13 research outputs found
Effective storage of electrons in water by the formation of highly reduced polyoxometalate clusters
Aqueous solutions of polyoxometalates (POMs) have been shown to have potential as high-capacity energy storage materials due to their potential for multi-electron redox processes, yet the mechanism of reduction and practical limits are currently unknown. Herein, we explore the mechanism of multi-electron redox processes that allow the highly reduced POM clusters of the form {MO3}y to absorb y electrons in aqueous solution, focusing mechanistically on the WellsâDawson structure X6[P2W18O62], which comprises 18 metal centers and can uptake up to 18 electrons reversibly (y = 18) per cluster in aqueous solution when the countercations are lithium. This unconventional redox activity is rationalized by density functional theory, molecular dynamics simulations, UVâvis, electron paramagnetic resonance spectroscopy, and small-angle X-ray scattering spectra. These data point to a new phenomenon showing that cluster protonation and aggregation allow the formation of highly electron-rich meta-stable systems in aqueous solution, which produce H2 when the solution is diluted. Finally, we show that this understanding is transferrable to other salts of [P5W30O110]15â and [P8W48O184]40â anions, which can be charged to 23 and 27 electrons per cluster, respectively
Non-Standard Errors
In statistics, samples are drawn from a population in a data-generating process (DGP). Standard errors measure the uncertainty in estimates of population parameters. In science, evidence is generated to test hypotheses in an evidence-generating process (EGP). We claim that EGP variation across researchers adds uncertainty: Non-standard errors (NSEs). We study NSEs by letting 164 teams test the same hypotheses on the same data. NSEs turn out to be sizable, but smaller for better reproducible or higher rated research. Adding peer-review stages reduces NSEs. We further find that this type of uncertainty is underestimated by participants
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Using Metal-Oxo Clusters to Expand Actinide Chemistry
A molecular approach to metal oxides allows to the study to fundamental bonding and formation behavior for such metals. Moreover, the molecular nature of such oxides or metal-oxo clusters allows the possibility of exploitation of the properties of the parent oxides. The tetravalent metals, in specific, (M=Zr, Hf, Ce, Th, U, Np, Pu) have been chemistâs choice in bottoms-up material design, catalysis, and elucidating reaction pathways in nature and in synthesis. Actinide-oxide clusters, colloids, and materials are particularly sought after and studied for 1) nuclear materials applications, 2) understanding environmental fate and transport of actinides, and 3) exploring the complex bonding behavior of open-shell f-elements. Synthesis of these metal-oxo cluster depend on controlling the hydrolysis and condensation reactions the govern oxide formation. Herein we demonstrate control over the reactions can be met by using counterions and strongly coordinating oxo-anions. As such we demonstrate three new crystal structure families, M6, M84, and M70, and how to access them. The study using three tetravalent metals (Th, U, Ce) additionally allows to probe differences between the metals in the tetravalent group
Supramolecular Assembly of CeIV-Oxo Sulfate Torus with Transition Metal Countercations
MIV molecular oxo-clusters of the f- and d-block
(M=Zr, Hf, Ce, Th, U, Np, Pu) have been prolific in bottoms-up material design,
catalysis, as well as understanding metal oxide assembly, dissolution and
surface reactivity in nature and in synthesis. Here we introduce Ce70,
a new CeIV wheel-shaped oxo-cluster, [CeIV(OH)36(O)64(SO4)60(H2O)10]4-,
isostructural with prior-reported U70. Like U70, Ce70
crystallizes into intricate frameworks with divalent transition metal
counter-cations (TMII), and also CeIV-monomer and sulfate
addenda ions
Bridging the Transuranics with Uranium(IV) Sulfate Aqueous Species and Solid Phases
International audienceIsolating isomorphic compounds of tetravalent actinides (i.e., Th IV , U IV , Np IV , and Pu IV) improve our understanding of the bonding behavior across the series, in addition to their relationship with tetravalent transition metals (Zr and Hf) and lanthanides (Ce). Similarities between these tetravalent metals are particularly illuminated in their hydrolysis and condensation behavior in aqueous systems, leading to polynuclear clusters typified by the hexamer [M IV 6O4(OH)4] 12+ building block. Prior studies have shown the predominance and coexistence of smaller species for Th IV (monomers, dimers, and hexamers) and larger species for U IV , Np IV , and Pu IV (including 38-mers and 70-mers). We show here that aqueous uranium(IV) sulfate also displays behavior similar to that of Th IV (and Zr IV) in its isolated solid-phase and solution speciation. Two single-crystal X-ray structures are described: a dihydroxide-bridged dimer (U2) formulated as U2(OH)2(SO4)3(H2O)4 and a monomer-linked hexamer framework (U-U6) as (U(H2O)3.5)2U6O4(OH)4(SO4)10(H2O)9. These structures are similar to those previously described for Th IV. Moreover, cocrystallization of monomer and dimer and of dimer and monomer-hexamer phases for both Th IV (prior) and U IV (current) indicates the coexistence of these species in solution. Because it was not possible to effectively study the sulfate-rich solutions via X-ray scattering from which U2 and U-U6 crystallized, we provide a parallel solution speciation study in low sulfate conditions, as a function of the pH. Raman spectroscopy, UV-vis spectroscopy, and small-angle X-ray scattering of these show decreasing sulfate binding, increased hydrolysis, increased species size, and increased complexity, with increasing pH. This study describes a bridge across the first half the actinide series, highlighting U IV similarities to Th IV , in addition to the previously known similarities to the transuranic elements
