Preparation And Characterization Of Nano-and Mesoporous Materials And Catalytic Properties

Mesoscopic system with dimension of 0.5~100 nm, in between the macroscopic and the microscopic systems, has been learned little for a long time due to limitations in both experimental and theoretical approaches. In the past decade, along with the developments of experimental and computer techniques, a series of unique properties of mesoscopic or mesostructured materials have been continuously unveiled differing from not only the macroscopic but also single molecules. These discoveries have sequentially surged worldwide highlights of studies on mesoscopic system. At the aspect of synthesis of materials, the first artificial preparation of metal consisting of so-called nanocrystal by H. Gleiter in 1984 and the first synthesis of ordered mesostructured materials in 1992 using self-organized surfactant as templates evoked researchers' great interests in study of nanomaterials and mesostructured materials. Nowadays, the research work in the rising research field is going deep and spreading out, novel nanomaterials or mesostructured materials have been continuously reported to be successfully synthesized;their novel physical and chemical properties such as some unique optic, electric, magnetic, acoustic and catalytic properties have been also revealed, which enrich the contents in scientific field of nanomaterials and mesostructured materials.Due to their abundant chemical and physical properties, transition metal phosphates and transition metal oxides, e.g. iron phosphate and cupric oxide, were selected in this dissertation to prepare nano- or mesostructured iron phosphate and cupric oxide, which had seldom been studied. Our efforts focused on synthesis of these materials with nanoparticles or mesostructure and also the study on their catalytic properties. The main contents of this dissertation are summarized as follows.1. Study on preparation methods of nanoparticles of iron phosphates and their catalytic properties.Several methods have been developed to prepare nanosized iron phosphates, including:(i) On the base of thorough analysis over traditional homogeneous precipitation method, a novel method for preparation of nanosized particles had been developed, employing an seemingly inhomogeneous operation but actually resulting inhomogeneous precipitation to succeed in obtaining nanosized iron phosphate particles with narrow size distribution. The method could also be applied in preparation of other compounds, for example, nanosized NiS particles with uniform size of 5~ 10 nm have been successfully prepared via this method.(ii) Sol-gel process using citric acid as ligand was used to prepare the nanosized iron phosphate, the influence of preparation parameters on average particle size had been studied in detail, and the optimum preparation conditions were obtained. After calcination at 600°C, iron phosphate catalyst with particle size of ~100 nm and surface area of 21 m2/g was obtained. This is a good result for transition metal phosphates for the surface area of normal iron phosphates was reported less than 8 m2/g.(iii) A modified homogeneous precipitation method was developed utilizing rapid decomposition of high concentration of urea under assistance of microwave and polyethylene glycol as protective agents. The study on influence of preparation conditions and calcined temperature on resulted particle size gave out the optimum preparation parameters. After calcination at 370°C to remove polyethylene glycol, nanosized iron phosphate catalysts with particle size of -14 nm and surface area of 150 m2/g was obtained.(iv) Catalytic activity of obtained nanosized iron phosphate catalyst was studied for selective oxidation of ethyl lactate to ethyl pyruvate. Compared with the normal iron phosphate catalyst prepared via conventional method, nanosized iron phosphate catalyst provided almost the same selectivity (>93%) but much higher activity;conversion of ethyl lactate on nanosized iron phosphate was as four times as the later. The results of characterizations indicated that differences in composition of surfaces and adsorption property together with surface area was account for the differences in catalytic activity for the two catalysts.2. Synthesis of ordered mesostructured iron phosphate. The ordered mesostructured iron phosphate was attempted to synthesized using CTABr, alkylamine, sodium dodecyl sulfate as cation, neutral, anion surfactants, and the influences of the synthesis conditions arid approaches to remove surfactants on the products were studied. The results can be summarized as the following paragraphs:(i) The