Zircon (ZrSiO4, I41/amd) can accommodate actinides, such as thorium, uranium, and plutonium. The zircon structure has been determined for several of the end-member compositions of other actinides, ...such as plutonium and neptunium. However, the thermodynamic properties of these actinide zircon structure types are largely unknown due to the difficulties in synthesizing these materials and handling transuranium actinides. Thus, we have completed a thermodynamic study of cerium orthosilicate, stetindite (CeSiO4), a surrogate of PuSiO4. For the first time, the standard enthalpy of formation of CeSiO4 was obtained by high temperature oxide melt solution calorimetry to be −1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO2 and SiO2 by 27.5 ± 3.1 kJ/mol. The metastability explains the rarity of the natural occurrence of stetindite and the difficulty of its synthesis. Applying the obtained enthalpy of formation of CeSiO4 from this work, along with those previously reported for USiO4 and ThSiO4, we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO4 are then determined with a discussion of future strategies for efficiently immobilizing Pu or minor actinides in the zircon structure.
A series of sintered UO2 pellets doped with lanthanide (Ce, Nd, Yb) elements were investigated using powder X-ray diffraction, Raman spectroscopy, thermogravimetric analyses and differential scanning ...calorimetry. A combination of electron microprobe and thermogravimetric analyses, for oxygen content, enabled precise determination of the hypostoichiometry for lanthanide-doped samples at 1 and 5 atom percent. Two Raman laser wavelengths (785 and 455 nm) have afforded greater sensitivity to spectroscopic signatures of the phonon bands (1LO and 2LO) associated with oxidation of (U1-yMy)O2-x and the anion defects introduced by lanthanide substitution. Oxygen hypostoichiometry forces a reduction in the average coordination number surrounding (U,M) sites, which is compensated by a decrease in U–O bond length, and concomitantly the lattice parameter, consistent with the obtained Raman spectra. The evolution of O/M ratio up to (U1-yMy)O2 after oxidation was also examined using Raman spectroscopy, revealing that the ‘defect band’, including a component attributed to oxygen vacancies (∼540 cm−1) and the 1LO phonon (∼575 cm−1) increased in intensity with increasing dopant concentration and upon oxidation. The lanthanide dopants inhibited oxidation to U3O8, most prominently for Yb 5 at%, having been delayed by ∼180 °C. Thermogravimetric analyses reveal an early oxidation feature that may be related to influx of O to satisfy hypostoichiometry up to (U1-yMy)O2, possibly stabilizing a U4O9 or U3O7 intermediate, delaying oxidation to U3O8.
•Oxidation characteristics for hypostoichiometric Ln-doped UO2-x revealed.•Two Raman lasers provide spectrally selective information on UO2-x.•This is the first oxidation study on hypostoichiometric Yb-doped UO2-x.•Yb-doping strongly inhibits U3O8 formation by 180 °C relative to pure UO2.•New spectroscopic and oxidation signatures compared with prior studies of UO2-x.
Intrinsic properties of a compound (e.g., electronic structure, crystallographic structure, optical and magnetic properties) define notably its chemical and physical behavior. In the case of ...nanomaterials, these fundamental properties depend on the occurrence of quantum mechanical size effects and on the considerable increase of the surface to bulk ratio. Here, we explore the size dependence of both crystal and electronic properties of CeO2 nanoparticles (NPs) with different sizes by state-of-the art spectroscopic techniques. X-ray diffraction, X-ray photoelectron spectroscopy, and high-energy resolution fluorescence-detection hard X-ray absorption near-edge structure (HERFD-XANES) spectroscopy demonstrate that the as-synthesized NPs crystallize in the fluorite structure and they are predominantly composed of CeIV ions. The strong dependence of the lattice parameter with the NPs size was attributed to the presence of adsorbed species at the NPs surface thanks to Fourier transform infrared spectroscopy and thermogravimetric analysis measurements. In addition, the size dependence of the t2g states in the Ce LIII XANES spectra was experimentally observed by HERFD-XANES and confirmed by theoretical calculations.
