催化剂既有粉状纳米TiO₂优良的光催化活性,同时通过外加磁场很容易实现催化剂的回收而具有负载型TiO₂光催化剂的特点。本文采用纳米磁性粒子和光催化剂纳米粒子复合的思路,制备出了在紫外光下具有催化活性的可磁分离的复合光催化剂,并在其基础上改变工艺条件制备出了在可见光下具有催化活性的可磁分离的复合光催化剂。同时,结合VSM、XRD、TEM、EDS、XPS、FT-IR、BET、DRS等实验手段,研究了复合光催化剂的结构与催化性能之间的关系。1.通过液相催化相转化的方法制得了分散性好、磁性强、化学性质和晶相稳定具有超顺磁性的NiFe₂O₄纳米磁性粒子。当初始反应物的浓度(Ni²⁺和Fe³⁺的总浓度)为0.6～1.5 mol·L^-1、体系的pH范围为8.5≤pH≤10.3、Fe²⁺离子催化剂的浓度与Fe³⁺离子浓度之比为0.02时,沸腾回流2～4 h,获得了磁性强的NiFe₂O₄纳米粒子。TEM和XRD的分析结果表明,该NiFe₂O₄纳米粒子为尖晶石结构,其粒径不超过5nm。微量Fe²⁺离子的存在对NiFe₂O₄纳米粒子的生成具有显著的催化作用,作者认为Fe²⁺离子的这种催化作用是通过溶解再结晶和固相转化的途径来实现的。2.通过化学沉积的方法对制备的NiFe₂O₄纳米磁性粒子进行SiO₂改性,制备了包覆二氧化硅膜的SiO₂/NiFe₂O₄（SN）纳米磁性粒子。然后将SN纳米磁性粒子和P-25纳米粒子复合制备出可磁分离的TiO₂/SiO₂/NiFe₂O₄（TSN）复合光催化剂,其在紫外光下对甲基橙溶液表现出了高的光催化活性。TEM和XRD的分析结果表明,SN纳米磁性粒子被P-25包裹形成了TiO₂壳。当SiO₂的含量为NiFe₂O₄质量的200%时,能通过化学沉积的方法在NiFe₂O₄纳米粒子的表面包覆一层较致密的SiO₂膜,其对复合光催化剂的磁性能和光催化性能的影响达到最佳。TSN复合光催化剂中,SiO₂膜中间层的引入极大地提高了复合光催化剂的光催化性能。SiO₂膜对光催化性能的改善是由于其阻止了光生电子和空穴向磁核NiFe₂O₄中迁移和复合。3.在通过化学沉积的方法制备的SN纳米磁性粒子上,负载具有可见光活性的TiO_2-xN_x纳米粒子,制备出可磁分离的TiO_<sub>2-xN_x/SiO₂/NiFe₂O₄（N-TSN）复合光催化剂,其能在可见光下（λ> 400 nm）降解甲基橙溶液。TEM和XRD的分析结果表明,SN纳米粒子黏附在TiO_<sub>2-xNx聚集体的表面形成N-TSN复合光催化剂。N-TSN复合光催化剂不仅比表面积较大、在可见光下具有光催化活性、重复使用光催化活性稳定,而且具有超顺磁性。4.在通过化学沉积的方法制备的SN纳米磁性粒子上,负载对可见光有较好吸收特性的Bi₁₂TiO₂0光催化剂纳米粒子,制备出具有可见光活性的可磁分离的Bi₁₂TiO₂0/SiO₂/NiFe₂O₄（BSN）复合光催化剂,其吸收边为450 nm左右。苯酚溶液的降解实验结果表明,BSN复合光催化剂在可见光下（λ> 400 nm）具有催化活性。TEM和XRD的分析结果表明,SN纳米粒子黏附在Bi₁₂TiO₂0聚集体的表面形成BSN复合光催化剂。5.在制备的NiFe₂O₄纳米磁性粒子基础上,首先通过反胶束的方法制备磁性SiO₂/NiFe₂O₄（m-SN）纳米球。然后通过化学沉积的方法在磁性m-SN纳米球的表面包覆一层TiO₂壳制备出蛋型结构的可磁分离的TiO₂/SiO₂/NiFe₂O₄（e-TSN）纳米球光催化剂,其在紫外光下对甲基橙溶液表现出了高的光催化活性。TEM和XRD的分析结果表明,NiFe₂O₄纳米粒子被完全包裹在单分散的二氧化硅纳米球内形成磁性m-SN纳米球,二氧化钛纳米粒子聚集体包覆在m-SN纳米球的表面形成不完美的二氧化钛壳层。当NiFe₂O₄的含量为SiO₂质量的15 wt%时,m-SN纳米球对e-TSN纳米球光催化剂的磁性能和光催化性能的影响达到最佳。e-TSN纳米球光催化剂循环使用几次后,其光催化性能仍然没有明显的降低。更多还原

【Abstract】 Semiconductor photocatalysts, mainly TiO₂, have attracted much attention in last decade because of their potential application in the removal of all kinds of organic and inorganic pollutants in air or water. In water and wastewater treatment field, a slurry type reactor is the most commonly applied method because of its high specific surface area and dispersion. However, the use of TiO₂ slurry reactor is still limited mainly due to difficult separation of TiO₂ particles from treated water. To overcome the problem, titania beads, TiO₂ based thin film, fiberglass loaded with titania, and encapsulated titania within a zeolite framework have been prepared and used in fixed bed. However, the activity of TiO₂ photocatalyst is reduced to a considerable extent in the application of these immobilizations because the effective surface area of photocatalysts decreases considerably. In addition, though titania shows excellent photocatalytic activity for removal of environmental pollutants under ultraviolet irradiation, it is a wide bandgap semiconductor （3.2 eV for anatase） and can only absorb about 5% of sunlight in the ultraviolet region, which greatly limits its practical applications. So, it is an urgent and important task to develop an easily separable titania photocatalyst with