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随着世界经济的迅速发展，环境污染和能源短缺问题日益严峻，水体中的重金属离子和有机污染物对人类健康和生态系统造成了严重威胁，因此，人们致力于寻求高效节能的材料制备技术，开发新型的环境友好功能材料，为环境治理和能源利用提供新的技术支撑。纳米二氧化钛（TiO2）具有无毒、廉价和化学稳定性等优点，广泛应用于吸附和光催化去除水中的污染物。但是，纳米TiO2禁带宽度较宽、量子效率较低，在制备和应用过程中存在的团聚和难以回收等问题限制了其工业应用。因此，减少材料在应用中的损失，控制团聚，提高纳米TiO2的吸附能力和光催化活性是本领域研究的重点和热点。首先，针对纳米TiO2在制备过程中存在工艺复杂、耗时长、产率低和需高温高压等问题，以硫酸氧钛为前驱体，采用常压微波辅助强化技术制备介孔纳米TiO2催化剂，探讨晶型结构和形貌特征等对吸附和光催化性能的影响，证实常压微波辅助水解技术制备介孔纳米TiO2的可行性。其次，针对TiO2纳米颗粒在应用过程中存在易团聚、流失及难回收等问题，以天然高分子材料-纤维素为模板和载体，通过微波辅助水解法将纳米TiO2原位地固定在纤维素表面，得到了具有多级结构的TiO2-纤维素复合材料，研究其对重金属离子Pb2+的吸附性能及机理。在此基础上，为提高TiO2-纤维素复合材料的吸附能力和利用率，实现连续吸附过程，通过湿法成网技术制备了TiO2-纤维素复合膜，研究其对低浓度重金属离子Pb2+的动态吸附行为。进一步地，为提高TiO2-纤维素复合材料的催化活性，通过高温碳化和活化处理，得到了具有特定性质和结构的氮掺杂TiO2-多孔碳纤维材料，并研究其对亚甲基蓝、苯酚和Cr（VI）的吸附及光催化性能。本论文取得的主要结论如下：（1）在不添加模板条件下，以硫酸氧钛为前驱体，采用微波辅助均相水解法，在短时间内（<30 min）制备介孔结构的纳米TiO2，随后经750 oC煅烧2 h得到具有锐钛矿/金红石混合晶型的TiO2纳米颗粒。通过TiOSO4:H2SO4体积比和反应温度等调控TiO2纳米颗粒的微观形貌和结构，发现体系中的H2SO4不仅影响TiO2的水解反应动力学，而且控制其成核和结晶过程。在TiOSO4:H2SO4体积比为5:3，反应温度为90 oC时，得到具有高比表面积、粗糙结构的介孔纳米TiO2，其在可见光下对亚甲基蓝的光催化降解速率是商用P25的6倍，表明该材料对亚甲基蓝染料具有良好的光催化效果。（2）以天然高分子-纤维素为模板和载体，通过微波辅助原位水解法将纳米TiO2均匀地负载在纤维素表面，得到具有多级结构的TiO2-纤维素复合材料。在相同条件下，反应系统含纤维素模板时，TiOSO4水解反应速率快，且得到的TiO2纳米颗粒尺寸小，这是因为纤维素表面的羟基作为成核位点，原位调控纳米TiO2的成核、结晶和生长过程，通过氢键等相互作用将尺寸约100 nm的纳米TiO2固定在纤维素上。负载后复合材料保持纤维素的多级结构特性，且由于TiO2的负载，使其具有介孔结构和较高的比表面积，对水中重金属离子Pb2+的吸附速率快且吸附量大。在5 min内达到吸附平衡，最高吸附量为42.5 mg/g，较纤维素提高了75.9倍。Pb2+与TiO2之间存在化学作用力，即TiO2表面的羟基通过形成Pb-O键参与吸附过程，加强对Pb2+的吸附。吸附动力学符合拟二级动力学模型，热力学符合Freundlich等温吸附模型。（3）为提高TiO2-纤维素复合材料的利用率和吸附能力，通过湿法成网技术制备孔径分布均匀的TiO2-纤维素复合膜，研究该复合膜对水中的低浓度重金属离子Pb2+的动态吸附行为。对不同初始浓度、流速和膜厚条件下的吸附数据进行数学模型拟合，发现所有的数据均符合Adams-Bohart和Dose-Response模型。根据Bed depth service time（BDST）模型，发现厚度小于0.01 cm的复合膜即可有效防止穿透。通过计算发现，直径为4 cm，厚度为0.65 mm的复合膜可处理150 L，初始浓度为50 μg/L的含Pb2+废水，并使其达到饮用水标准，说明该复合膜对低浓度重金属离子Pb2+具有较高的去除率和利用率。在水中存在多种共存阳离子时，对Pb2+具有优异的吸附选择性，且经多次再生-吸附后，仍具有较高的吸附能力，说明T-CF-M复合膜对吸附过滤低浓度的重金属离子Pb2+具有一定的应用潜力。（4）为得到高活性的纳米TiO2光催化剂，通过将TiO2-纤维素材料进一步氮气碳化和二氧化碳活化处理，得到氮掺杂TiO2-多孔碳纤维复合材料。在此过程中，纤维素不仅作为载体固定TiO2纳米颗粒，同时作为碳源，在碳化和活化过程中，转变为具有多孔结构的碳纤维。在不额外添加有机氮源和还原剂的条件下，利用纤维素碳化热解过程产生的氢气和形成的碳作为还原剂，将Ti4+还原成Ti3+，并将氮掺入TiO2晶格，形成氮掺杂TiO2。该复合材料具有大比表面积和孔体积，能够快速地吸附水中的有机污染物-苯酚、阳离子染料-亚甲基蓝及重金属离子-Cr（VI），对亚甲基蓝的吸附量是商用活性炭粉末的2.1倍。此外，由于氮和Ti3+掺杂形成新的能级结构，降低其禁带宽度。在紫外可见光和可见光照射下，可将苯酚氧化分解为无机小分子，将Cr（VI）还原Cr（III）。在紫外可见光下对苯酚和Cr（VI）的去除速率分别是商用P25的9.2和8.8倍，说明该复合材料具有良好的环境修复潜力。;With the rapid development of the world economy, a series of severe environmental pollution and energy shortage problems emerge. For example, heavy metal and organic pollutants in water pose serious threats to the human health and ecological systems. Therefore, people are committed to seeking high efficiency and energy-saving material fabrication technologies, developing novel and environmentally friendly functional materials, which can provide new technical support for environmental governance and energy use. Nano titanium dioxide has been widely used in adsorption, photocatalysis, solar cells and so on, due to its unique physicochemical properties such as non-toxicity, low cost and chemical stability. However, the wide band gap of nano-titanium dioxide, low quantum efficiency, tendency of agglomeration and material loss during the preparation and application processes, limit its practical applications. Therefore, reducing material loss in use, mitigating particle agglomeration, increasing adsorption capacity