Promotion of a Rh/SiO/sub 2/ catalyst with lanthana results in a partial coverage of the supported Rh crystallites with LaO/sub x/ islands. These moieties reduce the capacity of Rh to adsorb CO but have little effect on the chemisorption of H/sub 2/ because of spillover of H atoms from the exposed Rh sites onto the surface of the LaO/sub x/ islands. At elevated temperatures, the LaO/sub x/ islands promote the dissociation of CO. It is proposed that this process occurs preferentially at Rh sites located along the perimeter of the LaO/sub x/ islands. Support for this interpretation is provided by infrared spectra of CO adsorbed on lanthana-promoted Rh/SiO/sub 2/. 40 references.
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