Interface Properties Of Ag/Ceo₂and Ni/Ceo₂Model Catalysts

Ceria-supported metal catalysts are widely used in many important catalytic reactions, including automotive exhaust emission-control reactions, low-temperature water-gas shift reaction, and steam reforming of ethanol owing to the unique redox properties and oxygen storage capacity of ceria. Ceria-supported Ag and Ni catalysts are attractive for industrial applications due to their promising catalytic properties in these reactions, and, more importantly, their low cost and easy preparation. Therefore, the studies of the structures of Ag/CeO2and Ni/CeO2catalysts and their catalytic properties at the atomic-molecular level are very important since it helps to improve the catalysts’efficiency and develop new catalysts with high performances, which can help to solve the severe problems on energy crisis and environmental pollution. It turns out that the interfacial electronic structures and properties of the metal-ceria catalysts play an essential role in their applications. The studies on the model systems involving metal particles supported on planar oxide substrates under well-controlled conditions have been proved to be a useful approach to provide fundamental knowledge on the interfacial properties of metal/oxide systems. However, only a few reports are addressed on the electronic structures and properties of Ag/CeO2. In order to understand the interaction of Ag with the ceria support and the chemical state of supported Ag at the atomic-molecular level, we employed synchrotron radiation photoemission spectroscopy (SRPES) together with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) to investigate the growth, interfacial electronic structure, and thermal stability of vapor-deposited Ag nanoparticles on well-ordered CeO2(111) surfaces.The CO2activation or dissociation is the key step in the catalytic processes of CO2conversion, such as CO2reforming of methane and reverse water-gas shift. Several studies have shown that ceria-supported metal particle catalysts can promote CO2activation. However, the mechanism of CO2activation over metal/ceria catalysts is still unclear. In order to gain a deeper insight in the mechanism of CO2activation over the metal/ceria catalysts at the molecular level, a comprehensive study of CO2interaction with the ordered CeO2(111) and the Ni/CeO2(111) model catalysts as well as the interaction between Ni and CeO2(111), were performed by using XPS, reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The main results of this thesis can be summarized as follows:1. The growth, nucleation and thermal stability of Ag on CeO2(111) thin films as well as interfacial interaction between them were investigated by photoemission spectroscopy. Stoichiometric CeO2(111) thin films(-4nm) were grown on a clean Ru(0001) substrate, followed with the chemical vapor deposition of Ag nanoparticles. At room temperature, Ag initially grows as two-dimensional(2D) islands on the CeO2(111) thin films up to0.3ML, followed by three-dimensional (3D) growth with a number density of-4×10particles/cm. Compared to the Ag growth on the well-ordered fully oxidized CeO2(111) surface, Ag favors the growth of smaller particles with a larger particle density on the reduced or rougher ceria substrate surface with more defect sites. The binding energy of Ag3d increases when the Ag particle size decreases both on oxidized and reduced CeO2-x(111) surfaces, which are mainly due to the final state screening. The Auger parameter method was used to distinguish the initial and final state contributions to the particle size-dependent Ag3d core level shifts. The results indicate that the initial sate effects contribution can be ignored, suggesting that the interfacial interaction between Ag and CeO2(111) is rather weak. There is no electron transfer between Ag and ceria, even on the surface with increased number of oxygen vacancies. The slight reduction of the CeO2(111) and CeO2-x(111) surfaces upon Ag deposition has been observed, which may be ascribed to the reverse spillover of oxygen atoms on CeO2or CeO2-x at the Ag-CeO2/CeO2-x boundary to the Ag nanoparticles. Heating the Ag/CeO2(111) system leads to the significant sintering of Ag nanoparticles before they completely desorb from the CeO2(111) surface. The sintering temperature of Ag on CeO2-x(111) surface during annealing is slightly higher (-100K) than that of Ag on CeO2(111) surface, indicating that Ag nanoparticles are slightly more stable on the reduced ceria surface.2. The interaction of Ni with CeO2(111) and CeO2-x(111) thin films were investigated by XPS. Ni0is the predominate species observed upon deposition of Ni on CeO2at300K. However, a small amount of Ni+species is also found and an increased ratio of Ni2+to Ni0is observed with the decrease of Ni coverage. Moreover, CeO2is slightly reduced after Ni deposition onto the surface, suggesting that a strong interaction between Ni nanoparticles and ceria supports. This can be ascribed to the electron transfer from Ni adatoms to the CeO2thin film substrate. Upon heating, the Niz+species grows at the expense of metallic Ni. Considering the previous STM observation reported in the literature, the appearance of Ni2+features can be contributed to the formation of mixed Ce-Ni-O mixed oxide through the migration of ceria to the surfaces of Ni particles. Interestingly, metallic Ni is the only species on the reduced CeO2-x(111) thin films upon deposition of Ni. Upon heating of the Ni/CeO2-x(111) system, Ni nanoparticles experience significant sintering before Ag desorption from the surface. No oxidation of Ni nanoparticles is found. Using the Auger parameter method, it confirms that there is no electron transfer between Ni and CeO2-x. These results indicate that the chemical states of Ni nanoparticles on the ceria substrates strongly depend on the oxidation state of ceria and the temperature of substrate.3. The adsorption and activation of CO2on CeO2(111) and Ni/CeO2(111) films were studied by RAIRS and XPS. When CO2is exposed to the surface of clean CeO2(111) film at97K, the physisorbed linear CO2molecules and bidentate carbonate species(CO32-) are observed. With the increase of the reduction degree, i.e., the concentration of oxygen vacancies, of the ceria surface, the intensity of CO," band increases. CO2immediately dissociates into CO and O on the CeO2(111) surface covered with Ni nanoparticles upon the adsorption at97K, leading to the formation of Ni-CO adsorbates and partial oxidation of Ni nanoparticles. This dissociation activation of CO2is enhanced when the coverage of Ni increases, and inhibited when Ni nanoparticles on CeO2(111) are pre-oxidized. The results indicate that Ni naop articles play a crucial role in the dissociation of CO2. In contrast to the results reported for CO2adsorption on Ni single-crystals where the dissociation temperature was found to be higher than240K, the much lower dissociation temperature (-97K) for CO2on Ni nanoparticles supported on CeO2(111) suggests that the Ni/CeO2catalyst exhibits high activity toward CO2activation. Thermal treatment also activates dissociation of CO2on the reduced CeO2-x films, which gives rise to reoxidation of CeO2-x, that does not occur upon exposure to CO2at97K, At the same coverage of Ni, the desorption temperature of CO species on the Ni/CeO2-x (111) surface (-360K) is slightly higher than that on Ni/CeO2(111) surface (-300K). Moreover, the saturated adsorption capacity of CO2for Ni/CeO2-x is more stronger than that for Ni/CeO2(111). These results suggest that CO interacts with Ni/CeO2-x more strongly, particularly with the Ni nanoparticles on surface, which may be related to the smaller particle size and more Ni0species of Ni nanoparticles on the reduced CeO2-x(111) surface, as compared to the oxidized CeO2(111) surface.