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Synchrotron small angle X-ray scattering measurements on water and alkaline bromine aqueous solutions (XBr, with X = Li, Rb, or Cs) were carried out from ambient to supercritical conditions. The temperature was increased from 300 to 750 K along several isobars between 24 and 35 MPa. The correlation length and the structure factor were extracted from the data following the Ornstein-Zernike formalism. We obtained experimental evidence of the shift of the critical point and isochore and their dependence on the ions concentration (0.33 mol/kg and 1.0 mol/kg). We also observed that the size of the density fluctuations and the structure factor increase with the presence of the ions and that this effect is positively correlated with the atomic number of the cation. These behaviors were compared with ZnBr2 and NaCl systems from the literature. [ABSTRACT FROM AUTHOR]

The geometrical and electronic structure of the arsenious acid molecule As(OH)3 in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P=250 and 600 bars, T≤500 °C) using a new optical cell. The analysis of the XAS spectra is consistent with (1) a constant As-O distance, (2) an opening of the O-As-O angles within the C3V pyramidal structure in the range 30–200 °C. This structural evolution comes along with a small decrease of the partial charges of the atoms in the As(OH)3 molecule. The explanation invoked for both structural and electronic modifications observed is the weakening of the interactions, through hydrogen bonds, between the As(OH)3 complex and water molecules. This is a fingerprint of the similar weakening of hydrogen bonding interactions in the solvent itself. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A bent-crystal spectrometer based on the Rowland circle geometry has been installed and tested on the BM30b/FAME beamline at the European Synchrotron Radiation Facility to improve its performances. The energy resolution of the spectrometer allows different kinds of measurements to be performed, including X-ray absorption spectroscopy, resonant inelastic X-ray scattering and X-ray Raman scattering experiments. The simplicity of the experimental device makes it easily implemented on a classical X-ray absorption beamline. This improvement in the fluorescence detection is of particular importance when the probed element is embedded in a complex and/or heavy matrix, for example in environmental sciences. [ABSTRACT FROM AUTHOR]

Hydrothermal conversion of actinide oxalates has recently gained attention as an innovative fabrication route for nuclear fuels but has remained mainly limited to tetra- or tri-valent cations. We report herein the reductive conversion of mixtures of uranyl and oxalate ions into UO2+x oxides under mild hydrothermal conditions (T = 250 °C). A multi-parametric study first led to specifying the optimal conditions in terms of pH, oxalate/U ratio and duration to provide a quantitative precipitation of uranium in the hyper-stoichiometric dioxide form with pH = 0.8; R = noxalate/nU = 3, and t = 72 hours. Particularly, pH was evidenced as a key parameter, with 3 different compounds obtained over a range of only 0.4 units. The mechanism leading to the formation of UO2+x was then investigated thanks to an in situ XANES study. Analysis of the supernatant showed that U(VI) was quickly reduced into U(IV) thanks to the presence of oxalates and/or their decomposition products in solution, following first-order kinetics. Tetravalent uranium was then hydrolysed, leading to the precipitation of UO2+x as the only crystalline phase. This study thus demonstrates that the hydrothermal conversion of actinide oxalates into oxides is an extremely versatile tool that can be implemented in a large variety of chemical systems, particularly in terms of the oxidation state of the cations initially present in solution. [ABSTRACT FROM AUTHOR]

X‐ray ptychography and ptychographic computed tomography have seen a rapid rise since the advent of fourth‐generation synchrotrons with a high degree of coherent radiation. In addition to quantitative multiscale structural analysis, ptychography with spectral capabilities has been developed, allowing for spatial‐localized multiscale structural and spectral information of samples. The SWING beamline of Synchrotron SOLEIL has recently developed a nanoprobe setup where the endstation's first spectral and resonant ptychographic measurements have been successfully conducted. A metallic nickel wire sample was measured using 2D spectral ptychography in XANES mode and resonant ptychographic tomography. From the 2D spectral ptychography measurements, the spectra of the components of the sample's complex‐valued refractive index, δ and β, were extracted, integrated along the sample thickness. By performing resonance ptychographic tomography at two photon energies, 3D maps of the refractive index decrement, δ, were obtained at the Ni K‐edge energy and another energy above the edge. These maps allowed the detection of impurities in the Ni wire. The significance of accounting for the atomic scattering factor is demonstrated in the calculation of electron density near a resonance through the use of the δ values. These results indicate that at the SWING beamline it is possible to conduct state‐of‐the‐art spectral and resonant ptychography experiments using the nanoprobe setup. [ABSTRACT FROM AUTHOR]

This paper presents the development of a novel high-pressure/high-temperature reactor cell dedicated to the characterization of catalysts using synchrotron x-ray absorption spectroscopy under operando conditions. The design of the vitreous carbon reactor allows its use as a plug-flow reactor, monitoring catalyst samples in a powder form with a continuous gas flow at high-temperature (up to 1000 °C) and under high pressure (up to 1000 bar) conditions, depending on the gas environment. The high-pressure/high-temperature reactor cell incorporates an automated gas distribution system and offers the capability to operate in both transmission and fluorescence detection modes. The operando x-ray absorption spectroscopy results obtained on a bimetallic InCo catalyst during CO2 hydrogenation reaction at 300 °C and 50 bar are presented, replicating the conditions of a conventional microreactor. The complete setup is available for users and permanently installed on the Collaborating Research Groups French Absorption spectroscopy beamline in Material and Environmental (CRG-FAME) sciences and French Absorption spectroscopy beamline in Material and Environmental sciences at ultra-high dilution (FAME-UHD) beamlines (BM30 and BM16) at the European Synchrotron Radiation Facility in Grenoble, France. [ABSTRACT FROM AUTHOR]

X-ray ptychography is a coherent diffraction imaging technique based on acquiring multiple diffraction patterns obtained through the illumination of the sample at different partially overlapping probe positions. The diffraction patterns collected are used to retrieve the complex transmittivity function of the sample and the probe using a phase retrieval algorithm. Absorption or phase contrast images of the sample as well as the real and imaginary parts of the probe function can be obtained. Furthermore, X-ray ptychography can also provide spectral information of the sample from absorption or phase shift images by capturing multiple ptychographic projections at varying energies around the resonant energy of the element of interest. However, post-processing of the images is required to extract the spectra. To facilitate this, ProSPyX, a Python package that offers the analysis tools and a graphical user interface required to process spectral ptychography datasets, is presented. Using the PyQt5 Python open-source module for development and design, the software facilitates extraction of absorption and phase spectral information from spectral ptychographic datasets. It also saves the spectra in file formats compatible with other X-ray absorption spectroscopy data analysis software tools, streamlining integration into existing spectroscopic data analysis pipelines. To illustrate its capabilities, ProSPyX was applied to process the spectral ptychography dataset recently acquired on a nickel wire at the SWING beamline of the SOLEIL synchrotron. [ABSTRACT FROM AUTHOR]

Synchrotron small angle x-ray scattering measurements on water and zinc bromide ZnBr2 aqueous solutions were carried out from ambient to supercritical conditions. For both systems several isobars (between 285 and 600 bars) were followed beyond the critical isochore. The data were analyzed through an Ornstein–Zernike formalism in terms of correlation length and null angle structure factor. The results for pure water are in agreement with previously published values. Solutions of different electrolyte concentrations were studied. In each case, the values of the correlation length and null angle structure factor are larger than those of pure water. This effect is more pronounced for higher concentrations and/or for pressure closer to the critical point of pure water. This is in agreement with the shift of the critical point determined in the literature for NaCl solutions. Comparing these results to previous x-ray absorption measurements carried out on identical samples we propose the following two step sequence for ionic hydration up to supercritical conditions: (1) from ambient to about 300 °C, an increase of ion pairing and formation of multi-ionic complexes which can be correlated to the decrease of the dielectric constant; (2) an enhancement of the local solvation shell of ions due to the onset of the thermal density fluctuations at high temperature, leading to a screening effect between ions and inhibiting the ion pairing processes. [ABSTRACT FROM AUTHOR]

