| Ceria-based materials has been widely applied as supporters/promoters in the field of catalysis such as water gas shift (WGS), CO oxidation and three-way catalysis. Generally, the excellent catalytic performances of ceria-based catalysts are closely related to the surface strcuture of ceria (especially the concentration and distribution of oxygen vacancy), the ceria/metal interaction and the composition and structure of ceria/metal interface etc. The well-defined model catalyst, which has been prepared in-situ under Ultra-high Vacuum (UHV) condition, can be used as a model system for a detailed understanding of the reaciton mechanism of heterogeneous catalysis. In this thesis, the inverse ceria/metal model surfaces with various surface composition and structure have been prepared and characterized. Furthermore, the adsorption and reaction behavior of small probe molecules (including D2O CO and atomic D) has been investigated on the above inverse model catalysts. The main results are summarized as follows:(1) Adsorption/reaction behavior on Cu(111) and CuOx/Cu(111)The adsorption behavior of NO on clean Cu(111) and the effect of pre-covered oxygen species have been investigated. NO adsorbs dissociatively on Cu(111) surface even at low temperature and left NO(a) and O(a) on the surface. By controlling NO exposure and the substrate temperature, several different types of O(a) can be produced: the metastable species O528(O1s BE-528.9eV), chemisorbed species0529(O1s BE-529.5eV), chemisorbed species O531(O1s BE～531.0eV). In the presence of O531species, the relative occupation of different NO adsorption states was largely affected and most of the adsorbed NO(a) underwent dissociative desorption, releasing N2O and N2upon heating. In contrast, in the presence of O529species, the dissociative desorption of NO(a) was largely suppressed. Furthermore, the O529species show a more significant blocking effect on NO adsorption than the chemisorbed O531species.The CuOx/Cu(111) surface was prepared by the oxidation of Cu(111) under UHV condition and the adsorption behavior of atomic deuterium (D(g)) on the CuOx/Cu(111) was studied. Exposure of CuOx/Cu(111) to atomic D(g) even at low temperature (115K) can produce surface hydroxyl groups (D(g)+OL'OD(a))and leads to the formation of the adsorbed water(OD(a)+D(g)'D2O(a))at higher D(g) exposure. The adsorption of D(a) on the Cu+sites of CuOx was also observed. Compared to Cu(111), the diffusion of D(a) into the bulk phase was significantly suppressed for D(a) adsorption on CuOx/Cu(111). The stability of surface hydroxyl species was found to depend largely on the surface composition and oxidation/reduction state of CuOx/Cu(111).(2) The preparation and chemical properties of CeO2(111) model surfaceThe adsorption behavior of D2O and atomic D was investigated on the well-defined CeO2(111) thin film, which was prepared on Cu(111) substrate by physical vapor deposition. On the stoichiometric CeO2(111) surface, the adsorption of atomic D leads to the formation of surface hydroxyl and D2O as well as the reduction of Ce4+into Ce3+. For D(g) adsorption on the reduced CeO2-x(111), surface hydroxyls are the main adsorption products while the formation of D2O(a) is largely suppressed. Moreover, D2O adsorbs both molecularly and dissociatively on the stoichiometric CeO2(111) surface while dissociative adsorption is much preferred on the reduced CeO2-x(111). Furthermore, on reduced CeO2-x(111) surfaces, the stability of OD(a) was enhanced by the presence of oxygen vacancies. Upon heating, surface hydroxyls undergo two competing reaction pathways:one is the associative desorption of OD(a) releasing D2O and creating oxygen vacancies (OD(a)+OD(a)'D20(g)+Olattice+Ovacancy), and the other one is to produce D2via OD(a)+OD(a)'D2(g)+2Oiattice. The presence of oxygen vacancies in CeO2favors the reaction pathway of D2formation.The interaction between oxygen and CeO2(111) thin film was also investigated by X-ray Photoelectron Spectroscopy(XPS). O2hardly adsorbs on the stoichiometric CeO2(111) surface even at a low temperature of115K. In contrast, on reduced CeO2-x(111) film, two kinds of adsorbed oxygen species are formed:peroxide species(O22-) and superoxide species (O2-), which exhibit the O1s feature with the binding energy of531.7～532eV and533.6～534eV, respectively. By using atomic Dreduction and low-temperature Ar-sputtering, two types of reduced ceria films with different vacancies structure were prepared and the related oxygen adsorption behavior were compared. After O2adsorption on the D(g) reduced CeO2-x(111) film, the amount of superoxide(O2-) and peroxide(O22-) species are comparable while peroxide(O22-) species are predominant on the low-temperature Ar-sputtered ceria films. Furthermore, We also prepared a disorder polycrystalline CeO2thick layer(about28ML) on Au(110)-(1×2) substrate and some preliminary results of O2 adsorption behavior were obtained. Surprisingly, O2desorption was observed in TPD results with the desorption temperature of～230K upon O2exposure on CeO2/Au(110) at100K. In the case of D(g)-reduced CeO2-x film, additional O2desorption peaks were detected in the temperature range of250-400K. Although a clear assignment of these O2desorption peaks are not possible at moment, the TPD results may provide some evidences for the structure-dependent O2adsorption behavior on ceria film.(3)The preparation and surface chemistry of inverse CeO2/Cu(111) model surfaceThe inverse CeO2/Cu(111) model surface was firstly prepared with the ceria coverage of0.5ML. It was found that, by choosing the D(g) exposure and substrate temperature, the surface structure, composition and surface species of the inverse model surface can be controlled to some extent, on which the adsorption and reaction of CO, D2O and atomic D was studied. The partly-covered ceria layer mostly shows a "site-blocking" effect on the adsorption properties of Cu substrate. The adsorption behavior on the ceria layer is largely dependent on the thickness. Compared to the thick CeO2(111) film(10ML), the removal of the lattice oxygen (the reduction ability) is much more difficult on the thin CeO2(111) layer (0.5ML) upon D(g) exposure at higher temperature. Furthermore, the dissociative adsorption of D2O was largely suppressed on the reduced thin CeO2-x(111) layer(θ=0.5ML). The above thickness-dependent properties of ceria layer was ascribed to the structure of Cu-CeO2interface and the electronic modification of Cu substrate. In addition, by tuning the D(g) exposure and substrate temperature, a model system was successfully prepared, in which CO adsorbs on Cu substrate and OD on ceria layer, and the possibility of the interfacial CO+OD reaction was further examined. No CO2formation was found via the interfacial CO+OD reaction, which suggests that such reaction mechanism is not likely on the inverse CeO2/Cu(111) model catalyst under the studied experimental condition.In summary, the geometric/electronic structure and the adsorption/reaction properties of various CeO2/Cu(111) model surface were investigated in the thesis. The obtained results will deepen our understanding on the structure-activity relationship as well as the related reaction mechanism in the ceria-based catalytic systems. |
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