Probing the surface sites of CeO ₂ nanocrystals with well-defined surface planes via methanol adsorption and desorption

Methanol has been considered as a "smart" molecule in studying the surface sites of metal oxide catalysts. In this work, methanol was utilized to probe the nature of surface sites of ceria nanocrystals with defined surface planes (nanoshapes), including rods (containing {110}), cubes ({100}), and octahedra ({111}). The adsorption and desorption of methanol were followed by in situ IR and Raman spectroscopy as well as mass spectrometry. Upon methanol adsorption at room temperature, on-top, bridging and three-coordinate methoxy species are formed on the surface of rods and cubes, whereas only on-top methoxy is present on the octahedra surface. The distribution of the methoxy species is believed to be determined not only by the coordination status of surface Ce cations but also by the number of defect sites on the three nanoshapes. During the desorption process, the methoxy species are gradually dehydrogenated into H ₂ and CO via formate species as intermediates on the three ceria surfaces. A second intermediate, formyl species is also evident on the rods surface. The methoxy species are more reactive and less stable on the rods surface, which results in desorption of H ₂ and CO at lower temperature (<583 K) than on cubes and octahedra. A higher than stoichiometric H/CO ratio is observed in the methanol-TPD products, attributed to the retention of some amount of formate and carbonate species on the ceria nanoshapes, as revealed by in situ IR. A small amount of methanol and formaldehyde desorbs at low temperature (<423 K) on the three surfaces as a result of the disproportionation reaction of the methoxy species. The UV Raman and IR results indicate that the ceria nanoshapes are slightly reduced at room temperature upon methanol adsorption and become more reduced at higher temperatures during methanol desorption. The degree of reduction is found to be dependent on the surface structure of the ceria nanoshapes.