The Experts below are selected from a list of 210 Experts worldwide ranked by ideXlab platform
Jean-louis Hazemann - One of the best experts on this subject based on the ideXlab platform.
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Surface Science Approach to the Solid-Liquid Interface: Surface-Dependent Precipitation of Ni(OH)2 on a-Al2O3 Surfaces
Angewandte Chemie International Edition, 2012Co-Authors: Asma Tougerti, Isabelle Llorens, Francesco D'acapito, Emiliano Fonda, Jean-louis Hazemann, Yves Joly, Dominique Thiaudi Ère, Michel Che, Xavier CarrierAbstract:A molecular-scale understanding of the adsorption of metal ions on oxide surfaces is of fundamental importance for various scientific areas where the oxide/water interface plays a central role, such as corrosion science, preparation of heterogeneous catalysts, transport of contaminants in the environment. As a matter of fact, a fine-tuning of metal speciation during the first steps of heterogeneous catalyst preparation (oxide/water interface) will strongly control the final characteristics of the catalyst (activity/selectivity/stability).[1] However, a rational description of sorption mechanisms is generally made difficult by the ill-defined surface structure of high surface area oxides exposing a number of different sorption sites.[2] One way to mitigate this problem is to simplify the sorption system by using oriented single crystals that have a limited number of well-defined surface sites. This reductionist approach applied to aqueous deposition is almost inexistent in the field of surface science where most of the model studies use nonaqueous deposition techniques such as metal evaporation.[3] However, pioneering works of the research groups of Brown, Jr.[4] in the field of earth sciences and Niemantsverdriet,[5] Regalbuto,[6] or more recently Freund[7] in the field of catalysis have shown the tremendous importance of developing a surface science approach using a realistic aqueous phase deposition method. Aqueous deposition of NiII complexes on a-Al2O3 single crystals has been chosen in this work since this system has practical implications in heterogeneous catalysis for hydrotreating (removal of S, N, O, and metals from crude oil in refineries), hydrogenation, or steam reforming purposes.[8] a-Al2O3 single crystals are commercially available in two orientations, (0001) and (11¯ 02), exposing different types of surface sites partly mimicking the complexity of the surface chemistry of g-Al2O3, the standard catalytic support, that is not available as single crystals.[9] Moreover, both (0001) and (11¯02) a-Al2O3 orientations have been extensively characterized in the presence of water.[10] However, the use of single crystals with low surface area requires characterization of very low quantities of adsorbates. Hence, grazing-incidence X-ray absorption spectroscopy (GI-XAS) was chosen as the main molecular-scale characterization technique, since detection of a small amount of adsorbates is made possible by enhancement of the fluorescence intensity in the grazingincidence geometry.[11] Furthermore, the combined use of GI-XAS and oriented single crystals provide additional structural information on the local environment of the Absorbing Atom thanks to the synchrotron beam polarization. Actually, the measured number of neighbors (Nmeasured) will be three times higher than the actual number of neighbors (Nreal) when the chemical bond (directed along~r) is parallel to the electric field vector~e[Eq. (1), cosq=1].[12]
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a new view on gold speciation in sulfur bearing hydrothermal fluids from in situ x ray absorption spectroscopy and quantum chemical modeling
Geochimica et Cosmochimica Acta, 2009Co-Authors: Gleb Pokrovski, Jean-louis Hazemann, Boris Tagirov, Jacques Schott, Olivier ProuxAbstract:Despite the common belief that AuI complexes with hydrogen sulfide ligands (H2S/HS−) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous AuI–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local Atomic structure around the dissolved metal in S–NaOH–Na2SO4–H2SO4 aqueous solutions were characterized at temperatures 200–450 °C and pressures 300–600 bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the Absorbing Atom (Au) and its local Atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)2− is the dominant Au species in neutral-to-basic solutions (5.5 ⩽ pH ⩽ 8.5; H2O–S–NaOH) over a wide range of sulfur concentrations (0.2 < ΣS < 3.6 mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur Atoms are in a linear geometry around AuI at an average distance of 2.29 ± 0.01 A. At acidic conditions (1.5 ⩽ pH ⩽ 5.0; H2O–S–Na2SO4–H2SO4), the Au Atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)0 complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H2S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H2S concentrations higher than 0.1–0.2 mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of AuI complexes with these ligands (e.g., AuHS(SO2)0, Au(HS2O3)2−, Au(HSn)2−) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth.
