The Role of Alkalis in Orchestrating Uranyl-Peroxide Reactivity Leading to Direct Air Capture of Carbon Dioxide

Spectator ions have known and emerging roles in aqueous metal-cation chemistry, respectively directing solubility, speciation, and reactivity. Here, we isolate and structurally characterize the last two metastable members of the alkali uranyl triperoxide series, the Rb+ and Cs+ salts (Cs-U1 and Rb-U1). We document their rapid solution polymerization via small-angle X-ray scattering, which is compared to the more stable Li+, Na+ and K+ analogues. To understand the role of the alkalis, we also quantify alkali-hydroxide promoted peroxide deprotonation and decomposition, which generally exhibits increasing reactivity with increasing alkali size. Cs-U1, the most unstable of the uranyl triperoxide monomers, undergoes ambient direct air capture of CO2 in the solid-state, converting to Cs4[UVIO2(CO3)3], evidenced by single-crystal X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. We have attempted to benchmark the evolution of Cs-U1 to uranyl tricarbonate, which involves a transient, unstable hygroscopic solid that contains predominantly pentavalent uranium, quantified by X-ray photoelectron spectroscopy. Powder X-ray diffraction suggests this intermediate state contains a hydrous derivative of CsUVO3, where the parent phase has been computationally predicted, but not yet synthesized.

Gamma-Ray-Induced Formation of Uranyl Peroxide Cage Clusters

Aqueous solutions of lithium uranyl triperoxide, Li₄[UO₂(O₂)₃] (LiUT), were irradiated with gamma rays at room temperature and found to form the uranyl peroxide cage cluster, Li₂₄[(UO₂)(O₂)(OH)]₂₄ (Li-U₂₄). Raman spectroscopy and ¹⁸O labeling were used to identify the Raman-active vibrations of LiUT. With these assignments, the concentration of LiUT was tracked as a function of radiation dose. A discrepancy between monomer removal and cluster formation suggests that the reaction proceeds by the assembly of an intermediate. Non-negative matrix factorization was used to separate Raman spectra into components and resulted in the identification of a unique intermediate species. Much of the conversion appears to be driven by water radiolysis products, particularly the hydroxyl radical. This differs from the ¹⁸O-labeled copper-catalyzed formation of U₂₄, which progresses at a steady rate with no observation of intermediates. Li-U₂₄ in solution decomposes at high radiation doses resulting in a solid insoluble product similar to Na-compreignacite, Na₂(UO₂)₆O₄(OH)₆·7H₂O, which contains uranyl oxyhydroxy sheets.

Stability of Studtite in Saline Solution: Identification of Uranyl-Peroxo-Halo Complex

Hydrogen peroxide is produced upon radiolysis of water and has been shown to be the main oxidant driving oxidative dissolution of UO₂-based nuclear fuel under geological repository conditions. While the overall mechanism and speciation are well known for granitic groundwaters, considerably less is known for saline waters of relevance in rock salt or during emergency cooling of reactors using seawater. In this work, the ternary uranyl–peroxo–chloro and uranyl–peroxo–bromo complexes were identified using IR, Raman, and nuclear magnetic resonance (NMR) spectroscopy. Based on Raman spectra, the estimated stability constants for the identified uranyl–peroxo–chloro ((UO₂)(O₂)(Cl)(H₂O)₂^–) and uranyl–peroxo–bromo ((UO₂)(O₂)(Br)(H₂O)₂^–) complexes are 0.17 and 0.04, respectively, at ionic strength ≈5 mol/L. It was found that the uranyl–peroxo–chloro complex is more stable than the uranyl–peroxo–bromo complex, which transforms into studtite at high uranyl and H₂O₂ concentrations. Studtite is also found to be dissolved at a high ionic strength, implying that this may not be a stable solid phase under very saline conditions. The uranyl–peroxo–bromo complex was shown to facilitate H₂O₂ decomposition via a mechanism involving reactive intermediates.

Aqueous solutions containing UO₂²⁺ and H₂O₂ are stabilized by the presence of chloride. This is attributed to the formation of uranyl−chloro and uranyl−peroxo−chloro complexes preventing the precipitation of studtite. The existence of these complexes was confirmed using IR, Raman, and NMR spectroscopies.

Organic Functionalization of Uranyl Peroxide Clusters to Impact Solubility

Benzene-1,2-diphosphonic acid (Ppb) was introduced into the uranyl peroxide cluster system, resulting in three Ppb-functionalized uranyl peroxide clusters, (UO₂)₂₀(O₂)₂₀(C₆H₄P₂O₆)₁₀^40- (U₂₀Ppb₁₀), (UO₂)₂₆(O₂)₃₃(C₆H₄P₂O₆)₆^38- (U₂₆Ppb₆), and (UO₂)₂₀(O₂)₂₄(C₆H₄P₂O₆)₆^32- (U₂₀Ppb₆). Dissolution experiments were performed for the potassium salts of U₂₀Ppb₁₀ and U₂₆Ppb₆, which revealed the capacity of U₂₀Ppb₁₀ to dissolve in the organic solvent dimethyl sulfoxide (DMSO). Unlike U₂₀Ppb₁₀, the K salt of U₂₆Ppb₆ did not dissolve in DMSO but was more soluble in water, perhaps due to the lower proportion of Ppb ligands in its structure. In this work, U₂₀Ppb₁₀ and U₂₀Ppb₆ formed as potassium salts and both adopt the fullerene topology of previously reported U₂₀. U₂₀ contains 20 uranyl peroxide units and encapsulates 12 Na cations. It is not possible for unfunctionalized U₂₀ to incorporate 12 K cations owing to space constraints, as is the case in the new clusters reported here. Transformation of U₂₀Ppb₁₀ in water over time to produce U₂₄ was observed, possibly owing to its ability to incorporate K cations, which have been associated with the formation of U₂₄.

