Oxidation-Reduction Properties of Mixed Oxides in the Cerium–Uranium–Oxygen System

Abstract

The formation and the oxidation-reduction properties of mixed oxides in the Ce–U–O ternary system were examined by means of thermogravimetry and the X-ray diffraction technique. The mixed oxides were prepared by the reactions of CeO2 with U3O8 in air and in hydrogen. The composition of the products heated in air, followed by slow cooling to room temperature, was approximately expressed by this equation: yCeO2+(1–y)UO2.67. By X-ray diffraction, it was shown that the oxides formed at 1100 °C in air were mixtures of Ce0.6U0.4O2.3 and α-U3O8 in the y<0.6 range, while they were in single phases of solid solutions for y≥0.6. When the mixed oxides, MO2+X, where M indicates yCe+(1–y)U, were re-heated in air, weight loss due to oxygen liberation was observed above 500 °C, and it was enhanced with a rise in the temperature. The hydrogen reduction of the oxides proceeded in two steps; the first step was the reduction of MO2+X to MO2 below 600 °C, and the second one was the reduction of MO2 to MO2−X above 600 °C. The composition of the oxides obtained can be given by this equation: yCeO1.81+(1–y)UO2.00. Two separate linear relationships for the lattice parameter of the fluorite-type structure as a function of the cerium content were obtained; one is applicable to the solid solutions with y≤0.6, where the lattice parameter is nearly that of UO2, and the other to the solid solutions with y≥0.4. The latter obeys Vegard’s law between UO2 and CeO2. The oxides with y=0.4–0.6 were in the two-phase region of the solid solutions.

Oxidation behaviour of uranium in the internally gelated urania–ceria solid solutions – XRD and XPS studies

“…The XRD pattern of the oxidized product corresponding to the compositions of 8.5, 16.0 and 31.3 mol% Ce (100 AE Ce/(U + Ce)) was found to be a mixture of orthorhombic and cubic phases. The segregation of MO 2þx (M ¼ (U + Ce)) cubic phase observed during the oxidation of the mixed oxide containing 8.5 mol% Ce is in agreement with the results of Tagawa and Fujino [38] and is in contrast to the results reported by Nawada et al [32] wherein the limit of the M 3 O 8 singlephase region was found to be close to 12 mol% Ce. The phase separation observed in this study can be attributed to the long duration (24 h) of annealing of the sample in air as compared to 8-10 h reported in the earlier study [32].…”

“…In the case of U 0:915 Ce 0:085 O 2:67 , the U 4f 7=2 peak appeared at 380.9 eV of which the value is less than that of U(VI) in UO 3 [38] and more than that of U(V). Hence, U in this oxide exhibits an oxidation state higher than that in U 3 O 8 and lower than the one in UO 3 , which also supports the O/M result.…”

Influence of Sintering Conditions on the Structure and Redox Speciation of Homogeneous (U,Ce)O2+δ Ceramics: A Synchrotron Study

“…Nevertheless, while the values published by Tagawa et al . remained close to the Vegard’s law, those from Lorenzelli and Touzelin appeared to be strongly different and might probably be considered with caution (Figure a). Also, it is important to note that when the amount of cerium incorporated in the samples increased (typically, x Ce > 0.60), the unit cell parameter got closer to the Vegard’s law, indicating almost negligible deviation from the stoichiometry despite the conditions applied.…”

“…Variation of the unit cell parameters of U 1– x Ce x O 2±δ reported in the literature for (a) sub-stoichiometric samples with O/M < 2.00 (black square: Lorenzelli and Touzelin, blue triangle: Tagawa et al, and red circle: Markin et al); (b) stoichiometric samples with O/M = 2.00 (black square: Dorr et al, red circle: Krishnan et al, blue triangle: Martin et al, pink star: Krishnan et al, blue inverted triangle: Nagarajan et al, pink diamond: Kim et al, violet triangle: Cao et al, orange triangle: Markin et al, yellow hexagon: Kurosaki et al, green pentagon: Designan et al, blue hollow circle: Prieur et al, and brown hollow circle: Zinkevich et al; and (c) hyper-stoichiometric samples with O/M > 2.00: Nawada et al (black square: 873 K), (red circle: 1123 K), (blue triangle: 1273 K); green inverted triangle: Markin et al; and violet diamond: Tagawa et al The dashed lined represents the Vegard’s law extrapolated from pure UO 2 and CeO 2 data.…”

“…Indeed, laboratory PXRD experiments were generally performed shortly after the fabrication of the samples, while the latter were stored for several weeks prior to synchrotron measurements, probably leading to a limited oxidation of uranium at the surface. 59 green pentagon: Designan et al, 13 blue hollow circle: Prieur et al, 60 and brown hollow circle: Zinkevich et al; 61 and (c) hyperstoichiometric samples with O/M > 2.00: Nawada et al 53 (black square: 873 K), (red circle: 1123 K), (blue triangle: 1273 K); green inverted triangle: Markin et al; 49 and violet diamond: Tagawa et al 51 The dashed lined represents the Vegard's law extrapolated from pure UO 2 38 and CeO The impact of the O/M ratio and of the cerium content on the structure of U 1−x Ce x O 2+δ solid solutions was further evaluated by plotting the variation of the unit cell parameter of several samples as a function of the deviation from the O/M = 2.00 stoichiometry (Figure 8). In this aim, only S-PXRD results were considered for a matter of consistency as O/M ratios were determined from synchrotron measurements.…”