摘要
TiO_2半导体材料因具有无毒、无害、廉价、稳定、高催化性能等优点,而成为一种理想的光催化材料。但是由于其带隙能较大,只能被占太阳光不到5%的紫外光激发,太阳光利用率较低;此外,TiO_2表面产生的电子–空穴对复合几率较高,导致其量子效率较低。这些都限制了TiO_2的实际应用。因此研制具有较高光催化性能的新型光催化材料成为当前科研工作者的研究重点。另外,开发以太阳光为光源的高效光催化反应器是提高光催化体系能效和活性的另一重要途径。虽然传统的悬浮型光催化反应器构造设计简单,催化剂担载量高,但存在催化剂分离困难,光照不均匀,光利用率低等问题,从而使得这种反应器局限于实验室研究。因此,本文致力于设计和制备具有较高太阳光光催化活性的新型复合光催化材料,并研制一种新型高效的光催化反应器。通过各种分析测试方法对光催化材料的结构和形貌进行表征,并系统地考察其在新型反应器中的模拟太阳光催化性能。本论文具体的研究内容如下:
1.采用溶胶凝胶结合溶剂热技术制备了H_3PW_(12)O_(40)/TiO_2复合材料,采用浸渍提拉的方法把H_3PW_(12)O_(40)/TiO_2复合材料涂覆到光纤的表面,采用光沉积技术在光纤的表面制备了Ag/H_3PW_(12)O_(40)/TiO_2异质结薄膜复合材料。通过X射线粉末衍射仪(XRD)、傅里叶变换红外光谱仪(FT IR)、电感耦合等离子体原子发射光谱仪(ICP AES)、X射线光电子能谱仪(XPS)、透射电子显微镜(TEM)、场发射扫描电子显微镜(FESEM)、紫外可见漫反射光谱仪(UV vis/DRS)和N2吸附脱附等现代检测技术对复合光催化材料的组成结构、形貌、光吸收性质以及孔隙率进行了全面表征。通过在模拟太阳光的照射下,对染料罗丹明B(RB)和对硝基苯酚(4NP)的降解评价了各种催化剂薄膜的光催化活性;通过六次降解RB循环实验,评价了H_3PW_(12)O_(40)/TiO_2和Ag/H_3PW_(12)O_(40)/TiO_21薄膜的循环使用情况;把涂覆有催化剂薄膜的光纤垂直均匀地放置在石英反应器中,成功设计了新型的光纤反应器。在新型光纤反应器中,涂覆催化剂的光纤束不仅仅是催化剂薄膜的载体,而且又是光的传播媒介,因此,光纤反应器提高了光利用率,光纤反应器对RB和4NP的降解速率明显高于光纤水平放置仅作为催化剂载体时的光催化速率。新型高效的光催化剂与新型的光纤反应器相结合,大大提高了整个反应体系对污染物的降解能力。
2.分别采用热缩聚和溶剂热方法制备了石墨相C3N4(g-C_3N_4)和Bi_5Nb_3O_(15),以制备的g-C_3N_4和Bi_5Nb_3O_(15)为原料,通过研磨热处理的方法制备了g-C_3N_4/Bi_5Nb_3O_(15)复合光催化材料。采用XRD、FT IR、TEM、UV vis/DRS和荧光光谱(PL)等现代检测技术,对复合光催化材料的组成结构、形貌和光吸收性质等进行了系列表征。通过可见光下对染料甲基橙(MO)和对氯苯酚(4CP)的降解评价了制备的g-C_3N_4/Bi_5Nb_3O_(15)光催化材料的催化性能。结果表明,在适当的g-C_3N_4担载量下,g-C_3N_4/Bi_5Nb_3O_(15)的可见光催化活性显著优于纯g-C_3N_4和Bi_5Nb_3O_(15);通过自由基和空穴捕获实验,研究了4-CP降解过程中的主要活性物钟,提出了可能的降解机理;通过五次循环实验评价了制备的g-C_3N_4/Bi_5Nb_3O_(15)的循环使用情况。
通过光电化学实验和荧光光谱分析研究了g-C_3N_4/Bi_5Nb_3O_(15)复合光催化材料中光生电子和空穴的分离和迁移情况;通过g-C_3N_4/Bi_5Nb_3O_(15)复合材料能带结构的提出,理论上研究了光生电子和空穴分离和迁移的过程,为复合光催化材料的构筑提供了理论依据。
TiO_2semiconductor has been considered a kind of ideal photocatalytic material due to itsnon-toxic, low cost, high stability and superior photocatalytic properties. However, TiO_2canutilize no more than5%of the total solar energy impinging on the surface of the earth due toits wide bandgap. Moreover, the photogenerated electrons and holes of TiO_2can recombinerapidly, which results in the low quantum efficiency. These restrict the practical application ofTiO_2. Therefore, much effort has been devoted to developing more efficient and stablephotocatalysts. In addition, the design of highly efficient photoreactors is also crucial to thewide range of needs for environmental remediation and clean-up. Conventionally usedphotoreactors for liquid phase oxidation are based on the heterogeneous slurry system withsuspended catalyst. The design offers ease of construction and high catalyst loading. However,slurry reactors are limited to the laboratory scale for wastewater treatment because ofdifficulty of separation of photocatalyst nanoparticles from the treated water and nonuniformlight irradiation and low light utilization efficiency. Therefore, we devote ourselves to designand development of new and efficient composite photocatalyst with higher photocatalyticactivity and a new photoreactor. The photocatalytic materials were well characterized bymany techniques. And the photocatalytic activities were investigated systemically in thedesigned photoreactor under simulated sunlight irradiation.
