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  1. Hydrothermal solubility of Dy hydroxide as a function of pH and stability of Dy hydroxyl aqueous complexes from 25 to 250 °C

    Smith-Schmitz, Sarah E. ; Gysi, Alexander P. - Chemical Geology
    The rare earth elements (REE) have important applications in green energy technologies. The formation of mineral deposits in geologic systems commonly involves hydrothermal fluids which can mobilize the REE. However, the REE speciation is not well known as a function of pH. The thermodynamic properties of REE hydroxyl complexes used in geochemical models are based on the Helgeson-Kirkham-Flowers (HKF) equation of state parameters which were derived by extrapolation of low temperature experimental and estimated data. In this study, Dy hydroxide solubility experiments are combined with available literature data to improve these models from 25 to 250 °C and optimize themore » thermodynamic properties of Dy3+ and Dy hydroxyl complexes using GEMSFITS. Batch-type solubility experiments were conducted from 150 to 250 °C and at saturated water vapor pressure in perchloric acid solutions with initial pH values of 2 to 5 in 0.5 pH unit increments. The measured solubility of Dy hydroxide is retrograde with temperature and decreases with pH. The logarithm of total dissolved Dy molality ranges from –2.3 to –5.3 at 150 °C (pH 4.7–5.5), from –2.4 to –5.6 at 200 °C (pH 3.9–5.1), and from –3.7 to –6.9 at 250 °C (pH of 3.4 and 5.0). The optimized standard partial molal Gibbs energies of formation (ΔfG°T) derived for Dy3+ and DyOH2+ display a close to linear relationship with temperature, fitting with previous optimizations based on DyPO4 solubility data in the literature. A comparison of the optimized ΔfG°T values for aqueous Dy species with predictions from available HKF parameters indicates significant differences ranging from +11 to –26 kJ/mol between 25 and 250 °C. The experimental fits are used to derive the Dy hydroxide solubility products (Ks0) and formation constants for the hydrolysis of Dy (βn with n = 1 to 3; Dy3+ + nOH– = DyOHn3-n) as a function of temperature. The optimization method presented yields accurate thermodynamic properties for the Dy3+ aqua ions and the DyOH2+ species at the acidic to mildly acidic pH studied whereas more experimental work is needed at near-neutral and alkaline conditions to better constrain the other hydroxyl complexes. Furthermore, the optimized thermodynamic data have a significant impact on geochemical modeling of the mobility and solubility of REE minerals in acidic hydrothermal fluids.« less

  2. Raman spectroscopic study of anhydrous and hydrous REE phosphates, oxides, and hydroxides

    Hurtig, Nicole C. ; Gysi, Alexander P. ; Smith-Schmitz, Sarah E. ; ... - Dalton Transactions
    Rare earth elements (REE) include the lanthanides (La–Lu), Y, and Sc which are critical elements for the green energy transition. The REE show a decrease in ionic radii with increased atomic numbers, which results in a so-called lanthanide contraction systematically affecting crystal structures and mineral properties. Here we present a compilation of reference Raman spectra of ten REE sesquioxides (A-, B- and C-type), five REE hydroxides, eight xenotime-structured REE phosphate endmembers and two solid solutions, seven monazite-structured REE phosphate endmembers and two solid solutions and seven rhabdophane endmembers with up to five Ce1–xLREEx rhabdophane solid solutions (LREE = La–Gd). Ramanmore » mode assignment is based on a detailed literature review summarizing existing analytical work and theoretical calculations and systematic trends observed in this study by analyzing different REE-bearing solids. Here, the wavenumbers of the main REE-O Raman band systematically increase with decreasing ionic radii forming discrete linear trends within isostructural mineral groups, that can be used to estimate the REE-O mode in other solids with known REE-O coordination numbers. Photoluminescence using 266 nm, 532 nm and 633 nm excitation laser wavelengths for REE-bearing oxides, hydroxides, anhydrous and hydrous phosphates is also presented providing a new framework for identifying REE-phases in phosphate-bearing natural mineral deposits.« less

