Surface structure and catalytic activity of Pd and Fe oxide surfaces and thin films

Sammanfattning: Catalytic processes have tremendous importance in our everyday life. Most chemicals, pharmaceuticals, plastics etc. are, in fact, produced with the help of catalysts. What is probably even more important is that the toxic wastes produced in great amounts by modern factories and plants are recycled or transformed into less poisonous substances by means of catalytic units. One example from our everyday life is the exhaust of car engines and toxic emission of power plants. The products of fuel combustion, e.g. carbon monoxide, are extremely harmful for our health and environment. In order to reduce the negative impact, cleaning systems containing catalytically active materials are employed to handle toxic compounds.The reason why we need catalysts to transform one gas into another is that not all processes are favorable in nature and not all of them can proceed on their own. To continue with the carbon monoxide gas example, when the molecules of CO and the molecules of O2 are trapped together in a confined volume, they will simply coexist in a mixture preserving their identity. They do not react with each other because of the lack of energy required to initiate the reaction. The picture changes if a catalytically active material, e.g. platinum or palladium is introduced in the system. The gas molecules can then attach to the surface, where due to the interaction of electrons between the molecules and the substrate atoms, they can be activated and react with each other to produce CO2. This process is called heterogeneous catalysis because a solid catalyst is participating in the reaction of gases. Since heterogeneous catalytic reactions occur on the surface of a catalyst, it is clear that in order to handle greater amount of gases a more extended surface area is necessary. To achieve this, small particles of a catalytically active material, which all together have a much larger surface area than any extended bulk form, are deposited on a porous substrate, which is enclosed in a vessel attached to the emission line. Such catalytic units are widely produced and available for industrial use. In order to develop new, better and cheaper catalysts, however, the studies of the processes occurring on the surface of the active material are necessary. In particular, it is important to determine the atomic structure and its influence on the chemical and physical properties of the surface of the particles. The present dissertation reports on research work aimed at understanding the connection between structural properties of model catalysts and their catalytic performance. The samples used in the studies are extended atomically flat surfaces of crystals and thin films grown on them. The use of model catalysts is a way to overcome one of the two main problems arising in surface science — the material gap (between industrial catalysis and laboratory studies). It appears due to a restricted access to the particles in a real catalytic units. The second — the pressure gap — is constituted by the inability of most surface sensitive experimental techniques to operate under atmospheric pressures and, as a result, the requirement of ultra high vacuum conditions in an experiment. In the current work we employ a novel technique based on X-ray radiation, which can overcome the pressure restriction. The results reported in the dissertation show that when the surface of Pd acts as a catalyst in CO oxidation, a one-layer thick Pd oxide film forms on the surface and it is this compound that actually promotes the reaction. This film grows in thickness and finally loses the high degree of structural order upon increase of oxygen concentration. A stepped surface of Pd was also studied in a similar way in order to increase the complexity of the model system and approach closer to real catalysts. The steps on the surface mimic the edges of catalyst particles to a certain extent and allow to study the way they affect the reaction. Another model system studied in the current work is thin iron oxide films grown on Ag substrates. They were shown to perform well in catalytic oxidation processes. The reported results are therefore important for understanding of the catalytic performance of the studied structures. The special feature of such thin films is that, due to the difference between their atomic arrangement and that of the substrate, they can form completely different two-dimensional structures with different properties depending on the particular substrate and preparation conditions. The results of the current studies deliver insight into this dependence, which is necessary for the design of novel functional two-dimensional materials with desired properties.