Study On Preparation And Properties Of Layered Double Hydroxide Nanophotocatalysts

Layered double hydroxides (LDHs) have attracted extensive attention because of its structures and properties. The layered structure of LDHs makes it possible for many functional ions to be incorporated and the uniform dispersion of metal-organic anions in interlayer galleries or metal cations in layers extends the applications of layered materials to a wide variety of fields, especially as the catalyst and catalyst precursor. Meanwhile the electronic structure of transition metal offers many advantages in the areas of optical, magnetic and catalytic materials. Therefore, my dissertation aims to introduce transition metal as constituting elements to the layer board to synthesize LDHs with special structure and study the photocatalytic degradation application of organic dyes and we also prepared Fe3O4@SiO2@(Co/Cr)-LDH shell magnetic nanocomposites. The main research content can be divided into three parts:1. In this chapter, the transition metals cobalt, ferrum and chromium were introduced to synthesize the LDHs at pH=9-10, aging temperature of 60℃and aging time of 24 hours by coprecipitation using NaOH and Na2CO3 as the precipitator. The structure, surface morphology, specific surface area and preparation mechanism of the resulting sample were investigated. The result indicated that the crystalling phase of the product was single and the specific surface area was large, CO32- was the unique ion in the interlayer of LDHs. A solution of methyl orange with azo was used as simulated dye wastewater to study photocatalytic activity of LDHs on degradation of methyl orange at room temperature under visible light irradiation. The rate of photocatalysis degradation can be expressed in first-order kinetics model. The result was satisfactory and LDHs have potential as new type visible catalysts.2. The nanomaterial combines both catalytic capability and high separation efficiency, making it an ideal support for recoverable anionic functional materials. Herein, magnetic Fe3O4 crystalline nanoparticles were synthesized using a solvent-thermal method. By using the hydrolysis of tetraethyl orthosilicate, a silica-coated magnetite core was prepared following a sol-gel process. After that, Fe3O4@SiO2@(Co/Cr)-LDH shell nanocomposites deposited from a layered double hydroxide at aging temperature of 180℃and aging time of 8 hours by solvent-thermal method. Nanoparticle morphology and structure were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and infrared spectroscopic (FT-IR) techniques. Their magnetic saturation values were determined using a vibration sample magnetometer. The result indicated a typical layer by layer structure with a magnetite core, a silica layer in the middle layer, and an LDH shell nano structure in the outer layer can be clearly observed. The obtained product possesses high magnetization values and superparamagnetic property which provides a convenient means to remove nanoparticles from solution. A solution of methyl orange as the simulated dye wastewater, we investigate the photocatalytic activity of composite nanomaterial on degradation of methyl orange under visible light irradiation. The results indicated that the nanocomposites had perfect photocatalytic degradation activity of methyl orange. Remarkably, the photocatalyst still kept the decomposition efficiency up to 80% even after being used for four times.3. A co-precipitation method was used for the preparation of nanometer-size Ni/Cr-LDH particles. The LDHs were facile to be decomposed when heated and obtained the complex oxides. Because of their typical characteristics such as big specific surface area, even distribution of catalytic active cites and mild reduction condition of metallic cations, the complex oxides will have fine application prospects in some acid-base catalysis or redox catalysis reactions. The physicochemical performances of the LDHs and complex oxides were characterized by using the modern instrumentals of SEM, XRD, FT-IR and BET techniques. The thermal decomposition procedure is complicated and the properties of complex oxides are mainly determined by the composition of the precursor and calcination temperatures of thermal decomposition process. We investigated the photocatalytic activity of Ni/Cr-LDH and complex oxides on degradation of methyl orange under visible light irradiation. The mechanisms and the kinetics of photocatalysis degradation of methyl orange were also discussed and the degradation data were fitted to the pseudo first-order kinetic model. Attributed to the synergistic effect of the components, the complex oxide Ni(Cr)O showed the preferable catalytic performance. It is shown that the differences in catalytic performances of these samples are related to the structure and composition of the catalysts.