【作者】 李凡修；

【导师】 陆晓华；

【作者基本信息】 华中科技大学 ， 能源与环境工程， 2008， 博士

【摘要】 半导体多相光催化作为高级氧化技术之一正引起人们的日益重视。光催化反应过程中产生氧化性极强的羟基自由基(·OH),可将难降解有机污染物分解为二氧化碳和水,因此被应用于有机废水处理和空气净化研究中。在各种半导体光催化剂中,二氧化钛(TiO2)因其无毒、化学惰性、廉价、高效等优异特点,成为研究最多的光催化材料。TiO2较宽的禁带宽度和较低的量子效率是限制其实际应用的主要原因,因此高效率的TiO2光催化剂的研究仍是该领域的热点。改进制备方法是提高催化剂活性的途径之一,目前关于超声、微波辅助制备TiO2催化剂的研究较少,因此超声、微波辅助TiO2催化剂制备技术的研究,对于拓展纳米TiO2催化剂制备技术,促进该研究领域的发展具有实际意义。本文在总结和评述半导体光催化效率研究进展的基础上,针对目前纳米TiO2催化剂制备中存在的易团聚、分散性差的问题,采用超声、微波辅助法成功地制备了一系列纳米TiO2光催化剂,探讨了晶型结构对光催化活性的影响;研究了不同的制备条件对纳米TiO2的光催化活性的影响。在催化剂制备的基础上,研究了超声纳米TiO2光催化降解氯苯与活性艳红X-3B模拟废水的效果和机理。本文的主要研究工作如下:(1)以Ti(OC4H9)4为原料,采用超声辅助法制备了纳米TiO2,通过正交试验对制备过程中的主要因素:钛酸丁酯用量、水用量、乙醇用量、乙酸用量、超声反应时间等进行了优化,并以活性艳红X-3B模拟废水为处理对象,研究了纳米TiO2催化剂的光催化活性。实验表明,工艺条件对TiO2催化活性影响大小的顺序为:钛酸丁酯用量、乙酸用量、超声反应时间、水用量、乙醇用量。对TiO2催化剂的XRD和TEM表征表明,超声辅助制备的催化剂为单分散性颗粒,粒径为24.0nm,是以锐钛矿晶型为主的混晶结构。(2)采用超声辅助法在玻璃表面制备了均匀透明的Fe3+掺杂TiO2薄膜,并以对硝基氯苯模拟废水为对象进行了该薄膜的光催化性能研究。借助于XRD和SEM对Fe3+/TiO2薄膜进行了表征。样品经500℃煅烧2h后,6.0%Fe3+掺杂TiO2纳米薄膜是以锐钛矿为主晶型的混晶结构,平均粒径约为13.0nm,膜表面TiO2颗粒分布均匀,无开裂现象。实验结果表明,Fe3+离子掺杂可以明显改善TiO2薄膜的光催化活性,Fe3+/TiO2纳米薄膜降解对硝基氯苯模拟废水的降解率是未掺杂Fe3+的TiO2薄膜的2.1倍。(3)以Ti(SO4)2为原料,采用微波辅助水解法制备了纳米TiO2,探讨了Ti(SO4)2浓度、反应液pH值、煅烧温度等条件对TiO2粒径、晶型和光催化性能的影响。研究表明,经700℃煅烧处理的纳米TiO2为锐钛矿晶型,具有最高的光催化活性,光催化降解对硝基氯苯模拟废水,90min降解率为89.3%,是未经微波辅助处理TiO2催化剂降解率的1.3倍。硫酸根离子(SO42-)的存在一定程度上抑制了锐钛矿晶型向金红石晶型的转变,增加了TiO2催化剂的抗烧结性,所制备TiO2催化剂经800℃处理仍能保持单一锐钛矿晶型。(4)以Ti(SO4)2水溶液为前驱物,氨水为沉淀剂,采用微波辅助沉淀法制备了纳米TiO2催化剂。其最佳制备条件如下:硫酸钛浓度为0.2mol/L,反应终点的pH值为9.0,煅烧温度600℃。通过TEM、XRD对所制备TiO2催化剂进行了表征,结果表明,纳米TiO2光催化剂颗粒具有粒径小、分散性好、纯度高等特点。制备的纳米TiO2处理对硝基氯苯模拟废水,90min的降解率可达87.1%。以尿素沉淀剂为例,研究了表面活性剂十二烷基苯磺酸钠(SDBS)对纳米TiO2粒径的影响,结果表明,当SDBS质量分数为1.0%时,所制备TiO2催化剂粒径在20.8nm左右。在煅烧过程中,由于SDBS的包覆作用可有效地抑制晶粒的长大。(5)以Ti(SO4)2为原料,采用微波辅助反向沉淀法制备了晶粒尺寸为10.4nm的高活性Fe3+/TiO2催化剂,并运用XRD及TEM对其进行了表征。结果发现,Fe3+的掺杂抑制了TiO2粒径的长大,细化了晶粒,Fe3+掺入到TiO2的晶格中,引起了晶格的畸变和膨胀。掺杂Fe3+可显著地提高TiO2光催化活性,Fe3+最佳掺杂量为0.5%(wt)。以对硝基氯苯为模拟废水,研究了所制备催化剂TiO2的光催化活性及重复使用对光催化活性的影响,结果表明,紫外光照射90min对硝基氯苯降解率可达95.1%,催化剂重复使用4次后对硝基氯苯降解率仍能达到75.2%。掺杂Fe3+的TiO2光催化剂,其光催化性能的提高可归因于载流子复合率的降低。(6)有机污染物的超声降解和光催化降解均属于自由基历程的高级氧化技术,超声空化效应可以极大地改善TiO2催化剂表面的传质效果,因此二者耦合可能产生协同效应。以活性艳红X-3B为模拟废水,研究了催化剂用量、pH值、H2O2用量、光照距离、活性艳红X-3B浓度等对超声光催化降解的影响。以均匀实验设计方法确定了超声光催化降解浓度为50mg/L的活性艳红X-3B模拟废水的最佳条件。结果表明,TiO2用量为0.73g/L,H2O2投加量为0.33g/L,初始pH值为3.5时,活性艳红X-3B降解速度最快。超声光催化降解低浓度活性艳红X-3B的反应为一级反应,反应动力学行为符合Langmuir-Himshelwood方程,通过实验,采用初始浓度法确定了动力学方程中的反应速度常数k和Langmuir吸附平衡常数K。应用UV-Vis图谱及GC-MS分析,探讨了活性艳红X-3B的降解机理,据此推测了活性艳红X-3B的超声光催化降解可能的反应历程。(7)超声光催化降解氯苯模拟废水具有良好的效果,光催化与超声波之间存在声光协同作用。本研究考察了TiO2用量、氯苯的初始浓度、模拟废水pH值、H2O2用量等对超声光催化降解氯苯的影响。结果表明,pH值对超声光催化降解氯苯影响较小;H2O2用量、模拟废水初始浓度对氯苯降解影响较大。在TiO2用量为300mg/L、氯苯质量浓度为110.6mg/L、反应时间为90min的条件下,氯苯的降解率可达91.2%,在实验的基础上,探讨了超声光催化降解氯苯模拟废水的机理。(8)由于六氯苯(HCB)的水溶性较小,本研究采用使六氯苯预先吸附到催化剂表面的做法,研究了它在TiO2催化作用下的光降解。研究表明,HCB/TiO2量、Fe3+浓度、H2O2用量对六氯苯的光催化影响较大;TiO2表面覆盖度、体系pH值对六氯苯的光催化影响相对较小。在UV/TiO2/H2O2条件下,二氧化钛可有效地催化降解HCB,当表面覆盖度为4×10-5mol/g,H2O2用量16mmol/L时,室温条件下90min六氯苯脱氯率可达94.8%。更多还原

