Surface Studies of Model Systems relevant for Pd and Ag Catalysts

Abstract: This PhD thesis reports on investigations of the atomic scale structure of model catalysts relevant for the catalytic oxidation of CO and methane over Pd, and NOx reduction over silver-alumina, important reactions in automotive catalysts. By using complementary experimental techniques such as X-ray Photoelectron Spectroscopy, High Pressure X-ray Photoelectron Spectroscopy, Surface X-ray Diffraction, Low Energy Electron Diffraction, Infrared Spectroscopy, and Scanning Tunneling Microscopy, combined with theoretical calculations, the work presented in this thesis aims at an atomistic characterization of the active sites for silver-alumina and Pd model catalysts which may help to the design of new and improved catalysts. The results highlight the complexity of the different structures that may form upon oxygen interaction with Ag and Pd single crystal surfaces. The gas adsorption studies (CO, NO) on clean and oxidized silver surfaces give insights into the chemical properties of the surfaces, information important for the understanding of the reaction mechanism for the NOx reduction. We have been able to propose models for the adsorption sites for CO and NO on oxidized Ag surface and to propose a model system for the NOx reduction over silver-alumina. A particular surface orientation of bulk Pd oxide, the PdO(101) was found to be active for the oxidation of methane. Our studies allow us to reveal the active sites on the surface, information important for the design of better catalysts. Moreover, the results from the CO oxidation over Pd(100) give new insights into the reaction mechanism. The active phase switches between a surface with chemisorbed oxygen to a surface oxide when the oxygen partial pressure is increased.