Thermodynamics and Chemical Behavior of Uranyl Superoxide at Elevated Temperatures

Impacts of trace level chromium on formation of superoxide within uranyl triperoxide complexes

U(VI) peroxides are important within the nuclear fuel cycle, but reactive oxygen species (ROS) can form upon oxidation. Herein, we identified the spectral signatures of a U(VI) diperoxosuperoxide complex (KUPS-1) and observed that the transformation of U(VI) triperoxide (KUT-1) to superoxide forms occurred with trace-level Cr. U(VI) superoxide complexes were identified in EPR solution spectra without the use of spin-traps.

Electronic Structure and CO2 Reactivity of Group IV/V/VI Tetraperoxometalates

Tetraperoxo metal complexes are a category of dioxygen compounds with novel properties. One of their underconsidered applications is in direct air capture (DAC) reactions, whose study is of great interest in order to slow the effects of climate change. Through computational modeling, the present work considers a family of tetraperoxometalate complexes of the form [M(O2)4]x- that capture atmospheric CO2 to produce [MO(O2)2(CO3)]x- and O2. This reaction was experimentally documented with vanadium and serves as a model for analogous reactions with Group IV (x = 4; M = Ti, Zr, Hf), Group V (x = 3; M = V, Nb, Ta), and Group VI (x = 2; M = Cr, Mo, W) metal centers. Descriptors from density functional theory (DFT) calculations, including optimized structures, partial charges, and frontier orbital interactions, provide rationalization for predicted differences in reactivity. Of the nine complexes studied, [Ti(O2)4]4- and [W(O2)4]2-, respectively, represent the most and least efficient DAC reagents from their differing abilities to stabilize a bidentate peroxycarbonate (κ2-CO42-) intermediate in the proposed reaction mechanism.

Implementing vanadium peroxides as direct air carbon capture materials

Direct air capture (DAC) removal of anthropogenic CO2 from the atmosphere is imperative to slow the catastrophic effects of global climate change. Numerous materials are being investigated, including various alkaline inorganic metal oxides that form carbonates via DAC. Here we explore metastable early d0 transition metal peroxide molecules that undergo stabilization via multiple routes, including DAC. Specifically here, we describe via experiment and computation the mechanistic conversion of A3V(O2)4 (tetraperoxovanadate, A = K, Rb, Cs) to first a monocarbonate VO(O2)2(CO3)3-, and ultimately HKCO3 plus KVO4. Single crystal X-ray structures of rubidium and cesium tetraperoxovanadate are reported here for the first time, likely prior-challenged by instability. Infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), 51V solid state NMR (nuclear magnetic resonance), tandem thermogravimetry-mass spectrometry (TGA-MS) along with calculations (DFT, density functional theory) all converge on mechanisms of CO2 capture and release that involve the vanadium centre, despite the end product of a 300 days study being bicarbonate and metavanadate. Electron Paramagnetic Resonance (EPR) Spectroscopy along with a wet chemical assay and computational studies evidence the presense of ∼5% adventitous superoxide, likely formed by peroxide reduction of vanadium, which also stabilizes via the reaction with CO2. The alkalis have a profound effect on the stability of the peroxovanadate compounds, stability trending K > Rb > Cs. While this translates to more rapid CO2 capture with heavier alkalis, it does not necessarily lead to capture of more CO2. All compounds capture approximately two equivalents CO2 per vanadium centre. We cannot yet explain the reactivity trend of the alkali peroxovanadates, because any change in speciation of the alkalis from reactions to product is not quantifiable. This study sets the stage for understanding and implementing transition metal peroxide species, including peroxide-functionalized metal oxides, for DAC.

Solid-State Transformation of Uranyl Peroxide Materials through High-Level Irradiation

The solid-state transformation of sodium uranyl triperoxide (Na4(UO2)(O2)3·9H2O, NaUT) to sodium uranyl tricarbonate (Na4(UO2)(CO3)3) by radiolysis was observed for the first time. The exposure of NaUT to 3 MGy gamma irradiation resulted in partial breakdown of the peroxides forming a mixed peroxide and carbonate species. The effects of He-ion irradiation on NaUT were also investigated up to 225 MGy using both hydrated argon and dry argon. The complete conversion to the uranyl tricarbonate phase by 56 MGy was done using hydrated argon, while dry argon did not fully convert showing the importance of water in the system. He-ion irradiated NaUT samples all convert to the tricarbonate phase with time in air post radiation exposure. This transition was monitored via Raman spectroscopy, infrared spectroscopy (IR), and powder X-ray diffraction (PXRD) to further confirm the identity of the final product as the sodium uranyl tricarbonate, čejkaite. This transformation outlines a mechanism for the mobility of uranyl in natural environments and in the Hanford tanks.

Capture and Stabilization of the Hydroxyl Radical in a Uranyl Peroxide Cluster

Here we describe the synthesis and characterization of a new uranyl peroxide cluster (UPC), U60 Ox30 *, which captures and stabilizes oxygen-based free radicals for more than one week. These radical species were first detected with a nitroblue tetrazolium colorimetric assay and U60 Ox30 * was characterized by single crystal X-ray diffraction as well as infrared (IR), Raman, UV-Vis-NIR, and electron paramagnetic resonance (EPR) spectroscopies. Identification of the free radicals present in U60 Ox30 * was done via room temperature solid and solution state X-band EPR studies using spin trapping methods. The spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was definitive for identifying the free radicals in U60 Ox30 *, which are hydroxyl radicals (⋅OH) that are stable for up to ten days that also persist upon addition of the metalloenzymes catalase and superoxide dismutase. Addition of the spin trapping agent α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN) further confirmed the radicals were oxygen based, and deuteration experiments showed that the origin of the free radicals was from the decomposition of H2 O2 in water. These results demonstrate that highly oxidizing species such as the ⋅OH radical can be stabilized in UPCs, which alters our understanding of the role of free radicals present in spent nuclear fuel.

Mechanochemical activation and oxidation of U(IV)O2

Mechanochemical impact induces oxidation of U(IV)O₂ in presence of alkali metal peroxides Li₂O₂ and Na₂O₂ during liquid-assisted grinding, resulting in respective alkali U(VI) triperoxide phases (e.g. Na₄[UO₂(O₂)₃]·9H₂O), based on powder X-ray diffraction analysis, solid- and solution-phase vibrational spectroscopy.

Mechanochemical synthesis of crystalline U(vi) triperoxide solids

Mechanochemical reaction of UO3 with metal peroxides (M2O2) yields U(vi) triperoxide materials without producing radioactive solvent wastes.