Supramolecular Assembly of U(IV) Clusters and Superatoms
Superatoms are nanometer-sized molecules or particles that can form ordered lattices, mimicking their atomic counterparts. Hierarchical assembly of superatoms gives rise to emergent properties in superlattices of quantum-dots, p-block clusters, and fullerenes. Here, we introduce a family of uranium-oxysulfate cluster anions whose hierarchical assembly in water is controlled by two parameters; acidity and the countercation. In acid, larger LnIII (Ln=La-Ho) link hexamer (U6) oxoclusters into body-centered cubic frameworks, while smaller LnIII (Ln=Er-Lu &Y) promote linking of fourteen U6-clusters into hollow superclusters (U84 superatoms). U84 assembles into superlattices including cubic-closest packed, body-centered cubic, and interpenetrating networks, bridged by interstitial countercations, and U6-clusters. Divalent transition metals (TM=MnII and ZnII), with no added acid, charge-balance and promote the fusion of 10 U6 and 10 U-monomers into a wheelâshaped cluster (U70). Dissolution of U70 in organic media reveals (by small-angle Xray scattering) that differing supramolecular assemblies are accessed, controlled by TM-linking of U70-clusters. <br /
Snapshots of Ce70 Toroid Assembly from Solids and Solution
Crystallization at the solid-liquid interface is difficult to spectroscopically observe and therefore
challenging to understand and ultimately control at the molecular level. The Ce70-torroid
formulated [CeIV70(OH)36(O)64(SO4)60(H2O)10]
4-
, part of a larger emerging family of MIV70-
materials (M=Zr, U, Ce), presents such an opportunity. We have elucidated assembly mechanisms
by X-ray scattering (small-angle scattering and total scattering) of solutions and solids, as well as
crystallizing and identifying fragments of Ce70 by single-crystal X-ray diffraction. Fragments
show evidence for templated growth (Ce5, [Ce5(O)3(SO4)12]
10-
) and modular assembly from
hexamer (Ce6) building units (Ce13, [Ce13(OH)6(O)12(SO4)14(Î2Î)14]
6- and Ce62,
[Ce62(OH)30(O)58(SO4)58]
14-
). Ce62, an almost complete ring, precipitates instantaneously in the
presence of ammonium cations as two torqued arcs that interlock by hydrogen boding through
NH4
+, which can also be replaced by other cations, demonstrated with CeIII. Room temperature
rapid assembly of both Ce70 and Ce62, respectively, by addition of Li+ and NH4
+, along with ion?exchange and redox behavior, invite exploitation of this emerging material family in
environmental and energy applications
Solution and Solid State Structural Chemistry of Th(IV) and U(IV) 4âHydroxybenzoates
Organic
ligands with carboxylate functionalities have been shown to affect
the solubility, speciation, and overall chemical behavior of tetravalent
metal ions. While many reports have focused on actinide complexation
by relatively simple monocarboxylates such as amino acids, in this
work we examined ThÂ(IV) and UÂ(IV) complexation by 4-hydroxybenzoic
acid in water with the aim of understanding the impact that the organic
backbone has on the solution and solid state structural chemistry
of thoriumÂ(IV) and uraniumÂ(IV) complexes. Two compounds of the general
formula [An<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(H<sub>2</sub>O)<sub>6</sub>(4-HB)<sub>12</sub>]·<i>n</i>H<sub>2</sub>O [An = Th (<b>Th-1</b>) and U (<b>U-1</b>); 4-HB = 4-hydroxybenzoate]
were synthesized via room-temperature reactions of AnCl<sub>4</sub> and 4-hydroxybenzoic acid in water. Solid state structures were
determined by single-crystal X-ray diffraction, and the compounds
were further characterized by Raman, infrared, and optical spectroscopies
and thermogravimetry. The magnetism of <b>U-1</b> was also examined.
The structures of the Th and U compounds are isomorphous and are built
from ligand-decorated oxo/hydroxo-bridged hexanuclear units. The relationship
between the building units observed in the solid state structure of <b>U-1</b> and those that exist in solution prior to crystallization
as well as upon dissolution of <b>U-1</b> in nonaqueous solvents
was investigated using small-angle X-ray scattering, ultravioletâvisible
optical spectroscopy, and dynamic light scattering. The evolution
of U solution speciation as a function of reaction time and temperature
was examined. Such effects as well as the impact of the ligand on
the formation and evolution of hexanuclear UÂ(IV) clusters to UO<sub>2</sub> nanoparticles compared to prior reported monocarboxylate
ligand systems are discussed. Unlike prior reported syntheses of Th
and UÂ(IV) hexamers where the pH was adjusted to âŒ2 and 3, respectively,
to drive hydrolysis, hexamer formation with the HB ligand appears
to be promoted only by the ligand