Globular pattern formation of hierarchical ceria nanoarchitectures

Dissipative structures often appear as an unstable counterpart of ordered structures owing to fluctuations that do not form a homogeneous phase. Even a multiphase mixture may simultaneously undergo one chemical reaction near equilibrium and another one that is far from equilibrium. Here, we observed in real time crystal seed formation and simultaneous nanocrystal aggregation proceeding from CeIV complexes to CeO2 nanoparticles in an acidic aqueous solution, and investigated the resultant hierarchical nanoarchitecture. The formed particles exhibited two very different size ranges, resulting in further pattern formation with opalescence. The hierarchically assembled structures in solutions were CeO2 colloids, viz. primary core clusters (1-3 nm) of crystalline ceria and secondary clusters (20-30 nm) assembled through surface ions. Such self-assembly is widespread in multi-component complex fluids, paradoxically moderating hierarchical reactions. Stability and instability are not only critical but also complementary for co-optimisation around the nearby free energy landscape prior to bifurcation.

Speciation of Technetium Dibutylphosphate in the Third Phase Formed in the TBP/HNO3 Solvent Extraction System

The speciation of technetium in the nitric acid/dibutylphosphoric acid (HDBP)-n-dodecane system was studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and theoretical methods. Tetravalent technetium, produced by the hydrazine reduction of TcO4- in 3 M HNO3, was extracted by HDBP in n-dodecane (30% by volume). During extraction, the splitting of the organic phase into a heavy phase and a light phase was observed. EXAFS analysis is consistent with the presence of Tc(NO3)3(DBP)(HDBP)2 in the light phase and Tc(NO3)2(DBP)2(HDBP)2 in the heavy phase. Density functional theory calculations at the B3LYP/6-31G* level confirm the stability of the proposed species and indicate that stereoisomers -mer- and fac-Tc(NO3)3(DBP)(HDBP)2 for the light phase and cis- and trans-Tc(NO3)2(DBP)2(HDBP)2 for the heavy phase] could coexist in the system (in the n-dodecane solution). Mechanisms of formation of Tc(NO3)3(DBP)(HDBP)2 and Tc(NO3)2(DBP)2(HDBP)2 are proposed.

Structural Investigation of Technetium Dibutylphosphate Species Using X-ray Absorption Fine Structure Spectroscopy

The speciation of Tc after the extraction of Tc(IV) from H2O and 1 M HNO3 by dibutylphosphoric acid (HDBP) in dodecane has been studied by X-ray absorption fine structure (XAFS) spectroscopy. Results show the formation of dimeric species with Tc2O2 and Tc2O units, and the formulas [Tc2O2(DBP·HDBP)4] (1) and [Tc2O(NO3)2(DBP)2(DBP·HDBP)2] (2) were, respectively, proposed for the species extracted from H2O and 1 M HNO3. The interatomic Tc-Tc distances found in the Tc2O2 and Tc2O units [2.55(3) and 3.57(4) Å, respectively] are similar to the ones found in Tc(IV) dinuclear species. It is likely that the speciation of Tc(IV) in dodecane is due to the extraction of a species with a Tc2O unit for (2) and to the redissolution of a Tc(IV)-DBP solid for (1). The XAFS results for (1) and (2) were compared to that obtained for the extraction of Tc(IV) with TBP/HDBP/dodecane from 0.5 M HNO3, (3) which highlight the formation of Tc mononuclear nitrate species {i.e., [Tc(NO3)3(DBP)] or [Tc(NO3)2(DBP·HDBP)]}. These results confirm the importance of the preparation and speciation of the Tc(IV) aqueous solutions prior to extraction and how much this influences and drives the final Tc speciation in organic extraction. These studies outline the complexity of Tc separation chemistry and provide insights into the behavior of Tc during the reprocessing of used nuclear fuel.

Extraction of rare earth elements from aqueous solutions using the ionic liquid trihexyltetradecylphosphonium 3-hydroxy-2-naphthoate

The task-specific ionic liquid trihexyltetradecylphosphonium 3-hydroxy-2-naphthoate has been described as a suitable extraction agent for numerous metals from aqueous phases, while additionally providing reduced leaching into the used matrices. Here, we investigate the extraction properties of this extractant towards rare earth elements. Of these, La, Ce, Nd, Ho und Lu were chosen as a representative mix of light and heavy elements. Single- as well as double-element extractions were carried out under varying conditions regarding pH, temperature and extraction time. The highest extraction efficacies and minimalized precipitation of the respective metals were recorded at a pH of 2.5. Satisfactory extraction efficacies (>80%) were achieved already after 6 hours for the elements Ce, Nd and Lu in single-element extraction experiments at room temperature. Increased temperatures improved the extraction efficacy for Nd from 36% at 20 °C to 80% at 30 °C after only 2 hours. Surprisingly, this effect was not observed for Ce in single-element experiments. In double-element feed solutions containing both Ce and Nd, however, the time-dependant extraction efficacy of Ce mirrored that of Nd. The pH in the aqueous extraction matrix changed during the extraction, showing a positive correlation with the extraction efficacy and revealing the extraction mechanism to be via anion exchange. The leaching was in good agreement with literature values, showed a positive correlation with extraction efficacies, and ranged for all extractions between 0.8 and 1.2%. Remarkably, increasing the temperature from 20 °C to 30 °C had no significant influence on leaching.

