Fabrication And Properties Investigation Of Supported Noble-Metal Nanocatalysts

Due to their high catalytic activities and selectivities, supported noble-metal nanocatalysts have been wildly used in the field of industrial catalysis and organic synthesis field, however, this kind of nanocatalysts still suffers from several technical difficulties. First, how to choose and fabricate nano-sized supports of good quality; second, how to develop a simple and mild method to control the immobilization of noble metal nanoparticles (NPs) on the surface of supports; third, how to separate the nanocatalysts from the reaction media for recycling; fourth, how to fabricate novel atomically noble metal nanoclusters-based catalysts and thus investigate the inherent relationship between the metal structures and the catalytic properties in molecular level. Thus, this thesis is focusing on the synthesis of the supported noble-metal nanocatalysts, aiming at solving the synthetic difficulties mentioned above, fabricating a series of noble-metal nanoparticles and nanoclusters-based catalysts achieved by combining the hydrothermal method, solvothermal method, or electrospinning technique with an in situ reduction approach and impregnation method. The composition of nanocomposites, catalytic properties and recycling features have also been investigated, to reveal the effects of microstructures, and specific surface area of the support, the size, the structure and the loading amount of noble metals, and the interaction between the support and catalyst on the activity and stability of obtained nanocomposites catalysts. The research of this subject is expected to provide a theoretical an experimental data for designing and fabricating novel nanocomposites catalysts with superior catalytic activity and satisfying perfonnance of separation and recycle. The main results are listed as follow:Hierarchical nickel silicate nanoflowers (NSFs) have been successfully fabricated by a facile one-step solvothermal method, onto which the highly dispersed small silver nanoparticles (AgNPs) with diameter about 8.28 nm were immobilized through in situ reduction of Ag+ by Sn2+. The AgNPs/NSFs showed an enhanced catalytic activity in the catalytic reduction of 4-nitrophenol. The results showed that AgNPs attached on the supporter tightly, indicating the strong interaction between AgNPs and NSFs support. We also extended this in situ loading approach to achieve AuNPs/NSFs nanocomposites with controllable-sized AuNPs by manipulating the amount of chelating agent. The catalytic results showed that the size of AuNPs had a great effect on their catalytic activities. When we reduced the size of AuNPs to 2.7 nm, the highest activity were achieved. Furthermore, these AuNPs/NSFs could still exhibit high catalytic activity even after being reused for 5 times.Yttrium silicate hollow spheres (YSiO) have been successfully obtained by using sacrificial templates, the size, shell thickness and inner cavity of the YSiO nanosphere can be easily tuned. The effects of the experimental parameters (eg. chelating agent, size of the template, reactant ratios, reaction duration, reaction temperature and so on) on the obtained products and the formation mechanism of hollow structure were investigated. Moreover, we have successfully achieved the ytterbium silicate, erbium silicate, and thulium silicate hollow spheres by using the similar method, indicating the versatility of this method. The highly dispersed ultra-small AuNPs were immobilized on the surface of YSiO nanospheres through in situ reduction, the particle size of the AuNPs was 3.6 nm. After being reusing for 5 times, the nanocomposites still exhibited high catalytic activity, indicating the strong interaction between AgNPs and YSiO support which led to the excellent stability of the nanocomposites. Furthermore, This synthetic strategy can be extended to prepare magnetic nanocatalysts with yolk-shell structure, offering the benefit of facile recovery and regeneration of the catalysts under magnetic field.Hierarchical double-walled copper silicate hollow nanofibers (CSHNFs) assembled by ultrasmall nanotubes have been successfully synthesized by combination of electrospinning technique and hydrothemial method. We can also obtain the hierarchical double-walled nickel silicate (NSHNFs) and magnesium silicate (MSHNFs) hollow nanofibers assembled by ultrathin nanosheets. These nanofibers showed a characteristic of three-dimensional nanostructure in the micro level and network structure in the macro level. The highly dispersed ultra-small PtNPs were immobilized on the surface of CSHNFs through in situ reduction, the particle size of the PtNPs was 2.2 nm. The catalytic test demonstrated PtNPs/CSHNFs had a high catalytic activity on CO oxidation. After calcination at 450 ? the supported PtNPs didn't show any aggregation, indicating the excellent thermal stability of PtNPs/CSHNFs. AuNPs/NSFTNFs have also been obtained by a similar method, the particle size of the AuNPs was 2.6 nm. AuNPs/NSHNFs showed remarkable catalytic performances on catalytic hydrogenation even after being reused for 5 times. Furthermore, the AuNPs/NSHNFs could be easily recycled due to their unique net structures and the sedimentation because of gravity.A new size of Au nanoclusters has successfully been prepared though a kinetically controlled approach, UV-vis-NIR absorption spectrum of this new cluster showed four distinct absorption bands at 1230,795,490, and 430 nm, respectively. X-Ray crystallographic analysis showed that this nanocluster features a rod-like structure assembled by three icosahedral Au13 units in a linear fashion. The observed differences in the UV-vis-NIR absorption spectra of Au37, Au25, and Au13 indicated that the electronic nature of the individual Au13 icosahedron was preserved in bi-icosahedral Au25 nanocluster and the tri-icosahedral Au37 nanocluster, while new collective features also emerged; demonstrating the size evolution of the electronic and optical properties in the clusters of clusters series with increasing numbers of Au13 building blocks. We have also investigated the catalytic behaviors of Au25/TiO2 and Au37/TiO2 nanocomposites on CO oxidation. Compared with Au25/TiO2, Au37/Ti02 exhibited a higher activity, indicating the size of the particles playing an important role on catalytic activity. Moreover, the catalytic activity of Au37/TiO2 would become lower after the ligands being partially or completely removed from Au clusters, which may be ascribed to the aggregation of the ligand-off clusters on the surface of the TiO2.