I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[17053]
Abstract: Background: The bioavailable supply of copper to ruminants has long been problematic. Complexities in supply exist due to interactions with other dietary elements in the rumen, most notably with iron or molybdenum in combination with sulphur, which can result in copper binding preventing its absorption. The molybdenum-sulphur-copper interaction has been extensively studied over the years. However, very little is known about the iron-sulphur-copper interaction, especially its mode of action in the gastrointestinal tract. Methods In the present work digesta from the rumen and jejunum of sheep fed a high copper, sulphur and iron diet was analysed using X-ray absorption spectroscopy (XAS). Results: X-ray absorption fine structure (XAFS) and X-ray absorption near edge structure (XANES) indicated that all of the copper and iron had changed in bonding in the rumen and that the oxidation state of the elements had been reduced into a mix of Fe2+ & Fe3+ and Cu+ with some Cu0. Conclusion: The copper compounds were most likely to be thiol co-ordinated in line with Cu+ chemistry. Changes to the copper compounds took place in the jejunum, although thiols were still highly favoured the possible existence of a copper-iron-sulphur complex which also included oxygen and chloride was also observed. This possibly has some resemblance to the crystal structure of bornite.
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Jul 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Abstract: High charge synthetic Na-Mica-2 has been synthesized by the NaCl melt method. The Na+ cations in the interlayer space of the clay have been exchanged with Eu3+ ions by a cation exchange reaction. The correct incorporation of Eu3+ and further characterization of its coordination environment and location within the clay, as well as its optical properties, have been studied by means of x-ray diffraction, x-ray absorption (XANES and EXAFS), thermogravimetry and luminescence. Different structural scenarios for the incorporation of europium have been proposed and evaluated.
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May 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[21441]
Open Access
Abstract: Selenium (Se) is a toxic contaminant with multiple anthropogenic sources, including 79Se from nuclear fission. Se mobility in the geosphere is generally governed by its oxidation state, therefore understanding Se speciation under variable redox conditions is important for the safe management of Se contaminated sites. Here, we investigate Se behavior in sediment groundwater column systems. Experiments were conducted with environmentally relevant Se concentrations, using a range of groundwater compositions, and the impact of electron-donor (i.e., biostimulation) and groundwater sulfate addition was examined over a period of 170 days. X-Ray Absorption Spectroscopy and standard geochemical techniques were used to track changes in sediment associated Se concentration and speciation. Electron-donor amended systems with and without added sulfate retained up to 90% of added Se(VI)(aq), with sediment associated Se speciation dominated by trigonal Se(0) and possibly trace Se(-II); no Se colloid formation was observed. The remobilization potential of the sediment associated Se species was then tested in reoxidation and seawater intrusion perturbation experiments. In all treatments, sediment associated Se (i.e., trigonal Se(0)) was largely resistant to remobilization over the timescales of the experiments (170 days). However, in the perturbation experiments, less Se was remobilized from sulfidic sediments, suggesting that previous sulfate-reducing conditions may buffer Se against remobilization and migration.
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Apr 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[9970]
Open Access
Abstract: The local structural changes associated with the ZIF-8 framework flexibility upon nitrogen gas adsorption have been studied by in situ X-ray absorption spectroscopy (XAS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy. Different thermodynamic conditions (isobar and isotherm) have been used to explore the so-called “gate opening” transition in which the hexagonal pore windows of the sodalite cage open, increasing the accessible volume for gas adsorption. To elucidate the source of the spectral changes in the XANES region of the absorption spectra observed along the gas adsorption and through the transition from the closed to the open pore configuration, ab initio calculations have been performed. Our results demonstrate that the transition from the closed to the open pore configurations involves not only the rotation of the MeIM ligand but also a further bend of the Me group away from the plane defined by the IM ring. Additionally, the contribution of the N2 molecules adsorbed in the center of the 4-ring window has been included in the scattering model to fully reproduce the main features of the X-ray absorption spectra in the open pore configuration.
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Mar 2022
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Andree
Iemhoff
,
Maurice
Vennewald
,
Jens
Artz
,
Chalachew
Mebrahtu
,
Alexander
Meledin
,
Thomas E.