Selenium 0 (Se0) is a powerful anti-proliferative agent in cancer research. We investigated the impact of sub-toxic concentrations of Se0 functionalized nanoparticles (SeNPs) on prostate cancer PC-3 cells and determined their intracellular localization and fate. An in-depth characterization of functionalized selenium nanoparticles composition is proposed to certify that no chemical bias relative to synthesis issues might have impacted the study. Selenium is an extremely diluted element in the biological environment and therefore requires high-performance techniques with a very low detection limit and high spatial resolution for intracellular imaging. This was explored with state-of-the-art techniques, but also with cryopreparation to preserve the chemical and structural integrity of the cells for spatially resolved and speciation techniques. Monodisperse solutions of SeNPs capped with bovine serum albumin (BSA) were shown to slow down the migration capacity of aggressive prostate cancer cells compared to polydisperse solutions of SeNPs capped with chitosan. BSA coating could prevent interactions between the reactive surface of the nanoparticles and the plasma membrane, mitigating the generation of reactive oxygen species. The intracellular localization showed interaction with mitochondria and also a localization in the lysosome-related organelle. The SeNPs-BSA localization in mitochondria constitute a possible explanation for our result showing a very significant dampening of the PC-3 cell proliferation capabilities. The purpose of the use of sublethal compound concentrations was to limit adverse effects resulting from high cell death to best evaluate some cellular changes and the fate of these SeNPs on PC-3. Our findings provide new insight to further study the various mechanisms of cytotoxicity of SeNPs. [ABSTRACT FROM AUTHOR]

Metal encapsulation in zeolitic materials through one‐pot hydrothermal synthesis (HTS) is an attractive technique to prepare zeolites with a high metal dispersion. Due to its simplicity and the excellent catalytic performance observed for several catalytic systems, this method has gained a great deal of attention over the last few years. While most studies apply synthetic methods involving different organic ligands to stabilize the metal under synthesis conditions, here we report the use of metallosiloxanes as an alternative metal precursor. Metallosiloxanes can be synthesized from simple and cost‐affordable chemicals and, when used in combination with zeolite building blocks under standard synthesis conditions, lead to quantitative metal loading and high dispersion. Thanks to the structural analogy of siloxane with TEOS, the synthesis gel stabilizes by forming siloxane bridges that prevent metal precipitation and clustering. When focusing on Fe‐encapsulation, we demonstrate that Fe‐MFI zeolites obtained by this method exhibit high catalytic activity in the NH3‐mediated selective catalytic reduction (SCR) of NOx along with a good H2O/SO2 tolerance. This synthetic approach opens a new synthetic route for the encapsulation of transition metals within zeolite structures. [ABSTRACT FROM AUTHOR]

Metal encapsulation in zeolitic materials through one‐pot hydrothermal synthesis (HTS) is an attractive technique to prepare zeolites with a high metal dispersion. Due to its simplicity and the excellent catalytic performance observed for several catalytic systems, this method has gained a great deal of attention over the last few years. While most studies apply synthetic methods involving different organic ligands to stabilize the metal under synthesis conditions, here we report the use of metallosiloxanes as an alternative metal precursor. Metallosiloxanes can be synthesized from simple and cost‐affordable chemicals and, when used in combination with zeolite building blocks under standard synthesis conditions, lead to quantitative metal loading and high dispersion. Thanks to the structural analogy of siloxane with TEOS, the synthesis gel stabilizes by forming siloxane bridges that prevent metal precipitation and clustering. When focusing on Fe‐encapsulation, we demonstrate that Fe‐MFI zeolites obtained by this method exhibit high catalytic activity in the NH3‐mediated selective catalytic reduction (SCR) of NOx along with a good H2O/SO2 tolerance. This synthetic approach opens a new synthetic route for the encapsulation of transition metals within zeolite structures. [ABSTRACT FROM AUTHOR]

Critical to interpreting platinum chemical speciation using X-ray absorption spectroscopy (XAS) is the availability of reference spectra of compounds with known Pt redox and coordination. Here we compare different techniques for Pt LIII-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectral regions for a large set of Pt-O-Cl-S reference compounds of known structures. The measurements were conducted in HERFD (high-energy resolution fluorescence detection, high-resolution or HR) mode, as well as in two conventional modes such as transmission (TR) and nominal-resolution total fluorescence yield (TFY or NR). Samples analyzed here included Pt0 (TR), PtIIS (HR), PtIVS2 (TR), K2PtIICl4 (HR + TR), K2PtIVCl6 (HR + TR), PtIVO2 (HR + TR), C6H12N2O4PtII (HR + TR), and aqueous solutions of K2PtIICl4 and H2PtIVCl6 (NR + TR), as well as (NH4)2PtIV(S5)3 (HR + TR). XANES spectra in HERFD mode offer a better energy resolution than in conventional modes, allowing a more accurate identification of Pt redox state and coordination geometry. EXAFS spectra in all three modes for a given compound yield identical within errors values of Pt-neighbor interatomic distances and mean square relative displacement (MSRD, σ2) parameters. In contrast, both TR and NR spectra on the one hand and HR spectra on the other hand yield distinct amplitude reduction factor (S02) values, 0.76 ± 0.04 and 0.99 ± 0.07 (1 standard error), respectively. This study contributes to the development of an open-access XAS database SSHADE. [ABSTRACT FROM AUTHOR]

In situ X-ray absorption spectroscopy (XANES and EXAFS) was applied to study palladium speciation in model (Na,Li)Cl–HCl–HNO 3 –HClO 4 –H 2 O solutions and to measure the solubility of PdO(s) and PdS(s) in NaCl–HCl–H 2 O solutions up to 450 °C and 600 bar. In HNO 3 and HClO 4 solutions, the square planar Pd(H 2 O) 4 2+ cation with an average Pd–O distance of 2.00 ± 0.01 Å is the dominant form at 30–100 °C. At T > 100 °C, this cation undergoes hydrolysis resulting in Pd precipitation. In (Na,Li)Cl–HCl solutions Pd forms square planar PdCl n (H 2 O) 4− n 2− n complexes with Pd–O and Pd–Cl distances of 2.00–2.10 and 2.26–2.31 Å, respectively. At 30–250 °C our data are consistent with a mixture of PdCl(H 2 O) 3 + , PdCl 2 (H 2 O) 2 0 (aq) , PdCl 3 (H 2 O) − , and PdCl 4 2− , but at T > 250 °C PdCl 3 (H 2 O) − and PdCl 4 2− become dominant over a wide range of chloride concentration, from ∼0.03 to at least 9 mol/kg H 2 O. XANES and EXAFS analyses of these species distribution allowed derivation of the stability constant of the reaction PdCl 3 − + Cl − = PdCl 4 2− from 300 to 450 °C and 600 bar. These results, coupled with in situ PdO(s) and PdS(s) solubility measurements of this study and a revision of thermodynamic data for these solid phases, allowed generation of a self-consistent thermodynamic data set of the system PdS(s)–PdO(s)–PdCl 3 − –PdCl 4 2− . Our data indicate that Pd can be significantly mobilized as chloride complexes only in sulfide-free oxidizing geological settings (e.g., Chudnoe deposits in Russia, Waterberg deposits in South Africa, Jacutinga-type deposits in Brazil). By contrast, at typical pH (4–8), chlorinity (<10–15 wt% NaCl) and H 2 S contents (0.001–0.1 wt%) of most hydrothermal fluids, the absolute concentration of Pd–Cl complexes is too low to explain Pd enrichment in volcanogenic massive sulfide, modern seafloor sulfide, and porphyry Cu–Au–Mo deposits. Complexes with S-bearing ligands, very likely other than H 2 S/HS − , such as S 3 − and other polysulfide anions, are required to account for Pd hydrothermal mobility in these geological settings. [ABSTRACT FROM AUTHOR]

