Methanol adsorption on monocrystalline ceria surfaces

• IRRA spectra of ceria (1 1 0) and (1 1 1) single crystal surfaces exposed to methanol.
• DFT calculations of methoxy and methanol species on ceria (1 1 1) and ceria (1 1 0) surfaces.
• Assignment of adsorbate vibrational bands by combining experiment and theory.
• Monodentate methoxy species at ceria (1 1 0) yield a sharp IR band at 1108 cm−1.
• Methanol monolayers at ceria (1 1 1) yield two IR bands at 1085 and 1060 cm−1.

We have studied the adsorption of methanol on the (1 1 0) and (1 1 1) surfaces of bulk ceria (CeO2) single crystals using infrared reflection–absorption spectroscopy (IRRAS). To assign the experimentally observed features, density functional theory (DFT) has been applied. For ceria (1 1 0), only a single intense band at 1108 cm−1 is observed, which is assigned to a monodentate methoxy species. For the fully oxidized ceria (1 1 1) surface, a more complex spectrum has been obtained: the small band at 1108 cm−1 is attributed to methanol adsorption at a small amount of (1 1 0) facets exposed at step edges in the (1 1 1) surface. The strong bands at 1085 and 1060 cm−1 are assigned to a methanol monolayer that consists of H-bonded methoxide as well as molecularly adsorbed methanol species. These assignments are derived from molecular dynamics simulations for several adsorption structures with low and high methanol coverage. Surface defects were also considered. Our results reveal that a simple relation between the redshift in the methoxy CO stretching mode and the number of cerium cations coordinated to the methoxide oxygen atom (as often used in previous work) does not apply.

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