【Abstract】 Heterogeneous photocatalysis by semiconductor catalyst as one of the advanced oxidation processes (AOPs) is receiving increasing attention. Due to its high oxidation potential of the species, such as hydroxyl radials (·OH) generated in AOPs, refractory pollutants can be efficiently decomposed into H2O and CO2. Photocatalysis has been researched in the mineralization of organic pollutants in wastewater and air. Among various oxide semiconductor photocatalysts, titanium dioxide (TiO2), especially its anatase phase, is one of the most promising photocatalysts because of its high photocatalytic activity, high chemical stability, low cost and non-toxicity in the purification of air and wastewater. The broad band gap and low quantum efficiency limit its extensive use in environment purification, so the development of high efficiency TiO2 photocatalyst is still the hot-pot in this field.At present, modification of preparation methods is thought one of promising ways. However, less research had been reported in ultrasonic and microwave assisted synthesis of nano-TiO2 photocatalyst. So it will have far-reaching reality meaning to study above problems for application of nano-TiO2 photocatalyst.In this dissertation, the research progress about improving the quantum-efficiency of semiconductor photocatalyst has been summarized. Aiming at the problems in research and practical application of TiO2, such as easy agglomeration and bad dispersibility, a series of nanosized TiO2 photocatalysts was successfully prepared by ultrasonic and microwave assisted method. The effects of crystal phase and structure on activity were explored. On this basis, the sono-photocatalytic degradation efficiency and mechanism of chlorobenzene (CB) and reactive brilliant red X-3B simulated wastewater were studied. The detailed works are as follows:(1) The ultrasonic assisted preparation process of TiO2 photocatalyst was optimized using Ti(OC4H9)4 as precursor. Using the photodegradation rate of reactive brilliant red X-3B simulated wastewater as an indicator, the effects of the dosage of Ti(OC4H9)4, water, ethanol (C2H5OH), acetic acid (CH3COOH) and ultrasonic reaction time on the photocatalytic activity of TiO2 were investigated by using experimental methods of the L16(45) orthogonal design. The results of orthogonal design showed that the important sequence of the factor affecting TiO2 photocatalysis was dosage of Ti(OC4H9)4>dosage of acetic acid > ultrasonic reaction time >dosage of water>dosage of ethanol.The as-prepared TiO2 was characterized by XRD and TEM. The results showed that the TiO2 particles were of good single dispersion and had bi-phase contained anatase as the main phase and rutile phase, with about 24.0nm diameter.(2) The transparent Fe-doped nano-TiO2 films on the surface of glass were prepared by the ultrasonic assisted method. Photocatalytic activity of the films in p-nitrochlorobenzene (p-CNB) simulated wastewater was studied. The Fe-TiO2 films are characterized by XRD and SEM. The results indicated that the Fe3+ doped TiO2 film with an average diameter of 13.0nm contained anatase as the main phase and rutile phase after calcination at 500℃for 2h, and the films are uniform and have no cracking on the surface. The results showed that Fe-doping obviously influenced the photocatalytic activity, the mass fraction of 6.0% Fe2O3 for the Fe3+/doped films as catalyst can increase degradation rate of p-CNB by 2.1 times on comparison with that of undoped nano-TiO2.