Reduction properties of a model ceria catalyst at the microscopic scale

Cerium oxide is a rare earth metal oxide with versatile catalytic properties used for many industrial applications. The catalytic behavior of cerium oxide is based on its ability to store and release oxygen by reversibly changing its oxidation state between Ce4+ and Ce3+. The topic of this thesis is the examination of epitaxial, micrometer-sized, a few monolayers thick cerium oxide islands with (111) and (100) crystal surface terminations, prepared on copper single crystalline substrates. These cerium oxide islands serve as model systems for catalytic oxidation and reduction reactions. The formation of CeO2 islands of both surface orientations and their different reduction behaviors were observed simultaneously, in-situ, in realtime and under identical conditions with low energy and photoemission electron microscopy and X-ray absorption spectroscopy. Experiments on the reduction behavior of CeO2 (111) and (100) oriented islands were performed using two different approaches: the reduction in a hydrogen atmosphere and the reduction by deposition of metallic cerium in vacuum on top of the originally prepared islands. It was found that the (100) oriented islands became more reduced than the (111) islands in both cases. Analyzing the reduction kinetics of the cerium oxide reduction in hydrogen showed an overall higher reducibillity of the (100) terminated cerium oxide surface, which is not caused by a difference in kinetics between the two surface terminations but by a difference in thermodynamics. This demonstrates that the surface orientation of cerium oxide has a strong effect on its catalytic properties, which is in agreement with predictions by density functional theory calculations. A reduction experiment in hydrogen atmosphere was also performed on a platinum/cerium oxide system, which was prepared by the deposition of Pt on top of CeO2 (111) and (100) islands. The observed formation of highly reduced cerium oxide structures decorating the original islands indicated a mixture of platinum and cerium oxide and the formation of active centers for catalysis in these areas. For the (100) islands, the formation of the highly reduced structures was observed to a greater degree, again showing a behavioral difference between the two cerium oxide surface orientations.