Controllable Synthesis, Surface Modification And Application In Catalysis Of Low-dimensional Nanomaterials

Scientific researchers have been seeking to design and manufacture novel efficient catalysts for relieving energy and environmental issues. For gas-solid heterogeneous catalytic reaction, surface layer atoms of the solid catalysts evidently have different bonding state and electronic states from their internal atoms, which results in the stronger reactivity due to their surface atoms with unsaturated coordination, and thus, they are bound to play an important role in the catalytic reaction. Low-dimensional nanostructured materials with small particle size possess the larger volume percentage of surface atoms among their composition in comparison with bulk materials. Their surface roughness is increased to form the uneven atomic steps in their surface layer, which induces the increasing of the contact surface for the chemical reaction. They are used as a catalyst material to improve the catalytic efficiency, significantly. Therefore, low-dimensional nanomaterials have attracted the extensive interest in the application in the heterocatalysis fields. This thesis is based on the structural features of nanomaterials to modify their surface with special functional materials by different methods to control their catalytic properties. On the other hand, the precursors of the target nanomaterials are designed and synthesized by controlling the precursor composition, morphology and structure by the help of the hydrothermal synthesis technique. Thereby, low dimensional metal oxide nanomaterials with the unique structure and properties are fabricated, especially materials with cubic crystal structure, and used to explore its application in the field of catalysis.1. Carbon nanotubes (CNTs) filled with CeO2particles are prepared by wet impregnation assisted by capillary force. Compared to CeO2outside CNTs, these composites show superior catalytic performance of oxidative dehydrogenation (ODH) of ethylbenzene (EB) to styrene in the presence of CO2. Transmission electron microscopy, Temperature-programmed reduction, Raman and X-ray photoelectron spectroscopy are used to investigate the effect of CNT confinement on the catalytic performance of CeO2inside CNTs. The results indicate that CNT tubular structure results in strengthened interaction between CeO2and inner wall, which induces distortion and reducibility of CeO2lattices to promote the activation of surface lattice oxygen and the formation of oxygen vacancy. The activated surface oxygen and oxygen vacancy from CeO2-CNT composites play an important role in two-step ODH reaction by promoting reverse water-gas shift reaction. In addition, CeO2filled into shorter CNTs exhibits higher catalytic activities due to decreasing the diffusion resistance of reactants and products in CNT channels.2. The hydroxylation of multi walled carbon nanotubes (MWCNTs) is carried out by an alkali-mediated hydrothermal treatment. It is found that cross-section defects introduced by a prior ball-milling treatment act as the active sites for the hydroxylation and thus result in the production of much more hydroxyl groups. The hydroxylated MWCNTs induced by defects can be used as metal-free catalysts for the oxidative dehydrogenation (ODH) of ethylbenzene using CO2as a mild oxidant and show an interesting behaviour. Their catalytic activity is about twice that of the hydroxylated MWCNTs without the ball milling treatment due to the presence of more hydroxyl groups as active sites of the ODH. However, the ball-milled MWCNTs before hydroxylation and those treated with a strong oxidizing acid have no efficient active sites of the ODH and show very low catalytic activity. The research results reveal that the defects obtained by ball milling can induce the production of more hydroxyl groups on the surface of MWCNTs by hydroxylation and results in the substantial increasing of the catalytic activity for ODH.3. Mesoporous CeO2nanobelts have been synthesized by a facile hydrothermal route via controlling cation type and concentration of alkali without any surfactant or template. The key of the synthesis of CeO2nanobelts is forming the beltlike precursors in the presence of excessive NaOH during hydrothermal process. The enough OH-ions induce the two steps of Cannizzaro disproportionation reaction to result in conversion of formate partly into carbonate, and Na+ions with small ionic radius allow the coexistence of carbonate and formate in structure and accordingly are favorable to anisotropic growth of beltlike precursors. The increased hydrothermal temperature can promote the formation of carbonates and decrease the minimal required Na/Ce molar ratio. When Na+is substituted by the same concentration of K+or NH4+, the obtained precursor products are finally nanowires or irregular morphology. XRD measurement proves that the sodium ions do not enter the frame of CeO2nanobelts crystalline and can be easily removed by simple washing with deionzed water. After washing, the CeO2nanobelts with enlarged mesoporous pores show superior catalytic performance for CO oxidation compared with CeO2nanoparticles prepared with traditional methods.4. CeO2nanobelts, nanorods and nanowires have been synthesized by a formaldehyde-assisted hydrothermal system. It is found that the morphologies of these one-dimensional (1D) CeO2nanomaterials are dependent on the components of the corresponding precursors, which is achieved to control the reaction degree of Cannizzaro disproportionation tuned by Na/Ce molar ratio, hydrothermal temperature and type of strong alkali. The characterization results indicate that the morphologies of1D CeO2nanomaterials are closely correlated to their structural features, such as lattice cell parameters, specific surface area and pore volume. These1D nanomaterials show excellent morphology-dependent optical absorption and catalytic performance. From nanowires, nanorods to nanobelts, the visible absorption gradually decreases along with the increasing of band gap values, whereas the catalytic activity of CO oxidation increases along with lattice cell expansion.5. Hierarchical CeO2nanobundles are prepared by a cation-induced formaldehyde-carbonate hydrothermal route without any template or surfactant. It is found that for the synthesis of the CeO2nanobundle precursors, the formate is produced from formaldehyde via Cannizzaro disproportionation reaction and then renucleates with carbonate and cerium ions to grow anisotropically into the small nanorod products with formate and carbonate. The presence of ammonium ions leads to self-assembly of these small nanorod products into nanobundle precursors. After calcination, the obtained CeO2nanobundles possess smaller particle size and much more specific surface areas (130.4m2g-1) than CeO2nanoparticles prepared by a traditional precipitation method and thus, show much higher catalytic performance for CO oxidation.6. Visible-light-responsive titanium dioxide (TiO2) photocatalysts were fabricated by applying a low pressure H2-plasma treatment method on the commercial anatase TiO2nanopaticles. As compared to the pristine TiO2particles, the treated samples exhibit enhanced visible light absorption and excellent photocatalytic activity of degradation of methylene blue, especially under visible light (>400nm). As revealed by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) measurements, the H2-plasma treated samples have not any phase transition, or changes in particle size and chemical composition compared to the raw one. It is proposed that the H2-plasma treatment-induced defects states at the TiO2particles surface layer contribute to the enhanced visible light absorption, thereby greatly improving the photocatalytic activity.