lamellar mesostructured iron phosphate was successfully synthesized for the first time, by using respectively phosphoric acid and partially hydrolyzedFe(acac)3 as P and Fe sources, and CTABr as template. But the ordered lamellar structure collapsed after removal of the surfactants by calcination or extraction, (ii) Novel lamellar iron phosphates with crystalline or amorphous inorganic layer had been successfully synthesized using a long chain alkylamine as template,. It was found that the layer space changed linearly against the number of CH2 units in main chain of used alkylamine with a slope of -0.2 nm/CH2. Post-treatments showed that these synthesized lamellar iron phosphates were relatively stable, but their lamellar structure was difficult to transform to hexagonal structure, (iii) Lamellar and hexagonal mesostructured iron phosphates had been successfully synthesized by several experimental strategies with sodium dodecyl sulfate as template. The hexagonal mesostructured iron phosphate was synthesized with addition of hydrofluoric acid as synthetic agent. After ion exchange to remove surfactants, a novel hexagonal mesoporous iron phosphate with surface area of 254 m /g, pore volume of 0.21 cmVg and average pore size of 2.6 nm had been obtained. The synthetic mechanism and the role of hydrofluoric acid was also discussed, i.e., HF reacted with the iron phosphate precipitation to form ion pairs of [Fe+-P-O] H F , in which F banded to H via hydrogen bond. The F adjusted charge density of [Fe-P-O-H] , which made the later matching to the surface charge of surfactant micelles and sequentially resulted in the formation of ordered hexagonal mesostructured iron phosphate.3. Selective catalytic reduction of NO over nanometer and disordered mesoporous CuO/SiO2 composite oxide catalysts. A series of Cu-Si-0 composites with different ratio of CuO/SiO2 were prepared by impregnating SiO2 aerogel and commercial nanosized silicon oxide with cupric precursor, and by sol-gel method followed by super-critical drying. The obtained samples were characterized by XRI> TEM% BET^ H2-TPR and XPS techniques. The results of test for selective reduction of NO by CO suggested that different preparation way of catalysts led to different catalytic properties, which related to disperse state of copper species and interaction between the dispersed copper and the carriers. The copper species mainly existed in the form of big CuO particles in the catalysts prepared by sol-gel method, leading to low surface concentration of copper and then poor catalytic activity. The copper species interacted strongly with nanosized silicon oxide carrier, resulting in change in surroundings of the copper species such as rise in binding energy of Cu2p and lowercatalytic activity than that of catalysts prepared from SiO2 aerogel, in which copper species were deduced mainly in form of highly dispersed Cu2+ and nanoparticles of CuO.4. Synthesis and characterization of ordered mesoporous copper containing silica.A series of mesoporous copper containing silica were synthesized at room temperature using NaSiO3 and cuprammonia as Si and Cu sources and CTABr as template. To obtain ordered mesoporous composite, the copper content was found must be less than 16.8%, otherwise no ordered mesostructured products could be obtained. The formation mechanism of mesoporous copper containing silica was also discussed. It was speculated that composite mesoporous precipitated in form of sandwich-type, i.e. the copper oxide was in fact surrounded by silica. The test for selective catalytic reduction of NO over the resulted products showed double peaks of catalytic activity against reaction temperature, hasn' t reported in literature, suggesting an interesting catalytic property of the mesop—orous composite of Cu-Si oxides. The unique phenomenon was interpreted by the fluctuarion of copper valence during reactions.5. Synthesis of mesostructured copper oxides. A lot of attempts was tried to synthesize mesostructured copper oxides, without assistance of silica, using sodium dodecyl sulfate and CTABr as templates. It was found that mesostructured copper oxides with layer and hexagonal structure could be successfully synthesized under appropriate conditions with sodium dodecyl sulfate template. Novel hexagonal mesoporous copper oxide was obtained by ion exchange removing the templates. All the synthesized ordered mesostructured copper oxides, however, were thermally unstable. Maybe the thin wall of the mesostructure, the unstable valence state of copper ion and its notable Jahn-Teller effect, which resulted in the deformation of Cu-0 octahedral units, were the causes.