Gold mobilization, transfer, and concentration in the Earth's crust are controlled by hydrothermal sulfur- and chloride-bearing fluids. Yet the exact chemical identity, structure, and stability of Au-bearing species and, in particular, the respective contributions of the sulfide (HS−) and trisulfur ion ( S 3 ∙ − $\text{S}_{3}^{\bullet-}$) ligands to Au transport lack direct in situ evidence. Here we employed high energy resolution fluorescence detection X‑ray absorption spectroscopy (HERFD-XAS) on aqueous sulfate/sulfide/ S 3 ∙ − $\text{S}_{3}^{\bullet-}$ -bearing solutions at typical hydrothermal temperatures and pressures (T = 350 °C, P = 600 bar) to reveal differences in dissolved Au spectral signatures indicative of contrasting fluid-phase Au speciation as a function of acidity and redox conditions. Combined with in situ Au solubility measurements and quantum-chemical and thermodynamic modeling, our spectroscopic data provide direct evidence for the Au(HS)S͞3 and Au(HS)S͞2 complexes predominant at acidic-to-neutral and alkaline conditions, respectively. Our findings thus directly confirm a recent speciation scheme for Au in aqueous S-bearing fluids established using less direct methods, and highlight an important role of the trisulfur ion in gold mobilization and concentration in hydrothermal-magmatic deposits associated with subduction zones. More generally, our results show that HERFD-XAS enables the identification of structural and coordination features in metal complexes virtually unresolvable using classical XAS techniques. By avoiding limitations of less direct techniques, our integrated high-resolution spectroscopic approach opens perspectives for studies of the speciation and solubility of gold and other metals in high T-P fluids, and potentially silicate melts, inaccessible to direct observation in nature. [ABSTRACT FROM AUTHOR]

Abstract: The impact of deep-subsurface pressure conditions on microbial activity is still poorly constrained. In particular it is unknown how pressure of deep environments affects microbial transformations of iron. We investigated the effects of high hydrostatic pressure (HHP) on the rate and the extent of bacterial dissimilatory iron reduction (DIR). We employed a novel experimental setup that enables in situ monitoring of Fe oxidation state and speciation in bacterial cultures in an optimized HHP incubation system using X-ray Absorption Near-Edge Structure (XANES) spectroscopy. The iron-reducing bacterium Shewanella oneidensis MR-1 was incubated at 30°C with Fe(III) citrate and tryptone at pressures between 0.1 and 100MPa. For pressures up to 70MPa strain MR-1 (108 cellsml−1) was able to reduce all 5mM Fe(III) provided. Above 70MPa, the final amount of Fe(III) that MR-1 could reduce decreased linearly and DIR was estimated to stop at 109±7MPa. The decrease in the reduction yield was correlated with the dramatic decrease in survival (as determined by CFU counts) above 70MPa. The initial rate of DIR increased with pressure up to 40MPa, then decreased to reach zero at about 110MPa. Increased rates of DIR activity and relatively high growth rates for pressures below 40MPa would potentially ensure the maintenance of MR-1 in most of deep subsurface environments where moderate pressures occur, i.e. deep-sea environments. This study not only provides the first in situ quantitative results for microbial iron metabolism under HHP conditions but also sets the stage for future investigations of deep-sea pressure-adapted iron reducers. Moreover it demonstrates for the first time that XANES at the Fe K-edge is a powerful probe for in vivo monitoring of iron transformations in living microbial cultures. [Copyright &y& Elsevier]

Abstract: The spectroscopic properties of the deoxygenated truncated hemoglobin from the actinobacterium Thermobifida fusca have been investigated by means of extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and near infrared spectroscopies both at room and cryogenic temperatures. At room temperature the near infrared charge transfer band III occurs at 772nm, a value that is unusually high for a canonical deoxygenated hemoglobin species, and can only be found as a transient species after photolysis in vertebrate hemoglobins and myoglobins or under strongly dehydrating conditions. EXAFS and XANES quantitative analyses, carried out in parallel with deoxygenated horse myoglobin, revealed an unusually short iron–histidine distance 1.90±0.03Å, significantly shorter than the deoxygenated horse myoglobin distance of 2.11±0.02Å. These findings provide novel structural basis for discussing the fine structural geometry of the proximal site, and eventually mapping the coordinates of the metal with respect to the pyrrole nitrogens and the proximal histidine nitrogen. [Copyright &y& Elsevier]

Abstract: X-ray absorption spectroscopy measurements have been performed both at the Ga and Br K-edges on aqueous GaBr3 solutions. The isobaric experiments have been recorded at 30 MPa from ambient temperature to 670 K for two GaBr3 concentrations (0.017 and 0.17 mol/dm3). At room temperature, Ga3+ and Br− ions are fully solvated, surrounded by water O atoms at 1.97 Å (Ga–O) and 3.37 Å (Br–O). When the temperature is elevated, Ga3+ cations precipitate as gallium oxy-hydroxide colloids while Br− anions remain solvated. With a further increase in temperature, the gallium solid precipitates remarkably re-dissolved (25% and 50% for 0.017 and 0.17 mol/dm3 respectively), due to the formation of [GaBr n (H2O)4− n ](aq) 3− n (n =2, 3 or 4) tetrahedral complexes. [Copyright &y& Elsevier]

Abstract: Dissolution of synthetic argutite (GeO2, tetragonal, rutile-like) in pure water, and local atomic structure around Ge(IV) in aqueous solution were characterized by in situ X-ray Absorption Fine Structure (XAFS) spectroscopy at the Ge K-edge using a new high temperature–high pressure cell. XAFS transmission and fluorescence spectra were collected from germanic acid aqueous solutions of 0.02–0.09 mol Ge/kg H2O at ambient pressure and temperature, and from a 0.04 mol Ge/kg H2O solution at 400 bar from 20 to 450 °C. Spectra at all temperatures and concentrations exhibit the 1st shell contribution from 4±0.3 oxygen atoms at 1.75±0.01 Å, consistent with a tetrahedral structure of the Ge(OH)4 complex which is a chemical analog of Si(OH)4 and is the dominant Ge aqueous species, in agreement with previous solubility and potentiometric measurements. The constancy of Ge(OH)4 structural parameters over the wide temperature and fluid-density range is consistent with the strong covalent character of the Ge–(OH) bonds. A dissolution experiment on argutite in pure H2O was carried out at 400 bar from 200 to 500 °C by monitoring the height of the Ge absorption edge of transmission spectra. The solubility of GeO2, derived from a classical X-ray absorption relation using the absorption height values and fluid density, increases with increasing temperature up to 400 °C but decreases at 450 and 500 °C, similar to that observed for quartz. The new solubility data were analyzed using both a density model and the revised HKF equation of state to better constrain Ge(OH)4 thermodynamic properties. Calculations using these models indicate that despite strong similarities in the structure and energetics of aqueous germanic acid and silica, differences in the thermodynamic properties of germanate and silicate dissolution reactions may account for the increase of the Ge/Si ratio observed in high-temperature natural fluids. [Copyright &y& Elsevier]

The influence of aqueous silica on gallium(III) hydrolysis in dilute (2 × 10−4 ≤ mGa ≤ 5 × 10−3) and moderately concentrated (0.02 ≤ mGa ≤ 0.3) aqueous solutions was studied at ambient temperature, using high resolution X-ray absorption fine structure (XAFS) and nuclear magnetic resonance (NMR) spectroscopies, respectively. Results show that, in Si-free acidic solutions (pH < 3), Ga is hexa-coordinated with oxygens of H2O molecules and/or OH groups in the first coordination sphere of the metal. With increasing pH, these hydroxyl groups are progressively replaced by bridging oxygens (–O–), and polymerized Ga–hydroxide complexes form via Ga–O–Ga chemical bonds. In the 2.5–3.5 pH range, both XAFS and NMR spectra are consistent with the dominant presence of the Ga13 Keggin polycation, which has the same local structure as A113. Under basic pH (pH > 8), Ga exhibits a tetrahedral coordination, corresponding to Ga(OH)4− species, in agreement with previous NMR and potentiometric studies. Major changes in Ga hydrolysis have been detected in the presence of aqueous silica. Ga is tetra-coordinated, both in basic and acid (i.e., at pH > 2.7) Si-bearing solutions (0.01 ≤ mSi ≤ 0.2), and forms stable gallium–silicate complexes. In these species, Ga binds via bridging oxygen to 2 ± 1 silicons, with an average Ga–Si distance of 3.16 ± 0.05 A˚, and to 2 ± 1 silicons, with an average Ga–Si distance of 3.39 ± 0.03 A˚. These two sets of Ga–Si distances imply the formation of two types of Ga–silicate aqueous complex, cyclic Ga–Si2–3 species (formed by the substitution of Si in its tri-, tetra- or hexa-cyclic polymers by Ga atoms), and chainlike GaSi2–4 species (similar to those found for A1), respectively. The increase in the number of Si neighbors (a measure of the complex concentration and stability), in alkaline media, with increasing SiO2(aq) content and decreasing pH is similar to that for A1–Si complexes found in neutral to basic solutions. At very acid pH and moderate silica concentrations, the presence of another type of Ga–Si complex, in which Ga remains hexa-coordinated and binds to the silicon tetrahedra via the GaO6 octahedron corners, has also been detected. These species are similar to those found for Al3+ in acid solutions. Thus, as for aluminum, silicic acid greatly hampers Ga hydrolysis and enhances Ga mobility in natural waters via the formation of gallium–silicate complexes. [Copyright &y& Elsevier]

