Extensive oxidation treatments: ageing effects on a catalytic model system studied in UHV by STM

This thesis concerns a surface science approach for the investigation of the ageing process of a model catalyst. It combines extreme oxidation conditions with Ultra High Vacuum (UHV) compatible characterization techniques. Our model system was the surface oxide formed on Rh(110); to grow such oxide, we used three alternative oxygen sources, optimizing for each case the preparation recipe. When dosing molecular oxygen, pressures in the ? 10? 4 mbar range were used, therefore bridging, to some extent, the pressure gap. For characterization of the oxides we used mainly Scanning Tunneling Microscopy (STM) and Thermal Desorption Spectroscopy (TDS), providing atomic scale and desorption mechanism information, respectively. Low Energy Electron Microscopy (LEEM) and X-ray Photoelectron Specroscopy (XPS) complemented our measurements with large scale morphology and reactivity data and with chemically resolved results. To mimic real catalytic conditions, we setup an ageing protocol consisting of cycles of oxidation and annealing in UHV (up to more than ? 40), with each of the three oxygen sources. In this way, we were able to observe two di?erent kinds of ageing: a †œcontaminant-driven†� and an †œintrinsic†� one, caused by the iterative oxidation procedure. The latter is connected to the presence of a new species we detected on the (1 à- 1) surface obtained after the oxide desorption, that we named †œunits†� (or ageing ?ngerprints). By decreasing their number we were able to show that the intrinsic ageing is, at least partially, reversible. We could not uniquely determine the structure of the †œunits†�, but plausible models are proposed. ------------------------------------------------