and improving photocatalytic activity of nano-titanium dioxide, have become the major focuses of many research works.In this thesis, we first aim to solve the problems of easy particle agglomeration, high temperature and high pressure, long time and complicated procedures needed in the preparation process of nano-titanium dioxide. Microwave assisted strengthening technology was applied to prepare a series of titanium dioxide nanoparticles, where the inorganic reagent-titanyl sulfate was used as the precursor. The effects of crystal structure and morphology on adsorption and photocatalytic activity were investigated. Next, in order to reduce the loss of nanoparticles during application and minimize nanoparticle agglomeration, we used cellulose fibers as the template/substrate to grow and immobilize in situ the nano-titanium dioxide and obtain titanium dioxide-cellulose composite fibers. The composite was then explored for Pb2+ removal from water and the adsorption mechanism was studied. Subsequently, to improve the adsorption capacity and efficiency to achieve a continuous adsorption process, the titanium dioxide-cellulose composite filter media/membrane was prepared using wet-laid technology and then its use in flow-through adsorption of low-concentration Pb2+ was investigated. Furthermore, a titanium dioxide-porous carbon fiber material with specific structural properties was obtained by high-temperature carbonization and activation of the composite fibers for enhanced photocatalytic activity. The adsorption and photocatalytic property for hexavalent chromium, organic pollutants-phenol and methylene blue dyes were studied in detail.The main results achieved in this thesis are as follows:(1) Assisted by microwave irradiation, high surface area and mesoporous nano TiO2 photocatalysts were synthesized via fast hydrolysis (within 30 min) of inorganic titanyl sulfate. Followed by a post-annealing treatment at 700 oC, the nano TiO2 with mixed rutile and anatase crystallines were obtained. The morphology and structure of the TiO2 nanoparticles were controlled by the TiOSO4:H2SO4 volume ratio (pH) and reaction temperature. The H2SO4 added in the reaction system not only affected the hydrolysis kinetics, but also controlled the nucleation and crystallization processes. When the volume ratio of TiOSO4:H2SO4 was 5:3 and the reaction temperature was at 90 °C, mesoporous TiO2 nanoparticles with rough surface were obtained, and their photocatalytic degradation of methylene blue was evaluated under visible light. The results showed that the self-made TiO2 had a 6-fold enhancement in photocatalytic activity compared to commercial P25.(2) The cellulose fibers were used as templet to immobilize nano-TiO2 via in situ hydrolysis of titanium oxysulfate under microwave irradiation, and as a result, a hierarchically structured TiO2-cellulose composite was obtained. It was found that under the same experimental conditions, when cellulose was added to the reaction system, the reaction became faster and the size of the TiO2 nanoparticles became smaller, indicating that the hydroxyl groups on the surface of the cellulose served as the nucleation sites and regulated the crystallization of nano TiO2. As a result, TiO2 with a size of 100 nm was immobilized on cellulose surface by hydrogen bonds, which still