CO DISSOCIATION CHARACTERISTICS ON SIZE-DISTRIBUTED RHODIUM ISLANDS ON ALUMINA MODEL SUBSTRATES

The dissociation of CO on size-distributed Rh particles supported on a thin alumina film has been studied with high resolution X-ray Photoel ectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). Ads orbed CO dissociates upon heating to temperatures above 300 K. The dis sociation activity is dependent on the island size, exhibiting a maxim um for islands with around 1000 atoms. We have identified size-depende nt changes in the C Is photoelectron spectra for these CO-Rh systems o ccurring at temperatures lower than the onset of both the dissociation and desorption processes. These changes are interpreted as being due to adsorbed CO shifting into more highly coordinated sites. The dissoc iation activity is directly correlated to the availability of these si tes, where the observed dissociation is proposed to occur. These resul ts can be interpreted primarily in terms of the size and shape of the deposited Rh particles. (C) 1998 American Institute of Physics.