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An in situ X-ray absorption spectroscopy study of gold-chloride complexing in hydrothermal fluids
Chemical Geology, 2009Co-Authors: Gleb Pokrovski, Jean-louis Hazemann, Boris Tagirov, Jacques Schott, Elena Bazarkina, Olivier ProuxAbstract:Despite the growing body of experimental data on gold solubility in hydrothermal fluids, the identity, structure and stoichiometry of Au-bearing aqueous complexes remain poorly known. Here we present the first in situ measurements, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structures of AuIII and AuI chloride complexes at elevated temperatures and pressures (T–P) typical of natural hydrothermal conditions. The HAuCl4–NaCl–HCl–Au(s) and NaCl–H2SO4–Au(s) systems were investigated to 500 °C and 600 bar using a recently designed X-ray cell which allows simultaneous determination of the absolute concentration of the Absorbing Atom (Au) and its local Atomic environment in the fluid phase. XAFS data combined with Density Functional Theory quantum-chemical calculations of species structures and ab initio modeling of XANES spectra show that the AuIIICl4 − species is rapidly reduced to AuICl2 − at temperatures above 100–150 °C in acidic NaCl–HCl solutions. In the latter complex, two chlorine Atoms are aligned in a linear geometry around Au at an average distance of 2.267±0.004 Å. Our data provide the first direct structural evidence for AuCl2 − , which is the major Au-bearing species in acidic Cl-rich hydrothermal fluids over a wide T–P range, in agreement with previous solubility and Raman spectroscopy data. Total aqueous Au concentrations measured by XAFS in HAuCl4–HCl–NaCl and NaCl–H2SO4 solutions in the presence of Au(s) are, however, one to two orders of magnitude lower than those predicted by equilibrium thermodynamic calculations. This discrepancy is believed to be due to the combined effects of the cell properties, X-ray beam induced phenomena, and kinetic factors which may complicate the interpretation of high T–P spectroscopic data in redox-sensitive systems.
Olivier Proux - One of the best experts on this subject based on the ideXlab platform.
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a new view on gold speciation in sulfur bearing hydrothermal fluids from in situ x ray absorption spectroscopy and quantum chemical modeling
Geochimica et Cosmochimica Acta, 2009Co-Authors: Gleb Pokrovski, Jean-louis Hazemann, Boris Tagirov, Jacques Schott, Olivier ProuxAbstract:Despite the common belief that AuI complexes with hydrogen sulfide ligands (H2S/HS−) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous AuI–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local Atomic structure around the dissolved metal in S–NaOH–Na2SO4–H2SO4 aqueous solutions were characterized at temperatures 200–450 °C and pressures 300–600 bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the Absorbing Atom (Au) and its local Atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)2− is the dominant Au species in neutral-to-basic solutions (5.5 ⩽ pH ⩽ 8.5; H2O–S–NaOH) over a wide range of sulfur concentrations (0.2 < ΣS < 3.6 mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur Atoms are in a linear geometry around AuI at an average distance of 2.29 ± 0.01 A. At acidic conditions (1.5 ⩽ pH ⩽ 5.0; H2O–S–Na2SO4–H2SO4), the Au Atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)0 complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H2S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H2S concentrations higher than 0.1–0.2 mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of AuI complexes with these ligands (e.g., AuHS(SO2)0, Au(HS2O3)2−, Au(HSn)2−) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth.
