The Controlled Synthesis,2d Assembly Of Pd And Au Nanoparticles And Catalytic Performances Of The Corresponding2D/3D Supported Model Catalysts

Thanks to the advent of nanotechnology, catalysis science came into the new eraof nanocatalysis at the end of the last century. Since then, nanocatalysis has beendeveloping at a rapid pace and made much remarkable progress. However, as thetheoretical knowledge of catalysis is understood more, nanocatalysis faces huge newchallenges. The catalyst is a nanoscale engine, and in order to ensure its role inpromoting the progress and development of future science and technology, it isnecessary to control the composition and the structure of the catalyst precisely overlength scales from1nanometer to1micrometer. To achieve this goal, the primary taskof nanocatalysis is to prepare the nanocatalysts in a controlled way under differentdimensions and to develop suitable model catalysts. Nowadays, although a largenumber of catalytic meaningful research results have been made with the currentmodel catalysts, most model catalysis researches still depend on the ultra-highvacuum (UHV) system and the single crystal surface model catalysts. Even the adventof the in-situ and the elevated-pressure characterization approaches extends the modelcatalysis research, how to overcome the "material gap" and "pressure gap" betweenthe model catalysts and the industrial counterpart is still a great challenge, and todevelop new models catalyst is very urgent. On the other hand, because of thevarieties of the catalysts, the study on the "structure-performance" relationship overdifferent catalysts and catalytic systems has yet to be carried out. Based on thesituation of nanocatalysis stated above, in this dissertation, the two and threedimensional (2D and3D) model catalysts were prepared based on the palladium andgold nanoparticles (NPs) with controlled morphology and size, and characterized withAFM, XPS and other approaches. The two huge "gaps" between the model catalystsand the industrial ones are overcome further and the "structure-performance"relationship in the CO catalytic oxidation reaction is studied in detail. The mainresearch contents and the corresponding results are as follows:1. The gold (Au) NPs with different sizes were synthesized using the chemicalreducing method, and how the kinds and the adding way of the reducing agents, andthe concentrations of the precursors affect the size of the NPs have been investigated.The2D assembly of Au NPs was achieved with the aid of(3-Aminopropyl)trimethoxysilane (APTMS), and the effects of the factors in the assembly process on the coverage of the Au NPs on the surface of the silicon waferwere studied by the characterization of AFM. It is shown that, with NaBH4as thereducing agent, when the PVA/Au4+(mass ratio) was1.5/1and NaBH4/Au4+(moleratio) was5/1, the monodispersed gold nanoparticles were synthesized and theaverage size is3.0nm. When trisodium citrate acted as the reducing agent and theAu4+/trisodium citrate (mole ratio) value was1/4, the Au NPs with the average size of15.2nm could be synthesized.2D Au NPs assembly results show that compared to thesilicon wafer, it is much easier for Au NPs to be adsorbed on the mica surface. Whenthe silicon wafer was treated with the silane, Au NPs could be assembled on themodified silicon wafer with different distributions. To achieve the close-packedmonolayer of Au NPs, the concentrations of the APTMS-methanol solution and theAu NPs colloid should be1wt%and0.096mmol·L-1, respectively, and the assemblyprocess should be lasted for4h. Last but not the least, the AFM characterization onthe2D assembly of the Au NPs shows that AFM is one powerful method tocharacterize the2D supported model catalyst.2. Palladium (Pd) NPs with spherical and cubic morphology were successfullysynthesized with the chemical reducing method and the2D assembly of the Pd NPswas studied at the aid of AFM. It was found that, Pd NPs with small size can besynthesized under the following conditions: the total solution volume is200mL, theconcentrations of the precursor, NaBH4and CTAB are5,15and100mM,respectively, and the reaction is carried out in the UAMR system. The average size ofthe as-synthesized nanoparticles is2.8nm. When the ascorbic acid is the reducingagent, under the following conditions: the total solution volume is100mL, theconcentrations of the precursor, ascorbic acid are0.5and0.8mM, respectively, theCATB/Pd2+(mole ratio) was25/1and the reaction is carried out at95oC, themorphology of as-synthesized Pd NPs are cubic and the average diagonal length is20.5nm. After being removed the excess CATB in the colloid with the"precipitation-dispersion" method, the Pd NPs can be assembled on the surfaces of thefreshly-cleaved mica and the washed silicon wafer. The2D assembly results showthat it is easier for Pd NPs to be adsorbed on the hydrophilic surface and the2Dassembly can be achieved on the silicon wafer which is treated with Piranha solution.The2D assembly results of Pd NPs Laid the foundation for the followinginvestigation on the2D Pd supported model catalyst.3. Based on the Au and Pd NPs synthesized as stated above, the2D supported model catalysts with the silicon wafer as the2D support were prepared using the"dip-coating" method. The preliminary study on the CO oxidation over the2Dsupported model catalysts were carried out with the homemade evaluation reactor andsystem. It is found that, the homemade evaluation reactor and the mass spectroscopyanalysis method are suitable to study the CO oxidation over the as-prepared2Dsupported model catalysts. Over the2D Pd and Au supported model catalysts, e.g.Pd-C/silicon wafer, Pd-S/silicon wafer, Au/silicon wafer, Au-PVA/silicon waferand Au-film-commercial, the surface properties of the2D substrates and themorphology of the NPs can affect the chemisorptions of O. And the chemisorbed O isfavorable for the CO oxidation reaction. Among the five2D supported modelcatalysts, the Pd-S/MgO (100) possess the biggest ability to chemisorb O,consequently, it shows the best catalytic performance toward the CO oxidationreaction.4. Based on the Pd NPs synthesized as stated above, and three materials withdifferent basic properties, e.g. MgO, SiO2and TiO2as the support materials, the3Dsupported model catalysts were prepared using the impregnation method. The"morphology effect" and the "support effect" in the CO oxidation reaction over the3DPd supported model catalysts were investigated. The results show that, the COoxidation activity over the six catalysts increase in the following order: Pd-C/TiO2<Pd-C/SiO2<Pd-S/TiO2<Pd-S/SiO2<Pd-C/MgO <Pd-S/MgO. Among the sixcatalysts, Pd-S/MgO possess the best catalytic performance toward the CO oxidation.The "temperature-dependent" CO-DRIFT spectra and the CO-TPD profiles show that,the effect of the basic property of the support on the adsorption behaviors of COmolecules is the same for the three cubic and the three spherical Pd supportedcatalysts. CO molecules adsorb on the cubic and spherical Pd NPs in the bridgedmanner. Enhancement in the basic property of the support can improve the COadsorption over the surface of Pd0. The "donor-acceptor" and the "donor-backdonation" models can explain the "basicity effect" of the support materials on the COadsorption behavior over the3D Pd supported model catalysts. Over the3D Pdsupported model catalysts, the "support effect" and the "morphology effect" followthe "volcano curve" theory.