Morphology-controlled Synthesis Of Metal Oxide Nanostructures And Their Catalytic Applications

Metal oxide is one type of inorganic compounds widely existing in the natural world. Due to the unique physico-chemical properties and wide applications in many fields, it is of great importance to industry, agriculture and fundamental science. How to find and manufacture functional metal oxide materials with novel structures, and how to precisely control the crystal structures, sizes, and morphologies have become a hot topic in the current chemistry and materials science.The catalysts applied in energy and environment science have drawn more and more attention in recent years. Among the methods for volatile organic compounds elimination, catalytic oxidation has been recognized as one of the most effective pathways, in which catalyst is the key for the process. Hydrogen production via Bio-ethanol steam reforming is regarded as an environmental-beginning pathway to provide hydrogen source for fuel cell applications. The Co-based catalyst is one of the effective systems for the SRE of ethanol.In this thesis study, morphology effect of metal oxide nanostructures in catalytic combustion of benzene and steam reforming of ethanol processes has been investigated. The main conclusions are described as follows:1. In the present work, nano-sized copper oxides of different morphologies (nanoplatelet, nanorod, nanobelt, and nanocuboid) were synthesized through the solvothermal route. The catalytic benzene combustion was also studied to elaborate the effect of morphology of CuO nano-crystallites on the activity of benzene combustion. The results show that the specific activity of benzene combustion is determined by the mainly exposed crystal plane, the kind of surface atoms, and the density of active sites. Higher density of Cu2+ sites accounts for favorable activation of oxygen as well as benzene and the reaction between them. The activity of different crystal planes shows the following order:(200)> (111)> (011)> (001), in line with the density of Cu2+ sites and the amount of oxygen adspecies over the corresponding plane.2. Mn3O4 nanostructures of different morphologies (nanocube, nanooctahedra, and nanohexagon) have been synthesized by the hydrothermal route with different deposition agents, and the Au/Mn3O4 systems are derived via the deposition of Au nanoparticles (NPs) onto the corresponding Mn3O4 nanostructures. The obtained materials are applied for the catalytic combustion of benzene. The Mn3O4 nanostructures of different morphologies show difference in reactivity of lattice oxygen, and loading of Au NPs can enhance the reactivity of lattice oxygen of Mn3O4 substrate. In addition, there is difference in amount of oxygen adspecies and its reactivity on the Mn3O4 nanostructures of different morphologies. Au-loading can also increase the amount as well as reactivity of oxygen adspecies. It is observed that the morphology of Mn3O4 substrate can significantly affect the catalytic performance of benzene combustion. The deposition of Au NPs can further notably enhance the activity, which is resulted from the weakened Mn-O bond by the Au NPs with enriched electrons. Note that the oxygen species activated over the Au NPs can migrate onto the Mn3O4 substrate via a quick spillover process. The results of in-situ IR and O2-TPD indicate that the interfaces between the Au NPs and Mn3O4 substrate are important regions where the adsorbed oxygen or lattice oxygen of higher reactivity could more readily react with activated benzene molecules.3. Based on the Kirkendall effect in liquid phase, the Co3O4 nanotubes have been successfully prepared via controlled interface reaction between the precursor of COC2O4 nanorods and NaOH. The conditions used for this preparation route is mild, free from high pressure and specific template agent required. And the route is thought to be versatile for synthesizing other kind of inorganic nanotubes. The derived Co3O4 nanotubes show superior activity and stability in catalytic methane combustion. The higher exposure of reactive (112) crystal plane, and the higher reactivity of adsorbed and lattice oxygen species in greater amount over the Co3O4 NTs should account for its better performance.4. Co3O4 nanostructures of different morphologies have been synthesized via the hydrothermal and precursor calcination routes. (HR)TEM observations reveal that the Co3O4 nanostructures obtained under various conditions are uniform in morphology and size. The mainly exposed crystal planes are found to be (001), (111), (110), and (112) for nanocubes, nanooctahedras, nanobelts, and nanoplatelets, respectively. The results of H2-TPR indicate there is considerably difference in reactivity of lattice oxygen of the Co3O4 nanostructures with different morphologies. The results of steam reforming of ethanol suggest the morphology of Co3O4 nanostructures can intensively affect the conversion and product distribution. Among various Co3O4 nanostructures, the nanoplatelets outperform the other counterparts, giving the highest H2 selectivity of 74.5%(the theoretical value is 75%) and highest H2 yield (97.3%). The characterization of the used samples suggests that due to the different reactivity of lattice oxygen in the Co3O4 nanostructures, the sample composition changes gradually and individually under reaction atmosphere, which in turn has an impact on the product distribution (those easily reduced Co3O4 nanostructures are more favorable for the target reaction). It is also found that surface coking is the main cause for catalyst deactivation.