Probing Atomic Scale Structure and Catalytic Properties of Cobalt Oxide Model Catalysts

Sammanfattning: Cobalt oxides are known to be active catalysts for a number of chemical reactions, but very little is known about the atomic scale processesresponsible for the activity. The research presented in this thesis is focused on obtaining an atomic scale understanding of the chemistry of wellcharacterizedcobalt oxide model catalyst surfaces consisting of pristine and defective CoO and Co3O4 thin films with the (111) and (100)terminations supported by Ag(100), Ir(100), and Au(111) single crystal surfaces. The structure and the adsorption properties of probe moleculesonto these cobalt oxide model catalyst surfaces are studied under ultra-high vacuum conditions using the interplay of X-ray photoemissionspectroscopy (XPS), scanning tunneling microscopy (STM), and low energy electron diffraction (LEED). Further, high pressure XPS (HPXPS)is used to study the stability and phase transitions of the cobalt oxide model catalysts in more realistic gas environments. As a side project tothe work on cobalt oxide thin films the thesis gives a comprehensive spectroscopic picture of Ir(100) surface reconstructions and molecularadsorption onto these surfaces.The adsorption experiments of H2, CO, CO2, and H2O probe molecules give a detailed picture of the surface chemistry of Co oxide surfaces andit is demonstrated that Co ions naturally found on the surface of Co3O4(111) and Co3O4(100) thin films or artificially created on the CoO(111)surface are extremely important for chemical properties of the surface. Water dissociation, carbonate formation, weak adsorption of CO andCO2 are examples of processes that only take place in the presence of Co surface ions. The work at more realistic gas pressures in the mbarregime demonstrates that Co oxide thin films should be seen as dynamic films that easily change phase between the CoO and Co3O4 structurein response to the gas composition.To summarize, the work presented in this thesis is important for the fundamental understanding of cobalt oxide surfaces and their catalyticproperties, and hopefully, this fundamental understanding can be used to develop new and better cobalt oxide based catalysts.