Low-temperature scanning tunneling microscopy studies on model catalysts

Heterogeneous catalysis plays an important role in industrial synthesis as well as in environmental chemistry. Many heterogeneous catalysts consist of transition metals as active species, which are highly dispersed on an inert oxide support, such as alumina or silica. These catalysts have often very complex structures, which hamper a detailed understanding of decisive structural parameters and underlying reaction mechanisms. Thus, the investigation of well-defined model systems is very important to gain a fundamental understanding of the principles of heterogeneous catalysis. Within the scope of this work, a well-ordered, thin alumina film on NiAl(110) has been investigated by scanning tunneling microscopy (STM) and spectroscopy (STS) at 5 K. This film was established as model for bulk alumina supports in previous studies, though its exact structure remained unknown. The first part of this work aimed therefore to gain a deeper insight into the atomic structure of the film. Indeed, atomically resolved images of the alumina film could be obtained, whereby the symmetry of the displayed features varied dependent on the tunneling conditions. The assignment of these images to distinct atomic layers of the film was possible only after determination of its structure by DFT calculations [1]. According to these, three out of four oxide layers were visualized by STM: The surface oxygen layer, the surface aluminum layer and the interface aluminum layer. The symmetry of the interface oxygen layer can be derived directly from that. Based on this knowledge, a detailed analysis of antiphase domain boundaries (APDB) became possible, which are regularly occurring line defects in the oxide film. Defects play generally a central role in catalytic processes, especially due to the interplay between their geometric structure and electronic properties. Atomically resolved STM images of APDB and comparing DFT studies revealed that APDB are oxygen-deficient. This leads to the induction of three unoccupied defect levels within the oxide band gap, as determined by STS and confirmed theoretically. Often, oxygen vacancies induce occupied defect levels. The contrary effect that was observed here, is ascribed to an electron transfer into the NiAl substrate, acting as an electron reservoir. The second part of this thesis addressed the adsorption behavior of the alumina film toward single gold atoms, which were evaporated onto the sample at about 10 K. Prior to thermalization, gold atoms can move on the surface, thus dimers and small clusters are observed beside monomers. At low coverage, these clusters are often onedimensional (1D) chains, which have limited length, preferential orientation and a goldgold separation, which is about twice the distance observed in bulk gold. STM images show that gold adsorbs on top of aluminum, whereby only every second aluminum site is occupied in chains. The preferential orientation of dimers and chains with respect to the NiAl substrate clearly indicates a participation of the metal support in the gold binding. Special gold adsorption sites were identified as places, where the gold adatom, an aluminum atom of the oxide film and atoms of the NiAl support have a special arrangement with respect to each other (e.g. on top). The gold-oxide interaction was further characterized by STS and conductance imaging. Monomers exhibit an occupied and an unoccupied state. According to computational results on related systems, they might be related to the Au 6s level, which splits due to the interaction with the alumina film. For dimers, two unoccupied resonances are observed with symmetric and antisymmetric shape, respectively, clearly indicating an interaction between the gold atoms. As direct orbital overlap can be excluded because of their large separation, a substrate-mediated interaction is suggested. Both occupied and unoccupied states are detected for longer gold chains, and their symmetry confirms the hypothesis of large gold-gold separations. The chain formation is explained with the linear arrangement of favorable adsorptions sites, whereby the large separations might result from repulsive (e.g. Coulomb or polaronic) interactions of gold-induced defect states in the oxide. These results demonstrate that adsorption properties of thin oxide films can deviate significantly from bulk oxides. However, the metal adatom plays an important role beside the oxide film itself. For silver atoms on alumina / NiAl(110), for instance, no evidence for a participation of the metal support was observed. The influence of the metal support on the oxide-adatom interaction has therefore to be analyzed carefully for each adsorbate-substrate system to evaluate the model character of an oxide film for the corresponding bulk oxide. The third part of this work presents size-dependent STM studies on metal clusters (silver, palladium) deposited onto the thin alumina film on NiAl(110). Conductance spectra reveal usually a gap around the sample Fermi level and a series of equidistant peaks for both types of metal clusters, whereby the energy separation between the peaks increases with decreasing cluster size. Spectra series taken along the cluster show furthermore, that the peak positions shift to higher absolute energy with increasing distance from the cluster center. This finding was confirmed by conductance images, where the peaks appear as concentric circles of enhanced conductance with a diameter depending on the sample bias. The described observations are best explained by a Coulomb blockade effect. Another possibility, namely the interpretation of the peaks as quantized electronic levels, is also discussed, but cannot account for all experimental findings. Thus, the spectroscopic data reflect most likely no intrinsic properties of the metal clusters but are due to the specific behavior of a double barrier tunneling junction.