Weirich
,
Heinrich
Hartmann
,
Astrid
Besmehn
,
Matteo
Aramini
,
Federica
Venturini
,
Fred
Mosselmans
,
Georg
Held
,
Rosa
Arrigo
,
Regina
Palkovits
Diamond Proposal Number(s):
[26053, 26030]
Abstract: Stabilization of single metal atoms is a persistent challenge in heterogeneous catalysis. Especially supported late transitions metals are prone to undergo agglomeration to nanoparticles under reducing conditions. In this study, nitrogen-rich covalent triazine frameworks (CTFs) are used to immobilize iridium complexes. Upon reduction at 400°C, immobilized Ir(acac)(COD) on CTF does not form nanoparticles but transforms into a highly active Ir single atom catalyst. The resulting catalyst systems outperforms both the immobilized complex and supported nanoparticles in the dehydrogenation of formic acid as probe reaction. This superior performance could be traced back to decisive changes of the coordination geometry positively influencing activity, selectivity and stability. Spectroscopic analysis reveals an increase of electron density on the cationic iridium site by donation from the CTF macroligand after removal of the organic ligand sphere from the Ir(acac)(COD) precursor complex upon reductive treatment. This work demonstrates the ability of nitrogen moieties to stabilize molecular metal species against agglomeration and opens avenues for catalysts design using isolated sites in high-temperature applications under reducing atmosphere.
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Mar 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Abstract: Niobium oxides have a prominent role in many technological applications, including electrochemical energy storage where they are employed as anodes for lithium- and sodium-ion batteries, hybrid supercapacitors, electrocatalysts in vanadium redox flow batteries, and electrochemical support in fuel cells. Owing to the great complexity of the available coordination environment, they can host other transition metals without phase changes. In this paper, multiple techniques are employed, for the first time, to study the influence of iron cations on the structural and functional properties of monoclinic and orthorhombic FeNb11O29. The element-selectivity of X-ray absorption spectroscopy and Mössbauer spectroscopy demonstrated the disorder of Fe3+ cations over the six octahedral sites of FeNb11O29. Ab-initio Density Functional Theory calculations were used in tandem with spectroscopic techniques to confirm the cationic disorder of the material and link its implications to the conductivity investigated with electrochemical impedance spectroscopy. The paramagnetism given by Fe3+ was confirmed with muon spin relaxation spectroscopy. The differences between FeNb11O29 and other analogous niobium-based compounds were rationalized and discussed, in view of their electrochemical applications.
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Feb 2022
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Chris
Foster
,
Samuel
Shaw
,
Thomas
Neill
,
Nick
Bryan
,
Nick
Sherriff
,
Louise S.
Natrajan
,
Hannah
Wilson
,
Laura
Lopez-Odriozola
,
Bruce
Rigby
,
Sarah J.
Haigh
,
Yi-Chao
Zou
,
Robert
Harrison
,
Katherine
Morris
Diamond Proposal Number(s):
[17243, 21441]
Abstract: In the United Kingdom, decommissioning of legacy spent fuel storage facilities involves the retrieval of radioactive sludges that have formed as a result of corrosion of Magnox nuclear fuel. Retrieval of sludges may re-suspend a colloidal fraction of the sludge, thereby potentially enhancing the mobility of radionuclides including uranium. The colloidal properties of the layered double hydroxide (LDH) phase hydrotalcite, a key product of Magnox fuel corrosion, and its interactions with U(VI) are of interest. This is because colloidal hydrotalcite is a potential transport vector for U(VI) under the neutral-to-alkaline conditions characteristic of the legacy storage facilities and other nuclear decommissioning scenarios. Here, a multi-technique approach was used to investigate the colloidal stability of hydrotalcite and the U(VI) sorption mechanism(s) across pH 7–11.5 and with variable U(VI) surface loadings (0.01–1 wt %). Overall, hydrotalcite was found to form stable colloidal suspensions between pH 7 and 11.5, with some evidence for Mg2+ leaching from hydrotalcite colloids at pH ≤ 9. For systems with U present, >98% of U(VI) was removed from the solution in the presence of hydrotalcite, regardless of pH and U loading, although the sorption mode was affected by both pH and U concentrations. Under alkaline conditions, U(VI) surface precipitates formed on the colloidal hydrotalcite nanoparticle surface. Under more circumneutral conditions, Mg2+ leaching from hydrotalcite and more facile exchange of interlayer carbonate with the surrounding solution led to the formation of uranyl carbonate species (e.g., Mg(UO2(CO3)3)2–(aq)). Both X-ray absorption spectroscopy (XAS) and luminescence analysis confirmed that these negatively charged species sorbed as both outer- and inner-sphere tertiary complexes on the hydrotalcite surface. These results demonstrate that hydrotalcite can form pseudo-colloids with U(VI) under a wide range of pH conditions and have clear implications for understanding the uranium behavior in environments where hydrotalcite and other LDHs may be present.
