The carbon monoxide oxidation reaction over controlled catalyst structures based on gold/cerium dioxide thin films

Gold-based catalysts have attracted significant research interest due to their remarkably high activity for many important reactions, including the low-temperature CO oxidation reaction. Despite extensive studies, several fundamental issues in the understanding of CO oxidation over supported gold catalysts are unresolved. The locus and nature of active sites, including the role of the metal, metal oxide support, and their interface, remain controversial. The objectives of this thesis were to study CO oxidation on Au/CeO2 catalysts that were prepared using thin film techniques so as to form geometrically controlled structures in contrast to the typical supported nanoparticle systems. Using these structures enables one to evaluate the contribution of metal oxide support (CeO2), and locate and identify the active sites in the Au/CeO2 system. In particular, the specific contribution to the reaction by the metal/oxide interface, the role and nature of the vacancies in the oxide, the role of the dimensions of the Au and ceria, and the role of stress in the gold films have been investigated.;Two catalyst systems, each employing nanoscale films, were investigated. One was formed from a "single layer" of metal or oxide in the form of an extended surface that was then decorated with a thin layer of the other catalyst partner; the second approach was a study of multilayer metal/oxide structures to assess the role of the metal/metal oxide interface. More specifically the reaction sites formed at the three-phase boundary of metal, metal oxide and gas.;In the first case, a Au film (4.5nm) was prepared using vapor deposition on SiO2/Si (100) substrates. The gold coalesced into discrete islands with large lateral size (20-100nm) and was found to be only weakly active. The extremely low activity of gold with large lateral extended surface has been attributed to the lack of uncoordinated gold atoms on this type of gold surface. Subsequently decorating the gold with nanosized ceria created an active catalyst with an activity about three orders of magnitude higher. In addition, the activity scaled with the extent of the ceria decoration on the same type of gold surface. Comparison of the activities between this large-sized gold catalyst and the conventional nanosized gold catalysts reported in the literature showed that the two types of catalysts were not greatly different in activity. This result demonstrates that there is a reaction channel that is promoted by ceria, and is independent of the size of the gold. Nanoscale gold is not a necessary requirement for activity. Comparing Au films as-deposited and after annealing, with the same amount of ceria decoration, the as-deposited gold film was slightly more active than the annealed gold film, which indicates the roughness or stress in the gold plays a minor role in the activity. Properties such as the number of defects in ceria and the morphology/strain in gold were controlled for the films before decorating the surface with nanoparticles of the counter material. It was found that defects in ceria affected the activity. A sample with 24% Ce3+ was more than twenty times more active than one containing 7% Ce3+ with the same amount of Au decoration. The decreased activity of Nb-doped ceria provided evidence that the active defects in ceria are its oxygen vacancies.;Stability studies showed that the morphology change and the stress reduction in gold, the defect loss in ceria, and the surface contamination all contributed to activity loss with time-on-stream. However, key among these parameters is the ceria oxygen defect loss. The structure change such as the reduction in atomic level roughness and the defect loss in ceria were irreversible to treatments in He (up to 250 °C) and H2 (up to 200 °C) and accounted for over 90% activity loss of the catalysts in the model structure. Some of the surface contaminants can be removed by purging with inert gas at elevated temperature, however, the recovered activity was only a fraction of the initial activity. The deactivation rate was higher in the first few hours on stream, which was attributed to the structural change, especially loss of interaction at the interface between the gold and ceria, and the activity loss was over 50%. The long-term deactivation is attributed to further sintering of the gold. The deactivation continued even when only less than 1% of the initial activity remained.;Kinetic studies were performed on a relatively stable sample. After correcting for deactivation, the reaction order was found to be 0.62 for CO and 0.26 for O2. The results fell in the range of the values reported by other researchers. These results indicated that both CO and O2 were adsorbed on the surface during the reaction. Additional spectroscopic studies are recommended to identify the type of adsorbed oxygen.;The second structure examined used multilayer Au/CeO2 structures in the form of nanotowers with 10 micron cross-sectional areas but with alternating nano-layers of each phase. Each nanotower has an inert SiO2 layer on the top so only the edge of the multilayer was exposed to the gaseous reactants. By inserting inert layers between Au and ceria layers, the length of the exposed interface or the three-phase boundary of the gold/ceria can be changed independently of the total exposed surface area of gold and ceria and independently of the film thickness of each layer. Some nanotowers were prepared with gold film thicknesses corresponding to the typical dimension of gold in supported active nanosized catalysts. The nanotowers were characterized using AFM, SEM, and TEM. It was found that the three-phase boundary between any two layers is essentially a straight line and the activity scaled with the length of the three-phase boundary for nanotowers with the same structure (film thickness and surface treatment). Variable activity per unit length was observed, but this was a secondary, minor effect. The variability is a complex function of strain relaxation of gold with increased film thickness, annealing, and surface treatment with Ar+ sputtering, all of which might have affected the properties of the interface.;The overall finding of this thesis is that the strong interfacial interaction between a highly defective ceria and gold (of any size) creates the active gold sites for the CO oxidation reaction.;Key words: CO oxidation, gold, ceria, interface, nanoparticles, thin films...