Preparation, Catalytic Performance And Mechanism Of Several Metal Oxide Nanocatalysts

In the field of catalysis, the development of highly active catalyst is the research topic all over the world. A nanomaterial significantly enhance catalytic effieiency because of its small size, large specific surface area, many active sites, and the unique crystal structure and the the special surface properties（the high surface activity and surface energy）. It used as a catalyst greatly improves the reaction efficiency in a catalytic system. As a result, the research of a nanomaterial acting as a novel catalyst has become attractive in the field of catalysis. The material performance largely depends on its structure, the structure control of nanomaterials will help realize the adjustment of catalytic performanc, which will reveal the relationship between structure and performance of catalyst in a nanometer level and then provide methodological guidance and theoretical basis for developing a high efficient catalyst. Due to its unique physical and chemical properties, metal oxide nanomaterials exhibit a great application potential in the field of catalysis. Therefore, in view of metal oxide, bimetal oxide and metal oxide composite, this thesis introduces the controllable preparation of several kinds of metal oxide nanomaterial catalysts, their applications in model catalysis reactions, catalytic performance, and the relationship between catalyst structure and performance.1.（a） Nano-sized Ti O₂ solid acid with different exposed facets: The sulfated Ti O₂ solid acids exposed with（001） and（101） facets were prepared and well characterized. They were used as catalysts in an esterification reaction. Their catalytic activities were compared and the reasons for the differences in catalytic activity were analyzed. The effects of reaction conditions were studied, and reusabiliby of SO42-/T101 were discussed. It was found that the sulfate groups were dispersed in the surface of T001 and T101 with similar bidentation with Ti4+ ions. Compared with T001, more sulfate groups were binded on T001 surface, and then the higher acid concentration and more strong acid strength generated, leading to the higher catalytic activity of SO42-/T101 with esterification yield as high as 92.2 %. The SO42-/T101 had both Br?nsted and Lewis acidic sites. When the optimized reaction condition（reaction time: 150 min, reaction temperature: 120 oC, catalyst amout: 0.18 g, molar ratio of alcohol to acid: 1.2） was used, the highest catalytic activity was obtained for SO42-/T101. In the recycle experiment, the SO42-/T101 showed the decreased catalytic activity. The catalytic activity of recycle SO42-/T101 could be regenerated by the diluted sulfuric acid soaking followed by caicination.（b） Nano-sized Ti O₂ solid acid with different crystal phases: The sulfated Ti O₂ solid acids with different anatase/rutile phase contents were prepared and well characterized. They were used as catalysts in an esterification reaction. The relationships between crystal phase content and catalytic activity/surface acid concentration. It was found that the sulfate groups were dispersed on the surface of Ti O₂ with similar bidentation with Ti4+ ions. Under the per unit area, the increase of rutile phase content improve surface acid concentration, resulting in the enhancement of catalytic activity. The catalytic activity was positively linearly coordinated with rutile phase content. The SO42-/T1 had both Br?nsted and Lewis acidic sites. When the optimized reaction condition（reaction time: 90 min, reaction temperature: 120 oC, catalyst amout: 0.10 g, molar ratio of alcohol to acid: 1.2） was used, the higher catalytic activity was obtained for SO42-/T101. In the recycle experiment, the SO42-/T1 showed a stable reusability.2. High-surface-area nanocrystalline Zn Al₂O₄ was prepared by a one-step solvothermal method in different solvents and well characterized. Zn Al₂O₄ was developed as an ozonation catalyst in removal of a pollutant with a high concentration. The relationship between the density of surface hydroxyl group and catalytic activity was established. The reusability of Zn Al₂O₄ and effects of p H of solution were discussed. The catalytic mechanism was investigated. It was found that their surface areas are all higher than 195 m2/g and the highest surface area reaches to be 288 m2/g. The presence of Zn Al₂O₄ accelerated the degradation of a pollutant and the highest degradation efficiency was 78.8%. The