Probing the Surface Sites of CeO₂ Nanocrystals with Well-Defined Surface Planes via Methanol Adsorption and Desorption

Abstract

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.

Selective Hydrogenation of Phenol Catalyzed by Palladium on High-Surface-Area Ceria at Room Temperature and Ambient Pressure

“…The increased C−O vibrational energy relative to that of phenol is consistent with the formation of cerium-bound phenoxy owing to the strong interaction expected between cationic cerium and anionic oxygen. 63 This likely results in Figure S5 in the Supporting Information), which suggests new triply bridging hydroxyl groups are formed upon adsorption of phenol (Scheme 1c). 65 The phenol adsorption analysis suggests that coordinatively unsaturated cerium cations near hydroxyl groups may be active sites for the reaction.…”

“…The formation of phenoxy species is also consistent with the basic nature of cerium dioxide. 31 The C−O stretching frequency of ceriumbound phenoxy is expected to shift relative to that of phenol, 63 and the O−H bands of phenol should not be observed. The peaks in Figure 6d at 1587, 1479, and 1273 cm −1 correspond to adsorbed phenyl species ( Figure S3 in the Supporting Information).…”

Unraveling the structure sensitivity in methanol conversion on CeO2: A DFT+U study

“…Both HCHO and CO were found to be the main products on CeO 2 (100), while on the reduced CeO x (100) almost all methanol was converted to CO [34]. Temperature-programmed desorption (TPD) experiments on nanocrystals with different nanoshapes, namely octahedra, cubes and rods, showed that the latter is the most reactive shape (but non-selective to formaldehyde) one, evolving CO and H 2 at T < 583 K [35]. On the other hand, Zhou and Mullins reported that formaldehyde does not decompose on stoichiometric CeO 2 (111), but does evolve to CO and H 2 on the same reduced surface at temperatures above 500 K [36].…”

Effects of Morphology of Cerium Oxide Catalysts for Reverse Water Gas Shift Reaction

“…CO oxidation [23][24][25], NO reduction [26] and WGS reaction [27]. Ceria nanorods exhibited superior activity in oxidation of CO, 1,2-dichloroethane, ethyl-acetate and ethanol [19,[28][29][30], methanol conversion in presence of CO 2 [31], methanol [32] and acetaldehyde decomposition [33] compared to ceria cubes and octahedra. Moreover, shuttleshaped particles composed of closely packed ceria nanorods displayed higher activity for CO oxidation compared to ceria nanorods [34], which has been attributed to enhanced porosity, surface area and oxygen deficiency of these nanoshapes compared to ceria rods.…”