Use of methanol as an IR molecular probe to study the surface of polycrystalline ceria

Abstract

Methanol adsorption has been studied by IR spectroscopy on unreduced, microporous and non-microporous ceria samples presenting different surface areas. The spectra were assigned according to whether the so-formed methoxy species were on-top, doubly or triply bridged to cationic surface sites. Apparent molar absorption coefficients for the ν(OC) stretching mode were measured and found to be independent of the coordination of the methoxy species and of the reduction state of ceria. For a monolayer of irreversibly adsorbed methoxy species, the total ν(OC) band intensity was directly related to the external surface area, inferring the possibility of a spectroscopic measurement of this specific surface area. Analysis of the relative ν(OC) band intensities for on-top and doubly bridging species led to the conclusion that, for non-microporous samples contacted with O₂ at 673 K, then quenched to room temperature, the mean surface morphology was the same. The ν_s(CH₃) symmetric stretching frequency of adsorbed methoxy species was found to be sensitive to the degree of overall ceria surface reduction. The possibility of using the ν(OC) band of methoxy species to follow the ceria reduction was also investigated. Substitution of on-top methoxy species 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 been tentatively evaluated. Oxygen storage capacity, reduction percentages, as well as the relative contributions of external surface and bulk or micropores to reduction, have been evaluated from O₂ reoxidation at room temperature.

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