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An in situ X-ray absorption spectroscopy study of gold-chloride complexing in hydrothermal fluids
Chemical Geology, 2009Co-Authors: Gleb Pokrovski, Jean-louis Hazemann, Boris Tagirov, Jacques Schott, Elena Bazarkina, Olivier ProuxAbstract:Despite the growing body of experimental data on gold solubility in hydrothermal fluids, the identity, structure and stoichiometry of Au-bearing aqueous complexes remain poorly known. Here we present the first in situ measurements, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structures of AuIII and AuI chloride complexes at elevated temperatures and pressures (T–P) typical of natural hydrothermal conditions. The HAuCl4–NaCl–HCl–Au(s) and NaCl–H2SO4–Au(s) systems were investigated to 500 °C and 600 bar using a recently designed X-ray cell which allows simultaneous determination of the absolute concentration of the Absorbing Atom (Au) and its local Atomic environment in the fluid phase. XAFS data combined with Density Functional Theory quantum-chemical calculations of species structures and ab initio modeling of XANES spectra show that the AuIIICl4 − species is rapidly reduced to AuICl2 − at temperatures above 100–150 °C in acidic NaCl–HCl solutions. In the latter complex, two chlorine Atoms are aligned in a linear geometry around Au at an average distance of 2.267±0.004 Å. Our data provide the first direct structural evidence for AuCl2 − , which is the major Au-bearing species in acidic Cl-rich hydrothermal fluids over a wide T–P range, in agreement with previous solubility and Raman spectroscopy data. Total aqueous Au concentrations measured by XAFS in HAuCl4–HCl–NaCl and NaCl–H2SO4 solutions in the presence of Au(s) are, however, one to two orders of magnitude lower than those predicted by equilibrium thermodynamic calculations. This discrepancy is believed to be due to the combined effects of the cell properties, X-ray beam induced phenomena, and kinetic factors which may complicate the interpretation of high T–P spectroscopic data in redox-sensitive systems.
S. Della Longa - One of the best experts on this subject based on the ideXlab platform.
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x ray structure analysis of a metalloprotein with enhanced active site resolution using in situ x ray absorption near edge structure spectroscopy
Proceedings of the National Academy of Sciences of the United States of America, 2007Co-Authors: Alessandro Arcovito, M. Benfatto, Michele Cianci, Samar S Hasnain, Karin Nienhaus, Ulrich G Nienhaus, Carmelinda Savino, Richard W Strange, Beatrice Vallone, S. Della LongaAbstract:X-ray absorption spectroscopy is exquisitely sensitive to the coordination geometry of an Absorbing Atom and therefore allows bond distances and angles of the surrounding Atomic cluster to be measured with Atomic resolution. By contrast, the accuracy and resolution of metalloprotein active sites obtainable from x-ray crystallography are often insufficient to analyze the electronic properties of the metals that are essential for their biological functions. Here, we demonstrate that the combination of both methods on the same metalloprotein single crystal yields a structural model of the protein with exceptional active-site resolution. To this end, we have collected an x-ray diffraction data set to 1.4-Å resolution and Fe K-edge polarized x-ray absorption near edge structure (XANES) spectra on the same cyanomet sperm whale myoglobin crystal. The XANES spectra were quantitatively analyzed by using a method based on the multiple scattering approach, which yielded Fe-heme structural parameters with ±(0.02–0.07)-Å accuracy on the Atomic distances and ±7° on the Fe–CN angle. These XANES-derived parameters were subsequently used as restraints in the crystal structure refinement. By combining XANES and x-ray diffraction, we have obtained an cyanomet sperm whale myoglobin structural model with a higher precision of the bond lengths and angles at the active site than would have been possible with crystallographic analysis alone.
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Advances in the Theoretical Analysis of the XANES Xray Absorption Near Edge Structure Energy Region for Quantitative Structural Use
Physica Scripta, 2005Co-Authors: M. Benfatto, S. Della Longa, Paola D'angeloAbstract:XRay Absorption Near Edge Structure (XANES) spectroscopy is a powerful method to study the local structure around an Absorbing site of various types of matter. Analysis of the spectrum can give both electronic and structural information on the site around the Absorbing Atom. In this paper we present some recent theoretical advances that allow a complete quantitative analysis of the XANES energy region in term of structural parameters. Some examples will be presented in details with a complete discussion of the strengths and limitations of the method when applied to real systems.