(3) TiO2 were prepared by microwave assisted hydrolysis of Ti (SO4)2. The effects of pH value of Ti(SO4)2 solution, calcination temperature and Ti(SO4)2 concentration on the photocatalyst particle size, chemical properties, crystal form and transition temperature from anatase to rutile were examined. The catalyst calcined at 700℃has optimal photocatalytic performance. The photocatalytic degradation percentages of p-CNB reached 89.3% at 90min. When the temperature increased to 800℃, only anatase phase presented. The presence of SO42- restrains the transformation of anatase to rutile, strengthens the ability against sintering.(4) Nano-TiO2 powders were prepared with titanium sulphate as initial materials by microwave assisted precipitation method. The optimum synthesis condition for nano-TiO2 were obtained by the experiment: the concentrations of Ti(SO4)2 was 0.2mol/L, the pH value of the reaction end point was 9.0, the calcination temperature was 600℃. The as- synthesized TiO2 was characterized by means of XRD and TEM. The results indicated that the TiO2 samples possessed small size, good dispersity, and had a high pure crystalline phase etc. Photocatalytic properties of nano-TiO2 were researched by degradating the p-CNB simulated wastewater. The results showed that degradation rate of p-CNB reached 87.1% in 90min. Takes urea as the instance, nano-TiO2 powders had been successfully prepared by microwave assisted hydrolyzation-deposition, in which titanium sulfate solution was used as the precursor, urea and sodium dodecylbenzene sulfonate (SDBS) were utilized as precipitator and surfactant respectively. The prepared TiO2 samples were mainly consisted of anatase TiO2 with a uniform size of 20.8nm through the analysis of XRD and SEM. When the content of surfactant is 1.0wt %, the size of nano-TiO2 can be controlled effectively because of the coated effect of SDBS.(5) Fe3+-doped TiO2 photocatalysts with good quality, which crystallite sizes were 10.4nm could be readily prepared by the method of microwave assisted converse precipitation and characterized by XRD and TEM. It is found that Fe3+ doping restrains crystal size increasing. Fe3+ ions enter into the crystal lattice of TiO2 to lead to matrix distortion and lattice expansion.TiO2 photocatalysts have been also evaluated by the photocatalytic degradation of p-CNB in simulated wastewater. The results showed that the photocatalytic activity of the Fe3+-doped TiO2 photocatalyst was much higher than that of undoped TiO2, and the optimum value of Fe3+ doped amount should be at 0.5wt %, the enhanced photocatalytic activity might be attributed to an increase in the charge separation efficiency. The experimental results of the photocatalytic degrading p-CNB indicated that the degradation rate of p-CNB was 95.1% by the UV rays irradiation in 90min, and the degradation rate of p-CNB was 75.2% on the photocatalyst used repeatedly 4 times.