Unveiling the Cerium(III)/(IV) Structures and Charge-Transfer Mechanism in Sulfuric Acid

The Ce³⁺/Ce⁴⁺ redox couple has a charge transfer (CT) with extreme asymmetry and a large shift in redox potential depending on electrolyte composition. The redox potential shift and CT behavior are difficult to understand because neither the cerium structures nor the CT mechanism are well understood, limiting efforts to improve the Ce³⁺/Ce⁴⁺ redox kinetics in applications such as energy storage. Herein, we identify the Ce³⁺ and Ce⁴⁺ structures and CT mechanism in sulfuric acid via extended X-ray absorption fine structure spectroscopy (EXAFS), kinetic measurements, and density functional theory (DFT) calculations. We show EXAFS evidence that confirms that Ce³⁺ is coordinated by nine water molecules and suggests that Ce⁴⁺ is complexed by water and three bisulfates in sulfuric acid. Despite the change in complexation within the first coordination shell between Ce³⁺ and Ce⁴⁺, we show that the kinetics are independent of the electrode, suggesting outer-sphere electron-transfer behavior. We identify a two-step mechanism where Ce⁴⁺ exchanges the bisulfate anions with water in a chemical step followed by a rate-determining electron transfer step that follows Marcus theory (MT). This mechanism is consistent with all experimentally observed structural and kinetic data. The asymmetry of the Ce³⁺/Ce⁴⁺ CT and the observed shift in the redox potential with acid is explained by the addition of the chemical step in the CT mechanism. The fitted parameters from this rate law qualitatively agree with DFT-predicted free energies and the reorganization energy. The combination of a two-step mechanism with MT should be considered for other metal ion CT reactions whose kinetics have not been appropriately described.

Aggregation Behavior of Nitrilotriacetamide (NTAmide) Ligands in Thorium(IV) Extraction from Acidic Medium: Small-Angle Neutron Scattering, Fourier Transform Infrared, and Theoretical Studies

Two tripodal amides obtained from nitrilotriacetic acid with n-butyl and n-octyl alkyl chains (HBNTA(L_I) and HONTA(L_II), respectively) were studied for the extraction of Th(IV) ions from nitric acid medium. The effect of the diluent medium, i.e., n-dodecane alone and a mixture of n-dodecane and 1-decanol, onto aggregate formation were investigated using small angle neutron scattering (SANS) studies. In addition, the influence of the ligand structure, nitric acid, and Th(IV) loading onto ligand aggregation and third-phase formation tendency was discussed.The L_I/L_II exist as monomers (aggregarte radius for L_I: 6.0 Å; L_II:7.4 Å) in the presence of 1-decanol, whereas L_II forms dimers (aggregarte radius for L_II:9.3 Å; L_I does not dissolve in n-dodecane) in the absence of 1-decanol. The aggregation number increases for both the ligands after HNO₃ and Th(IV) loading. The maximum organic concentration (0.050 ± 0.004 M) of Th(IV) was reached without third-phase formation for 0.1 M L_I/L_II dissolved in 20% isodecanol +80% n-dodecane. The interaction of 1-decanol with L_II and HNO₃/Th(IV) with amidic oxygens of L_I/L_II results in shift of carbonyl stretching frequency, as shown by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) studies. The structural and bonding information of the Th-L_I/L_II complex were derived from the density functional theoretical (DFT) studies. The molecular dynamics (MD) simulations suggested that the aggregation behavior of the ligand in the present system is governed by the population of hydrogen bonds by phase modifier around the ligand molecules. Although the theoretical studies suggested higher Gibbs free energy of complexation for Th⁴⁺ ions with L_I than L_II, the extraction was found to be higher with the latter, possibly due to the higher lipophilicity and solubility of the Th-L_II aggregate in the nonpolar media.