Platinum group elements (PGE) are considered to be very poorly soluble in aqueous fluids in most natural hydrothermal-magmatic contexts and industrial processes. Here, we combined in situ X-ray absorption spectroscopy and solubility experiments with atomistic and thermodynamic simulations to demonstrate that the trisulfur radical ion S3 •-forms very stable and soluble complexes with both PtII and PtIV in sulfur-bearing aqueous solution at elevated temperatures (~300 °C). These Pt-bearing species enable (re)mobilization, transfer, and focused precipitation of platinum up to 10,000 times more efficiently than any other common inorganic ligand, such as hydroxide, chloride, sulfate, or sulfide. Our results imply a far more important contribution of sulfur-bearing hydrothermal fluids to PGE transfer and accumulation in the Earth's crust than believed previously. This discovery challenges traditional models of PGE economic concentration from silicate and sulfide melts and provides new possibilities for resource prospecting in hydrothermal shallow crust settings. The exceptionally high capacity of the S3 •-ion to bind platinum may also offer new routes for PGE selective extraction from ore and hydrothermal synthesis of noble metal nanomaterials. [ABSTRACT FROM AUTHOR]

Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, Q act , and the stress exponent, n , for SPS of nearly stoichiometric UO 2. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate Q act and n , respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare Q act. The average values were Q act =96 kJ/mol (CHR), Q act = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250−450 kJ/mol), creep (350−550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol). This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS. [ABSTRACT FROM AUTHOR]

• B impurities on Co drastically affect dry reforming of methane (DRM) performance. • DFT simulations show that B remained only in interstitial cavities of Co. • Incorporation of B into Co induced acceleration of both CH 4 and CO 2 activation. • The resultant kinetic balance between surface C and O atoms prevents deactivation. Stable operations for catalytic dry reforming of methane (DRM) are essential for industrial applications. High stability for syngas production can be achieved via a kinetic balance between formation of carbon species and their removal by oxygen species on the metal surface, which clears the surface for further reaction steps. This study reports highly stable performance by a boron-doped cobalt catalyst as a non-noble-metal coking-free catalyst. Although the precise location of doped boron could not be identified experimentally because of its low concentration, density functional theory calculations suggested that interstitial boron (B) is most likely present in the subsurface region of cobalt (Co) surfaces. B-doping was shown both experimentally and computationally to increase the reactivity of Co catalysts toward both methane (CH 4) and carbon dioxide (CO 2). Moreover, B-doping was found to balance the amounts of surface C and O and maintain the reduced state of Co surfaces while in a steady-state. Nevertheless, a negative kinetic order with respect to CO 2 partial pressure indicates that steady-state surface coverage of oxygen species originating from CO 2 dissociation was prevalent on B-doped Co, consistent with the coking-free nature of the catalyst. This study introduces a promising Co–B catalyst design for controlling metal surface reactivity toward DRM and relevant catalytic reactions. [ABSTRACT FROM AUTHOR]

The volcanic degassing of halogens, and especially of the heavier Br and I, received increased attention over the last 20 years due to their significant effect on atmospheric chemistry, notably the depletion of stratospheric ozone. While the effect of melt composition on halogen diffusion, solubility, or fluid-melt partitioning in crustal magma chambers has been thoroughly studied, structural controls on halogen incorporation in silicate melts remain poorly known, with only few studies available in simplified borosilicate or haplogranite compositions. Here, we demonstrate that high-energy resolution fluorescence detection X -ray absorption spectroscopy (HERFD-XAS) with a crystal analyzer spectrometer (CAS) is well-suited for the study of Br speciation in natural volcanic glasses which can contain lower Br concentrations than their laboratory analogs. Especially, HERFD-XAS results in sharper and better-resolved XANES and EXAFS features than previously reported and enables detection limits for EXAFS analysis down to 100 ppm when previous studies required Br concentrations above the 1000 ppm level. XANES and EXAFS analyses suggest important structural differences between synthetic haplogranitic glass, where Br is surrounded by Na and next-nearest oxygen neighbors, and natural volcanic glasses of basaltic to rhyodacitic compositions, where Br is incorporated in at least three distinct sites, surrounded by Na, K, or Ca. Similar environments, involving both alkali and alkaline earth metals have already been reported for Cl in Ca-bearing aluminosilicate glass and our study thus underlines that the association of Br with divalent cations (Ca2+) has been underestimated in the past due to the use of simplified laboratory analogs. Overall, similarities in Cl and Br structural environments over a large array of compositions (46-67 wt% SiO2) suggest that melt composition alone may not have a significant effect on halogen degassing and further support the coupled degassing of Cl and Br in volcanic systems. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy (XAS) has been used to unveil the bromide ion local coordination structure in 1-alkyl-3-methylimidazolium bromide [Cnmim]Br ionic liquids (ILs) with different alkyl chains. The XAS spectrum of 1-ethyl-3-methylimidazolium bromide has been found to be different from those of the other members of the series, from the butyl to the decyl derivatives, that have all identical XAS spectra. This result indicates that starting from 1-buthyl-3-methylimidazolium bromide the local molecular arrangement around the bromide anion is the same independently from the length of the alkyl chain, and that the imidazolium head groups in the liquid ILs with long alkyl chains assume locally the same orientation as in the [C4mim]Br crystal. With this study we show that the XAS technique is an effective direct tool for unveiling the local structural arrangements around selected atoms in ILs. [ABSTRACT FROM AUTHOR]

[Display omitted] • DFT determines the thermodynamic states of oxychlorinated Pt single atoms on γ − Al 2 O 3. • The nature of Pt single atoms depends on calcination and XAS conditions. • EXAFS with wavelet analysis and DFT solves the first and second shell of Pt single atoms. • HERFD-XANES is interpreted with FDMNES-DFT simulations of Pt single atom models. • FDMNES simulations reveal that XANES is sensitive to the number of Cl ligands. • A reliable approach is provided to identify Pt single atoms and their local environment. The existence and nature of oxidized Pt single atoms (SA) are unraveled on two γ-Al 2 O 3 supported Pt catalysts (0.3 wt%) containing two chlorine contents (0.1 and 1.4 wt%) characterized by HR-HAADF-STEM and HERFD-XANES/EXAFS after a calcination treatment at 520 °C. Pt SA are revealed as PtO x Cl y complexes where the number of oxygen (x) and chlorine (y) ligands directly depends on the chlorine content. In order to interpret these experimental observations and provide coherent atomic structures for these PtO x Cl y complexes, the thermodynamic stability of the oxidized Pt SA species supported on the (1 0 0) surface model of γ-Al 2 O 3 is determined by density functional theory (DFT) calculations as a function of the calcination conditions determined by the temperature, and partial pressures of H 2 O, HCl and O 2. It is shown that various ligands such as Cl, OH, O (from the alumina surface or as extra-ligands) and H 2 O are coordinated to Pt depending on the conditions and chlorine content. Various square planar and square-based pyramidal PtO x Cl y species such as PtO 2 , PtOCl 2 , PtCl 4 are found to be stable in calcination conditions, while octahedral Pt(OH) x Cl 4-x (H 2 O) are stabilized in XAS recording conditions. The features of these DFT simulated octahedral Pt SA species are successfully compared to XAS data. On one hand, based on thermodynamics arguments, we proposed models of supported complexes in agreement with EXAFS coordination numbers and distances (Pt-O, Pt-Cl and Pt-Al). On the other hand, the simulated XANES spectra at the Pt L 3 -edge for these complexes match well with the experimental HERFD-XANES and reveal that the bump feature observed between 11575 eV and 11585 eV (post-edge signal) is a tracer of the amount of Cl coordinated to Pt SA. [ABSTRACT FROM AUTHOR]

