Agglomeration of supported rhodium on model catalysts

In heterogenous catalysis the size of the active metal aggregates is very i mportant not only because of the large surface-to-bulk ratio of small clust ers, but also because of the different intrinsic activity of clusters of va rying size. Using X-ray photoelectron spectroscopy (XPS) and low-energy ion -scattering spectroscopy (ISS) we investigated the change in size of suppor ted rhodium clusters on a model catalyst system during temperature treatmen ts in ultrahigh vacuum. The model catalyst consisted of a thin silicon oxid e layer, prepared by chemical vapor deposition on a molybdenum substrate, a nd a nominal load of about two monolayers of rhodium (Rh/SiO2/Mo). During a nnealing at 773 K, ISS showed an agglomeration of rhodium and an increase o f the cluster height by a factor of four. The changes in intensity and bind ing 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 proced ures allowed a consistent description of the growth of rhodium clusters as a function of temperature. The influence of vanadium oxide, a catalytic pro motor material, on the rhodium agglomeraton was studied for a system with t wo monolayers of vanadium oxide (Rh/VOx/SiO2/Mo). (C) 2000 Elsevier Science B.V. All rights reserved.