Surface Structures Of Ag/CeO₂ Model Catalysts And Their Changes By Cu Or Zr Addition

CeO2 has been widely used in industrial applications as catalyst or catalyst support due to its superior oxygen storage capacity and unique redox properties. CeO2 supported Ag catalysts are of particular interests owing to their importance in technical applications for many chemical reactions such as automotive exhaust treatment reactions, preferential oxidation of CO in the presence of H2, methane oxidation reaction and oxidative decomposion of formaldehyde. However, the interaction between Ag and CeO2, the interfacial structures and the thermal stability as well as their determining factors of Ag/CeO2 catalysts are largely unknown. In order to gain a fundamental understanding of these issues, we employed model catalyst system to investigate the morphology, interfacial electronic structure, and thermal stability of vapor-deposited Ag nanoparticles on well-ordered CeO2(111) surfaces by using scanning tunneling microscopy (STM) together with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The catalytic activity of ceria-supported metal catalysts are known to degrade with time in the reaction environment. This problem can be solved by introducing the second metal into Ag/CeO2 to establish the ceria supported bimetallic system, resulting in much stronger thermal stability of ceria supported bimetallic nanoparticles upon heating. Moreover, the catalytical activity of ceria based bimetallic catalysts is found to be considerly better than the monometallic ones because of the synergistic effect However, the modulation mechanism in terms of how the second metal enhance the catalytical performance of metaVceria catalysts is still unclear. In order to probe the nature the synergistic mechanism of Ag and one another metal supported on ceria at the atomic-molecular level, we employed model systems consisting of vapor-deposited Cu-Ag and Zr-Ag bimetallic clusters supported on CeO2(111) to investigate the interaction between ceria and zirconium as well as copper, and their influences on the growth and sintering behaviors of Ag nanoparticles by using XPS and STM.In this dissertation, we have systemically investigated above-mentioned scientific problems to probe the interfacial structures and the thermal stability Ag/CeO2 and their changes by Cu or Zr addition at atomic-molecular level. The will provide us a promising pathway to design catalysts at fundamental level with higher activity and stability, which could be beneficial to solve the severe problems on energy crisis and environmental pollution.The main achievements in this thesis are summarized as follows:1. The growth and structures of Ag nanopartieles on CeO2-x(111)thin films with different thicknesses, morphologies and reduction degrees have been systematically studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The CeO2-x(111) thin films were epitaxially grown onCu(111). With increasing the ceria thin film thickness, the size of the terraces decreases along with the increase of the number of open monolayers and defects. In most cases Ag exhibits three-dimensional (3D) growth with constant particle densities on the CeO2-x(111) surfaces at 300 K. Ag mainly populates the sites at the ceria-ceria step edges instead of ceria terraces, independent of the thicknesses but influenced by the reduction degree of the ceria films. On the fully oxidized ceria films, the particle density is directly proportional to the number of step edges of ceria, which is related to its thickness on Cu(111). On the slightly reduced ceria. films which were prepared by annealing the fully oxidized ceria films in ultrahigh vacuum, single surface oxygen vacancies and their linear agglomerates are observed; but they do not anchor Ag particles during Ag deposition. While on the strongly reduced ceria films produced by decreasing the oxygen pressure during ceria film growth, large defect sites related to surface and subsurface oxygen vacancies are found; they can anchor the Ag nanopartieles, leading to the random distribution of Ag nanoparticles on ceria terraces upon deposition. Upon heating, the Ag nanoparticles undergo serious sintering before desorption at 800 K on the fully oxidized CeO2 films. While on the reduced ceria films, the sintering and desorption processes are slowed down at the same annealing temperatures as those on CeO2. Moreover, Ag nanopartieles exhibit stronger thermal stability on strongly reduced ceria surface compared to that on slightly reduced ceria surface. These results suggest that the defects on reduced ceria surfaces can enhance the thermal stability of Ag nanopartieles during annealing.2. The interaction of Cu with 2 nm thick CeO2(111)thin films was investigated by XPS and STM. Cu follows the two-dimensional (2D) growth mode on CeO2 surface at room temperature. Cu nanopartieles were found randomly dispersed on CeO2 surface without showing any preferential nucleation. Cu is oxidized to Cu2+ upon the deposition of Cu on CeO2 at 300 K, accompanied with the partial reduction of CeO2 thin film. These results indicate a strong interaction between Cu and CeO2, leading to the charge transfer from Cu adatom to CeO2 thin film substrate. Further deposition of Cu above 1 ML gives rise to the increase of metallic Cu. Upon heating, Cu nanoparticles undergo sintering with Ostwald ripening process, which means large Cu islands grow at the expense of small Cu nanoparticles. Further annealing leads to the desorption of Cu from the ceria surface at 700 K. Sequentially deposited Ag on Cu/CeO2 surface was found exclusively anchored on top of Cu clusters, which results in a higher particle density and smaller particle size of Ag nanoparticles compared to those on pure CeO2 surface. This result indicates that the existence of Cu can increase the dispersion of Ag and in turn its surface area. Besides, the thermal stability of Ag particles was also enhanced on Cu/CeO2 surface upon annealing. For comparison, we also investigated the influence of Ag on Cu/CeO2 system by depositing Cu on to Ag/CeO2 surface. It was found that Cuadatoms adsorb on the exposed ceria surface instead of the Ag nanoparticles, implying that the growth of Cu on CeO2 surface can be hardly influenced by the pre-deposited Ag nanoparticles.3. Submonolayer coverage of Zr was deposited onto 2 nm-thick well-ordered CeO2(111)thin films at different substrate temperatures under uftrahigh vacuum conditions. The structures, morphologies and interfacial electronic properties of Zr/ceria were investigated by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The strong Zr-CeO2 interaction leads to a two-dimensional (2D) growth of Zr on the CeO2 surface and the formation of a homogeneous Zr-O-Ce mixed oxide layer at room temperature, accompanied with the partial reduction of CeO2 substrate surface. Heating the Zr/CeO2(111) system leads to the sintering of the zirconia cluster before they; vanish from CeO2 surface. Besides, these zirconia clusters exhibit structure change upon heat treatment above 800 K. A disturb hexagonal structure emerges on the top of zirconia clusters along with the increase of the zirconia cluster height. Such hexagonal structure could be better observed upon deposition ofZr on CeO2(111) surface at 900 K under ultra-high vacuum (UHV) condition, which can be ascribed to the formation of ZrO2 (2×2)/((?)3 ×(?)) reconstruction. Hence, our STM and XPS results demonstrate the reverse oxygen spillover from ceria to zirconia clusters at high temperature, which leads to the further reduction of ceria and the structure change of Zr/CeO2 interface layer from Zr-O-Ce to O-Zr-O-Ce. Further depositing Ag on to the mixed oxides surface results in a higher particle density and smaller particle size of Ag nanoparticles compared to those on pure CeO2 surface. In addition, it is found that the sintering and desorption processes of Ag particles were retarded on such mixed oxide surface upon annealing. These results indicate that introducing Zrinto Ag/CeO2 system not only increase the dispersion but also enhance the thermal stability of ceria supported Ag nanoparticles.