IR STUDY OF CARBON-DIOXIDE AND CARBON-MONOXIDE ADSORPTION ONTO CERIA - EFFECT OF THE REDUCTION STATE OF CERIA

The adsorption of CO2 and CO was studied by FT-IR spectroscopy at room temperature on ceria either evacuated at 823 K (ceria A) or on ceria H-2-reduced at 823 K (ceria B). CO2 adsorption onto ceria A and B led to the formation of hydrogen carbonate, of bidentate and polydentate carbonate species. Moreover, on ceria B, the formation of complex carboxylate-like species was suggested ; their thermal dismutation would explain the reoxidation of ceria B by heating under vacuum. Carbonate species were also formed by CO adsorption onto ceria A ; coordination of CO on weak Lewis acid sites, characterized by a nu(CO) band at 2170 cm-1, also occured. Two types of formiate species were produced by CO adsorption onto ceria B. They were spectroscopically characterized by the H --> D and O-16 --> O-18 isotopic substitutions. They differed from those occuring from the thermal decomposition of preadsorbed methoxy species adsorbed on ceria A. All these formate species are ionic and present the C2v symmetry. Isotopic exchange studies evidenced formate CO bond ruptures before stabilizing at surface saturation ; a mobile formyl species may be postulated as intermediate.