Unveiling a Photoinduced Hydrogen Evolution Reaction Mechanism via the Concerted Formation of Uranyl Peroxide

We present a density functional theory study for the photochemical water oxidation reaction promoted by uranyl nitrate upon sunlight radiation. First, we explored the most stable uranyl complex in the absence of light. The reaction in a dark environmen proceeds through the condensation of uranyl monomers to form dimeric hydroxo-bridged species, which is the first step toward a hydrogen evolution reaction (HER). We found a triplet-state-driven mechanism that leads to the formation of uranyl peroxide and hydrogen gas. To describe in detail this reaction path, we characterized the singlet and triplet low-lying states of the dimeric hydroxo-bridged species, including minima, transition states, minimal energy crossing points, and adiabatic energies. Our computational results provide mechanistic insights that are in good agreement with the experimental data available.

Dynamics of Cation-Induced Conformational Changes in Nanometer-Sized Uranyl Peroxide Clusters

Conformational changes of the pyrophosphate (Pp)-functionalized uranyl peroxide nanocluster [(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]^48- ({U₂₄Pp₁₂}), dissolved as a Li/Na salt, can be induced by the titration of alkali cations into solution. The most symmetric conformer of the molecule has idealized octahedral (O_h) molecular symmetry. One-dimensional ³¹P NMR experiments provide direct evidence that both K⁺ and Rb⁺ ions trigger an O_h-to-D_4h conformational change within {U₂₄Pp₁₂}. Variable-temperature ³¹P NMR experiments conducted on partially titrated {U₂₄Pp₁₂} systems show an effect on the rates; increased activation enthalpy and entropy for the D_4h-to-O_h transition is observed in the presence of Rb⁺ compared to K⁺. Two-dimensional, exchange spectroscopy ³¹P NMR revealed that magnetization transfer links chemically unique Pp bridges that are present in the D_4h conformation and that this magnetization transfer occurs via a conformational rearrangement mechanism as the bridges interconvert between two symmetries. The interconversion is triggered by the departure and reentry of K (or Rb) cations out of and into the cavity of the cluster. This rearrangement allows Pp bridges to interconvert without the need to break bonds. Cs ions exhibit unique interactions with {U₂₄Pp₁₂} clusters and cause only minor changes in the solution ³¹P NMR signatures, suggesting that O_h symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D_2h molecular symmetry, implying that while solvated, this cluster is in equilibrium with a more symmetric form. These results highlight the unusually flexible nature of the actinide-based {U₂₄Pp₁₂} and its sensitivity to countercations in solution.

Effects of H2O2 Concentration on Formation of Uranyl Peroxide Species Probed by Dissolution of Uranium Nitride and Uranium Dioxide

Dissolution of uranium materials in alkaline aqueous conditions containing H₂O₂ results in uranyl peroxide species in solution, including anionic uranyl peroxide cage clusters. Uranyl peroxide cage clusters are generally highly soluble in water, where they persist as aqueous macroanions. Previous studies indicate that uranyl cluster speciation and dissolution of uranium materials is impacted by the concentration of alkali metal in solution, but in these studies, high concentrations of H₂O₂ were used. Herein, the role of hydrogen peroxide concentration is examined relative to the dissolution of powdered UN and UO₂. Lower initial H₂O₂ concentrations reduce dissolution of UO₂ and UN and tend to produce simple (small) uranyl peroxide species rather the highly soluble uranyl peroxide clusters. H₂O₂ availability will have implications for uranyl speciation and solubility where spent nuclear fuel is in contact with water and where alkaline peroxide conditions are used in dissolution of nuclear material.

In situ Raman spectroscopy of uranyl peroxide nanoscale cage clusters under hydrothermal conditions

The behaviours of two uranyl peroxide nanoclusters in water heated to 180 °C were examined by in situ Raman spectroscopy.

The lithium–water configuration encapsulated by uranyl peroxide cage cluster U24

Lithium cations encapsulated within the U₂₄ nanocapsule are in square pyramidal and octahedral coordination environments imposed by the topology of the cluster, whereas lithium outside the cages are in a tetrahedral coordination environment.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

Homoleptic U(iii) and U(iv) amidate complexes

Homoleptic U(iv) and U(iii) amidate complexes have been isolated and characterized; these species undergo an unusual and reversible change in coordination number upon reduction/oxidation.

Captivation with encapsulation: a dozen years of exploring uranyl peroxide capsules

Uranyl peroxide cages are an extensive family of topologically varied self-assembling nanoscale clusters with fascinating properties and applications.

Single-Crystal Time-of-Flight Neutron Diffraction and Magic-Angle-Spinning NMR Spectroscopy Resolve the Structure and 1H and 7Li Dynamics of the Uranyl Peroxide Nanocluster U60

Single-crystal time-of-flight neutron diffraction has provided atomic resolution of H atoms of H₂O molecules and hydroxyl groups, as well as Li cations in the uranyl peroxide nanocluster U₆₀. Solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to confirm the dynamics of these constituents, revealing the transportation of Li atoms and H₂O through cluster walls. H atoms of hydroxyl units that are located on the cluster surface are involved in the transfer of H₂O and Li cations from inside to outside and vice versa. This exchange occurs as a concerted motion and happens rapidly even in the solid state. As a consequence of its large size and open hexagonal pores, U₆₀ exchanges Li cations more rapidly compared to other uranyl nanoclusters.