【作者】 刘引烽；

【导师】 孟中岩； 桑文斌；

【作者基本信息】 上海大学 ， 材料学， 2004， 博士

【摘要】 光合作用是自然界对二氧化碳实现光还原反应的杰作。如何利用光催化剂对二氧化碳实现还原是目前催化研究领域的重要课题,也是向自然学习、模拟生物过程的重要课题。它将废弃的二氧化碳资源加以利用,减少其对环境的污染,减轻温室效应,同时充分运用太阳能能源,是实现自然生态循环、保护环境的有效途径。纳米半导体显著的量子尺寸效应,使其具有较强的光催化活性:而通过调节纳米晶粒尺寸可以改变其光学响应,从而提高太阳能利用率,因此已成为光催化领域的研究热点。本文以此为背景,综合分析了该领域的研究与发展,运用本课题组首先提出的配位转化法制备了纳米光催化剂MS（M=Cd,Zn）,并对二氧化碳进行光催化还原反应,系统地研究了配位转化法制备纳米MS的原理和动力学、纳米MS的结构与制备条件间的关系、以及二氧化碳光催化还原反应机理及其影响因素等。 本文选择甲壳胺、聚乙烯醇、聚氧乙烯和聚丙烯腈共聚物等极性聚合物作为配位体,与金属离子M²⁺（M=Cd,Zn）进行配位,并引入Na₂S溶液作为硫源,采用配化转化方法制备了不同聚合物基体保护的纳米硫化物MS（M=Cd,Zn）。对配位转化法原理、制备动力学、纳米微粒的结构和尺寸研究证明,配位转化法对于制备含有过渡金属元素化合物纳米材料具有普适性。采用间接显色方法跟踪测定了溶液中硫离子浓度的变化,率先研究了制备过程动力学。根据Fick扩散定率,基于非恒源及存在化学反应的扩散过程,提出了硫离子在配合物膜中的扩散模型,建立了其动力学方程,并进行模拟计算,模拟曲线与实验结果相当吻合,由此估算出在PVA/Zn²⁺配合物膜中硫离子的扩散系数D为9.92×10^-6cm²/s;配合物转化反应的速率常数k为16.1mol/Ls,反应活化能△E=61.7KJ/mol。根据动力学研究结果,首次提出可以由配位比对纳米硫化物微粒尺寸进行预测,在亚浓溶液中制备纳米ZnS微粒时,其品粒尺寸与聚合物的聚合度成正比,而与配位比成反比。预测的结果与实际测定值相符。更多还原

【Abstract】 Photosynthesis is a natural phenomenon of photocatalytic reduction for carbon dioxide. It is a key project to achieve the artificial photoreduction of CO2 using photocatalysts in the field of catalysis. It is not only an effective way to improve ecological circulation, but also an important way to put in practice of biomimetic synthesis. As we know, this photoreduction utilizes both the clean solar energy and the source of CO2 waste which gives nse to the environmental pollution and the greenhouse effect. Nanometer-sized semiconductors have high catalytic activities because of their obvious quantum effect. Moreover their optical response can be adjusted by the particle size, therefore the nanometer-sized semiconductors have recently been used as high effective photocatalyst for exploiting the solar energy. Based on the consideration mentioned above, in this dissertation, a critical review of the development of researches in these fields is given and the content of this work has been proposed. Adopting the coordination transformation method, which was firstly presented by our group, to prepare nanometer-sized photocatalysts MS(M=Cd, Zn), the principle and the kinetics of preparation, the dependence of the structure upon the preparation conditions, the mechanism and influence factors of photocatalytic reduction of carbon dioxide were investigated.Nanometer-sized sulfides MS (M=Cd, Zn) were prepared by the coordination transformation method. Some polar polymers including chitosan (CTN), polyvinyl alcohol (PVA), polyethylene oxide (PEO) and polyacrylonitrile (PAN) were selected as polymer ligands to be coordinated with metal ions M2+ (M=Cd, Zn) and the polymer-metal complexes were then transformed to nanometer-sized semiconductors MS through introducing Na2S solution as the sulfide source By analyzing the principles, researching on the kinetics of the process, and characterizing the structures and the dimension of MS particles, the universality of this method for preparing nanometer-sized materials was demonstrated. The kinetics of preparation of the nanometer-sized MS particles, which was carried out by an indirect chromogenic measurement to determine the variation of the concentration of sulfide ions, has been developed. According to the Fick’s first diffusion law, the kinetics equations were established based on a mathematical simulation of the diffusion process with a non-constant source, and especially, accompanying chemical reactions taking place The simulated curve is in good agreement with the experimental results. The diffusion coefficient D of sulfide ion in PVA-Zn2+ complex film is 9.92 × 10-6 cm2/s by combining the kinetics equation and the experimentaldata. The constant of reaction rate and the active energy of the transformation from PVA-Zn2+ complex to ZnS particles are 16.1 mol/ Ls and 61.7 KJ/mol, respectively. According to the kinetics results, it was proposed that the size of the nanoparticles obtained by coordination transformation method can be predicted. The nanocrystalhte size is proportional to the degree of polymerization of the polar polymer and inversely proportional to the coordination number. It was found that the predicted results coincide with the measurement values.From the IR spectra tracing the process, coordination bond was also found between the polymer and the nanometer-sized MS particles obtained by the coordination transformation method. The mechanism of the process was then proposed. The dominant crystallites of MS particles are demonstrated to be sphalerite structure by X-ray diffraction analysis. And their size is demonstrated to be in the nanoscale since the band-edge wavelength is considerably blue shifted in UV absorption spectra and the diffraction lines are obviously broadened in XRD patterns. It is implied that coordination transformation method can be used to prepare homogeneous, size controllable and stable nanoparticles. In addition, the particulate size was determined by XRD, TEM and UV spectroscopy, respectively. Among the different ways for evaluating the size of the nanoparticles, the value obtained by EMA (effective-mass approximation) method is close to that obtained by TEM, while the value by Scherrer equation based on XRD is obviously smaller than that by TEM. It is manifested that the polymers containing strong-coordination groups have better behavior to protect nanoparticles than those with only poor coordination groups. It is shown that the bigger the coordination number, the more the blue shift of the band edge absorption, the smaller the particle size should be. Small particles with great amount of blue shift were also obtained under low concentration of Na2S solution, short time of the complex film dipping in the Na2S solution and low temperature of the transformation reaction. But the concentration and the dipping time only in a certain range would affect the size of the nanoparticles.The photocatalysts of SiO2/PVA/MS was prepared by coordination transformation method with silica supporter. The photoreduction of CO2 induced by photocatalysis was investigated The gas chromatogram of the photocatalytic products indicated that there were oxygen produced. The main products catalyzed by nanometer-sized ZnS were methanol, ethanol, formaldehyde and a little nitromethane. The maximum amount of 10-4mol/(0.1 gCat 24h) of the organic products was obtained Meanwhile the main products catalyzed by nanometer-sized CdS were methanol, formaldehyde and a little nitromethane. The amount was 10 5mol/(0.1gCat 24h). In the presence of Na2SO3 reactant the yield of organic compounds would increase and there was a lot of acetaldehyde produced in the system. It indicated that the catalytic ability of nanometer-sized photocatalyst CdS is weaker than that of nanometer-sized ZnS, and that the ability, however, would be enhanced by reductants Based on the resultants and the characteristics of the volume change of the system, the photocatalytic更多还原