visible light activity.The nanosized magnetic particles can solve the difficulty of photocatalyst separation from the treated water by applying an external magnetic field due to its magnetism property. In this study, magnetically separable photocatalysts with UV and visible light activity have been prepared through the combination of the nanosized magnetic particles and nanosized photocatalyst particles. The morphology and structure of the samples have been characterized using analytical techniques of VSM, XRD, TEM, EDS, XPS, FT-IR, BET and DRS etc. The relationship between the microstructure and photocatalytic properties was investigated. Some conclusions have been made as follows:1. NiFe₂O₄ nanoparticles were prepared by liquid catalytic phase transformation method at low temperature. The prepared samples show the characteristics of excellent dispersion, high magnetic property, stable crystalline phase and the superparamagnetic nature. The magnetic property of the prepared samples is very strong, when the action conditions are 0.6～1.5 mol·L^-1 of total concentration, 8.5～10.3 of pH value, 0.02 of mole ratio for Fe²⁺/ Fe³⁺ and 2～4 h of boiling and refluxing time. The minute Fe²⁺ ions play the role of remarkable catalysis for the preparation of NiFe₂O₄ nanoparticles. The results of TEM and XRD testing indicate that the crystalline phase of prepared NiFe₂O₄ nanoparticles is spinel phase, and their size is less than 5 nm. The dissolution/reprecipitation and solid-state transformation mechanisms explain the reason why the minute Fe²⁺ ions play the role of remarkable catalysis.2. Silica-coated NiFe₂O₄ nanoparticles based on the prepared NiFe₂O₄ nanoparticles were prepared by a chemical precipitation method. A magnetically separable photocatalyst TiO₂/SiO₂/NiFe₂O₄ （TSN） with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic SiO₂/NiFe₂O₄ （SN） dispersion and P-25 titania were mixed, sonificated, refluxed, separated, dried, and calcined, showing high photocatalytic activity for the degradation of methyl orange in water under UV irradiation. Transmission electron microscope （TEM） and X-ray diffractometer （XRD） were used to characterize the structure of photocatalyst TSN, indicating that the magnetic SN particle was compactly enveloped by P-25 titania, and TiO₂ shell was formed. The effect of a thin SiO₂ layer between NiFe₂O₄ and TiO₂ shell on the magnetic property and photocatalytic activity of photocatalyst TSN is least when the weight ratio of SiO₂/NiFe₂O₄ is 2:1. In photocatalyst TSN, a thin SiO₂ layer between NiFe₂O₄ and TiO₂ shell prevented effectively the leakage of charges from TiO₂ particles to NiFe₂O₄, which gave rise to the increase in photocatalytic activity.3. Silica-coated NiFe₂O₄ nanoparticles based on the prepared NiFe₂O₄ nanoparticles were prepared by a chemical precipitation method. A magnetically separable nitrogen-doped photocatalyst TiO_2-xN_x/SiO₂/NiFe₂O₄ （N-TSN） with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic SiO₂/NiFe₂O₄ （SN） dispersion and the visible-light-active photocatalyst TiO_2-xN_x were mixed, sonificated, dried, and calcined at 400°C, showing photocatalytic activity for the degradation of methyl orange in water under visible light irradiation （λ> 400 nm）. Transmission electron microscope （TEM） and X-ray diffractometer （XRD） were used to characterize the structure of photocatalyst N-TSN. The results indicated that the magnetic SN nanoparticles adhered to the surface of TiO_2-xN_x congeries. The prepared photocatalyst N-TSN show the characteristics of high specific surface area, photocatalytic activity under visible light irradiation, stable photocatalytic activity after several cycles and the superparamagnetic nature.4. Silica-coated NiFe₂O₄ nanoparticles based on the prepared NiFe₂O₄ nanoparticles were prepared by a chemical precipitation method. A magnetically separable photocatalyst Bi₁₂TiO₂₀/SiO₂/NiFe₂O₄ （BSN） with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic SiO₂/NiFe₂O₄ （SN） dispersion and the visible-light-active photocatalyst Bi₁₂TiO₂₀ prepared by a simple coprecipitation processing were mixed, sonificated, dried, and calcined at 550°C. The spectrum of UV-Vis absorption indicated that its absorption edge was at 450 nm. The prepared photocatalyst BSN showed photocatalytic activity for the degradation of phenol in water under visible light irradiation （λ> 400 nm）. Transmission electron microscope （TEM） and X-ray diffractometer （XRD） were used to characterize the structure of photocatalyst BSN. The results indicate that the magnetic SN nanoparticles adhere to the surface of Bi₁₂TiO₂0 congeries. The prepared photocatalyst BSN show the characteristics of photocatalytic activity under visible light irradiation and the superparamagnetic nature.5. Magnetic SiO₂/NiFe₂O₄ （m-SN） nanospheres based on the prepared NiFe₂O₄ nanoparticles were prepared by reverse micelle technique. A magnetically separable TiO₂/SiO₂/NiFe₂O₄ （e-TSN） photocatalyst nanosphere with egg-like structure was prepared by chemical precipitating TiO₂ onto the surface of m-SN nanospheres, showing high photocatalytic activity for the degradation of methyl orange in water under UV irradiation. Transmission electron microscope （TEM） and X-ray diffractometer （XRD） were used to characterize the structure of e-TSN photocatalyst nanospheres, indicating that nickel ferrite core nanoparticles were completely enveloped into monodisperse silica nanospheres as carrier and titania nanoparticles aggregates coated onto the surface of m-SN nanospheres to form a imperfect TiO₂ shell for photocatalysis. Magnetic m-SN nanospheres can be prepared by reverse micelle technique when the weight ratio of NiFe₂O₄/SiO₂ is 15 wt%, the effect of NiFe₂O₄ nanoparticles on the magnetic property and photocatalytic activity of e-TSN photocatalyst nanospheres being least. The photocatalytic activity of the recycled e-TSN photocatalyst nanospheres has no noticeable change after several cycles under UV irradiation.更多还原