USE OF METHANOL AS AN IR MOLECULAR PROBE TO STUDY THE SURFACE OF POLYCRYSTALLINE CERIA

Methanol adsorption has been studied by IR spectroscopy on unreduced, microporous and non-microporous ceria samples presenting different sur face areas. The spectra were assigned according to whether the so-form ed methoxy species were on-top, doubly or triply bridged to cationic s urface sites. Apparent molar absorption coefficients for the v(OC) str etching mode were measured and found to be independent of the coordina tion of the methoxy species and of the reduction state of ceria. For a monolayer of irreversibly adsorbed methoxy species, the total v(OC) b and intensity was directly related to the external surface area, infer ring the possibility of a spectroscopic measurement of this specific s urface area. Analysis of the relative v(OC) band intensities for on-to p and doubly bridging species led to the conclusion that, for non-micr oporous samples contacted with O-2 at 673 K, then quenched to room tem perature, the mean surface morphology was the same. The v(s)(CH3) symm etric stretching frequency of adsorbed methoxy species was found to be sensitive to the degree of overall ceria surface reduction. The possi bility of using the v(OC) band of methoxy species to follow the ceria reduction was also investigated. Substitution of on-top methoxy specie s by doubly bridging ones indicated the known surface reduction at 673 K, while the reduction of other doubly bridging species sites at 873 K was related to some subsurface reduction. Surface O vacancies have b een tentatively evaluated. Oxygen storage capacity, reduction percenta ges, as well as the relative contributions of external surface and bul k or micropores to reduction, have been evaluated from O-2 reoxidation at room temperature.