Small angle x-ray scattering experiments were performed in order to investigate the semiconductor-metal and metal-nonmetal transitions that take place in supercritical fluid selenium. At the semiconductor to metal transition, we do not observe any density fluctuations indicating the homogeneity of the fluid at this scale. On the other hand, we observe the onset of density fluctuations and a sharp density variation in the pressure-temperature range of the metal-nonmetal transition. We show that the supercritical domain can be separated into two regions: a crossover takes place between a liquidlike density region and a gaslike one. The onset of density fluctuations, i.e., the development of rare and dense domains, forbids any percolation and induces the loss of the metallic character. [ABSTRACT FROM AUTHOR]

• Atomically dispersed Fe in mesoporous nitrogen doped carbon is prepared. • Atomically dispersed Fe boosts the catalytic performance of CO 2 electroreduction. • High stability and reusability are achieved. • The local coordination sphere of Fe is a FePc structure with H 2 O/OH moieties. Mesoporous nitrogen-doped carbon nanoparticles with atomically dispersed iron sites (named mesoNC-Fe) are synthesized via high-temperature pyrolysis of an Fe containing ZIF-8 MOF. Hydrolysis of tetramethyl orthosilicate (TMOS) in the MOF framework prior to pyrolysis plays an essential role in maintaining a high surface area during the formation of the carbon structure, impeding the formation of iron (oxide) nanoparticles. To gain inside on the nature of the resulting atomically dispersed Fe moieties, HERFD-XANES, EXAFS and valence-to-core X-ray emission spectroscopies have been used. The experimental spectra (both XAS and XES) combined with theoretical calculations suggest that iron has a coordination sphere including a porphyrinic environment and OH/H 2 O moieties responsible for the high activity in CO 2 electroreduction. DFT calculations demonstrate that CO formation is favored in these structures because the free energy barriers of *COOH formation are decreased and the adsorption of *H is impeded. The combination of such a unique coordination environment with a high surface area in the carbon structure of mesoNC-Fe makes more active sites accessible during catalysis and promotes CO 2 electroreduction. [ABSTRACT FROM AUTHOR]

• Ultrafine Pt succeeded photocatalytic overall water splitting without back-reaction. • Pt clusters less than 100 atoms remained oxidized on SrTiO 3. • The oxidized Pt is insensitive to CO poisoning for water splitting. The metal cluster size in supported metal catalysts impacts the oxidation state of the metal atoms, coordination capability, and finally the catalytic activity—especially when the number of atoms becomes countable. The correlation between metal oxidation state and its catalytic consequences for ultrafine Pt was studied for photocatalytic overall water splitting using a Pt/SrTiO 3 (photo)catalyst. A distinctive change in catalytic behavior and oxidation state was observed below 100 Pt-atom clusters at ∼2 nm. Combining density functional theory (DFT) and experimental characterizations including X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the smaller Pt clusters obtained by the surface organometallic route (under 100 Pt atoms) were predominantly oxidized and selectively performed photocatalytic water splitting selectively without activation of the water-forming back-reaction from H 2 and O 2. When the Pt clusters obtained by classical impregnation were larger than 2 nm, they remained metallic (Pt0) and were active for both water splitting and the competing thermal water formation back reaction. In addition, Pt0 clusters are poisoned in the presence of CO, whereas highly dispersed ultra-fine oxidized Pt clusters are insensitive to CO. This paper presents evidence of ultrafine PtO x (below approximately 2 nm clusters) that are insensitive to coordination of various gas identities (H 2 , O 2 , CO), resulting in efficient and selective photocatalytic overall water splitting. [ABSTRACT FROM AUTHOR]

The alarming atmospheric concentration and continuous emissions of carbon dioxide (CO2) require immediate action. As a result of advances in CO2 capture and sequestration technologies (generally involving point sources such as energy generation plants), large amounts of pure CO2 will soon be available. In addition to geological storage and other applications of the captured CO2, the development of technologies able to convert this carbon feedstock into commodity chemicals may pave the way towards a more sustainable economy. Here, we present a novel multifunctional catalyst consisting of Fe2O3 encapsulated in K2CO3 that can transform CO2 into olefins via a tandem mechanism. In contrast to traditional systems in Fischer-Tropsch reactions, we demonstrate that when dealing with CO2 conversion (in contrast to CO), very high K loadings are key to activate CO2 via the well-known 'potassium carbonate mechanism'. The proposed catalytic process is demonstrated to be as productive as existing commercial processes based on synthesis gas while relying on economically and environmentally advantageous CO2 feedstock. [ABSTRACT FROM AUTHOR]

• Au in sulfide-sulfate solutions is complexed with the HS−, S 3 · - and S 2 · - ions. • In As-poor systems invisible gold in pyrite is present as Au(0) and Au(I). • Au(I) in pyrite occurs as chemisorbed S-Au-S n moieties similar to those in the fluid. • Au(I) uptake by pyrite from fluid is enhanced with decreasing temperature. • Au(I) pyrite/fluid partition coefficients allow predictions of gold tenors in pyrite ore. The most characteristic feature of hydrothermal deposits of gold is its intimate association with pyrite. Microscopically visible gold occurs in pyrite ore as metal particles of >0.1 µm in size, together with so called "invisible" gold, undetectable by conventional microscopic methods. The chemical, redox and structural state of this invisible gold and the mechanisms of its incorporation into pyrite remain both inconsistent and controversial since the dawn of economic geology. To clarify these issues, we performed laboratory experiments to simulate interactions of gold-bearing sulfur-rich hydrothermal fluids with arsenic-free pyrite at temperatures from 350 to 450 °C and pressures from 400 to 700 bar, typical of the formation conditions of many types of gold deposits. Gold solubility was measured in these fluids as a function of sulfur speciation and acidity. Gold redox and structural state in pyrite was characterized by high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), together with more traditional analytical techniques such as scanning electron microscopy (SEM), x-ray diffraction (XRD), electron probe micro analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and inductively coupled plasma atomic emission spectrometry (ICP-AES). Results show that dissolved Au in sulfide-sulfate solutions forms complexes with hydrogen sulfide, and tri- and di-sulfur radical ions whose amounts depend mostly on the fluid pH and total sulfur concentration. Invisible gold in pyrite occurs as Au metal submicron- to nano-sized particles and chemically bound Au(I) in the form of (poly)sulfide clusters composed on S-Au-S linear units, similar to those in aqueous complexes. Our findings contest the common belief that Au(I) substitutes for Fe and/or S in the structure of As-poor pyrite. The partition coefficient of Au(I) between pyrite and the fluid, D py/fl , is determined to be 0.15 ± 0.07 at 450 °C in a wide range of Au fluid phase concentrations (10–1000 ppm), but much higher D py/fl values, between 10 and 50, are found at 350 °C. These Au partitioning trends coupled with the new data on Au molecular environment in pyrite suggest a control of Au(I) incorporation in the mineral by a chemisorption step. Extrapolated to Au contents of hydrothermal fluids of the Earth's crust which are typically below 1 ppm, our D py/fl values reproduce fairly well the natural Au tenors in As-poor pyrites (∼0.1–1 ppm Au), which are 100–1000 times lower than those typically observed in arsenian pyrites and arsenopyrites (10–1000 ppm Au at As tenors of 0.01–10 wt%). Our results thus indirectly highlight a key role played by arsenic in gold enrichment in As-bearing iron sulfide ore, a role that yet remains to be fully understood and quantified. [ABSTRACT FROM AUTHOR]

