【作者】 邹建；

【导师】 高家诚；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2009， 博士

【摘要】 纳米TiO₂由于具有较高的光催化活性,化学性能稳定,无毒等优点,成为了最受人们关注的光催化材料之一。但纳米TiO₂具有较宽的禁带宽度,只能吸收太阳光中5%左右的紫外线,而太阳光中近45%的可见光在处理污染物中却得不到有效利用,因此太阳能的利用率较低,这就限制了其实际使用范围;另外,TiO₂半导体载流子的复合率较高,因此量子效率较低。光催化剂作为光催化技术的核心部分,是决定光催化技术能否实际应用的关键。因此,如何提高TiO₂的光催化效率以及拓展TiO₂吸收光谱到可见光区是光催化研究领域最具挑战性的两大课题。为了拓展纳米TiO₂吸收光谱范围,金属离子掺杂、非金属离子掺杂、半导体复合以及染料敏化等手段被采用,并也取得了一定的成效。H₂O₂作为一种有效的电子捕获剂,在提高催化剂的光催化活性中得到了成功的应用。而本文用H₂O₂为敏化剂,对自制的不同体系纳米TiO₂进行敏化,成功地实现了催化剂的可见光化,并用现代表征技术对催化剂结构、性能等进行了系统表征。用H₂O₂敏化自制的纳米TiO₂实现了催化剂的可见光化,其对可见光的吸收值可达550nm;在可见光区,金红石敏化后具有更强的吸收,但锐钛矿对可见光的吸收更稳定。H₂O₂敏化后,样品表面的吸附水和表面羟基减少,同时样品表现出拉曼增强效应。敏化样品对底物的降解都具有选择性,在可见光激发下对甲基橙降解很差,但能够有效降解亚甲基蓝。混晶样品具有最高的光催化活性,金红石光催化活性最差,这主要是因为混晶样品在可见激发下所产生的羟基自由基最多,而金红石样品能够生成的羟基自由基最少。IR,Raman和XPS结果表明,酸化处理后样品表面有大量的硫酸根物种存在。TiO₂粉末的制备方法不同硫酸根物种的存在方式和吸附量也不同。硫酸根物种的存在,尤其是活性硫酸根物种,将在催化剂表面形成大量的表面酸位。这些表面酸位有利于吸附底物,样品用H₂O₂敏化后能够形成更多的过氧配合物,因此在可见光区具有更强的吸收,并且这些配合物在可见光照射下能够稳定存在。升高煅烧温度和延伸煅烧时间可以引起硫酸根物种的分解,这将降低催化剂对H₂O₂的吸附,从而降低催化剂对可见光的吸收。硫酸酸化处理后,H₂O₂敏化不会降低催化剂表面吸附水和表面羟基;一步法制备的SO₄^2-/TiO₂敏化后,由于形成了四面体配位钛离子,表面吸附水和表面羟基还有增加的趋势。H₂O₂敏化的各种SO₄^2-/TiO₂催化剂都能够有效降解甲基橙,比纯TiO₂具有更高的光降解能力,而一步法制备的SO₄^2-/TiO₂还能有效降解亚甲基蓝。硫酸根物种在催化剂表面的配位方式影响催化剂的光催化活性,螯合配位方式更有利于光催化降解。活性硫酸根物种的存在对样品的光催化活性起到关键作用。B酸位可被配合物基态的空穴氧化生成羟基自由基,从而使样品具有高的光催化活性,这个空穴转移过程也是表面羟基配合物能够稳定存在的根本所在。直接超声分散Ti（OH）₄沉淀可以制备中性TiO₂溶胶,水热处理后溶胶的结晶度得到提高。IR和XPS表明水热处理后溶胶表面吸附水增加而表面羟基减少。大量的吸附水将阻隔溶胶粒子与外界的接触,因此,H₂O₂敏化后,水热处理溶胶表面形成的过氧配合物减少,溶胶的可见光吸收减弱,并且对亚甲基蓝的吸附也降低。水热处理溶胶具有更高的紫外光催化活性,但H₂O₂敏化后,水热处理溶胶的紫外和可见光催化活性都比水热前低。这主要是由于大量的吸附水直接影响了水热溶胶体系中羟基自由基的生成。通过在高分散的纳米SiO₂表面负载纳米TiO₂获得了高分散的纳米TiO₂催化剂。SiO₂与TiO₂以Ti-O-Si键结合。负载的TiO₂为无定型,煅烧和改变负载方法可以到结晶的纳米TiO₂。TiO₂的负载量将影响催化剂的分散性和敏化后的可见光光吸收强度;TiO₂量低于30%,催化剂分散性很好,而高于30%,催化剂出现团聚。随着TiO₂量的增加,H₂O₂敏化后催化剂的可见光吸收强度也增加,但TiO₂含量达到20%后,TiO₂对可见光吸收影响趋势减弱;SiO₂的存在可以增加过氧配合物在可见光照射下的稳定性。酸化处理将提高样品的可见光吸收。TiO₂含量也将影响样品光催化活性,当TiO₂含量达到20%～30%时,样品光催化活性最好。而酸化和热处理都将降低甲基橙的褪色率。但酸化处理样品可以使甲基橙完全降解,而未酸化处理样品在甲基橙褪色过程中有新的小分子化合物生成。TiO₂/SiO₂具有的高光催化活性源于H₂O₂与TiO₂/SiO₂界面处的Ti离子形成的配合物。更多还原

