Preparation Principle And Synthetic Kinetics And The Photocatalysis Of The Nanosized MS (M=Cd, Zn) Prepared By Coordination Transformation

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...