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M. Datler:
"CO oxidation reaction on palladium - platinum model catalysts: the role of the support";
Betreuer/in(nen): Y. Suchorski; Institut für Materialchemie (E165), 2013; Abschlussprüfung: 30.10.2013.

The aim of the present thesis was to study factors which might in uence the catalytic behaviour of model catalysts in the CO oxidation reaction under high vacuum conditions. This topic was chosen for two main reasons: although there already is a lot of data concerning the individual catalytic components of the three-way catalyst which is used in millions of cars to decrease the harmful pollutant emissions of the combustion engines, we still do not know much about the interactions between the catalytically active particles or details of the interaction between the support and the catalyst. Secondly, the good understanding of the Langmuir-Hinchelwood mechanism of the CO oxidation reaction, which is based on a considerable number of studies, provides the hope to reveal details of the interactions mentioned above.
The studies were performed using a photoemission electron microscope (PEEM). A PEEM focuses photoelectrons which are emitted from a sample onto a phosphorous screen to image the sample surface in the low µm-range. As the contrast of PEEM images depends on the number of photoelectrons which are emitted from a certain area of the sample surface, changes in the local image brightness result mainly from changes of the surface coverage by adsorbates, e.g. CO or oxygen.
Reactions following the Langmuir-Hinchelwood mechanism show a bistable behaviour under certain reaction conditions: e.g. the CO oxidation reaction shows two steady states and a region of bistability; at increasing CO pressure in the reactor, the kinetic transition from the oxygen-covered surface (steady state of high reactivity) to the CO-covered surface (steady state of low reactivity) takes place at the transition point A, while the reverse kinetic transition from an CO- to a oxygen-covered surface takes place at B. Between A and B there is the area of bistability, where the sample can be either reactive or inactive at exactly the same reaction conditions (temperature and partial pressures of CO and oxygen), depending on its prehistory.
In the present work the kinetic transitions during the CO oxidation reaction were monitored by a PEEM to obtain information about the local catalytic behaviour of the studied samples. A set of transition points which were obtained at the same partial oxygen pressure, in a T/pCO parameter space is called kinetic phase diagram. Such a kinetic phase diagram characterises the catalytic behaviour of a sample in a certain parameter range. The data concerning the kinetic behaviour of the samples in the CO oxidation reaction were obtained from isothermal measurements (temperature and partial oxygen pressure were held constant while the partial CO pressure was changed cyclically) as well as from isobaric ignition-extinction measurements (partial oxygen and partial CO pressure were held constant while the temperature was changed cyclically) which simulate a "cold-start" of a real catalyst.
To receive new insights into the interaction between the catalytic particles and their supports and to reveal the role of the surface morphology in the catalytic behaviour, a Pd/Pt model system sample was prepared by impregnating Pd powder into a Pt foil. Another model system was prepared by impregnating the same Pd powder as in the Pd/Pt model system into an Al2O3 foil, while a pure polycrystalline Pt and a pure polycrystalline Pd foil served as reference samples.
To directly compare the catalytic properties of Pd with those of Pt, a certain region of the Pd/Pt model system sample, where two Pd powder agglomerates and di erent domains of the Pt foil were visible, was chosen. Two low Miller index domains, namely Pt(100) and Pt(110), were determined by comparing the local PEEM image intensities at different adsorbate coverages. The kinetic phase diagrams obtained for these domains show a good agreement with the previous studies for the domains of the same crystallographic orientation of a pure polycrystalline Pt foil. In turn, the kinetic phase diagrams for the Pd powder agglomerates, which were locally obtained, diff er signi cantly from those for the Pt domains, i.e. that the Pd and Pt components of the combined Pd/Pt system sample retain their independence in respect to the CO oxidation under the present (high vacuum) conditions.
The comparison of the data received for the two Pd powder agglomerates of the Pd/Pt model system sample, which diff er in size and shape, shows that the catalytic reactivity of Pd powder agglomerates is not in uenced by variations of size and shape in the µ mscale. Further, a phenomenon which has not been described in the literature yet, could be observed: the occurrence of reaction fronts on powder surfaces. The comparison of the velocities of the reaction fronts on the Pd part of the Pd/Pt sample and the Pd part of the Al2O3 sample with those on a sputtered Pd foil whose surface has (due to the arti cially created steps and defects) a morphology similar to that of the powder surfaces, shows, that the supports have a signi cant in uence on the behaviour of the reaction fronts. Finally, the striking di erence between the catalytic behaviour of a sputtered Pd foil and the pressed Pd powder which was supported by an Al2O3 foil could be explained: the oxide-metal interaction between the oxidic supporting material Al2O3) and the impregnated Pd powder changes the catalytic behaviour of the catalytically active Pd particles signi cantly. Additionally, the comparison of the Pd part of the Pd/Pt model system with a sputtered Pd foil shows that the Pt foil used as support for the same Pd powder has no signi cant in uence on its catalytic behaviour.

PEEM, Catalysis, CO oxidation, Pd, Pt, foil, powder