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Zusammenfassung:
RuCu/SiO2 catalysts were characterized by temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and IR spectroscopy of adsorbed carbon monoxide. The catalysts contained 5 wt% Ru and varying amounts of Cu up to a maximum atomic ratio Ru : Cu of 1 : 1.6. At this composition accessible Ru sites could not be detected. Model calculations indicated that Ru particles might be encapsulated by one to two atomic layers of Cu at this composition. The CO stretching frequency of adsorbed CO on this material appeared at higher values than it did on pure Cu/SiO2 which should indicate a charge transfer from Cu to Ru. This conclusion is supported by the CO carbonyl spectra of CO adsorbed on materials containing less Cu. When Ru sites were detected by their characteristic CO stretching bands besides Cu-CO species, the Ru-CO stretching frequency was shifted to lower values as compared to the frequency observed for pure Ru/SiO2. This is consistent with the suggested charge transfer from Cu to Ru. Experiments with 12CO/13CO mixtures indicated that dipole-dipole interactions occur between CO oscillators adsorbed on Ru sites in the bimetallic materials at sufficiently low Cu content. This suggests that the bimetallic particles expose islands of uncovered Ru surface large enough for the dipole-dipole coupling effects to occur.
