4 research outputs found
Strain Field in Ultrasmall Gold Nanoparticles Supported on Cerium-Based Mixed Oxides. Key Influence of the Support Redox State
Using a method that combines experimental
and simulated Aberration-Corrected
High Resolution Electron Microscopy images with digital image processing
and structure modeling, strain distribution maps within gold nanoparticles
relevant to real powder type catalysts, i.e., smaller than 3 nm, and
supported on a ceria-based mixed oxide have been determined. The influence
of the reduction state of the support and particle size has been examined.
In this respect, it has been proven that reduction even at low temperatures
induces a much larger compressive strain on the first {111} planes
at the interface. This increase in compression fully explains, in
accordance with previous DFT calculations, the loss of CO adsorption
capacity of the interface area previously reported for Au supported
on ceria-based oxides
Improving the Redox Response Stability of Ceria-Zirconia Nanocatalysts under Harsh Temperature Conditions
By
depositing ceria on the surface of yttrium-stabilized zirconia
(YSZ) nanocrystals and further activation under high-temperature reducing
conditions, a 13% mol. CeO<sub>2</sub>/YSZ catalyst structured as
subnanometer thick, pyrochlore-type, ceria-zirconia islands has been
prepared. This nanostructured catalyst depicts not only high oxygen
storage capacity (OSC) values but, more importantly, an outstandingly
stable redox response upon oxidation and reduction treatments at very
high temperatures, above 1000 Ā°C. This behavior largely improves
that observed on conventional ceria-zirconia solid solutions, not
only of the same composition but also of those with much higher molar
cerium contents. Advanced scanning transmission electron microscopy
(STEM-XEDS) studies have revealed as key not only to detect the actual
state of the lanthanide in this novel nanocatalyst but also to rationalize
its unusual resistance to redox deactivation at very high temperatures.
In particular, high-resolution X-ray dispersive energy studies have
revealed the presence of unique bilayer ceria islands on top of the
surface of YSZ nanocrystals, which remain at surface positions upon
oxidation and reduction treatments up to 1000 Ā°C. Diffusion of
ceria into the bulk of these crystallites upon oxidation at 1100 Ā°C
irreversibly deteriorates both the reducibility and OSC of this nanostructured
catalyst
Imaging Nanostructural Modifications Induced by Electronic MetalāSupport Interaction Effects at Au||Cerium-Based Oxide Nanointerfaces
A variety of advanced (scanning) transmission electron microscopy experiments, carried out in aberration-corrected equipment, provide direct evidence about subtle structural changes taking place at nanometer-sized Au||ceria oxide interfaces, which agrees with the occurrence of charge transfer effects between the reduced support and supported gold nanoparticles suggested by macroscopic techniques. Tighter binding of the gold nanoparticles onto the ceria oxide support when this is reduced is revealed by the structural analysis. This structural modification is accompanied by parallel deactivation of the CO chemisorption capacity of the gold nanoparticles, which is interpreted in exact quantitative terms as due to deactivation of the gold atoms at the perimeter of the Au||cerium oxide interface
Critical Influence of Redox Pretreatments on the CO Oxidation Activity of BaFeO<sub>3āĪ“</sub> Perovskites: An in-Depth Atomic-Scale Analysis by Aberration-Corrected and in Situ Diffraction Techniques
A BaFeO<sub>3āĪ“</sub> (Ī“ ā 0.22) perovskite
was prepared by a solāgel method and essayed as a catalyst
in the CO oxidation reaction. BaFeO<sub>3āĪ“</sub> (0.22
ā¤ Ī“ ā¤ 0.42) depicts a 6H perovskite hexagonal
structural type with Fe in both III and IV oxidation states and oxygen
stoichiometry accommodated by a random distribution of anionic vacancies.
The perovskite with the highest oxygen content, BaFeO<sub>2.78</sub>, proved to be more active than its lanthanide-based counterparts,
LnFeO<sub>3</sub> (Ln = La, Sm, Nd). Removal of the lattice oxygen
detected in both temperature-programmed oxidation (TPO) and reduction
(TPR) experiments at around 500 K and which leads to the complete
reduction of Fe<sup>4+</sup> to Fe<sup>3+</sup>, i.e. to BeFeO<sub>2.5</sub>, significantly decreases the catalytic activity, especially
in the low-temperature range. The analysis of thermogravimetric experiments
performed under oxygen and of TPR studies run under CO clearly support
the involvement of lattice oxygen in the CO oxidation on these Ba-Fe
perovskites, even at the lowest temperatures. Atomically resolved
images and chemical maps obtained using different aberration-corrected
scanning transmission electron microscopy techniques, as well as some
in situ type experiments, have provided a clear picture of the accommodation
of oxygen nonstoichiometry in these materials. This atomic-scale view
has revealed details of both the cation and anion sublattices of the
different perovskites that have allowed us to identify the structural
origin of the oxygen species most likely responsible for the low-temperature
CO oxidation activity