Exploring the Redox Properties of Ce1-xUxO2±Î´ (x ≤ 0.5) Oxides for Energy Applications.

The Ce-U-O system, forming a solid solution in the fluorite structure, has gained much attention due to its unique properties. Mixed fluorite oxide powders of Ce1-xUxO2±Î´ compositions were found to be particularly active for H2 production through thermochemical water splitting. In the present work, we explore the reduction-oxidation properties of the mixed oxides with x = 0.1, 0.25, and 0.5. We report a particularly high oxygen storage capacity (OSC) for x ≥ 0.25 and show that the oxygen extracted from these mixed oxides is of a different origin than that extracted from CeO2. While in ceria, oxygen is extracted from the tetrahedral sites, leading to the formation of oxygen vacancies, the extracted oxygen in Ce1-xUxO2±Î´ (x ≥ 0.25) is essentially excess oxygen in the fluorite lattice (which spontaneously penetrates the oxide under ambient or oxidative conditions). This property, which is clearly related to the change in the valency of the U cations, is apparently responsible for the higher OSC and the lower activation energy for oxygen extraction from the mixed oxides compared to ceria. The mixed oxide powders are shown to be structurally stable, retaining their fluorite structure following reduction under Ar-5%H2 or oxidation in air until 1000 °C. The presented results provide new insights into the Ce-U-O system which may be exploited for future technical applications, as a catalyst for thermochemical water splitting, or as a solid electrolyte in solid oxide fuel cells.

Cerium metal oxidation studied by IR reflection-absorption and Raman scattering spectroscopies.

Oxidation of cerium metal is a complex process which is strongly affected by the presence of water vapor in the oxidative atmosphere. Here, we explore, by means of infrared reflection-absorption spectroscopy (IRRAS) and Raman scattering spectroscopies, thin oxide films, formed on cerium metal during oxidation, under dry vs ambient (humid) air conditions (∼0.2% and ∼50% relative humidities, respectively) and compare them with a thin film of CeO2deposited on a Si substrate. Complementary analysis of the thin films using x-ray diffraction and focused ion beam-scanning electron microscopy enables the correlation between their structure and spectroscopic characterizations. The initial oxidation of cerium metal results in the formation of highly sub-stoichiometric CeO2-x. Under dry air conditions, a major fraction of that oxide reacts with oxygen to form CeO∼2, which is spectroscopically detected by Raman scatteringF2gsymmetry mode and by IRAASF1usymmetry mode, splitted into doubly-degenerate transverse optic and mono-degenerate longitudinally optic (LO) modes. In contrast, under ambient (humid) conditions, the oxide formed is more heterogenous, as the reaction of CeO2-xdiverges towards the dominant formation of Ce(OH)3. Prior to the spectral emergence of Ce(OH)3, hydrogen ions incorporate into the highly sub-stoichiometric oxide, as manifested by Ce-H local vibrational mode detected in the Raman spectrum. The spectroscopic response of the thin oxide layer thus formed is more complex; particularly noted is the absence of the LO mode. It is attributed to the high density of microstructural and compositional defects in the oxide layer, which results in a heterogenous dielectric nature of the thin film, far from being representable by a single phase of CeO∼2.

Defect Chemistry of Oxides for Energy Applications.

Oxides are widely used for energy applications, as solid electrolytes in various solid oxide fuel cell devices or as catalysts (often associated with noble metal particles) for numerous reactions involving oxidation or reduction. Defects are the major factors governing the efficiency of a given oxide for the above applications. In this paper, the common defects in oxide systems and external factors influencing the defect concentration and distribution are presented, with special emphasis on ceria (CeO2 ) based materials. It is shown that the behavior of a variety of oxide systems with respect to properties relevant for energy applications (conductivity and catalytic activity) can be rationalized by general considerations about the type and concentration of defects in the specific system. A new method based on transmission electron microscopy (TEM), recently reported by the authors for mapping space charge defects and measuring space charge potentials, is shown to be of potential importance for understanding conductivity mechanisms in oxides. The influence of defects on gas-surface reactions is exemplified on the interaction of CO2 and H2 O with ceria, by correlating between the defect distribution in the material and its adsorption capacity or splitting efficiency.