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the mxan procedure a new method for analysing the xanes spectra of metalloproteins to obtain structural quantitative information
Journal of Synchrotron Radiation, 2003Co-Authors: M. Benfatto, S. Della Longa, C. R. NatoliAbstract:The first quantitative analyses are reported of the Fe K-edge polarized X-ray absorption near-edge structure (XANES) of a single crystal of the iron protein carbonmonoxy-myoglobin (MbCO) and of its cryogenic photoproduct Mb*CO. The CO-Fe-heme local structure has been determined using a novel fitting procedure, named MXAN, which is able to fit the XANES part (from the edge to about 200 eV) of experimental X-ray absorption data. This method is based on the comparison between the experimental spectrum and several theoretical spectra that are generated by changing the relevant geometrical parameters of the site around the Absorbing Atom. The theoretical spectra are derived in the framework of the full multiple-scattering approach. The MXAN procedure is able to recover information about the symmetry and Atomic distances, and the solution is found to be independent of the starting conditions. The extracted local structure of Mb*CO includes an Fe-CO distance of 3.08 (7) A, with a tilting angle between the heme normal and the Fe-C vector of 37 (7) degrees and a bending angle between the Fe-C vector and the C-O bond of 31 (5) degrees
Haifeng Zhao - One of the best experts on this subject based on the ideXlab platform.
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alternative difference analysis scheme combining r space exafs fit with global optimization xanes fit for x ray transient absorption spectroscopy
Journal of Synchrotron Radiation, 2017Co-Authors: Fei Zhan, Haifeng ZhaoAbstract:: Time-resolved X-ray absorption spectroscopy (TR-XAS), based on the laser-pump/X-ray-probe method, is powerful in capturing the change of the geometrical and electronic structure of the Absorbing Atom upon excitation. TR-XAS data analysis is generally performed on the laser-on minus laser-off difference spectrum. Here, a new analysis scheme is presented for the TR-XAS difference fitting in both the extended X-ray absorption fine-structure (EXAFS) and the X-ray absorption near-edge structure (XANES) regions. R-space EXAFS difference fitting could quickly provide the main quantitative structure change of the first shell. The XANES fitting part introduces a global non-derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR-XAS difference analysis of Fe(phen)3 spin crossover complex and yielded reliable distance change and excitation population.
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alternative difference analysis scheme combining r space exafs fit with global optimization xanes fit for x ray transient absorption spectroscopy
arXiv: Chemical Physics, 2016Co-Authors: Fei Zhan, Haifeng ZhaoAbstract:Time-resolved X-ray absorption spectroscopy (TR-XAS), based on laser-pump/X-ray probe method, is powerful in capturing the change of geometrical and electronic structure of the Absorbing Atom upon excitation. TR-XAS data analysis is generally performed on the laser-on minus laser-off difference spectrum. Here we present a new analysis scheme for the TR-XAS difference fitting in both the Extended X-Ray Absorption Fine Structure (EXAFS) and the X-ray Absorption Near Edge Structure (XANES) region. R-space EXAFS difference fitting could quickly give main quantitative structure change of the first shell and provide reliable constraint on the range of the variables in XANES fit. XANES fitting part introduces global non-derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR-XAS difference analysis of Fe(phen)3 spin crossover complex and yielded the reliable distance change and the excitation population.
K. Baberschke - One of the best experts on this subject based on the ideXlab platform.
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Probing local magnetic disorder by investigating spin dependent photoelectron scattering
Journal of Applied Physics, 1998Co-Authors: Heiko Wende, John W. Freeland, V. Chakarian, Yves Idzerda, L. Lemke, K. BaberschkeAbstract:We present here systematic temperature dependent magnetic extended x-ray absorption spectroscopy (MEXAFS) measurements. This fine structure can be explained by the spin dependent scattering of the photoelectron and yields information about the local magnetic surroundings of the Absorbing Atom. We find a strong temperature dependence at temperatures far below the Curie temperature. This shows that the intensity of the MEXAFS signal is not only determined by the value of the magnetic moment but also by the relative vibrations of the backscattering Atom to the Absorbing Atom.
