Temperature programmed desorption was used to study the reaction of CH3OH on several different
ceria-based model catalysts. These catalysts consisted of a CeO2(111) single crystal and thin ceria films
supported on α-Al2O3(0001) and yttria-stabilized zirconia (100). The results of this study demonstrate that
the reaction of CH3OH on CeO2 surfaces is highly structure sensitive and depends on crystallographic
orientation, the concentration of surface oxygen vacancies, and the oxidation state of surface cerium cations.
The primary decomposition pathway for methoxide intermediates adsorbed on surface oxygen vacancy
sites is dehydrogenation to produce H2CO and surface hydroxyl groups. The surface hydroxyl groups then
either react with additional methoxides to reform CH3OH or react to produce H2O. In contrast, methoxides
adsorbed on partially reduced ceria surfaces, possibly on Ce3+ sites, undergo complete dehydrogenation
to CO and H2.
The interaction of sulfur with ceria under highly reducing conditions was investigated. The phase boundary between CeO 1.83 and Ce 2 O 2 S was determined for temperatures between 873 and 1073 K. This data was used to derive an empirical equation for ΔG f º of Ce 2 O 2 S in this temperature range. This equation along with thermodynamic data for cerium oxides and sulfides obtained form the literature was used to predict Ce-O-S phase diagrams at 873 and 973 K. These phase diagrams provide insight into the mechanism of the deactivation of ceria-based catalysts by sulfur under reducing conditions.
The morphology and reducibility of vapor-deposited ceria films supported on yttria-stabilized zirconia (100) (YSZ(100)) and α-Al 2 O 3 (0001) single crystals were studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The results of this study show that the gas environment has a significant effect on the structure of the ceria films on both substrates. CeO 2 films on α-Al 2 O 3 (0001) were found to be stable in a reducing environment at temperatures up to 1000K, but underwent agglomeration and reaction with the support to form CeAlO 3 upon annealing at 1273 K in air. Heating CeO 2 /YSZ(100) in air at 1273 K caused the ceria thin film to agglomerate into bar-shaped features which were re-dispersed by subsequent annealing in vacuum. Interactions at the CeO 2-YSZ interface were also found to dramatically enhance the reducibility of ceria films supported on YSZ(100).
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