MICROSTRUCTURAL AND CHEMICAL-PROPERTIES OF CERIA-SUPPORTED RHODIUM CATALYSTS REDUCED AT 773-K

This work reports on the chemical and microstructural characterization of a series of Rh/CeO2 catalysts. The volumetric adsorption and FTIR spectroscopic studies show the occurrence of considerable changes in the chemistry of the H-2-(Rh/CeO2) system upon increasing the reduction temperature from 623 to 773 K. However, the H/Rh values determined from hydrogen isotherms at 191 K suggest that the rhodium microcrystals remain chemically active after reduction at 773 K. The major effect of the high-temperature reduction treatment would be the blocking of the spillover process, much slower on the catalysts reduced at 773 K. In agreement with this, no dramatic effect of the reduction temperature on the catalytic activity for benzene hydrogenation has been found. These observations contrast with some recent results in the literature, in accordance with which ceria-supported metal catalysts exhibit the so-called SMSI effect. The high-resolution transmission electron microscopy (HRTEM) micrographs reported here for Rh/CeO2 catalysts show clean, well-faceted rhodium particles. No evidences of metal decoration phenomena like those earlier reported for Rh/TiO2 could be obtained from them. However, HRTEM images of the metal/support interface reveal an epitaxial growth of the rhodium microcrystal on ceria. The structural nature of this epitaxial relationship is rather singular leading to the regular appearance of rhodium dislocations at the metal/support interface. In addition to several other observations also based on the HRTEM images, the occurrence of such an epitaxy might well be interpreted as being due to some kind of strong metal-support interaction. The ensemble of results presented here suggests that the nature of this rhodium-ceria interaction is different from that referred to as the classic SMSI effect.