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Synthesis, spectroscopy and characterization of titanium dioxide based photocatalysts for the degradative oxidation of organic pollutants

机译：用于降解有机污染物的二氧化钛基光催化剂的合成，光谱学和表征

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摘要

Heterogeneous photocatalysis is one of the most promising techniques for water treatment. Titanium dioxide (TiO2) is the most used catalyst in photo-induced reactions, mainly due to its high photocatalytic activity, chemical stability and inexpensiveness. In this thesis is described the synthesis, spectroscopy and characterization of TiO2 based photocatalysts, and their application to the detoxification of contaminated water containing organic pollutants.;The first part of this work is focused on the preparation and characterization of nanocrystalline TiO2, and its use in the photocatalytic degradation of a model compound - phenol - by ultraviolet (UV) irradiation. Materials were synthesized by means of an acid catalyzed sol-gel technique using an alkoxide precursor (titanium isopropoxide). By controlling the temperature of the calcination step, materials with different crystalline, spectroscopic and morphologic properties are obtained. TiO2 obtained at 673K, almost pure anatase is the most active catalyst, while pure rutile, obtained at 973K, showed poor photocatalytic efficiency. The catalytic materials are extensively characterized by several microscopic, spectroscopic and calorimetric techniques. The photocatalytic degradation process is characterized in terms of several operational parameters, namely pH of the medium, catalyst loading, phenol initial concentration, presence of oxidant species, oxygen partial pressure and photon flow. A modified Langmuir-Hinshelwood model with a pseudo-steady state approach is used for kinetic analysis. The dependence of both, kinetic and adsorption equilibrium constants, on light intensity is demonstrated.;In order to increase the TiO2 photo-efficiency, in particular in the visible spectrum, composite photocatalysts using different supports are synthesized by means of a modified sol-gel technique. Materials such as zeolites, mesoporous molecular sieves, activated carbon, activated carbon fibers and multi-walled carbon nanotubes are used as supports. The prepared catalysts are tested in the photocatalytic degradation of phenol by visible irradiation.;The introduction of carbon based materials results in a positive effect in the photodegradation of phenol. This effect, quantified by means of a synergy factor, is more pronounced when carbon nanotube (CNT)-TiO2 composite catalysts are used. The beneficial effect is explained by the action of carbon nanotubes as photosensitizers, injecting electrons into the conduction band of TiO2.;Due to the improved performance of CNT-TiO2 composite, a more detailed study on its properties is described. Catalysts with different CNT content are tested in the photocatalytic degradation of phenol under UV, UV-Visible and Visible irradiation wavelengths. The effect of the introduction of carbon nanotubes into the titania matrix is more obvious when irradiating in the Visible range, confirming the photosensitizing nature of their action. An optimal CNT to TiO2 mass ratio of 1:5 is observed. Also, carbon nanotubes surface chemistry was found to be important in the