(6) Sonolysis and photocatalysis of organic pollutions are all of advanced oxidation technologies following the same free radical mechanism, on the other hand, ultrasonic cavitation can greatly improve the mass transfer between photocatalyst and liquid. Therefore, a synergetic effect may be expected by coupling these two technologies. The effects of dosage of catalyst, pH value, the distance from light resource to reactor, H2O2 dosage and initial reactive brilliant red X-3B concentration on sono-photocatalytic degradation were examined. The optimal conditions of the degradation of X-3B were determined by the uniform experiment design as follow: the mass concentration of TiO2 added is 0.73g/L, the initial pH value of the simulated wastewater is 3.5, and the dosage of H2O2 is 0.33g/L. Kinetic equation of sono-photocatalytic degradation of X-3B was established and verified. The results showed that the sono-photocatalytic degradation reactions of low-concentration X-3B was the first-order kinetics model, and the degradation reactions were in accordance with Langmuir-Hinshelwood equation well, and the reaction rate constant and adsorption equilibrium constant in the equation were determined by means of initial concentration method. In order to investigate the mineralization activity of X-3B under sono-photocatalytic system, UV-Vis and GC-MS analysis were used to investigate the degradation mechanism. It is proved by combining reference conclusions that and a possible sono-photocatalytic degradation mechanism was inferred.(7) The synergistic effects between ultrasound and photocatalytic degradation processes are significant. The sono-photocatalytic degradation of chlorobenzene (CB) simulated wastewater was investigated using the TiO2 prepared by microwave assisted method as photocatalyst. The effects of the dosage of TiO2, the initial concentration of CB, the dosage of H2O2 and the pH value of the simulated wastewater on the degradation of CB were tested. The results indicated that the CB underwent a fast degradation. In the process, the pH value of the simulated wastewater had little effect on the photodegradation rate of CB, while the dosage of H2O2 and the initial concentration of CB affected photodegradation rate greatly. The degradation rate of CB simulated wastewater is over 91.2% when the mass concentration of TiO2 is 300mg/L, CB concentration is 110.6 mg/L and the reaction time is 90min. Based on the results of the experiment. The mechanism of sono-photocatalytic degradation for chlorobenzene simulated wastewater was also discussed.(8) HCB has extremely low solubility in water. In this work, the photocatalytic degradation of HCB was examined with a preadsorption method for the first time. HCB was first preadsorbed on the TiO2 particles (HCB/TiO2) and then suspended in an aqueous solution. Almost all the HCB molecules were adsorbed on the surface of TiO2 during the whole reaction process due to its extremely low solubility in water and strong adsorption on TiO2. This is a good way to investigate the photodegradation of insoluble organic compounds. The influencing factors such as surface coverage of TiO2, dosage of HCB/TiO2, pH value, dosage of H2O2 and concentration of Fe3+ were studied. The pH value of the dispersion and the surface coverage had little effect on the photooxidation rate of HCB catalyzed by TiO2, while the dosage of HCB/TiO2, addition of Fe3+ and H2O2 affected the photocatalytic degradation rate significantly. The dechlorination rate of HCB surpassed 94.8% within 90min under the experimental conditions when surface coverage for TiO2 is 4×10-5 mol/g and the dosage of oxidant H2O2 is 16 mmol/L.更多还原