2 research outputs found
Promotion of Ceria Catalysts by Precious Metals: Changes in Nature of the Interaction under Reducing and Oxidizing Conditions
By depositing ceria over supported precious metal (PM)
catalysts
and characterizing them with in situ diffuse reflectance UV (DR UV)
and in situ Raman spectroscopy, we have been able to prove a direct
correlation between a decrease in ceria band gap and the work function
of the metal under reducing conditions. The PMāceria interaction
results in changes on the ceria side of the metalāceria interface,
such that the degree of oxygen vacancy formation on the ceria surface
also correlates with the precious metal work function. Nevertheless,
conclusive evidence for a purely electronic interaction could not
be provided by X-ray photoelectron spectroscopy (XPS) analysis. On
the contrary, the results highlight the complexity of the PMāceria
interaction by supporting a spillover mechanism resulting from the
electronic interaction under reducing conditions. Under oxidizing
conditions, another effect has been observed, namely, a structural
modification of ceria induced by the presence of PM cations. In particular,
we have been able to demonstrate by in situ Raman spectroscopy that,
depending on the PM ionic radius, it is possible to create PMāceria
solid solutions. We observed that this structural modification prevails
under an oxidizing atmosphere, whereas electronic and chemical interactions
take place under reducing conditions
Redispersion of Gold Supported on Oxides
Although many gold heterogeneous catalysts have been
shown to exhibit
significant activity and high selectivity for a wide range of reactions
in both the liquid and gas phases, they are prone to irreversible
deactivation. This is often associated with sintering or loss of the
interaction of the gold with the support. Herein, we report on the
use of methyl iodide as a method of dispersing gold nanoparticles
supported on silica, titania, and alumina supports. In the case of
titania- and alumina-based catalysts, the gold was transformed from
nanometer particles into small clusters and some atomically dispersed
gold. In contrast, although there was a drop in the gold particle
size on the silica support following CH<sub>3</sub>I treatment, the
size remained in the submicrometer range. The structural changes were
correlated with changes in the selectivity and activity for ethanol
dehydration and benzyl alcohol oxidation. From these observations,
it is clear that this treatment provides a method by which deactivated
gold catalysts can be reactivated via redispersion of the gold