【作者】 刘英；

【导师】 尹双凤；

【作者基本信息】 湖南大学 ， 化学工程与技术， 2016， 硕士

【摘要】 利用光催化选择性氧化烷烃和芳烃类化合物制备高附加值醛、酮化合物具有非常重要的意义。目前该方面的研究主要存在的不足包括催化效率低、催化剂范围窄、需要利用紫外光或有毒溶剂等缺点。Bi2WO6作为一种价格低廉、简单易得的光催化剂,因具有良好的可见光响应、适当的能带位置和层状结构等性质而备受光催化研究者的关注。近年来,Bi2WO6已在光催化氧化醇类化合物的研究中表现出较好的光催化性能。本文一方面用简单的水热法制备花状Bi2WO6,探索其在可见光下选择性氧化甲苯类化合物的应用；另一方面通过调节制备条件,合成不同结构和性质的Bi2WO6,探究制备条件对其形貌结构等性质的影响,并尝试揭示Bi2WO6催化剂结构与选择性氧化性能之间的关系。本论文的主要研究内容及创新性结果归纳如下：1.利用简单的水热法,合成了由片状自组堆积而成的花状结构Bi2WO6,并以该花状Bi2WO6为催化剂,实现了在温和的条件下(常温、常压、无溶剂及可见光)高选择性氧化甲苯合成苯甲醛(选择性为96%)。底物拓展的研究结果显示,花状Bi2WO6对甲苯衍生物的选择性氧化都具有较好的活性,其氧化作用主要发生在饱和a碳的碳氢键上。通过自由基控制实验对反应机理进行了探究。研究结果表明,与大部分研究报道的光催化选择性氧化机理不同的是,该体系中羟基自由基和超氧自由基的参与很少,空穴是关键的活性物质,其直接氧化甲苯形成苄基自由基的过程是反应的控速步骤,O2进攻苄基自由基生成过氧中间体,最终脱水形成苯甲醛。2.通过调节Bi2WO6的制备条件(晶化温度和时间、pH值及HNO3加入的量),获得了具有不同性质的Bi2WO6光催化剂。采用粉末X-射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X-射线光电子能谱(XPS)和紫外-可见漫反射(UV-vis DRS)等表征手段,研究制备条件对催化剂的形貌结构、组成等性质的影响。结果表明,较高的晶化温度(>140℃)或在中性、碱性条件下均有利于形成高结晶度和强可见光吸收的Bi2WO6;加入HNO3主要通过影响晶体生长过程中离子的溶解度,从而改变催化剂的结晶性质和表面原子组成。考察所制Bi2WO6光催化选择性氧化甲苯合成苯甲醛的反应活性,并探究其结构与光催化性能的构-效关系。结果表明,在形成具良好可见光吸收的Bi2WO6晶体的基本条件下,(200)晶面的暴露和表面O原子的浓度是影响催化剂的选择性氧化性能的重要因素,其中催化剂表面的W-O、Bi-O缺陷和吸附氧有利于提高其选择性氧化性能。更多还原

【Abstract】 The selective oxidation of alkanes or aromatics to gain value-added aldehydes and ketones products is of great significant. Currently, there exist some main drawbacks of researches in this area, including low catalytic efficiency, narrow catalysts, and using of ultraviolet irradiation or noxious solvent. Bismuth tungstate (Bi2WO6) as a low cost and easily prepared photocatalyst, has attracted a lot attention due to its good visible light response, suitable band location and unique layered structure. Recently, Bi2WO6 was found displaying a good photocatalytic activity in the oxidation reactions of alcohols. In this paper, on the one hand, a flower-like Bi2WO6 was synthesized by a simple hydrothermal method, and it was investigated for selective oxidation of toluene derivatives. On the other hand, through adjusting the preparation conditions of Bi2WO6, a series of catalysts were synthesized with different structures and properties. The effects of different synthesis conditions on the catalyst’s properties such as morphology structure were explored. Furthermore, we tried to reveal the structure-activity relationship between the properties and selective oxidation activity of the as-prepared Bi2WO6. The main research contents and innovative results of the paper are summarized as follows:1. A flower-like Bi2WO6 consisting of nanosheet was prepared by a simple hydrothermal method. High selective oxidation of toluene into benzaldehyde was achieved (sel.96%) with the flower-like Bi2WO6 under mild conditions, e.g. room temperature and pressure, solvent-free, visible light irradiation. The results from substrate expansion showed that flower-like Bi2WO6 performs all good activity to toluene derivatives, and only the a-carbon of the C-H bond could be activated in this reaction system. In addition, the investigation of reaction mechanism in the photocatalytic system has demonstrated that the main active species are photogenerated holes, while hydroxyl radicals and superoxide radicals are little involved in, which is different from most reported studies. The plausible mechanism for the photocatalytic selective oxidation of toluene has also been proposed base on the results: toluene is directly oxidized to benzal raidicals by holes as a rate-determining step, then O2 reacts with the benzal radicals to form peroxy intermediate, and benzaldehyde is produced after dehydration finally.2. Through adjusting the preparation conditions of Bi2WO6, e.g. hydrothermal temperature, time, pH value, addition amount of HNO3, a series of Bi2WO6 photocatalysts were obtained with different properties. Power X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), X-ray photoelectron spectroscopic (XPS), UV-vis diffuse reflectance spectra (UV-vis DRS) were utilized for analysing the influences of synthesis conditions on the structure and properties. The characterization results showed that high temperature (>140℃) or under neutrality, alkaline condition are beneficial to synthesize Bi2WO6 with high crystallinity and strong visible light absorption ability; the addition of HNO3 could change the composition of surface atoms by affecting the solubility of ions in the crystal growth process. The selective oxidation of toluene into benzaldehyde has been performed to explore the structure-activity relationship of Bi2WO6. Results showed that with the basic properties of good crystallization and visible light absorption, the selective oxidation activity of Bi2WO6 may be concerned with (200) crystal plane and the concentration of oxygen atoms on the catalyst surface, the defects of W-O, Bi-O and the adsorbed oxygen is favourable to improve the photocatalytic activity for toluene oxidation reaction.更多还原