【作者】 郭英娜；

【导师】 袁星； 郭伊荇；

【作者基本信息】 东北师范大学 ， 环境科学， 2010， 博士

【摘要】 光催化氧化技术是近几十年发展起来且有望成为21世纪环境污染控制与治理的理想技术。作为环境净化技术,光催化过程是通过化学氧化的方法,把有机污染物矿化分解为水、二氧化碳和无毒害的无机酸。光催化剂有可能直接利用太阳光作为激发光源来驱动氧化-还原反应,从能源利用角度来讲,这一特征使光催化技术更具有开发潜力。TiO2光催化剂由于受到自身电子结构的局限性,难以有效利用太阳光。因此,本论文致力于设计新型非TiO2基高效光催化剂,采用水热技术合成出一系列铋系层状氧化物,并对其进行表征;系统研究了不同合成条件对光催化材料晶型结构与表面形貌的影响,考察了该系列复合光催化剂在模拟太阳光辐射下的光催化性能及其对污染物的降解机理,具体研究内容如下:1、形貌控制合成BiVO4及其光催化性能研究采用水热方法,无需添加任何模板和表面活性剂,通过控制水热时间、反应体系的酸度和初始反应物物质的量的比等条件,可控合成了不同晶型(四方白钨矿、四方锆石矿和单斜白钨矿)和形貌(片状、树枝状和花状)的BiVO4。采用X–射线粉末衍射仪、扫描电子显微镜、透射电子显微镜、拉曼光谱仪、紫外–可见漫反射光谱仪和X–射线光电子能谱仪对BiVO4的晶相结构、形貌以及光吸收性质等进行了表征;通过对染料罗丹明B(RB)和亚甲基兰(MB)在紫外及可见光下的降解评价其光催化活性。研究结果表明,在所有被测的样品中,与具有四方白钨矿和四方锆石矿结构的BiVO4相比,几种不同形貌单斜白钨矿结构的BiVO4在降解RB和MB时,均表现出较高的紫外及可见光光催化活性,证明了晶相结构是影响BiVO4光催化活性的主要因素;此外,这几种不同形貌的BiVO4样品在紫外光(? > 254 nm)下的催化活性相差不大,但在可见光(? > 420 nm)照射下催化活性各不相同,其中,具有花状形貌的BiVO4催化活性最高,说明形貌也是影响光催化活性的重要因素。2、Bi5Nb3O15的制备及其光催化性能研究采用较为温和的水热方法,合成了以传统的高温固相法很难得到的亚稳相Bi5Nb3O15。通过X–射线衍射仪、透射电镜、扫描电子显微镜、紫外–可见漫反射光谱和X–射线光电子能谱对产物进行表征,系统研究了水热时间,反应体系的酸度和初始反应物物质的量的比等制备条件的变化对产物组成、晶相结构以及粒子尺寸和光吸收性质的影响。研究结果表明,在pH为9、nBi : nNb = 1 : 0.7和保持200 ?C水热反应24 h后,可以得到粒子尺寸在100 nm左右及晶型较为完美的Bi5Nb3O15。在模拟太阳光(? > 320 nm)照射下降解染料甲基橙,2 h后降解基本完全;为了进一步证明Bi5Nb3O15是自身具有优异的可见光催化活性,而非来自染料的光敏化作用,本文考察了Bi5Nb3O15对在可见区无吸收的除草剂2,4–D的降解行为,结果表明,光照30 min后降解率即可达到90%。3、Ag掺杂Bi5Nb3O15的制备及其光催化性能研究采用光沉积法成功制备了高分散的及不同Ag担载量(1~20%)的Ag/Bi5Nb3O15,研究了其对水溶液中溴代阻燃剂四溴双酚–A(TBBPA)的降解行为,考察了辐射光源的波长和反应体系的pH值对Ag/Bi5Nb3O15光催化降解TBBPA的影响,并与纯Bi5Nb3O15及商用TiO2(Degussa P25)作了比较。研究结果表明,反应体系为中性、Ag的担载量为10%时,催化剂在模拟太阳光(? > 320 nm)辐射下光催化活性较高,尤其在? > 420 nm时仍具有明显高于Bi5Nb3O15和Degussa P25在此波长下的活性。这种优异的光催化活性一方面归结为Bi5Nb3O15自身的层状结构,由于受到中间层[Bi2O2]2+正电荷的强烈吸引,使得钙钛矿结构中的NbO6八面体结构发生畸变,这种特殊的层状结构能够有效抑制电子与空穴的复合,从而提高光催化效率;另一方面,光沉积法可使Ag均匀沉积在Bi5Nb3O15的表面和层间,不易发生团聚;均匀沉积的Ag具有强捕获电子能力和表面等离子体共振(SPR)效应,可提高其光催化活性。此外,对Ag/Bi5Nb3O15的循环使用进行了研究,通过Ag的再次光沉积处理,使催化剂在四次循环使用后对TBBPA的降解率仍然可以达到80%以上。通过对TBBPA降解过程中检测到的中间产物进行分析,提出了其可能的降解机理,为TBBPA在环境中的转化和归趋研究提供了科学依据。更多还原

