| Rare-earth metals and their compounds have been the focus of many investigations in recent years, owing to their characteristic activity and selectivity for a range of catalytic reactions. Particularly, cerium/ceria and samarium/samaria, have been used as components in various catalytic applications for a number of chemical process. It has been investigated that the catalytic efficiency of combined steam and carbon dioxide reforming of methane and N2O decomposition and reduction can be remarkably enhanced via modification of AI2O3support with Ce or CeO2respectively. In addition, Sm2O3has also been reported to be an effective promoter to the Al2O3catalysts in oxidative coupling of methane. In order to improve the performance of such catalysts, it is necessary to gain a more fundamental understanding of the structural and physical properties of rare-earth metals/Al2O3catalysts at an atomic level. It turns out that the surface/interface electronic structures and preperties of the metal/oxides catalysts play an important role in theri applications. In fact, most metal oxides are insulators or wide-bandgap semiconductors (such as Al2O3with a large bandgap of roughly9eV) that have very low electrical conductivities. Therefore, it is difficult to use surface-science techniques that are based on detection of electrons/ions to characterize and study the surface and interface properties of metal/bulk oxide systems because of photoinduced surface charging effects. To overcome this difficulty, surfaces of ordered ultrathin oxide films on metal single crystals are often used as model surfaces for bulk oxides. In this study, the interaction between oxygen and Sm on Al2O3/Ni3Al(111) surfaces as well as the growth and electronic structure of Ce on Al2O3/Ni3Al(111) film under high vacuum conditions are investigated by X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoemission spectroscopy (SRPES).The main results of this thesis could be summarized as follows:1. In UHV system, three controlled doses of Sm were deposited on Al2O3at300K, namely10ML,1.5ML and0.3ML. At Sm coverage higher than one monolayer, exposure of oxygen to the Sm films leads to the formation of both samarium peroxide (O22-) states and regular samarium oxide (O2-) states. In contrast, when exposing O2to Sm film less than one monolayer on Al2O3, no O22-can be observed. Meanwhile, metallic Sm and Sm2+are oxidized to Sm3+upon O2exposure. On the other hand, further oxidation of the Ni3Al(111) substrate, resulting in an increase of the alumina film thickness is observed after O2exposure to1.5ML Sm/Al2O3at room temperature.2. The oxygen-saturated Sm (10ML/1.5ML)/Al2O3was subsequently annealed to high temperatures in UHV, respectively. The metastable O22" states dissociate, supplying active O atoms which can diffuse through the Al2O3thin films to further oxidize the underneath Ni3Al(111) substrate. Comparing with10ML Sm covered Al2O3, O22-is more stable on1.5ML Sm/Al2O3, suggesting that the strong interaction between Sm and Al2O3substrate may help to stabilize O2at Sm/Al2O3interface.3. Adsorption of Sm on the Al2O3surface in an oxygen atmosphere at300K leads to the formation of both O2-and O2-states. Predictably, only the growth of the Sm3+signal is observed, whereas there is no Sm+signal. It confirms the idea that oxygen stabilizes the trivalent configuration of Sm atoms. Thickening of the alumina film has been detected by XPS during annealing simultaneous adsorption (O2/Sm) on Al2O3surface. We suggest that Sm, presumably in its peroxide form, acts as a catalyst for the reoxidation of the metal substrate below the alumina film by supplying the reactive oxygen species via dissociation at elevated temperatures. But we still could not exclude the possibility that samarium introduced structure imperfection in the oxide lattice and changed the electronic structure of the oxide, which promoted oxygen uptake and diffusion.4. The interfacial chemical reactions and electronic structure of vapor-deposited Ce on a well-ordered Al2O3/Ni3Al(111) ultrathin film under high vaccum conditions have been studied preliminarily using SRPES. Our results indicate that the interaction of Ce with Al2O3thin films leads to an initial oxidation of Ce at room temperature. The oxidation state of Ce+is observed, which can be attributed to the electron transfer from Ce adatoms to the Al2O3/Ni3Al(111) substrate. Finally, the metallic Ce atoms appears at high coverages. Annealing the4.25ML Ce/Al2O3film at T≤750K. results in further oxidation of the metallic Ce. |
PDF Full Text Request