Fluorescence Line-Narrowing Spectroscopy as a Tool to Monitor Phase Transitions and Phase Separation in Efficient Nanocrystalline Ce_<i>x</i>Zr_1–<i>x</i>O₂:Eu³⁺ Catalyst Materials

Despite the wide range of industrial applications for ceria-zirconia mixed oxides (Ce_<i>x</i>Zr_1–<i>x</i>O₂), the complex correlation between their atomic structure and catalytic performance is still under debate. Catalytically interesting Ce_<i>x</i>Zr_1–<i>x</i>O₂ nanomaterials can form homogeneous solid solutions and, depending on the composition, show phase separation under the formation of small domains. The characterization of homogeneity and atomic structure of these materials remains a major challenge. High-resolution emission spectroscopy recorded under cryogenic conditions using Eu³⁺ as a structural probe in doped CeZrO₂ nanoparticles offers an effective way to identify the different atomic environments of the Eu³⁺ dopants and, subsequently, to monitor structural parameters of the ceria-zirconia mixed oxides. It is found that, in stoichiometric CeZrO₂:Eu³⁺, phase separation occurs at elevated temperatures beginning with the gradual formation of (pseudo)­cubic crystallites in the amorphous materials at 500 °C and a sudden phase separation into tetragonal, zirconia-rich and cubic, ceria-rich domains over 900 °C. The presented technique allows us to easily monitor subtle changes even in amorphous, high surface area samples, yielding structural information not accessible by conventional techniques such as X-ray diffraction (XRD) and Raman. Moreover, in reference experiments investigating the reducibility of largely unordered Ce_0.2Zr_0.8O₂:Eu³⁺, the main reduction peak in temperature-programmed reduction measurements appeared at exceptionally low temperatures below 200 °C, thus suggesting the outstanding potential of this oxide to activate catalytic oxidation reactions. This effect was found to be dependent on the amount of Eu³⁺ dopant introduced into the CeZrO₂ matrix as well as to be connected to the atomic structure of the catalyst material

Low-Lanthanide-Content CeO2/MgO Catalysts with Outstandingly Stable Oxygen Storage Capacities: An In-Depth Structural Characterization by Advanced STEM Techniques

International audienceA novel CeO2/MgO catalyst with low ceria loading has been synthesized. This catalyst showed unique redox properties compared with conventional high and low surface area CeO2. Advanced (scanning) transmission electron microscopy techniques revealed the presence of a variety of highly dispersed ceria nanostructures: isolated CeOx entities, CeO2 clusters, as well as fairly small (<5nm) CeO2 nanoparticles. More interestingly, this CeO2/MgO catalyst showed outstanding stability in its redox response against high temperature aging treatments. Thus, after reduction in hydrogen at 950 degrees C and further oxidation at 500 degrees C, CeO2 reduction effects took still place at low temperatures, and no significant loss of oxygen storage capacity (OSC) was detected. Unique ceria-bilayer nanostructures were found and characterized in the aged catalyst. Their peculiar structural and chemical properties seem to be responsible for the large improvement observed in the stability of the redox response