Structures And Interface Properties Of ZrO₂ Supported Cu, Ag, Co And Ni Model Catalysts

Among various metal oxides, zirconia is of particular interests and has received widespread attention thanks to its ideal mechanical and chemical stability. In modern industry, zirconia has been widely used in various fields, for instance, high-temperature fuel cells, gas sensors, passivating coatings for metal anticorrosion, insulator in metal-oxide-semiconductor devices. In fact, one of the fields where zirconia has been technically employed to a large extent is heterogeneous catalysis. It has been well-demonstrated that zirconia can be used either as an active phase or a support material in heterogeneous catalysis. Cu/ZrO2 is well-known as the catalyst for the methanol steam reforming reaction.In addition, catalysts consisting of Ag or Cu supported on zirconia have also been investigated for the synthesis of methanol from CO2 and H2. Moreover, Co/ZrO2 and Ni/ZrO2 catalysts have a significant application in Fischer-Tropsch synthesis, low-temperature CO oxidation, NOx reduction, carbon dioxide reforming of methane and steam reforming of ethanoL Thus, a fundamental atomic-scale understanding of the interaction of metal with ZrO2 is essential for the development of highly efficient ZrO2-supported metal catalysts. The main results of this thesis are summarized in the following:(1) The growth, electronic structure, and thermal stability of Ag nanoparticles on thin ZrO2(111) film surfaces were investigated by low-energy electron diffraction (LEED), synchrotron radiation photoemission spectroscopy (SRPES), and X-ray photoelectron spectroscopy (XPS). The ordered ZrO2(111) film was prepared by evaporating Zr metal using an e-beam evaporator onto a Pt(111) surface in O2 atmosphere at 550 K, followed by annealing in ultrahigh vacuum A three-dimensional growth mode with a nearly constant particle density of ～2.0×1012 particles/cm2 was found for Ag growth on ZrO2(111) at 300 K. XPS results indicate that the binding energy (BE) of Ag 3ds/2 shifts to a higher BE up to 0.5 eV when decreasing the Ag coverages. The Auger parameter analysis shows that the primary contribution to the Ag 3d core level BE shift is the final state effect, indicating a very weak interaction between Ag nanoparticles and ZrO2(111). In addition, two different coverages (0.3 and 1.0 ML) Ag on ZrO2(111) are annealed to elevated temperatures. Thermal stability experiments demonstrate that Ag particles undergo serious sintering before they desorb from the zirconia surface.(2) The adsorption and decomposition of methanol on zirconia-supported Cu and Ag model catalysts were investigated by temperature programmed desorption (TPD), XPS and SRPES. On all the ZrO2(111),Cu/ZrO2(111) and Ag/ZrO2(111)surfaces, methanol adsorption at 110 K formed multilayers at high exposures. Upon heating, the multilayer methanol desorbed at 140 K, accompanied with the formation of chemisorbed methoxy groups. Further increasing the temperature led to the decomposition of methoxy on all surfaces via different pathway. On the ZrO2(111) surface, methoxy decomposed above 500 K to produce CO. However, on the Cu/ZrO2(111)surfaces, Cu coverage dependent thermal evolution of methoxy was investigated:at low coverage of Cu(0.1 ML), except for the formation of surface formate through the partial oxidation of methoxy, the dehydrogenation of the methyl group of methoxy also occurred. White athigh coverage of Cu (1 ML), only the dehydrogenation process could be found. In contrast, on the Ag/ZrO2(111) surfaces, the methoxy recombines to methanol and desorbed from the surface, so the adsorption of methanol is reversible.(3) The growth mode, interfacial interaction, and thermal stability of Co and Ni nanoparticles on ordered ZrO2(111) film surfaces were investigating by XPS, ultraviolet photoelectron spectroscopy (UPS) and LEED. The results indicate that both Co and Ni follow atypical Stranski-Krastanov growth mode on ZrO2(111) films, with a break-point at 0.7 and 0.5 ML, respectively. The interfacial interactions of Co/ZrO2(111) andNi/ZrO2(111) model catalysts are relatively strong. There is a full charge transfer taking place between Co and ZrO2 substrate when Co at very low coverage (0.05 ML), namely the Co is oxidized to Co2+. The Auger parameter analysis shows that the Ni 2p binding energy shift at Ni low coverage (0.05 and 0.1 ML) is caused by partial charge transfer between Ni and ZrO2(111) films, with forming Niδ+. Thus, the interfacial interaction of Co/ZrO2(111) model catalyst is stronger than Ni/ZrO2(111). Two different coverages of Co/ZrO2(111) and Ni/ZrO2(111) model catalysts are chosen to investigate the thermal stability (Co:0.5 and 1.0 ML, Ni:0.05 and 0.5 ML). During the annealing process, both Co and Ni are gradually oxidized to oxides. At the sometime, cobalt and nickel also diffuse into the ZrO2 lattice. Duo to Co is more active than Ni, when the coverage of Co and Ni is same (0.5 ML), with increasing the temperature, Co is fully oxidized to Co2+ while only a portion of Ni is oxidized to Ni2+.