导　　师： 唐绍裘

学科专业： H0502

授予学位： 博士

作　　者： ;

机构地区： 湖南大学

摘　　要： 本文采用酸催化溶胶-凝胶法制备了纳米TI02光催化剂，系统研究了热处理工艺对纳米TI02光催化活性的影响;考察了非金属元素包括氮、硫、纳米碳黑、活性炭等通过掺杂、改性或负载方式对纳米TI02的晶体结构、相变以及光催化活性的影响，并探讨了不同非金属对纳米TI02的作用机理。最后，比较了3种不同的稀土离子掺杂对纳米TI02的光催化活性的影响以及对不同染料分子的光催化降解特性，并探讨了掺杂机理。主要结论如下: 1.用盐酸催化溶胶-凝胶法制备了纳米TI02光催化剂。原料摩尔配比为钛酸丁酯:无水乙醇:水:盐酸=1:27:2.8:0.24时有利于获得较好的纳米TI02颗粒。用羟丙基纤维素(HPC)为表面活性剂，利用其较强的空间位阻作用阻止TI02纳米颗粒在制备过程及高温下热处理时团聚，从而获得了单分散的纳米TI02颗粒。通过XRD、 TG-DSC研究发现，将纳米TI02置于氮气中密闭热处理时，由于氮气分子对 TI02晶体的影响，使TI02在低温下(400℃)开始由锐钛矿相向金红石相转变，而在空气中热处理则要到650℃左右才发生锐钛矿相向金红石相转变。在氮气中可实现较低温热处理TI02即可达到较高光催化活性。XPS和DRS研究发现在氮气中进行热处理时由于微量的N原子进入到表面的TI02晶格中取代部分的O原子产生了氧缺陷，这些缺陷易成为光活性中心，导致吸收光子的数量增多，带隙变窄，对吸收光波长向可见光区方向移动，从而提高了可见光催化活性。FTIR分析发现随着热处理温度的升高，TI02表面的经基基团逐渐减少，从而导致光催化活性的降低。在空气和氮气中热处理过程中晶粒长大激活能分别为:15.99KJ/MO1和8.84 KJ/MOL。晶粒在高温热处理过程为扩散控制过程。 2.以硫脲为掺杂剂制备均分散硫掺杂纳米TI02光催化剂。XRD分析发现硫掺杂使TI02的晶粒尺寸减小，抑制了晶粒的增长，并使锐钛矿相向金红石相转变的温度提高到650℃以上。当硫脲与钛酸丁酯摩尔比为3.5，经500℃热处理后获得了粒径为12 NM的单分散的纳米颗粒。通过XRD, RAMAN、XPS和DRS等表征方法研究发现在高温热处理过程中由于微量的S4＋离子掺入纳米TI02的晶格中取代了部分TI4+离子，使TI02晶体结构发生形变，形成了亚稳定态的TI1-XSX02,并在带隙中形成了定域能级，从而提高了T102光催化剂对光的吸收强度并向可见光区迁移。 3.用酸催化溶胶-凝胶法制备碳黑改性纳米T102复合光催化剂。纳米碳黑改性使TI02在较低温度下发生相变;在400℃时热处理后的晶粒尺寸为7.3NM，表明纳米碳黑改性能抑制晶粒的增长，且量子尺寸效应明显。UV-VIS吸收发现碳黑改性纳米TI02催化剂的吸收带边发生红移。另外，由于碳黑是一种半导体材料，用它作为改性剂可以与纳米TI02形成复合光催化剂。碳黑在光催化剂中作为光敏剂对TI02进行敏化作用，从而进一步提高了纳米TI02在可见光区的催化活性。 4.对活性炭负载纳米TI02光催化剂分别在氮气和空气中进行相同的热处理制度后发现由锐钛矿相向金红石相的相变温度为前者低于后者。光催化降解实验表明:在N2气氛中经400℃热处理的活性炭负载TI02光催化剂的光催化活性达到了最高，且高于空气中热处理的光催化剂样品。这是由于该温度下活性炭的强吸附性与TI02的光活性的协同作用达到最佳，其对亚甲基蓝溶液的光催化去除能力最高为97.7﹪;而在空气中550℃锻烧后才使其光催化去除能力达到最佳为96.7﹪。利用活性炭强吸附性能将亚甲基兰分子吸附到催化剂表面和锐钦矿相纳米TI02自身的高催化活性，两者之间的协同作用是提高光催化效率的关键因素。 5.以钛酸丁酯为原料合成了DY203-TIO2, CE02- TI02和GD203-TI02复合光催化剂。由于部分稀土离子掺杂进入TI02晶格中而形成了钛取代位，导致TI02晶体结构畸变，并形成了晶体缺陷导致电荷不平衡。为了使电荷达到平衡，在催化剂的表面吸附一些OH-，在紫外光照射下OH-被氧化为OH-，而这些OH-能与表面的被吸附的物质发生氧化还原反应，从而抑制了光生载流子的复合。以甲基橙和亚甲基蓝为目标降解物通过紫外可见光光谱分析发现掺杂1.25﹪ GD203的光催化剂对甲基橙的降解效率最高，而掺1.25﹪OCEO2的光催化剂对亚甲基蓝具有最好的降解活性。因此，不同稀土掺杂的纳米TI02对不同的有机物具有选择性降解活性。 6.通过以上不同非金属元素和稀土元素掺杂纳米TI02的研究结果表明，由于掺杂或改性机理的不同，在一定条件下选用不同非金属元素或稀土元素掺杂或改性纳米TI02都是有利于提高催化剂的光催化活性。 Nano titanium dioxide, used as a photocatalyst, can degrade the trace organic compounds to non-toxic substances and can kill bacteria and virus, therefore, it is of great value in the practical applications. However, it suffered from the disadvantages that the nano titanium dioxide cannot show the excellent photocatalytic activity until irradiated with UV light and the utilization of the solar rays is too limited and ineffective, which hinders its widespread applications to a great extent. In order to improve the photocatalytic activity and make full use of the solar rays, the nano titanium dioxide must be modificated. With these in mind, the main purpose of the paper is to prepare nano TiO/_2 doped with several non-metal elements and rare-earth elements and improve the photocatalytic activity in visible light by means of controling the particle size of nano titanium dioxide and the phase transformation process, changing the crytal structure of titanium dioxide, along with suppressing the recombination of photogenerated electrons and holes. The nano titanium dioxide powders were synthesized by sol-gel method with the use of hydrochloric acid as the catalyst. The