CO has been adsorbed in doses at room temperature by ceria which had been reduced with CO and then evacuated at 873 K, or only evacuated at this temperature, as well as by a Pd/CeO, catalyst, H,-reduced and evacuated at 823 K. From comparison of the observed IR spectra with literature results concerning matrixisolated Cs,CO, species, the formation of two carbonite COZspecies adsorbed over reduced ceria may be inferred. One of these species, with Czv geometry and with IR bands at around 1300 and 1160 cm-', was formed before the other one, with a C, structure and characterized by IR bands at 1270, 1071 and 771 cm-'. A band at 1465 cm-' was only tentatively assigned to a third carbonite species. COZspecies ultimately lead to C0:-
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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