Graphical abstract The active site for the methane dehydroaromatization forms at reaction conditions in an initial phase where no desired products are observed yet. To characterize the activated Mo inside the pores of HZSM-5 zeolite, a site-sensitive quasi-insitu strategy was developed. A CO-treatment of the as-synthesized catalyst was employed as an alternative way to create the active site, which was then characterized using CO IR, 13C MAS NMR and DNP. Together with theory, these results point to the (oxy-)carbidic nature of the Mo-site. Highlights • A quasi-in situ strategy to distinguish among different Mo sites. • Direct spectroscopic evidence for two kinds of well-defined species as well as nanoparticles. • Mo nanoparticles contribute to the deactivation of the catalyst. • Mo has an oxidation state between 4+ and 6+. • The Mo-site does not transform further once benzene is formed. Abstract The active sites on the methane dehydroaromatization (MDA) catalyst Mo/HZSM-5 are very hard to characterize, because they are present in various geometries and sizes and only form under reaction conditions with methane at 700 °C. To address these issues an experimental strategy is presented that enables distinguishing different active sites for MDA present on Mo/HZSM-5 and helps determining the Mo charge, nuclearity and chemical composition. This approach combines a CO pretreatment to separate the active Mo site formation from coke formation, quasi-in situ spectroscopic observations using DNP, 13C NMR, CO IR and theory. This allows the discrimination between three different types of active sites. Distinct spectroscopic features were observed corresponding to two types of mono- or dimeric Mo (oxy-)carbide sites as well as a third site assigned to Mo 2 C nanoparticles on the outer surface of the zeolite. Their formal Mo oxidation state was found to be between 4+ and 6+. Dynamic nuclear polarization (DNP) measurements of samples carburized in CO as well as in CH 4 confirm the assignment and also show that accumulated aromatic carbon covers the bigger Mo nanoparticles on the outer surface of the zeolite, causing deactivation. It was previously observed that after an initial period where no desired products are formed yet, benzene starts slowly forming until reaching its maximum productivity. Direct observation of the active site with 13C NMR confirmed that Mo-sites do not transform further once benzene starts forming, meaning that they are fully activated during the period where no desired products are observed yet. Therefore the slow increase of the benzene formation rate cannot be attributed to a further transformation of Mo sites. [ABSTRACT FROM AUTHOR]

Well-defined silica supported bimetallic catalysts Pt 100- x Pd x were prepared by Surface Organometallic Chemistry (SOMC) and Ionic-Exchange (IE) methods. For all investigated catalysts, iso -butane reaction with hydrogen under differential conditions led to the formation of methane and propane, n-butane, and traces of iso -butylene. The total reaction rate decreased with increasing the Pd loading for both catalysts series as a result of decreasing turnover rate of both isomerization and hydrogenolysis. In the case of Pt 100- x Pd x (SOMC) catalysts, the experimental results in combination with DFT calculations suggested a selective coverage of Pt (1 0 0) surface by agglomerated Pd atoms like “ islands ”, assuming that each metal roughly keeps its intrinsic catalytic properties with relatively small electron transfer from Pt to Pd in the case of Pt-rich sample and from Pd to Pt in the case of Pd-rich sample. For the PtPd catalysts prepared by IE, the catalytic behavior could be explained by the formation of a surface alloy between Pt and Pd in the case of Pd-rich sample and by the segregation of a small amount of Pd on the surface in the case of Pt-rich sample, as demonstrated by TEM, EXAFS and DFT. The catalytic results were explained by a structure activity relationship based on the proposed mechanism of C H bond and C C bond activation and cleavage for iso -butane hydrogenolysis, isomerization, cracking and dehydrogenation. [ABSTRACT FROM AUTHOR]

Dry reforming of methane (DRM) proceeds via CH 4 decomposition to leave surface carbon species, followed by their removal with CO 2 -derived species. Reactivity tuning for stoichiometric CH 4 /CO 2 reactants was attempted by alloying the non-noble metals Co and Ni, which have high affinity with CO 2 and high activity for CH 4 decomposition, respectively. This study was focused on providing evidence of the capturing surface coverage of the reactive intermediates and the associated structural changes of the metals during DRM at high temperature using in-operando X-ray absorption spectroscopy (XAS). On the Co catalysts, the first-order effects with respect to CH 4 pressure and negative-order effects with respect to CO 2 pressure on the DRM rate are consistent with the competitive adsorption of the surface oxygen species on the same sites as the CH 4 decomposition reaction. The Ni surface provides comparatively higher rates of CH 4 decomposition and the resultant DRM than the Co catalyst but leaves some deposited carbon on the catalyst surface. In contrast, the bimetallic CoNi catalyst exhibits reactivity towards the DRM but with kinetic orders resembling Co catalyst, producing negligible carbon deposition by balancing CH 4 and CO 2 activation. The in-operando X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements confirmed that the Co catalyst was progressively oxidized from the surface to the bulk with reaction time, whereas CoNi and Ni remained relatively reduced during DRM. Density functional theory (DFT) calculation considering the high reaction temperature for DRM confirmed the unselective site arrangement between Co and Ni atoms in both the surface and bulk of the alloy nanoparticle (NP). The calculated heat of oxygen chemisorption became more exothermic in the order of Ni, CoNi, Co, consistent with the catalytic behavior. The comprehensive experimental and theoretical evidence provided herein clearly suggests improvement to the catalyst design protocol by selecting the appropriate composition of Co-Ni alloy. [ABSTRACT FROM AUTHOR]

The ISO-standardized RHIZOtest is used here for the first time to decipher how plant species, soil properties, and physical-chemical properties of the nanoparticles and their transformation regulate the phytoavailability of nanoparticles. Two plants, tomato and fescue, were exposed to two soils with contrasted properties: a sandy soil poor in organic matter and a clay soil rich in organic matter, both contaminated with 1, 15, and 50 mg·kg-1 of dissolved Ce2(SO4)3, bare and citrate-coated CeO2 nanoparticles. All the results demonstrate that two antagonistic soil properties controlled Ce uptake. The clay fraction enhanced the retention of the CeO2 nanoparticles and hence reduced Ce uptake, whereas the organic matter content enhanced Ce uptake. Moreover, in the soil poor in organic matter, the organic citrate coating significantly enhanced the phytoavailability of the cerium by forming smaller aggregates thereby facilitating the transport of nanoparticles to the roots. By getting rid of the dissimilarities between the root systems of the different plants and the normalizing the surfaces exposed to nanoparticles, the RHIZOtest demonstrated that the species of plant did not drive the phytoavailability, and provided evidence for soil-plant transfers at concentrations lower than those usually cited in the literature and closer to predicted environmental concentrations. [ABSTRACT FROM AUTHOR]

Conducting catalytic methane decomposition over Fe catalysts is a green and economic route to produce H 2 without CO/CO 2 contamination. Fused 65 wt% and impregnated 20 wt% Fe catalysts were synthesized with different additives to investigate their activity, whereas showing Fe-Al 2 O 3 combination as the best catalyst. Al 2 O 3 is speculated to expose more Fe 0 for the selective deposition of carbon nano tubes (CNTs). A fused Fe (65 wt%)-Al 2 O 3 sample was further investigated by means of H 2 -TPR, in-situ XRD, HRTEM and XAS to conclude 750 °C is the optimized temperature for H 2 pre-reduction and reaction to obtain a high activity. Based on density functional theory (DFT) study, a reaction mechanism over Fe catalysts was proposed to explain the formation of graphite from unstable supersaturated iron carbides decomposition. A carbon deposition model was further proposed which explains the formation of different carbon nano materials. [ABSTRACT FROM AUTHOR]