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Feb 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Weikai
Xiang
,
Nating
Yang
,
Xiaopeng
Li
,
Julia
Linnemann
,
Ulrich
Hagemann
,
Olaf
Ruediger
,
Markus
Heidelmann
,
Tobias
Falk
,
Matteo
Aramini
,
Serena
Debeer
,
Martin
Muhler
,
Kristina
Tschulik
,
Tong
Li
Diamond Proposal Number(s):
[25636]
Open Access
Abstract: The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability. Optimising the performance of electrocatalysts requires atomic-scale information, but it is difficult to obtain. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized Co2FeO4 and CoFe2O4 nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine Co2FeO4. The interfaces of Co-rich and Fe-rich nanodomains of Co2FeO4 become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe2O4. However, the activity of Co2FeO4 drops considerably due to concurrent irreversible transformation towards CoIVO2 and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for CoFe2O4 after OER, except for surface structural transformation towards (FeIII, CoIII)2O3. Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER.
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Jan 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[17472]
Open Access
Abstract: Uranium isotopic signatures can be harnessed to monitor the reductive remediation of subsurface contamination or to reconstruct paleo-redox environments. However, the mechanistic underpinnings of the isotope fractionation associated with U reduction remain poorly understood. Here, we present a coprecipitation study, in which hexavalent U (U(VI)) was reduced during the synthesis of magnetite and pentavalent U (U(V)) was the dominant species. The measured δ238U values for unreduced U(VI) (∼−1.0‰), incorporated U (96 ± 2% U(V), ∼−0.1‰), and extracted surface U (mostly U(IV), ∼0.3‰) suggested the preferential accumulation of the heavy isotope in reduced species. Upon exposure of the U-magnetite coprecipitate to air, U(V) was partially reoxidized to U(VI) with no significant change in the δ238U value. In contrast, anoxic amendment of a heavy isotope-doped U(VI) solution resulted in an increase in the δ238U of the incorporated U species over time, suggesting an exchange between incorporated and surface/aqueous U. Overall, the results support the presence of persistent U(V) with a light isotope signature and suggest that the mineral dynamics of iron oxides may allow overprinting of the isotopic signature of incorporated U species. This work furthers the understanding of the isotope fractionation of U associated with iron oxides in both modern and paleo-environments.
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Jan 2022
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[24074, 21441, 13559]
Open Access
Abstract: Neptunium (237Np) is an important radionuclide in the nuclear fuel cycle in areas such as effluent treatment and the geodisposal of radioactive waste. Due to neptunium’s redox sensitivity and its tendency to adsorb strongly to mineral phases, such as iron oxides/sulfides, the environmental mobility of Np can be altered significantly by a wide variety of chemical processes. Here, Np interactions with key iron minerals, ferrihydrite (Fe5O8H·4H2O), goethite (α-FeOOH), and mackinawite (FeS), are investigated using X-ray Absorption Spectroscopy (XAS) in order to explore the mobility of neptunyl(V) (Np(V)O2+) moiety in environmental (radioactive waste disposal) and industrial (effluent treatment plant) scenarios. Analysis of the Np LIII-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) showed that upon exposure to goethite and ferrihydrite, Np(V) adsorbed to the surface, likely as an inner-sphere complex. Interestingly, analysis showed that only the first two shells (Oax and Oeq) of the EXAFS could be modelled with a high degree of confidence, and there was no clear indication of Fe or carbonate in the fits. When Np(V)O2+ was added to a mackinawite-containing system, Np(V) was reduced to Np(IV) and formed a nanocrystalline Np(IV)O2 solid. An analogous experiment was also performed with U(VI)O22+, and a similar reduction was observed, with U(VI) being reduced to nanocrystalline uraninite (U(IV)O2). These results highlight that Np(V) may undergo a variety of speciation changes in environmental and engineered systems whilst also highlighting the need for multi-technique approaches to speciation determination for actinyl (for example, Np(V)O2+) species.
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Jan 2022
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