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Abstract

In heterogeneous catalysis the size of the active metal aggregates is very important not only because of the large surface-to-bulk ratio of small clusters, but also because of the different intrinsic activity of clusters of varying size. Using X-ray photoelectron spectroscopy (XPS) and low energy ion-scattering spectrometry (ISS) we investigated the change in size of supported rhodium clusters on the model catalyst system during temperature treatments in ultrahigh vacuum. The model catalyst consisted of a thin silicon oxide layer, prepared by chemical vapour deposition on a molybdenum substrate, and a nominal load of about two monolayers of rhodium (Rh/SiO2/Mo). During annealing at 773 K, ISS showed an agglomeration of rhodium and an increase of the cluster height by a factor of four. The changes in intensity and binding energy of the photoelectron spectra during heating up to 873 K (30 K h-1) can be interpreted with a simple agglomeration model. The three procedures allowed a consistent description of the growth of rhodium clusters as a function of temperature. The influence of vanadium oxide, a catalytic promoter material, on the rhodium agglomeration was studied for a system with two monolayers of vanadium oxide (Rh/Vox/SiO2/Mo).

Additional information

Authors: KOHL A, Max-Planck-Institut f³r Plasmaphysik, EURATOM-Association, Garching bei M³nchen (DE);LABICH S, Max-Planck-Institut f³r Plasmaphysik, EURATOM-Association, Garching bei M³nchen (DE);TAGLAUER E, Max-Planck-Institut f³r Plasmaphysik, EURATOM-Association, Garching bei M³nchen (DE);KNÍZINGER H, Institut f³r Physikalische Chemie der Universitõt M³nchen, M³nchen (DE);KNIZINGER H, Institut f?r Physikalische Chemie der Universit?t M?nchen, M?nchen (DE)
Bibliographic Reference: Article: Surface Science 454-456 (2000) pp. 974-978
Availability: Surface Science (Journal)
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