photo-efficiency of the obtained composite catalyst. CNT with a moderate amount of surface oxygenated groups results in more efficient catalytic activity. Carboxylic and phenol groups are believed to be anchoring points for titania particles. An excess of surface groups leaded to the agglomeration of TiO2 particles and, therefore, to a decrease in the photo-efficiency of the resulting composite catalyst. Besides phenol, the photocatalytic oxidation of other monosubstituted benzene derivatives such as aniline, nitrobenzene and benzoic acid, is investigated using visible irradiation with both TiO2 and CNT-TiO2 catalysts. The efficiency of CNT-TiO2 catalysts in the photocatalytic oxidation of mono-substituted aromatic compounds appears to depend on aromatic ring activating/deactivating properties of the substituents.;The photo-efficiency of the CNT-TiO2 catalyst is studied using 4-chlorophenol as model compound. The effect of several operation parameters, such as catalyst load, pH of the medium, concentration of hydrogen peroxide and concentration of substrate is described. CNT-TiO2 catalyst is also tested in the photodegradation of other para-substituted phenols, such as 4-aminophenol, 4-hydroxybenzoic acid and 4-nitrophenol. A relationship between the Hammet constant of each compound and its degradability by TiO 2 and CNT-TiO2 photocatalysts is demonstrated. These results appeared to be in line with the electron donor/withdrawing properties of the substituent groups in the phenol molecule.;Finally, an exploratory study on the photocatalytic oxidation of clofibric acid, a lipid regulator drug, under visible irradiation using nanocrystalline TiO2 is described as a possible initiator for future work.
机译：非均相光催化是最有前途的水处理技术之一。二氧化钛（TiO2）是光诱导反应中最常用的催化剂，主要是由于其高的光催化活性，化学稳定性和廉价。本文描述了TiO2基光催化剂的合成，光谱学和表征，以及它们在对有机污染物污染的水中进行解毒中的应用。这项工作的第一部分着眼于纳米TiO2的制备，表征及其用途。紫外线（UV）辐射降解模型化合物-苯酚的过程通过使用醇盐前体（异丙醇钛）的酸催化溶胶-凝胶技术合成材料。通过控制煅烧步骤的温度，可以获得具有不同晶体，光谱学和形态学性质的材料。在673K下获得的TiO2（几乎是纯锐钛矿）是活性最高的催化剂，而在973K下获得的纯金红石型则表现出较差的光催化效率。催化材料通过几种显微，光谱和量热技术得到了广泛的表征。光催化降解过程的特征在于几个操作参数，即介质的pH，催化剂负载，苯酚初始浓度，氧化剂种类的存在，氧分压和光子流量。使用伪稳态方法的改进的Langmuir-Hinshelwood模型用于动力学分析。证明了动力学平衡常数和吸附平衡常数对光强度的依赖性。为了提高TiO2的光效率，特别是在可见光谱中，通过改性溶胶-凝胶合成了使用不同载体的复合光催化剂技术。诸如沸石，中孔分子筛，活性炭，活性炭纤维和多壁碳纳米管的材料用作载体。对制备的催化剂进行了可见光辐射下苯酚的光催化降解测试。碳基材料的引入对苯酚的光降解具有积极作用。当使用碳纳米管（CNT）-TiO2复合催化剂时，通过协同因子量化的这种效果更加明显。碳纳米管作为光敏剂，将电子注入到TiO2的导带中，可以解释其有益作用。由于CNT-TiO2复合材料的性能提高，对其性能进行了更详细的研究。测试了具有不同CNT含量的催化剂在UV，UV-Visible和Visible照射波长下苯酚的光催化降解。在可见光范围内辐照时，将碳纳米管引入二氧化钛基质的效果更加明显，这证实了其作用的光敏性质。观察到最佳的CNT与TiO2质量比为1：5。另外，发现碳纳米管的表面化学对获得的复合催化剂的光效率很重要。具有适量的表面氧化基团的CNT可以提高催化活性。羧基和酚基被认为是二氧化钛颗粒的固定点。过量的表面基团导致TiO 2颗粒的团聚，因此导致所得复合催化剂的光效率降低。除苯酚外，还使用可见光照射TiO2和CNT-TiO2催化剂研究了其他单取代苯衍生物（例如苯胺，硝基苯和苯甲酸）的光催化氧化。 CNT-TiO2催化剂在单取代芳族化合物的光催化氧化中的效率似乎取决于取代基的芳环活化/失活性质。;以4-氯苯酚为模型研究了CNT-TiO2催化剂的光效率。复合。描述了几种操作参数的影响，例如催化剂负载，介质的pH，过氧化氢的浓度和底物的浓度。 CNT-TiO2催化剂还用于其他对位取代苯酚（如4-氨基苯酚，4-羟基苯甲酸和4-硝基苯酚）的光降解测试。证明了每种化合物的Hammet常数与其在TiO 2和CNT-TiO2光催化剂上的降解能力之间的关系。这些结果似乎与酚分子中取代基的电子给体/吸体性质相符。最后，描述了使用纳米晶态TiO2在可见光照射下对脂质调节剂氯纤维酸的光催化氧化的探索性研究。作为未来工作的可能发起者。

著录项

  • 作者

    Silva, Cláudia Sofia Castro Gomes da.;

  • 作者单位

    Universidade do Porto (Portugal).;

  • 授予单位 Universidade do Porto (Portugal).;
  • 学科 Chemical engineering.
  • 学位 Ph.D.
  • 年度 2008
  • 页码 226 p.
  • 总页数 226
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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