Knowledge of the mobility of Rare Earth Elements (REE) in crustal fluids is important for understanding both the formation of REE deposits and their use as geochemical tracers. This is particularly true for europium (Eu), which exists in both Eu(II) and Eu(III) states in natural fluids, making it a sensitive probe of hydrothermal redox processes. Currently, our understanding of Eu complexation in hydrothermal fluids relies mainly on extrapolations from room-temperature data, as there are no high-temperature experimental data available for Eu(II) species. In this study we conducted in-situ synchrotron X-ray absorption spectroscopy (XAS) experiments to investigate the complexing of Eu(II) and Eu(III) in chloride- and bromide-rich acidic solutions and Eu(III) sulphate complexes, at 35–400 °C, 600 bar. For Eu(III) chloride complexes, the total number of ligands decreased and the ratio of H 2 O:Cl ligands in the first coordination shell decreased with increasing temperature. For the Eu(II) system, our data confirm Sverjensky's (1984, EPSL 67, 70–78) theoretical predictions that Eu(II) species become increasingly stable (relative to Eu(III)) at elevated temperatures. However the new data indicate that complexation between Eu(II) and chloride is much weaker than Haas's (1995, GCA 59, 4329–4350) theoretical estimates. The thermodynamic properties of Eu(II) chloride complexes have been reassessed in light of the new data. Thermodynamic calculations reveal that the identity of the Eu(III) aqueous complexes (e.g., chloride or hydroxide species) have a significant impact on the oxidation state of Eu in hydrothermal fluids at elevated temperatures. [ABSTRACT FROM AUTHOR]

This paper presents a coherent X-ray-scattering imaging technique using a multipixel energy-dispersive system. Without any translation, the technique produces specific 1D image from data recorded by a single CdZnTe detector pixel using subpixelation techniques. The method is described in detail, illustrated by a simulation and then experimentally validated. As the main considered application of our study is breast imaging, this validation involves 2D imaging of a phantom made of plastics mimicking breast tissues. The results obtained show that our system can specifically image the phantom using a single detector pixel. For the moment, in vivo breast imaging applications remain difficult, as the dose delivered by the system is too high, but some adjustments are considered for further work. [ABSTRACT FROM AUTHOR]

To understand the fission products behaviour during a Nuclear Severe Accident (NSA), simulated high burnup UO 2 fuels produced by sintering at high temperature (SIMFUEL) were submitted to thermal treatments in conditions representative of the beginning of a Pressurized Water Reactor (PWR) severe accident. The samples made of UO 2 doped with eleven fission products (FP) surrogates were thus thermally treated from 400 ​°C up to 1000 ​°C at different oxygen potentials (up to −290 ​kJ ​mol (O2) −1 at 1000 ​°C). The samples were characterized before and after the heat treatments by Secondary Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDX) and X-ray Absorption Spectroscopy (XAS) at the MARS beamline, SOLEIL Synchrotron and at the FAME-UHD beamline, ESRF. Initially the SIMFUEL is constituted of a UO 2 matrix, a metallic phase composed of Mo, Pd, Ru, Rh and Tc, and an oxide phase with perovskite structure depicted as (Ba, Sr)(Zr, U, RE)O 3 where RE stands for Rare Earths. Around 1000 ​°C, it was found that Mo, initially contained in the metallic inclusions, oxidizes into MoO 2. In the same conditions, a part of the oxidized Mo interacts with the perovskite phase containing Ba to form BaMoO 4. These results will be useful to validate the theoretical models used in calculation codes and to assess thermodynamic databases, which are one of the main sources of errors when calculating a NSA sequence involving molten fuel. The study on UO 2 -based SIMFUEL allowed describing the formation of BaMoO 4 during thermal treatments representative the early stage of a severe nuclear accident sequence. Schematic mechanism for the evolution of the metallic and oxide precipitates at ΔG O2 ​= ​−290 ​kJ ​mol (O2) −1 at 1000 °C. [Display omitted] • Evolution of the perovskite phase studied by XANES coupled with SEM-EDX analyses. • Reaction between Mo and Ba at the periphery of the oxide precipitates. • Formation of BaMoO 4 around 1000 ​°C at −290 ​kJ ​mol−1. [ABSTRACT FROM AUTHOR]

The manganese-nitronyl-nitroxide two-dimensional coordination polymer {[Mn2(NITIm)3]ClO4}n (1) (NITImH = 2-(2-imidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxide-1-oxyl) undergoes an unusual hysteretic thermo-induced valence tautomeric transition near room temperature, during which the manganese(II) ions are oxidized to manganese(III) and two of the three deprotonated radicals (NITIm-) are reduced to their diamagnetic aminoxyl form (denoted NITRed2-). Upon cooling, the high-temperature species {[MnII2(NITIm)3]ClO4}n (1HT) turns into the low-temperature species {[MnIII2(NITRed)2(NITIm)]ClO4}n (1LT) around 274 K, while on heating the process is reversed at about 287 K. This valence tautomeric phenomenon is supported by temperature-dependent magnetic susceptibility measurements, differential scanning calorimetry (DSC), crystal structure determination, UV-vis absorption, X-ray absorption (XAS), and emission (XES) and electron paramagnetic resonance (EPR) spectroscopies in the solid state. [ABSTRACT FROM AUTHOR]

Original high-pressure glass-like carbon windows developed for x-ray spectroscopy applications are presented. The scientific and technological background of this new technical development is exposed, in particular the limitations of our existing beryllium windows in the context of x-ray absorption spectroscopy (XAS) measurements of solutions with very low solute concentrations at hydrothermal conditions (0.1-200 MPa, 30-600 °C). The benefits of glass-like carbon are exposed, notably its non-crystalline character, the absence of impurities which has been verified by micro-fluorescence laboratory measurements, and its non-toxicity which makes its machining safer. Finite elements mechanical calculations and experimental pressure tests were conducted to determine the pressure limits of windows with two different geometries: cylindrical (thickness 0.5 mm) and inversed-dome shape (thickness 0.5 mm at the tip of the dome). The former break at 150 MPa and the latter show no sign of rupture at 400 MPa. Recent XAS measurements conducted with the new dome shaped windows are presented to show the advantages of the design that allow for the detection of very low concentrations in the transmission mode (down to 30 ppm) and the acquisition of fluorescence XAS spectra in diluted solutions at high pressure. Eventually the perspectives of this original development are discussed. [ABSTRACT FROM AUTHOR]

Current models of the formation and distribution of gold deposits on Earth are based on the long-standing paradigm that hydrogen sulfide and chloride are the ligands responsible for gold mobilization and precipitation by fluids across the lithosphere. Here we challenge this view by demonstrating, using in situ X-ray absorption spectroscopy and solubility measurements, coupled with molecular dynamics and thermodynamic simulations, that sulfur radical species, such as the trisulfur ion S3-, form very stable and soluble complexes with Au+ in aqueous solution at elevated temperatures (>250 °C) and pressures (>100 bar). These species enable extraction, transport, and focused precipitation of gold by sulfur-rich fluids 10-100 times more efficiently than sulfide and chloride only. As a result, S3- exerts an important control on the source, concentration, and distribution of gold in its major economic deposits from magmatic, hydrothermal, and metamorphic settings. The growth and decay of S3- during the fluid generation and evolution is one of the key factors that determine the fate of gold in the lithosphere. [ABSTRACT FROM AUTHOR]

The VO x/SiO2 catalysts for oxidative dehydrogenation of propane were synthesized by a simple grafting method. The VOCl3 was first grafted at the surface of SiO2, which was dehydrated at different temperature (from 200 to 1000 °C). The formed grafted complexes were then calcined in air, leading to the formation of VO x/SiO2 catalysts. The synthesized catalysts were characterized by nitrogen adsorption, SEM, Raman spectroscopy, temperature-programmed reduction, and extended X-ray absorption fine structure analysis. The SiO2 pretreatment temperature has an evident effect on the loading and dispersion of VO x on SiO2, which finally affects their catalytic performance. High SiO2 treatment temperature is beneficial to dispersing the vanadium oxide species at the SiO2 surface. These materials are efficient catalysts for the catalytic oxidative dehydrogenation of propane to propylene. The best selectivity to propylene is achieved on the VO x/SiO2-(1000) catalyst. The high selectivity and activity are well maintained for three days catalytic reaction. [ABSTRACT FROM AUTHOR]

