Preparation And Characterization Of Supported Metal Catalysts With Controlled Structure And Size

This dissertation focuses on using plasma and surface science technology to study the structure and size effects in supported metal catalysts. The prepared and characterized metal catalysts range from monometallic Pd, Pt, Au, Ag, Co to bimetallic Co-Rh. The investigated catalytic reactions cover Suzuki coupling, methane combustion and Fischer-Tropsch synthesis. In order to fully characterize the catalysts and catalytic processes, a series of techniques were employed, including X-ray photoelectron spectroscopy (XPS), N2 sorption, small-angle and wide-angle X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive x-ray spectroscopy (EDX), NH3 temperature-programmed desorption (NH3-TPD), pyridine adsorption-infrared spectroscopy (Py-IR), CH4 temperature-programmed reduction (CH4-TPR), low energy ion scattering spectroscopy (LEIS) and so on.Highly dispersed metal (Pd, Pt) nanoparticles and uniformly distributed metal (Au, Ag) nanowires have been synthesized in ordered mesoporous molecular sieve SBA-15 via conventional incipient wetness impregnation followed by novel glow discharge plasma reduction. The diameter of the metal nanoparticles and the metal nanowires can be tailored according to the pore diameter of SBA-15 host while the population of the metal nanoparticles and the length of the metal nanowires can be tuned by varying the metal loading amount. Initial results indicate that the plasma reduced Pd/SBA-15 sample is very suitable for the Suzuki reactions between aryl halide and arylboronic acid. The advantage of this new synthesis method is that the process is simple, fast, green and economic.The beneficial effect of plasma treatment on the catalytic performance of Pd-based catalysts in methane combustion was investigated with the ordered mesoporous molecular sieve Al-MCM-41 as the model support. The results obtained confirm that palladium oxide (PdO) is the active phase. Plasma treatment enhances the acidity of the catalyst and improves the dispersion of PdO particles, which lead to a higher initial activity. The better stability for plasma treated Pd-based catalyst is attributed to the stronger interaction between palladium oxide and the molecular sieve support. Under ultrahigh vacuum conditions, the Co-Rh bimetallic catalysts were mimiced by two types of models, Co-Rh thin films and Co-Rh/SiO2 dispersed clusters. LEIS and line scan EDX were employed to characterize the atomic composition on the bimetallic surface. The characterization shows that a stable Co-Rh alloy is formed after high temperature treatment. Co is substantially enriched on the alloy surface compared with the total fraction value. This study demonstrates the efficacy of the various described preparation and characterization methodologies for the bimetallic model catalysts.Co/SiO₂ model catalysts were prepared and tested to investigate the intrinsic particle size effect in Fischer-Tropsch synthesis. The combination of kinetic data and XPS spectra demonstrates that no intrinsic particle size effect is observed for the metallic Co particles with size in the range of 3.5-10.5 nm. For the rather smaller Co particles (1.4 -2.5 nm), the metallic Co is easily oxidized by the water vapor, which leads to a lower TOF and a higher CH₄ selectivity. The clear picture developed by this model catalyst research versus the scattered data from technical catalyst studies highlights the significant importance of exploring the particle size effect or structure sensitivity on the well-defined surface.