Ozone-Facilitated Formation of Uranyl Peroxide in Humid Conditions

Metaschoepite, [(UO2)8O2(OH)12](H2O)10, maintained in a high relative humidity (RH) environment with air initially transformed into an intermediate phase that subsequently was replaced by the peroxide phase studtite, [(UO2)(O2)(H2O)2](H2O)2, over the course of 42 days, as observed using Raman and infrared spectroscopy and powder X-ray diffraction. Addition of atmospheric ozone vastly increased the rate and extent of the transformation to studtite but only in a high-RH atmosphere. Owing to its strong affinity for peroxide, uranyl reacted with hydrogen peroxide as it formed and precipitated stable studtite. In this work, we provide a previously unidentified source of hydrogen peroxide and make a case for the re-examination of storage systems where the consequences of atmospheric ozone are not considered

Structural and Morphological Influences on Neptunium Incorporation in Uranyl Molybdates

The in situ incorporation of pentavalent neptunium has been studied in the structurally related uranyl molybdate frameworks (NH₄)₄[(UO₂)₅(MoO₄)₇]­(H₂O)₅ and (NH₄)₂[(UO₂)₆(MoO₄)₇]­(H₂O)₂ prepared under similar synthetic conditions. The presence of Np­(V) was confirmed by UV–vis–NIR spectroscopy in the first compound, whereas Np­(VI) was identified in the second based on the observation of a unit-cell contraction and the lack of a spectral signature for Np­(V). The incorporation of neptunium does not affect the overall structure of the host compound based on the crystallographic unit-cell parameters. Neptunium appears to preferentially incorporate in the structure of (NH₄)₂[(UO₂)₆(MoO₄)₇]­(H₂O)₂ due to the formation of Np­(VI) during synthesis, although higher total uptakes were observed in (NH₄)₄[(UO₂)₅(MoO₄)₇]­(H₂O)₅ due to a higher initial concentration of neptunium in solution despite maintaining the same ratio of U:Np

Hybrid Uranyl-Carboxyphosphonate Cage Clusters

Two new hybrid uranyl-carboxyphosphonate cage clusters built from uranyl peroxide units were crystallized from aqueous solution under ambient conditions in approximately two months. The clusters are built from uranyl hexagonal bipyramids and are connected by employing a secondary metal linker, the 2-carboxyphenylphosphonate ligand. The structure of cluster <b>A</b> is composed of a ten-membered uranyl polyhedral belt that is capped on either end of an elongated cage by five-membered rings of uranyl polyhedra. The structure of cluster <b>B</b> consists of 24 uranyl cations that are arranged into 6 four-membered rings of uranyl polyhedra. Four of the corresponding topological squares are fused together to form a sixteen-membered double uranyl pseudobelt that is capped on either end by 2 topological squares. Cluster <b>A</b> crystallizes over a wide pH range of 4.6–6.8, while cluster <b>B</b> was isolated under narrower pH range of 6.9–7.8. Studies of their fate in aqueous solution upon dissolution of crystals by electrospray ionization mass spectrometry (ESI-MS) and small-angle X-ray scattering (SAXS) provide evidence for their persistence in solution. The well-established characteristic fingerprint from the absorption spectra of the uranium­(VI) cations disappears and becomes a nearly featureless peak; nonetheless, the two compounds fluoresce at room temperature

Hybrid Uranyl-Carboxyphosphonate Cage Clusters

Two new hybrid uranyl-carboxyphosphonate cage clusters built from uranyl peroxide units were crystallized from aqueous solution under ambient conditions in approximately two months. The clusters are built from uranyl hexagonal bipyramids and are connected by employing a secondary metal linker, the 2-carboxyphenylphosphonate ligand. The structure of cluster <b>A</b> is composed of a ten-membered uranyl polyhedral belt that is capped on either end of an elongated cage by five-membered rings of uranyl polyhedra. The structure of cluster <b>B</b> consists of 24 uranyl cations that are arranged into 6 four-membered rings of uranyl polyhedra. Four of the corresponding topological squares are fused together to form a sixteen-membered double uranyl pseudobelt that is capped on either end by 2 topological squares. Cluster <b>A</b> crystallizes over a wide pH range of 4.6–6.8, while cluster <b>B</b> was isolated under narrower pH range of 6.9–7.8. Studies of their fate in aqueous solution upon dissolution of crystals by electrospray ionization mass spectrometry (ESI-MS) and small-angle X-ray scattering (SAXS) provide evidence for their persistence in solution. The well-established characteristic fingerprint from the absorption spectra of the uranium­(VI) cations disappears and becomes a nearly featureless peak; nonetheless, the two compounds fluoresce at room temperature

Structure and Reactivity of X‑ray Amorphous Uranyl Peroxide, U₂O₇

Recent accidents resulting in worker injury and radioactive contamination occurred due to pressurization of uranium yellowcake drums produced in the western U.S.A. The drums contained an X-ray amorphous reactive form of uranium oxide that may have contributed to the pressurization. Heating hydrated uranyl peroxides produced during <i>in situ</i> mining can produce an amorphous compound, as shown by X-ray powder diffraction of material from impacted drums. Subsequently, studtite, [(UO₂)­(O₂)­(H₂O)₂]­(H₂O)₂, was heated in the laboratory. Its thermal decomposition produced a hygroscopic anhydrous uranyl peroxide that reacts with water to release O₂ gas and form metaschoepite, a uranyl-oxide hydrate. Quantum chemical calculations indicate that the most stable U₂O₇ conformer consists of two bent (UO₂)²⁺ uranyl ions bridged by a peroxide group bidentate and parallel to each uranyl ion, and a μ₂-O atom, resulting in charge neutrality. A pair distribution function from neutron total scattering supports this structural model, as do ¹H- and ¹⁷O-nuclear magnetic resonance spectra. The reactivity of U₂O₇ in water and with water in air is higher than that of other uranium oxides, and this can be both hazardous and potentially advantageous in the nuclear fuel cycle