CHARACTERISATION OF RARE EARTH OXIDE SUPPORTED METAL CATALYSTS. STUDY OF SOME CERIA SUPPORTED RHODIUM PHASES

The present work reportson the characterisation of several ceria supported rhodium catalysts. The selected reduction temperature were: 623 K and 773 K; the metal loading: 1% and 2.5%. The surface area of the two starting ceria samples used as support were: 20 m(2).g(-1) (LS) and 110 m(2).g(-1)(HS), respectively. The apparent H/Rh and CO/Rh were found to be quite different, which according to the results of the TPD study can be interpreted as due to the occurrence of strong adsorption of hydrogen on the support. Upon reduction at 773 K the adsorption capabilities are notably reduced. The effect is much stronger in the case of the phases based on HS ceria. For Fth/CeO2(LS) catalysts, the metal dispersions determined by HRTEM agree fairly well with those estimated from CO adsorption. By contrast, over Rh/CeO2(HS) catalysts. the values determined by TEM were found to be larger than those deduced from chemisorption. Taking into account that the reduction treatment leading to the Rh/CeO2(HS) induces strong sintering phenomena on the support and that this effect is negligible in the case of Rh/CeO2 (LS), the results above have been interpreted assuming that the inhibition of the chemisorptive capacity of RhfCeO(2)(HS) reduced at 773 K is primarily due to the encapsulation of the metal particles associated to the sintering occurred at the support. Reoxidation of the phases reduced at 773 K did not allow to recover the chemisorptive properties of the catalysts, which confirms that no classic SWSI occurs in the present case.