Intrinsic properties of a compound (e.g., electronic structure, crystallographic structure, optical and magnetic properties) define notably its chemical and physical behavior. In the case of ...nanomaterials, these fundamental properties depend on the occurrence of quantum mechanical size effects and on the considerable increase of the surface to bulk ratio. Here, we explore the size dependence of both crystal and electronic properties of CeO
nanoparticles (NPs) with different sizes by state-of-the art spectroscopic techniques. X-ray diffraction, X-ray photoelectron spectroscopy, and high-energy resolution fluorescence-detection hard X-ray absorption near-edge structure (HERFD-XANES) spectroscopy demonstrate that the as-synthesized NPs crystallize in the fluorite structure and they are predominantly composed of Ce
ions. The strong dependence of the lattice parameter with the NPs size was attributed to the presence of adsorbed species at the NPs surface thanks to Fourier transform infrared spectroscopy and thermogravimetric analysis measurements. In addition, the size dependence of the t
states in the Ce L
XANES spectra was experimentally observed by HERFD-XANES and confirmed by theoretical calculations.
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•NU-1000 was synthesized and used in the oxidative desulfurization of fuel.•The removal of dibenzothiophene reached 100% under the optimized conditions.•NU-1000 exhibited high ...stability in the reaction system.•The high efficiency of NU-1000 was benefited from its mesoporous structure.
NU-1000 was synthesized and used as a catalyst in oxidative desulfurization (ODS) of model fuel. The oxidation of dibenzothiophene was studied to explore the effect of different reaction parameters (temperature, time, catalyst and oxidant loading) on the ODS performance. Under the optimized conditions, the conversion of dibenzothiophene reached 100% within 180 min. The removal of other organosulfur compounds, 4,6-dimethyldibenzothiophene, 3-methylbenzothiophene, and benzothiophene, was also examined. It was found that the reactivity is highly related to the structure of the compounds. The NU-1000 exhibited excellent stability in the reaction system and can be reused for four cycles without obvious loss of activity. The high efficiency and robustness render NU-1000 promising potential in the application of deep desulfurization of fuels.
Raman spectroscopy is one of the most useful techniques for studying the structure of UO2 and changes due to specific defects caused by doping, changes in stoichiometry, irradiation, or heating under ...oxidizing conditions. In this paper, we illustrate several aspects of the application of Raman techniques to the study of UO2, including the use of wavelength‐dependent excitation (455, 532, and 785 nm) to assess the effects of doping (Nd, Th, and Zr), ion irradiation, and in situ heating and oxidation (UO2 to U3O8). Additionally, we show examples of how correlative microscopy is possible using electron backscatter diffraction combined with Raman maps of specific vibration bands or of laser‐induced luminescence generated by rare‐earth dopants in the matrix. For each of these applications, we suggest optimal excitation wavelengths that vary depending on the desired data. Blue (455 nm) excitation tends to promote oxidation even at low powers, but because Raman spectra change little with doping, irradiation‐induced changes are easier to observe. Green (532 nm) excitation is optimal for observing electron–phonon resonance effects in UO2 and offers a good compromise for high‐temperature oxidation experiments, delivering high‐quality spectra for both UO2 and U3O8. Infrared (785 nm) excitation is best for observing “defect” bands associated with doping in UO2, as changes with irradiation are small. Raman spectroscopy is particularly suited for studying the stability of UO2 towards oxidation in the presence of dopants simulating fission products, where electron–phonon resonant effects, dopant ion luminescence, and mapping can be used together to investigate structural rearrangement as a function of temperature. These techniques can offer insight into microstructural changes in UO2 fuels at higher burnups envisioned in future reactors.
Raman spectroscopy is very useful for studying the structural evolution of UO2 pellets that accumulate defects and fission product elements during and after use as nuclear fuel. Here, we illustrate spectroscopic differences obtainable with wavelength‐dependent excitation (455, 532, and 785 nm), for assessing effects of trivalent and tetravalent doping, ion irradiation, and in situ heating and oxidation (UO2 to U3O8). We also show the benefits of mapping and correlative microscopy, including use of Raman laser‐induced luminescence of rare‐earth ions.
Two new zirconium MOFs, WSU-6 and WSU-7, were synthesized through postsynthetic modifications. In both cases, linker insertion was conducted on a MOF consisting of eight-connected (8-c) Zr6 cluster ...and four-connected (4-c) ETTC linker, WSU-5, which possesses the uncommon 4, 8-c scu-c topology. The insertion of 1, 4-benzenedicarboxylate into the MOF formed the new 4, 12-c mjh topology, WSU-6. Interestingly, when 2, 6-naphthalenedicarboxylate was inserted, WSU-7 can be formed, which possesses a new 4, 14-c jkz topology. WSU-7 contains very rare 14-c Zr6 secondary building units (SBUs) and is the first MOF to have a Zr6 SBUs with connectivity greater than 12. The three Zr-MOFs were structurally characterized, and the photoluminescence properties of the materials were also studied.
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