Agglomeration of supported rhodium on model catalysts

In heterogenous 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 spectroscopy (ISS) we investigated the change in size of supported rhodium clusters on a model catalyst system during temperature treatments in ultrahigh vacuum. The model catalyst consisted of a thin silicon oxide layer, prepared by chemical vapor 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 promotor material, on the rhodium agglomeraton was studied for a system with two monolayers of vanadium oxide (Rh/VOx/SiO2/Mo).