METAL SUPPORT INTERACTION IN PD/CEO2 CATALYSTS - FOURIER-TRANSFORM INFRARED STUDIES OF THE EFFECTS OF THE REDUCTION TEMPERATURE AND METAL LOADING .1. CATALYSTS PREPARED BY THE MICROEMULSION TECHNIQUE

The effect of the temperature of reduction by H-2 of Pd/CeO2 catalysts on the accessibility of the metal to carbon monoxide adsorption has been estimated from a study of the integrated intensity of the nu(CO) absorption bands. No adsorption occurred when the reduction temperature was higher than 673 K; the behaviour of the catalysts, which present crystallites of the same morphology, appeared to be independent of the Pd loading (0.5, 2.5, 5 wt.%). The adsorption could be completely recovered when the catalysts were first reoxidized at 723 K and subsequently reduced by H-2 at 423 K: this characterized the occurrence of a strong metal-support interaction (SMSI). Study of the nu(CO) absorption bands corresponding to CO adsorption on the (100) and (111) planes tended to show only a weak selectivity of the (100) planes in the SMSI phenomenon. The effect of the reduction on the ceria support was established from the study of methanol and tetracyanoethylene adsorption; surface ceria reduction was enhanced owing to the presence of palladium metal.