【Abstract】 Nanosize TiO₂ particles are used widely as one of the photocatalysts due to their chemical stability, non-toxicity and high activity, but its high activity can be acquired only under ultraviolet light with a wavelength of 400 nm or less at room temperature due to its broad band gap. Since ultraviolet （UV） light is only 3-5% part of the solar spectrum, the photocatalytic activity of TiO₂ can not be sufficiently activated under solar light irradiation, which strongly limits the use of solar spectra as a source for photodecomposition of pollutant. In addition, the recombination for large numbers of charge carriers would occur in the volume or on the surface of TiO₂. Therefore, the extension of the photoactive wavelength region of TiO₂ into the visible region and improvement of quantum efficiency are desirable for popularizing more TiO₂ photocatalyst, especially under solar light for industrial areas or poor interior lighting illumination in living spaces. For this purpose, pure TiO₂ has been modified by various ways such as impurity doping, inorganic compound and dye sensitization to obtain visible light reactivity. Hydrogen peroxide often was used widely as an electron acceptor in photocatalytic degradation reaction. However, in this paper, H₂O₂ was used as a sensitizer to modify the TiO₂, and the attempt succeeded in extending the optical absorption edge of TiO₂ into the visible region.TiO₂ nanoparticles prepared by hydrolysis of TiCl4 were sensitized with H₂O₂, resulting in absorbing visible light up to 550 nm. Compared with anatase, the stronger visible photoabsorption could be observed for rutile nanopaiticles treated with H₂O₂, but the visible irradiation faded the yellow rutile more easily. The adsorbed water and surface hydroxyl group on TiO₂ nanoparticles treated with H₂O₂ would decrease. The dyes could be degraded selectively. Methylene blue （MB） could be degraded efficiently for all used samples, but a poor activity for decompositions of methyl orange （MO）.The mixture phase of anatase and rutile showed the highest photoactivity, and the photoactivity for rutile was most low, due to that more hydroxyl radicals were detected in the suspension of mixture phase of anatase and rutile.The results from XPS, Raman and IR indicated that an amount of SO₄^2- species anchored on the surface of sulfated TiO₂, which resulted in a large number of Br?nsted and Lewis acidic sites on the surface of TiO₂, especially for active sulfate species. More surface chemisorptions centers for some reactants can be facilitated due to surface acidic sites, and these chemisorptions centers also serve hydrogen peroxide. So, the sulfated TiO₂ sensitized with H₂O₂ can adsorb more hydrogen peroxide to form more peroxo-titanium complexes, resulting in more intensive Vis absorption, and the acidic sites could also stabilize the peroxo-titanium complexes. Calcinations at high temperature would lead to the decomposition of a large amount of active sulfate species, which decreased Vis absorption for the used samples treated with H₂O₂.Sensitizing with H₂O₂ to sulfated TiO₂ hardly reduced the adsorbed water and surface hydroxyl group, and more surface hydroxyl groups were observed for sulfated TiO₂ from one-step hydrolysis of boiling TiCl4 solutions due to formation of Ti ions in tetrahedral coordination.All sulfated TiO₂ could degrade efficiently MO, and the sulfated TiO₂ from one-step hydrolysis also could degrade efficiently MB. Photoactivity differed from sulfate species with different coordination to the surface of TiO₂. Higher Vis photoactivity occurred to sulfated TiO₂ with cheating bidentate sulfate species. The active sulfate species were mostly responsible for high Vis photoactivity due to formation of B acidic sites. The photoexcited holes could directly react with B acidic sites on the catalyst surface to produce hydroxyl radicals proven to be powerful oxidants in degrading organics, which would stabilize peroxo-titanium complexes.TiO₂ sol could be prepared via directly sonicating the Ti（OH）4 precipitate. Sol with better crystallization was acquired by hydrothermal treatment. The results from IR and XPS indicated that hydrothermal treatment could increase the absorbed water but reduce surface hydroxyl groups. A large numbers of adsorbed water isolate sol particles from reaction surroundings, so hydrothermal sol sensitized with H₂O₂ could absorb less visible light and adsorb also less MB. The hydrothermal sol had more high photoactivity under ultraviolet radiation; After sensitizing with H₂O₂, UV and Vis photoactivity both was reduced for hydrothermal sol, because the adsorbed water would restrain generation of hydroxyl radicals.Highly dispersive nano TiO₂ particles could obtained by loading TiO₂ particles on the surface of the dispersive nanosize SiO₂ particles by deposition method.TiO₂ loaded on the surface of SiO₂ by Ti-O-Si bond.TiO₂ loaded on the surface of SiO₂ were amorphous, but the crystal TiO₂ were got by calcining and changing the way of loading TiO₂.The catalyst could disperse well if the content of TiO₂ was less 30% in the catalyst; And the content of TiO₂ exceeded 30%, resulting in agglomeration of catalyst. With the increase of TiO₂, the Vis absorption for catalyst treated with H₂O₂ increased.After the content of TiO₂ exceeded 20%, the increase for the Vis absorption become milder. An amount of SiO₂ also could stabilize the peroxo-titanium complexes on the surface of catalyst. The sulfated catalyst could absorb more visible light. The catalyst had highest Vis photoactivity when the content of TiO₂ was about 20-30%. The sulfation and thermal treatment both decreased photoactivity of catalyst. However, sulfated catalyst could degrade MO completely, and other catalyst just discolored MO, due to the generation of small organic molecules. The peroxo-titanium complexes between TiO₂ and SiO₂ particles resulted in high Vis photoactivity for TiO₂/SiO₂ composite catalyst treated with H₂O₂.更多还原