In a photocatalytic suspension system with a powder semiconductor, the interface between the photocatalyst semiconductor and catalyst should be constructed to minimize resistance for charge transfer of excited carriers. This study demonstrates an in-depth understanding of pretreatment effects on the photocatalytic O2 evolution reaction (OER) activity of visible-light-responsive Ta3N5 decorated with CoOx nanoparticles. The CoOx/Ta3N5 sample was synthesized by impregnation followed by sequential heat treatments under NH3 flow and air flow at various temperatures. Various characterization techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS), were used to clarify the state and role of cobalt. No improvement in photocatalytic activity for OER over the bare Ta3N5 was observed for the as-impregnated CoOx/Ta3N5, likely because of insufficient contact between CoOx and Ta3N5. When the sample was treated in NH3 at high temperature, a substantial improvement in the photocatalytic activity was observed. After NH3 treatment at 700 °C, the Co0-CoOx core-shell agglomerated cobalt structure was identified by XAS and STEM. No metallic cobalt species was evident after the photocatalytic OER, indicating that the metallic cobalt itself is not essential for the reaction. Accordingly, mild oxidation (200 °C) of the NH3-treated CoOx/Ta3N5 sample enhanced photocatalytic OER activity. Oxidation at higher temperatures drastically eliminated the photocatalytic activity, most likely because of unfavorable Ta3N5 oxidation. These results suggest that the intimate contact between cobalt species and Ta3N5 facilitated at high temperature is beneficial to enhancing hole transport and that the cobalt oxide provides electrocatalytic sites for OER. [ABSTRACT FROM AUTHOR]

In a photocatalytic suspension system with a powder semiconductor, the interface between the photocatalyst semiconductor and catalyst should be constructed to minimize resistance for charge transfer of excited carriers. This study demonstrates an in-depth understanding of pretreatment effects on the photocatalytic O2 evolution reaction (OER) activity of visible-light-responsive Ta3N5 decorated with CoOx nanoparticles. The CoOx/Ta3N5 sample was synthesized by impregnation followed by sequential heat treatments under NH3 flow and air flow at various temperatures. Various characterization techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS), were used to clarify the state and role of cobalt. No improvement in photocatalytic activity for OER over the bare Ta3N5 was observed for the as-impregnated CoOx/Ta3N5, likely because of insufficient contact between CoOx and Ta3N5. When the sample was treated in NH3 at high temperature, a substantial improvement in the photocatalytic activity was observed. After NH3 treatment at 700 °C, the Co0-CoOx core-shell agglomerated cobalt structure was identified by XAS and STEM. No metallic cobalt species was evident after the photocatalytic OER, indicating that the metallic cobalt itself is not essential for the reaction. Accordingly, mild oxidation (200 °C) of the NH3-treated CoOx/Ta3N5 sample enhanced photocatalytic OER activity. Oxidation at higher temperatures drastically eliminated the photocatalytic activity, most likely because of unfavorable Ta3N5 oxidation. These results suggest that the intimate contact between cobalt species and Ta3N5 facilitated at high temperature is beneficial to enhancing hole transport and that the cobalt oxide provides electrocatalytic sites for OER. [ABSTRACT FROM AUTHOR]

A high-pressure/high-temperature cell dedicated to Raman spectroscopy is presented. The P and T ranges are optimized for the study of supercritical fluids (T ≤ 975 K, P ≤ 200 MPa); the P and T parameters are controlled independently. The autoclave was adapted from a previous cell (designed for X-ray absorption and scattering spectroscopies) to answer the specific requirement of Raman spectroscopy. Original high pressure windows and sample cell have been developed to ensure both pressure resistance and transparency for the optical laser and Raman scattering beams. An optical integration of the autoclave in the Raman spectrometer setup was designed to perform in situ Raman experiments on fluids. The spectroscopic measurements can be combined with visual observation, which is particularly useful for the study of multi-phasic systems. As examples of the efficiency of the set-up and its scientific opportunities, we present Raman measurements on water, carbon dioxide and mixed H 2 O–CO 2 system obtained from ambient to supercritical conditions. [ABSTRACT FROM AUTHOR]

Platinum nanoclusters highly dispersed on γ-alumina are widely used as heterogeneous catalysts. To understand the chemical interplay between the Pt nanoparticles, the support, and the reductive atmosphere, we performed X-ray absorption near edge structure (XANES) in situ experiments recorded in high energy resolution fluorescence detection (HERFD) mode. Spectra are assigned by comparison with simulated XANES spectra on models obtained by molecular dynamics (DFT-MD). We propose platinum cluster morphologies and quantify the hydrogen coverages compatible with XANES spectra recorded at variable hydrogen pressures and temperatures. Using cutting-edge methodologies to assign XANES spectra, this work gives unequalled atomic insights into the characterization of supported nanoclusters. [ABSTRACT FROM AUTHOR]

Platinum nanoclusters highly dispersed on g-alumina are widely used as heterogeneous catalysts. To understand the chemical interplay between the Pt nanoparticles, the support, and the reductive atmosphere, we performed X-ray absorption near edge structure (XANES) in situ experiments recorded in high energy resolution fluorescence detection (HERFD) mode. Spectra are assigned by comparison with simulated XANES spectra on models obtained by molecular dynamics (DFT-MD). We propose platinum cluster morphologies and quantify the hydrogen coverages compatible with XANES spectra recorded at variable hydrogen pressures and temperatures. Using cutting-edge methodologies to assign XANES spectra, this work gives unequalled atomic insights into the characterization of supported nanoclusters. [ABSTRACT FROM AUTHOR]

The partitioning of Zr between high P-T aqueous fluids and melts has been investigated in situ in the haplogranite-H2O and haplogranite-(F)-H2O systems to assess the mobilization of high field strength elements (HFSE) in magmatic-hydrothermal processes in subduction zones. The partition coefficients DZrf/m were determined from Zr concentrations measured in situ by synchrotron X-ray fluorescence (SXRF) in both aqueous fluids and F-free or F-bearing hydrous haplogranite melts equilibrated in diamond-anvil cells at 575 to 800 °C and 0.3 to 2.4 GPa. This experimental approach eliminates the need for internal or external calibrations of the SXRF signal and/or post-mortem analysis of the melt phase, hence decreasing the total uncertainties onDZrf/m below 16%. Above 0.6 GPa, Zr partitions favorably into the hydrous silicate melt in both F-free and F-bearing systems, with DZrf/m that range between 0.19 ± 0.02 and 0.38 ± 0.03. However, the relatively high DZrf/mvalues indicate that alkali-silica rich aqueous fluids generated by metamorphic devolatilization may contribute significantly to the recycling of HFSE in subduction zones. The efficient uptake of Zr (and likely other HFSE) by subduction zone fluids, regardless of their nature (aqueous fluid, hydrous melt, or supercritical fluid), supports the idea that the typical HFSE depletion recorded in arc magmas does not result from their incompatibility in water-rich slab-derived fluids but most probably originates from complex fluid-melt-rock interactions occurring at the slab interface and within the mantle wedge. At shallow crustal pressure conditions (800 °C and 0.3 GPa), Zr partitions reversely into the aqueous fluid in the presence of fluorine (DZrf/m= 1.40 ± 0.10) as observed for Nb at similar conditions by Webster et al. (1989). The enrichment of the aqueous phase in HFSE (Zr, Nb) at shallow crustal conditions is likely related to the enhanced peralkalinity of low pressure, F-bearing aqueous fluid with temperature, that provides the favorable conditions for their mobilization via the formation of HFSE-O-Si/Na clusters. This mechanism may control the enrichment in HFSE (and plausibly other rare metals such as REE) in early magmatic fluids exsolved from granitic melts, leading to the formation of HFSE-enriched aggregates in shallow magmatic-hydrothermal environments (e.g., Strange Lake and Thor Lake Nechalacho deposit, Canada; Galineiro complex, Spain). [ABSTRACT FROM AUTHOR]