Deciphering the Crystal Structure of a Scarce 1D Polymeric Thorium Peroxo Sulfate

Chronicles of plutonium peroxides: spectroscopic characterization of a new peroxo compound of Pu(IV)

Although hydrogen peroxide (H2O2) has been highly used in nuclear chemistry for more than 75 years, the preparation and literature description of tetravalent actinide peroxides remain surprisingly scarce. A new insight is given in this topic through the synthesis and thorough structural characterization of a new peroxo compound of Pu(IV).

Micrometric drilling of (meta-)studtite square platelets formed by pseudomorphic conversion of UO2 under high-frequency ultrasound

Pseudomorphic transformations are related to chemical conversions of materials while conserving their shape and structural features. Structuring ceramic shapes this way can be used to tailor the physico-chemical properties of materials that can benefit particular applications. In the context of spent nuclear fuel storage interacting with radiolysis products, the sonochemical behavior of powdered UO2 was investigated in dilute aqueous solutions saturated with Ar/(20 %)O2 (20 °C). Optimized parameter settings enabled the complete conversion of UO2 micrometric platelets into uranyl peroxide precipitates, referred to as (meta-)studtite [(UO2(O2)(H2O)2)xH2O] with x = 2 or 4. While the most acidic conditions yielded elongated crystal shapes in agreement with a dissolution/reprecipitation mechanism, softer conditions allowed the pseudomorphic transformation of the platelet shape oxide suggesting a complex formation mechanism. For specific conditions, this unprecedented morphology was accompanied with the formation of a hole in the platelet center. Investigations revealed that the formation of the drilled polymorphs is related to a perfect blend of H+, in-situ generation of H2O2 and high-frequency ultrasound, and is most probably related to the sono-capillary effect. These insights pave the way for new sonochemical approaches dedicated to the preparation of material polymorphs tailoring specific structural properties.

Structure-sensitive epoxidation of dicyclopentadiene over TiO2 catalysts

Epoxidation of dicyclopentadiene (DCPD) is studied on a series of TiO₂ catalysts using hydrogen peroxide as an oxidant. DCPD derivatives have applications in several areas including polymer, pharmaceutical and pesticide products. The control of selectivity leading to the desired product is important for many of these applications. Using experimental and computational studies, we show that the surface crystalline phases of TiO₂ play crucial roles not only in the formation of peroxo species but also in the selective epoxidation of two different CC double bonds in DCPD.

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

Due to the increased attention given to actinide nanomaterials, the question of their structure-property relationship is on the spotlight of recent publications. Plutonium oxide (PuO₂) particularly plays a central role in nuclear energetics and a comprehensive knowledge about its properties when nanosizing is of paramount interest to understand its behaviour in environmental migration schemes but also for the development of advanced nuclear energy systems underway. The element plutonium further stimulates the curiosity of scientists due to the unique physical and chemical properties it exhibits around the periodic table. PuO₂ crystallizes in the fluorite structure of the face-centered cubic system for which the properties can be significantly affected when shrinking. Identifying the formation mechanism of PuO₂ nanoparticles, their related atomic, electronic and crystalline structures, and their reactivity in addition to their nanoscale properties, appears to be a fascinating and challenging ongoing topic, whose recent advances are discussed in this review.

Sonochemistry of actinides: from ions to nanoparticles and beyond

Sonochemistry studies chemical and physical effects in liquids submitted to power ultrasound. These effects arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species. In principle, each cavitation bubble can be considered as a microreactor initiating chemical reactions at mild conditions. In addition, microjets and shock waves accompanied bubble collapse produce fragmentation, dispersion and erosion of solid surfaces or particles. Microbubbles oscillating in liquids also enable nucleation and precipitation of nanosized actinide compounds with specific morphology. This review focuses on the versatile sonochemical processes with actinide ions and particles in homogenous solutions and heterogenous systems. The redox reactions in aqueous solutions, dissolution or precipitation of refractory solids, synthesis of actinide nanoparticles, and ultrasonically driving decontamination are considered. The guideline for further research is also discussed.

Characterization of a Hexanuclear Plutonium(IV) Nanostructure in an Acetate Solution via Visible-Near Infrared Absorption Spectroscopy, Extended X-ray Absorption Fine Structure Spectroscopy, and Density Functional Theory

A new hexanuclear plutonium cluster has been stabilized in aqueous media with acetate ligands. To probe the formation of such a complex structure, visible-near infrared (vis-NIR) absorption spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) were combined. The presence of Pu₆O₄(OH)₄(CH₃COO)₁₂ species in solution was first detected by vis-NIR and EXAFS spectroscopy. To confirm unambiguously this structure, EXAFS spectra were simulated from ab initio calculations. Debye-Waller factors and structural parameters were derived from DFT calculations. A large number of 5f electrons were treated as valence or core electrons using small- and large-core relativistic effective pseudopotentials. It is possible to reproduce accurately the EXAFS spectrum of the octahedral hexamer cluster at both levels of calculations. Further DFT and EXAFS calculations were performed on clusters of lower or higher nuclearities and of different geometries using the 5f-core approximation. The result shows that trimer, tetramer, flat hexamer, and even 16-mer clusters exhibit different EXAFS patterns and confirm the very specific octahedral hexanuclear EXAFS signature.

Coprecipitation of actinides peroxide salts in the U-Th and U-Pu systems and their thermal decomposition

Uranium and plutonium coconversion process constitutes a continuous subject of interest for MOx fuel fabrication. Among the various routes considered, chemical coprecipitation by salt effect has been widely investigated regarding...

Sonochemical dissolution of nanoscale ThO2 and partial conversion into a thorium peroxo sulfate

The influence of the sample morphology and experimental conditions towards the sonochemical dissolution of nanoscale ThO₂ samples in sulfuric acid media is described. Significant sonochemical dissolution rates and yields are observed at 20 kHz under Ar/O₂ atmosphere in dilute 0.5 M H₂SO₄ at room temperature, contrasting with the generally-reported high refractory behavior for ThO₂. The dissolution of ThO₂ combines the physical effects driven by acoustic cavitation phenomenon, the complexing affinity of Th(IV) in sulfuric medium and the sonochemical generation of H₂O₂. These sonochemical conditions further allow the observation of the partial conversion of ThO₂ into a scarce Th(IV) peroxo sulfate with 1D morphology resulting from one or both following processes: dissolution/reprecipitation or formation of an intermediate Th(IV) surface complex.