Molecular Forces in Liquid-Liquid Extraction

The phase transfer of ions is driven by gradients of chemical potentials rather than concentrations alone (i.e., by both the molecular forces and entropy). Extraction is a combination of high-energy interactions that correspond to short-range forces in the first solvation shell such as ion pairing or complexation forces, with supramolecular and nanoscale organization. While the latter are similar to the long-range solvent-averaged interactions in the colloidal world, in solvent extraction they are associated with lower characteristic lengths of the nanometric domain. Modeling of such complex systems is especially complicated because the two domains are coupled, whereas the resulting free energy of extraction is around k_BT to guarantee the reversibility of the practical process. Nevertheless, quantification is possible by considering a partitioning of space among the polar cores, interfacial film, and solvent. The resulting free energy of transfer can be rationalized by utilizing a combination of terms which represent strong complexation energies, counterbalanced by various entropic effects and the confinement of polar solutes in nanodomains dispersed in the diluent, together with interfacial extractant terms. We describe here this ienaics approach in the context of solvent extraction systems; it can also be applied to further complex ionic systems, such as membranes and biological interfaces.

A HNO3 -Responsive Aqueous Biphasic System for Metal Separation: Application towards CeIV Recovery

An acidic aqueous biphasic system (AcABS) presenting a desired and reversible phase transition with HNO₃ concentration and temperature was developed herein as an integrated platform for metal separation. The simple, economical, and fully incinerable (C,H,O,N) AcABS composed of tetrabutylammonium nitrate ([N₄₄₄₄ ][NO₃ ])+HNO₃ +H₂ O was characterized and presented an excellent selectivity towards Ce^IV against other rare earth elements and transition metals from both synthetic solutions and nickel metal hydride (NiMH) battery leachates. The acid-driven self-assembly of AcABS bridges the gap between traditional ABS and liquid-liquid extraction whilst retaining their advantageous qualities, including compatibility with highly acidic solutions, water as the primary system component, the avoidance of organic diluents, rapid mass transfer, and the potential integration of the leaching and separation steps.

Structures and Free Energies of Cerium Ions in Acidic Electrolytes

The Ce³⁺/Ce⁴⁺ redox potential changes with the electrolyte, which could be due to unequal anion complexation free energies between Ce³⁺ and Ce⁴⁺ or a change in the solvent electrostatic screening. Ce complexation with anions and solvent screening also affect the solubility of Ce and charge transfer kinetics for electrochemical reactions involving waste remediation and energy storage. We report the structures and free energies of cerium complexes in seven acidic electrolytes based on Extended X-ray Absorption Fine Structure, UV-vis, and Density Functional Theory calculations. Ce³⁺ coordinates with nine water molecules as [Ce(H₂O)₉]³⁺ in all studied electrolytes. However, Ce⁴⁺ complexes with anions in all electrolytes except HClO₄. Thus, our results suggest that Ce⁴⁺-anion complexation leads to the large shifts in standard redox potential. Long range screening effects are smaller than the anion complexation energies but could be responsible for changes in the Ce solubility with acid.

Influence of pH on CeIV-[AsIIIW9O33]9− association for the formation of hexanuclear cerium(iv) oxo-hydroxo-clusters stabilized by trivacant polyanions

Influence of pH on Ce^IV-AsW₉O₃₃ association leads to the formation of four crystalline compounds incorporating classical and distorted hexanuclear cerium clusters.

Influence of Light and Temperature on the Extractability of Cerium(IV) as a Surrogate of Plutonium(IV) and its Effect on the Simulation of an Accidental Fire in the PUREX Process

Modeling of plutonium(IV) behavior during an accidental fire in a reprocessing plant was considered using various non-radioactive metallic surrogates. Among those elements, cerium(IV) was supposed to be a suitable candidate due to possible formation of a complex with TPB, but its extractability and stability have not been studied previously under representative plutonium uranium reduction extraction (PUREX) conditions. In this work, we investigated the chemical analogy between cerium(IV) and plutonium(IV) in this extractive process and combustion thereof. Distribution ratios are reported for acidities of 1-4 mol L^-1 in equal volumes of nitric acid and a 30:70 mixture of tributylphosphate and hydrogenated tetrapropylene. The influences of light, temperature, and extraction time were studied by UV-vis spectroscopy. The results showed that cerium(IV) is extracted quantitatively but is reduced over time to cerium(III) in the organic mixture. Spectrophotometric investigations of this reaction kinetics revealed an apparent rate constant k of 0.021 ± 0.002 mol^0.5 L^0.5 min^-1 at 298 K and an apparent fractional reaction order of 0.5. The activation energy of this reduction was found to be around 82 ± 2 kJ mol^-1 by the Arrhenius plot method. The combustion of mono- and biphasic solutions prepared with a cerium(IV) concentration of 10 g L^-1 revealed that the extracted complexes, Ce₂O·6NO₃·3TBP_(org) or Ce₄O₄·8NO₃·6TBP_(org), are reduced during the combustion. Compositions of the resulting ashes and soot were analyzed and highlighted the presence of pyrophosphates and polycyclic aromatic hydrocarbons, with some traces of cerium. Ce(IV) is not suitable to represent Pu(IV) from a chemical point of view in HNO₃/TBP-HTP solutions.