hydroxypropyl cellulose /(HPC/) was utilized as the surfactant for the first time. According to the experimental results, the optimum molar rate of the materials was shown as follows: Ti/(OC4H9/)4: C4H9OH: H2O: HCl = 1:27:2.8:0.24. HPC, used as the surfactant, provided strong steric stabilization in the sol-gel process, which can decrease the collision frequency between particles, baffle the growing of nano TiO/_2 particles and effectively undermine the further conglomeration of grains in the heat treated process. We studied the influence of heat treatment in different atmospheres /(air and nitrogen/) on the phases of titanium dioxide, the size of the particles as well as the photocatalytic activity. The results demonstrated that the transformation of titanium dioxide from anatase to rutile calcined in nitrogen took place at the temperature of 400 ℃,which is lower than that treated in air/(650 ℃/). The results of X-ray diffractometer /(XRD/),X-ray photoemission spectrum /(XPS/) and diffuse reflectance spectra /(DRS/) revealed that during the heat-treated process in nitrogen, the oxygen atoms of TiO/_2 were replaced by nitrogen atoms to form the compounds of TiO/_2-xNx, which caused the formation of oxygen defects. The defects can easily become the centers of photoactivity in TiO/_2, resulting in the increasing of the quantity of light absorbed, and the bandgap broadening. Therefore, the N doping caused the absorption edge of TiO2 to shift into the visible light region. Further photocatalytic experiments proved that TiO2 treated at lower temperature in nitrogen possessed higher photocatalytic activity than that treated in air. The activiation energy of nano-TiO2 calcined in air and nitrogen for crystal growth are 15.99 kJ·mol-1 and 8.84 kJ·mol-1, respectively. The growing process of TiO2 crystals is controlled by diffusion. S-doped uniform TiO2 photocatalysts were prepared using thiourea as the doping agent for the first time in China. The influence of sulfur doping on the crystal structure of nano TiO2, the energy level state and the surface state were investigated. The doping mechanism was also discussed. The results of photocatalytic degradation methylene blue demonstrated that the doped TiO2 exhibited the highest photocatalytic activity when the mole ratio of thiourea and tetrabutyltitanate/[Ti /(OC4H9/) 4/] is 3.5 and the doped TiO2 was calcined at 500℃ for 2 h. The results of crystal structure analysis showed that the growth of nano TiO2 crystal particles was suppressed effectively and the crystal grains were refined after sulfur doping. The transformation of TiO2 from anatase phases to rutile was also restrained and the temperature of transformaton is higher than 650℃. The results of X-ray photoelectron spectroscopy /(XPS/) showed that the trace of sulfur ions /(S4+/) substituted partially for the lattice titanium ions /(Ti4+/), which resulted in the localized crystal deformation of TiO2 and the bandgap between valence band and conduction band narrowed. The strong absorption of the S-doped TiO2 for visible light was observed from the results of diffusion reflectance spectra /(DRS/). The doping technique and the