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The effect of the temperature of reduction by H, of Pd/CeO, catalysts on the accessibility of the metal to carbon monoxide adsorption has been estimated from a study of the integrated intensity of the v(C0) 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, at 423 K : this characterized the occurrence of a strong metal-support interaction (SMSI). Study of the v(C0) 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 t h e ceria support was established from t h e study of methanol and tetracyanoethylene adsorption ; surface ceria reduction was enhanced owing to t h e presence of palladium metal.

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