1. H_3PW_(12)O_(40)/TiO_2composite photocatalytic material was prepared by combination ofthe methods of sol gel and hydrothermal treatment at a lower temperature. AndH_3PW_(12)O_(40)/TiO_2-coated optical fibers were prepared by dip coating method. The Agnanoparticles were photodeposited on the surface of H_3PW_(12)O_(40)/TiO_2films coated on theoptical fibers. The photocatalysts were characterized by X ray diffraction (XRD), FourierTransform Infrared Spectrometer (FT IR), Inductively Coupled Plasma Atomic EmissionSpectrometer (ICP AES), X ray photoelectron spectroscopy (XPS), transmission electronmicroscopy (TEM), field emission scanning electron microscopy (FESEM), UV Vis diffusereflectance spectra (UV Vis/DRS) and N2adsorption/desorption. Their photocatalyticactivities were evaluated by degradation of aqueous RB and4NP under the simulatedsunlight irradiation. The recyclability of the H_3PW_(12)O_(40)/TiO_2and Ag/H_3PW_(12)O_(40)/TiO_21filmwas evaluated through six consecutive catalytic cycles. The optical fiber reactor was designedby puting H_3PW_(12)O_(40)/TiO_2or Ag/H_3PW_(12)O_(40)/TiO_21film coated optical fiber bundlevertically in the quartz reactor. In this design, the optical fibers act as not only supporter of thecomposites film but also the medium of light transmission. Therefore, this new type optical fiber reactor enhances the light use efficiency, which results to the enhanced photodegradationefficiency.
2. g-C_3N_4and Bi_5Nb_3O_(15)were prepared by polycondensation and hydrothermaltreatment method respectively. g-C_3N_4/Bi_5Nb_3O_(15)composite photocatalytic materials wereprepared by a simple milling heat treatment method with g-C_3N_4and Bi_5Nb_3O_(15)as rawmaterials. The g-C_3N_4/Bi_5Nb_3O_(15)composites were characterized by XRD, FT IR, TEM,UV vis/DRS and PL. The photocatalytic activities of g-C_3N_4/Bi_5Nb_3O_(15)were evaluated bydegradation of aqueous MO and4CP under the visible light irradiation. At proper g-C_3N_4loading, the photocatalytic activity of g-C_3N_4/Bi_5Nb_3O_(15)outperforms pure g-C_3N_4andBi_5Nb_3O_(15). The main active species yielded in the g-C_3N_4-and g-C_3N_4/Bi_5Nb_3O_(15)-catalyzed4CP degradation systems were also investigated by the free radical and hole scavengingexperiments. Accordingly, the photodegradation mechanism was given. Andg-C_3N_4/Bi_5Nb_3O_(15)–70was chosen to evaluate the recyclability of the g-C_3N_4/Bi_5Nb_3O_(15)photocatalysts by five times’RB degradation reaction.
The separation and transportation of photogenerated electrons and holes wereinvestigated by photoelectrochemistry experiments and fluorescence spectra analysis. Thespecific process of the transportation of photogenerated carriers was studied by the bandstructure, which provided theoretical basis for the design of composite photocatalysts.
引文
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