investigation on the mechanism proposed here were described for the first time in China. The TiO2 photocatalyst modified with carbon black was prepared. The results based on the X-ray diffraction /(XRD/) indicated that the transformation temperature from anatase to rutile was reduced to 400-500℃ . Carbon black modification suppressed the growth of nano titanium dioxide crystals. The size of TiO2 crytals was about 7.3 nm when calcined at 400℃ in air, resulting in the quantum size effect enhanced. Moreover, nano carbon black used as a photosensitizer can sensilize titanium dioxide to the much broader visible portion of the spectrum and can further improve the catalytic activity of TiO2 in visible light. The photocatalytic activity mechanism was proposed as follows: *2 22 2 2S ?h?γ → S ?T?iO ?→ TiO g? + S + ?O?→ S + + TiO +Og? Activated carbon, used as an excellent carrier, possesses strong adsorption ability. The organic molecules can be absorbed to the surrounding of catalysts and form a circumstance with relatively high concentration of organic molecules. The organic molecules further diffused to the surface of nano-titanium dioxide and were quickly absorbed on the surface of TiO2, which provided a good organic circumstance for improving the photocatalytic activity of TiO2. The research on the photocatalytic activity of activated carbon loaded nano TiO2 prepared in different atmospheres /(air and nitrogen/) was carried out. The results indicated that the phase transformation of TiO2 from anatase to rutile calicined in nitrogen is lower than that treated in air. The results of photocatalytic degradation of methylene blue brought out that the highest removal rate of the catalyst was obtained when calcined at 400℃ in N2 which is higher than that treated in air at the same temperature. However, the highest removal rate of the catalyst in air was obtained when calcined at the temperature of 550℃. The improvement in the photocatalytic activity of the catalysts derived from the synergistic mechanism between activated carbon with the high adsorption ability and nano TiO2 itself with the high catalytic activity. Three kinds of catalysts including Dy2O3-TiO2, CeO2-TiO2 and Gd2O3-TiO2 were prepared using tetrabutyltitanate/[Ti/(OC4H9/)4/] as the starting material. The photocatalytic activity of the catalysts was found to be improved which may be associated with the fact that the structure deformation of TiO2 took place and the crystal size of the TiO2 particles was restrained after the rare earth elements doped into the crystal lattice. Additionally, some rare earth dioxide was converd on the surface of nano-TiO2 particles, which resulted in the separation of electrons and holes. We found that the degradation rate on methylene blue is highest with the use of 1.25 wt /% CeO2-TiO2, and that the degradation rate on methyl orange, however, is highest by using 1.25 wt /% Gd2O3-TiO2. Hence, we concluded the photocatalyst doped with different rare earth elements showed a selective photocatalytic activity to different degradation agents.

关 键 词： 纳米材料 二氧化钛 掺杂改性 溶胶 凝胶法 光催化剂 废水处理 染料分子 光催化降解

分 类 号： [X703.5 O643.36 TB383]

领　　域： [环境科学与工程] [理学] [理学] [一般工业技术]