density of surface hydroxyl group had positive linear correlation with the degradation efficiency of a pollutant. Zn Al₂O₄ revealed a good reusability in ozonation and showed a stable activity in a wide p H range from 3.3 to 9.3. In the ozonation with Zn Al₂O₄, the Lewis acid sites are reactive center for catalytic ozonation and the highly active hydroxyl radicals were the major oxidation species leading to the enhanced removal of a pollutant in bulk solution.3. Magnetic Ni Fe₂O₄ nanopolyhedron and nanosphere were synthesized by different methods and well characterized. They were developed as ozonation catalysts in removal of a pollutant with a high concentration. The reaction among Ni Fe₂O₄, ozone and pollutant was analyzed. From several aspects involving catalyst structure, reactive center, surface electron transfer and the reaction with ozonation, their difference in activity was discussed. The kinetics of Ni Fe₂O₄ in ozonation was investigated and the relationship between kintic constant and the density of surface hydroxyl groups was established. The catalytic mechanism was investigated and the role of water was elaborated. The separation and reusability of Ni Fe₂O₄ were evaluated in ozonation. It was found that Ni Fe₂O₄ nanopolyhedron showed higher catalytic activity than Ni Fe₂O₄ nanosphere. The more Lewis acid sites leading to more surface hydroxyl groups and chemisorbed water and the stronger surface electron transfer, combined with the beneficial structure, resulted in the enhanced interaction with ozone leading to the higher catalytic activity of Ni Fe₂O₄ nanopolyhedron. This interaction between catalyst and ozone was proved to play a vital role in the heterogeneous catalytic ozonation. The catalytic ozonation of Ni Fe₂O₄ followed the first order kinetic mode and the kinetic constants for ozone decomposition and pollutant degradation were positively and linearly correlated with the amount of surface hydroxyl groups. Ozone first reacted with surface hydroxyl group and the generated hydroxyl radicals accelerated the degradation of a pollutant. In this process, the Lewis acid sites were reactive center for catalytic ozonation. The water molecule was an accelerator not an inhibitor for catalytic reaction over Ni Fe₂O₄. The Ni Fe₂O₄ nanocatalyst could be easily and efficiently separated from the reaction mixture with an external magnet. In the recycle experiments, the gradual depositon of intermediate carboxylic acid compounds on the surface led to the decreased catalytic activity of Ni Fe₂O₄. The catalytic activity could be completely recovered by the calcination method and ozonation method.4. The nanocomposites of Ce O₂ on Ti O₂ nanotube were prepared and well characterized. They are developed as peroxidase-like mimics. The effect of Ce O₂ amount was discussed. The reaction among Ce O₂/NT-Ti O₂, H2O₂ and TMB was investigated. At the same time, Ti O₂ nanowire/nanorod/nanopariticle supported Ce O₂ were prepared. Their catalytic activities were compared and the reasons for their difference were analyzed. The effects of reaction condition were studied. This method is proposed to be applied in H2O₂ and glucose detections. It was found that Ce O₂/NT-Ti O₂ had the peroxidase-like activity. When the molar ratio of Ce/Ti was 0.1, the highest activity was obtained that was much higher than that for Ce O₂/NW-Ti O₂, Ce O₂/NR-Ti O₂, Ce O₂/NP-Ti O₂ with similar molar ratio of Ce/Ti. The optimized p H and temperature were 4 and 45 oC, respectively. Kinetic analysis indicated that the catalytic behavior of Ce O₂/NT-Ti O₂@0.1 was in accordance with typical Michaelis-Menten kinetics. The measured Km values over H2O₂ and TMB were 0.094 and 0.04 m M that were lower than those in other reported nanomaterials based peroxidase mimics, indicating the higher affinity to H2O₂ and TMB for Ce O₂/NT-Ti O₂@0.1. The Ce3+ sites were confirmed as the catalytic active sites for the catalytic reaction. The first interaction of surface Ce O₂ with H2O₂ chemically changed the surface state of Ce O₂ by transforming Ce3+ sites into surface peroxide species causing adsorbed TMB oxidation. Ce O₂/NT-Ti O₂@0.1 had the highest concentration of Ce3+ thus leading to the best peroxidase-like activity. Based on the high activity of Ce O₂/NT-Ti O₂@0.1, a simple method was provided for colorimetric detection of H2O₂ and glucose with the detection limits of 3.2 μM and 6.1 μM, respectively. And this method can be used to determine accurately the glucose content in serium.