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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:-

Adsorption of CO, used as infrared molecule probe, was performed at room temperature or at low temperature (140 K), on either chlorine-free or chlorine-containing ceria, in order to investigate both the chlorine effect and the ceria reduction state. On ceria and Pd/ceria catalysts, chlorine acts as an electronegative ligand. Its effect extends to the raft-like structure of palladium in Pd/CeO 2 catalysts. Moreover, ceria chlorination prevents palladium from CO clustering. CO adsorption on the chlorine-containing Pd/CeO 2 catalyst led to some water formation, tentatively assigned to an incomplete reduction of microporous ceria sites. Moreover, oxalatelike species were evidenced and considered as impurities on residual reduced carbonate sites.

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