retained the hierarchical structure of cellulose even after loaded with TiO2. The composite was characterred with mesoporous structure and high specific surface area, which led to rapid adsorption of Pb2+ from water with a maximum capacity of 42.5 mg/g. The adsorption fitted Freundlich isotherms and was pseudo-second order in kinetics. By XPS analysis, it was found that chemical interaction between Pb2+ and TiO2 occurred to form the Pb-O bond and an inner sphere complex that helped accelerate the adsorption. In addition, the adsorbent was easily regenerated for a number of times without significant reduction in its adsorption performances.(3) To improve the utilization efficiency and adsorption capacity of TiO2-cellulose composite, the material was wet-laid into a filter membrane with relatively uniform pore size districution for the removal of low-concentration Pb2+ in water through adsorptive filtration. The influences of initial concentration, flow rate and membrane thickness on breakthrough curves were studied. The adsorption behavior was found to be independent of the initial Pb2+ concentration and flow rate, but related to the thickess of the membrane. According to the Bed Depth Service Time (BDST) model, a filter membrane with a thickness of less than 0.01 cm would be sufficient to effectively prevent solute penetration. With a volume of merely 2.07 cm3, the membrane could treat 150.0 liters of water containing 50 μg/L of Pb2+ to the drinking water standard, indicating the high utilization and removal efficiency of the membrane. In addition, the membrane was also Pb2+ selective over co-existing cations, and could be easily regenerated and reused without obvious reduction in performance.(4) To improve the photocatalytic performance of TiO2-cellulose, where cellulose was used both as a template and a carbon source, hierarchically structured and porous N-doped TiO2-porous carbon fibers were synthesized by carbonizing the material at 800 °C and activating at 850 °C. In this case, cellulose not only acted as a substrate to bond TiO2 nanoparticles, but also functioned as a carbon source that was converted into carbon with a porous structure during carbonization and activation. More interestingly, it was discovered that without using an additional organic nitrogen source and a reducing agent, Ti4+ was reduced to Ti3+ and N was doped into TiO2 lattice to form N doped TiO2. This happened because the automatic generation of H2 and carbon that acted as reducing agents in the process of cellulose carbonization pyrolysis in the nitrogen atomsphere. The obtained carbon fibers with high specific surface area and large pore volume displayed rapid adsorption rate and high adsorption capacity of methylene blue, phenol and Cr (VI). In addition, due to the formation of a new energy level structure by N and Ti3+ doping, the composite material was endowed with a narrow band gap and effectively photo-catalyzed the oxidation of phenol and the reduction of Cr (VI) under UV-vis or visible light irradiation. The removal of phenol and Cr (VI) were 9.2 and 8.8 times than commercial P25 under UV-visible light, respectively, indicating the material’s great potential in environmental remediation.
|张锦菊. 微波辅助制备二氧化钛基功能复合材料及其在水处理中的应用[D]. 中国科学院大学,2019.|
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