Spectroscopic Investigation of Surface-Dependent Acid–Base
Property of Ceria Nanoshapes
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Abstract
In addition to their well-known redox
character, the acid–base
property is another interesting aspect of ceria-based catalysts. Herein,
the effect of surface structure on the acid–base property of
ceria was studied in detail by utilizing ceria nanocrystals with different
morphologies (cubes, octahedra, and rods) that exhibit crystallographically
well-defined surface facets. The nature, type, strength, and amount
of acid and base sites on these ceria nanoshapes were investigated
via in situ IR spectroscopy combined with various probe molecules.
Pyridine adsorption shows the presence of Lewis acid sites (Ce cations)
on the ceria nanoshapes. These Lewis acid sites are relatively weak
and similar in strength among the three nanoshapes according to the
probing by both pyridine and acetonitrile. Two types of basic sites,
hydroxyl groups and surface lattice oxygen are present on the ceria
nanoshapes, as probed by CO<sub>2</sub> adsorption. CO<sub>2</sub> and chloroform adsorption indicate that the strength and amount
of the Lewis base sites are shape dependent: rods > cubes >
octahedra.
The weak and strong surface dependence of the acid and base sites,
respectively, are a result of interplay between the surface structure
dependent coordination unsaturation status of the Ce cations and O
anions and the amount of defect sites on the three ceria nanoshapes.
Furthermore, it was found that the nature of the acid–base
sites of ceria can be impacted by impurities, such as Na and P residues
that result from their use as structure-directing reagent in the hydrothermal
synthesis of the ceria nanocrystals. This observation calls for precaution
in interpreting the catalytic behavior of nanoshaped ceria where trace
impurities may be present