A Telescoping View of Solute Architectures in a Complex Fluid System

Short- and long-range correlations between solutes in solvents can influence the macroscopic chemistry and physical properties of solutions in ways that are not fully understood. The class of liquids known as complex (structured) fluids-containing multiscale aggregates resulting from weak self-assembly-are especially important in energy-relevant systems employed for a variety of chemical- and biological-based purification, separation, and catalytic processes. In these, solute (mass) transfer across liquid-liquid (water, oil) phase boundaries is the core function. Oftentimes the operational success of phase transfer chemistry is dependent upon the bulk fluid structures for which a common functional motif and an archetype aggregate is the micelle. In particular, there is an emerging consensus that mass transfer and bulk organic phase behaviors-notably the critical phenomenon of phase splitting-are impacted by the effects of micellar-like aggregates in water-in-oil microemulsions. In this study, we elucidate the microscopic structures and mesoscopic architectures of metal-, water-, and acid-loaded organic phases using a combination of X-ray and neutron experimentation as well as density functional theory and molecular dynamics simulations. The key conclusion is that the transfer of metal ions between an aqueous phase and an organic one involves the formation of small mononuclear clusters typical of metal-ligand coordination chemistry, at one extreme, in the organic phase, and their aggregation to multinuclear primary clusters that self-assemble to form even larger superclusters typical of supramolecular chemistry, at the other. Our metrical results add an orthogonal perspective to the energetics-based view of phase splitting in chemical separations known as the micellar model-founded upon the interpretation of small-angle neutron scattering data-with respect to a more general phase-space (gas-liquid) model of soft matter self-assembly and particle growth. The structure hierarchy observed in the aggregation of our quinary (zirconium nitrate-nitric acid-water-tri-n-butyl phosphate-n-octane) system is relevant to understanding solution phase transitions, in general, and the function of engineered fluids with metalloamphiphiles, in particular, for mass transfer applications, such as demixing in separation and synthesis in catalysis science.

The coordination chemistry of CmIII, AmIII, and AcIII in nitrate solutions: an actinide L3-edge EXAFS study

Understanding actinide(iii) (AnIII = CmIII, AmIII, AcIII) solution-phase speciation is critical for controlling many actinide processing schemes, ranging from medical applications to reprocessing of spent nuclear fuel. Unfortunately, in comparison to most elements in the periodic table, AnIII speciation is often poorly defined in complexing aqueous solutions and in organic media. This neglect - in large part - is a direct result of the radioactive properties of these elements, which make them difficult to handle and acquire. Herein, we surmounted some of the handling challenges associated with these exotic 5f-elements and characterized CmIII, AmIII, and AcIII using AnIII L3-edge X-ray absorption spectroscopy (XAS) as a function of increasing nitric acid (HNO3) concentration. Our results revealed that actinide aquo ions, An(H2O) x 3+ (x = 9.6 ± 0.7, 8.9 ± 0.8, and 10.0 ± 0.9 for CmIII, AmIII, and AcIII), were the dominant species in dilute HNO3 (0.05 M). In concentrated HNO3 (16 M), shell-by-shell fitting of the extended X-ray fine structure (EXAFS) data showed the nitrate complexation increased, such that the average stoichiometries of Cm(NO3)4.1±0.7(H2O)5.7±1.3 (1.1±0.2)-, Am(NO3)3.4±0.7(H2O)5.4±0.5 (0.4±0.1)-, and Ac(NO3)2.3±1.7(H2O)8.3±5.2 (0.7±0.5)+ were observed. Data obtained at the intermediate HNO3 concentration (4 M) were modeled as a linear combination of the 0.05 and 16 M spectra. For all three metals, the intermediate models showed larger contributions from the 0.05 M HNO3 spectra than from the 16 M HNO3 spectra. Additionally, these efforts enabled the Cm-NO3 and Ac-NO3 distances to be measured for the first time. Moreover, the AnIII L3-edge EXAFS results, contribute to the growing body of knowledge associated with CmIII, AmIII, and AcIII coordination chemistry, in particular toward advancing understanding of AnIII solution phase speciation.

Structural investigations on uranium(vi) and thorium(iv) complexation with TBP and DHOA: a spectroscopic study

Spectroscopic studies were carried out to understand the complexation of U(vi) and Th(iv) with tri-butyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) in different non-aqueous solvents.

Square supramolecular assemblies of uranyl complexes in organic solvents

Uranyl, ligand and solvent interactions lead to unique supramolecular assembly.