【Abstract】 Photocatalytic oxidation technology developed in recent decades, and it is expected to become the ideal of environmental pollution control and treatment technology in the 21st century. As an environmental purification technology, photocatalytic process is the mineralization of organic pollutants by chemical oxidation techniques, which is decomposed into water, carbon dioxide and non-toxic inorganic acid. The sunlight can be used by photocatalyst to drive the oxidation-reduction reaction as the excitation source. The feature makes photocatalytic technology be more potential value to apply in energy. Due to the limitation of its electronic structure, TiO2 photocatalyst is difficult to use sunlight effectively. Therefore, we focus on design and development of new and efficient non-TiO2-based photocatalyst with visible-response by hydrothermal technique. A series of Bi-based oxides with layered structure are synthesized and characterized. We investigate the relationship between prepared conditions and the crystal phase and morphology of products, examine the photocatalytic activity under the visible light irradiation, and explain the degradation mechanism.1. Controllable fabrication of bismuth vanadates and their photocatalytic activityBismuth vanadates (BiVO4) with various crystal structures (tetragonal scheelite, tetragonal zircon and monoclinic scheelite) and morphologies (sheet-like, dendritic-like and flower-like) were controllably fabricated by using a mild additive-free hydrothermal treatment process by controlling the hydrothermal time, pH value and molar ratio of Bi to V in the starting materials. The crystal structures, morphologies, and photophysical properties of the products were well-characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman scattering spectra (Raman), UV-Vis diffuse reflectance spectra (UV-Vis/DRS) and X-ray photoelectron spectroscopy (XPS). Subsequently, their UV- as well as visible-light photocatalytic performances was evaluated via dyes rhodamine B (RB) and methylene blue (MB) degradation. The results show that compared with tetragonal scheelite and tetragonal zircon BiVO4, monoclinic scheelite BiVO4 has the highest UV- as well as visible-light photocatalytic activity toward dyes RB and MB degradation among all tested BiVO4 powders, confirming that the crystal structure of the BiVO4 predominantly affects its photocatalytic activity. In addition, several monoclinic scheelite BiVO4 products with different morphologies exhibit similar photocatalytic activities under UV light irradiation (? > 254 nm). However, the photocatalytic activities of them under visible light irradiation (? > 420 nm) are different. Monoclinic scheelite BiVO4 with a flower-like morphology shows the highest UV- as well as visible-light photocatalytic activity toward dyes RB and MB degradation, comfirming the morphology also play an important role to its photocatalytic activity.2. Preparation of Bi5Nb3O15 and their photocatalytic activityWe successfully synthesize orthorhombic Bi5Nb3O15 by the mild hydrothermal method at the first time, which is difficult to obtain by the traditional high temperature solid state reaction. The products were characterized by XRD, FESEM, TEM, UV-Vis/DRS and XPS. The crystal structure, particle size and optical absorption properties of Bi5Nb3O15 influenced by hydrothermal time, pH value, and molar ratio of initial materials were investigated. The results show that Bi5Nb3O15 with perfect crystal structure was obtained at pH = 9, nBi : nNb = 1 : 0.7 and hydrothermal treatment at 200 ?C for 24 h. High crystallinity and little particle size are favorable to improve photocatalytic activity. Dyes methyl orange (MO) are totally decomposed afte 2 h irradiation under simulated sunlight (? > 320 nm). To further prove Bi5Nb3O15 is excellent visible light photocatalytic activity, rather than from the photosensitive dye, the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), which has no absorption in visible range, is selected as model contaminant. The degradation rate can reach 90% afte 30 min irradiation.3. Preparation of Ag/Bi5Nb3O15 and their photocatalytic activityAg/Bi5Nb3O15 with different loading (1~20%) was successfully prepared by photo-deposition method. The visible-light photocatalytic activity is examined by the degradation of tetrabromobisphenol-A (TBBPA) at different range of wavelengths and pH values, and the results were compared with Degussa P25. The appropriate reaction system is neutral. The best loading of Ag is 10%. Ag/Bi5Nb3O15 (10%) exhibits higher photocatalytic activity under simulated sunlight irradiation (? > 320 nm). Especially ? > 420 nm, the activity of Ag/Bi5Nb3O15 (10%) is significantly higher than Bi5Nb3O15 and Degussa P25. On the one hand, this enhanced photocatalytic activity can mainly attribute to the layered structure of Bi5Nb3O15. The [Bi2O2]2+ layer in middle has positive charge, which can strongly attracted NbO6 octahedron. The distortion of NbO6 octahedral structure in layered structure can effectively suppress the recombination of electronics and holes, and improve photocatalytic efficiency. On the other hand, compared with other methods of doping noble metal, the Ag atoms can be well-proportioned on the catalyst surface and layer, and avoid aggregate. It is favorable to enhance photocatalytic activity. The noble metal Ag has strong ability to capture electronic and surface plasmon resonance (SPR), which can improve the visible-light photocatalytic activity. In addition, the method of catalyst recycling is also studied. Based on the detection of intermediate products, we deduce the degradation mechanism of TBBPA. It is significant to study the transformation of TBBPA in the environment.更多还原