  3. The crystal structure of feitknechtite (β-MnOOH) and a new MnOOH polymorph

    Post, Jeffrey E. ; Heaney, Peter J. ; Ilton, Eugene S. ; ... - American Mineralogist
    Studies suggest that feitknechtite (β-MnOOH) is a prevalent, and perhaps necessary, intermediate phase during the synthesis of birnessite-like phases, the abiotic oxidation of Mn2+, and the transformation of biogenic hexagonal phyllomanganates to more complex Mn oxides in laboratory and natural systems. Researchers have generally described feitknechtite as consisting of pyrochroite-like (or cadmium iodide-like) Mn-O octahedral layers, but a detailed crystal structure has not been reported. For this work, we used TEM/SAED and powder XRD and Rietveld refinements to derive the unit cell and, for the first time, report a complete structure description for feitknechtite (β-MnOOH). Rietveld refinements were also completedmore » for three natural feitknechtite/hausmannite samples, and time-resolved synchrotron XRD experiments were used to follow the thermal transformation of feitknechtite to hausmannite. Additionally, we identified and report the structure for a second, and perhaps novel, MnOOH polymorph (proposed designation ε-MnOOH), mixed with the synthetic feitknechtite, that is similar to β-MnOOH but with a different layer stacking.« less

  4. Oxygen vacancy-rich amorphous FeNi hydroxide nanoclusters as an efficient electrocatalyst for water oxidation

    Cao, Youhai ; Su, Yang ; Xu, Liangliang ; ... - Journal of Energy Chemistry
    Here, a one-pot strategy is presented to directly synthesize amorphous FexNiy hydroxide nanoclusters (denoted as ANC-FexNiy, <2 nm) with oxygen vacancies induced by ionic liquids. The ANC-FexNiy catalyst presents abundant catalytic sites and high intrinsic conductivity. As such, the optimized ANC-Fe1Ni2 exhibits high activity in oxygen evolution reaction (OER) with a Tafel slope of 39 mV dec–1 and an overpotential of 266 mV at 10 mA cm–2. Notably, the optimized ANC-Fe1Ni2 shows an extraordinarily large mass activity of 3028 A gFeNi–1 at the overpotential of 300 mV, which is ~24-fold of commercial RuO2 catalyst. The superior activity of these FexNiymore » hydroxide nanoclusters is ascribed to (i) the amorphous and distorted structure with abundant oxygen vacancies, and (ii) enhanced active site density by downsizing the ANC-FexNiy clusters. This strategy provides a novel route for enhancing OER electrocatalytic performance and highly encouraging for the future application of amorphous metal hydroxides in catalysis.« less

  5. Applications of Alkali Metal Hydroxide Hydrofluxes to the Synthesis of Single-Crystal Ternary Actinide Oxides

    Thomas, Evan L. ; Stegman, Samantha ; Skanthakumar, Suntharalingam ; ... - Chemistry - A European Journal
    Hydrofluxes are hydrated salts with melting points well below that of the dehydrated salt and boiling points well above that of water, affording a reaction medium, in which mild temperatures and pressures can be accessed for the synthesis of materials. Herein, the use of alkali metal hydroxide hydrofluxes for the synthesis of single crystal α-Na2NpO4 is described, and the single crystal X-ray structure of α-Na2NpO4, along with its X-ray absorption spectra and vibrational spectra, is reported. The ability to synthesize complex oxides of the actinides, in particular, transuranium materials, under mild conditions will serve to advance our ability to exploremore » the structure–property relationships of the f elements.« less

  6. Rational Design of Nickel Hydroxide-Based Nanocrystals on Graphene for Ultrafast Energy Storage

    Zhao, Bote ; Zhang, Lei ; Zhang, Qiaobao ; ... - Advanced Energy Materials
    Compact, light, and powerful energy storage devices are urgently needed for many emerging applications; however, the development of advanced power sources relies heavily on advances in materials innovation. Here, the findings in rational design, one-pot synthesis, and characterization of a series of Ni hydroxide-based electrode materials in alkaline media for fast energy storage are reported. Under the guidance of density functional theory calculations and experimental investigations, a composite electrode composed of Co-/Mn-substituted Ni hydroxides grown on reduced graphene oxide (rGO) is designed and prepared, demonstrating capacities of 665 and 427 C g-1 at current densities of 2 and 20 Amore » g-1, respectively. The superior performance is attributed mainly to the low deprotonation energy and the facile electron transport, as elaborated by theoretical calculations. When coupled with an electrode based on organic molecular-modified rGO, the resulting hybrid device demonstrates an energy density of 74.7 W h kg-1 at a power density of 1.68 kW kg-1 while maintaining capacity retention of 91% after 10,000 cycles (20 A g-1). In conclusion, the findings not only provide a promising electrode material for high-performance hybrid capacitors but also open a new avenue toward knowledge-based design of efficient electrode materials for other energy storage applications.« less
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