Monodisperse Mesoporous Microspheres Of Nanogold/Silica As Highly Efficient Catalysts For Liquid-Phase Reactions

Since the discovery of low temperature catalytic activities of nanogold, supported goldcatalysts have found applications in a number of important chemical reactions such as water-gasshift reaction, selective oxidations and hydrogenations. To increase the catalytic performance,the gold nanoparticles used to prepare supported catalysts should be small and highly dispersedin the support. There are several methods for the preparation of supported gold catalystsavailable in the literature, but each of them has limitations. The impregnation method yields Aunanoparticles larger than30nm. In deposition–precipitation and co-precipitation methods, theinfluencing factors are various and difficult to control. The chemical vapor deposition and directcurrent magnetron sputtering need special instruments. Gold colloids have been used asprecursors to prepare supported gold catalysts, with the advantage that the size of gold particlescan be tailored before their deposition on the support and the Au nanoparticle aggregationduring post-synthesis treatment can be eliminated. To our knowledge, however, there is noreport on the preparation of attrition-resistant catalysts using Au nanoparticles as a precursor.The goal of this thesis is to develop novel methods for preparation of supported nano-goldcatalysts with uniform particle sizes and tunable pore sizes. The approach is based onpolymerization of urea/formaldehyde （UF） in the presence of gold and silica nanoparticles toyield UF/SiO₂/Au composite particles followed by calcination to provide monodispersedmesoporous SiO₂/Au microspheres. The specific objectives are threefold: i) preparation andcharacterization of gold colloids; ii) synthesis and characterization of mesoporousnanogold-silica composite microspheres; iii) evaluation of catalytic performance of thecomposites obtained.First, the gold colloids of5²0nm were synthetized through the citrate reduction andcitrate/tannins reduction method. The gold colloids stabilized by citrate/tannic acid exhibitedbetter stability against aggregation at about pH2at which the polymerization ofurea/formaldehyde is carried out and therefore were chosen as precursors to be used in thepreparation of SiO₂/Au composite microspheres.Second, the mesoporous gold-silica composite microspheres were synthesized through atwo-step procedure and the morphological, structural and textural characteristics were examined.The UF/SiO₂/Au composites were prepared by the condensation of urea and formaldehyde in thepresence of Si sols and gold colloids. Removal of UF by calcination produces mesoporousSiO₂/Au composites. The effect on particle properties of gold colloids used, pH and the amount of Si sols added were investigated. The particle size, dispersity and the loading of Au NP in thesupports, the specific surface area and the pore structure of the composites were characterizedby UV-Vis spectrum, HRTEM, XRD, HRSEM atomic absorption spectrometer（AAS） and N2adsorption-desorption. Au NPs were highly dispersing in the support, and the Au content can becontrolled. Besides, the composites were spherical with the diameter of2μm, large specificsurface area and pore volume.Finally, the catalytic performance of the composites was evaluated using a model system inwhich the reduction of p-nitroaniline by sodium borohydride aqueous solution. The compositeshad effectively been employed as a catalyst for the reduction of p-nitroaniline. Thepseudo-first-order kinetics with respect to p-nitroaniline could be applied to this experimentalsystem. The rate constant k for the catalyst with2.18%Au loading was0.45min^-1.In summary, monodisperse mesoporous nanogold/silica microspheres can be prepared via atwo step procedure based on polymerization and calcination. Because of their high mechanicalstrength, low mass transfer resistance and good catalytic activity, these nanogold/silicacomposites are expected to find widespread applications in liquidphase reactions.