XAS Measurements Research Articles

A High-Spin Iron(IV)–Oxo Complex Supported by a Trigonal Nonheme Pyrrolide Platform

We report the generation and characterization of a new high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform. Oxygen-atom transfer to [(tpa(Mes))Fe(II)](-) (tpa(Ar) = tris(5-arylpyrrol-2-ylmethyl)amine) in acetonitrile solution affords the Fe(III)-alkoxide product [(tpa(Mes2MesO))Fe(III)](-) resulting from intramolecular C-H oxidation with no observable ferryl intermediates. In contrast, treatment of the phenyl derivative [(tpa(Ph))Fe(II)](-) with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)-oxo complex [(tpa(Ph))Fe(IV)(O)](-) that has been characterized by a suite of techniques, including mass spectrometry as well as UV-vis, FTIR, Mössbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide signatures for the iron-oxo chromophore, and Mössbauer and XAS measurements establish the presence of an Fe(IV) center. Moreover, the Fe(IV)-oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity studies show that the high-spin ferryl tpa(Ph) complex is capable of mediating intermolecular C-H oxidation as well as oxygen-atom transfer chemistry.

Zn induced structural aggregation patterns of β-amyloid peptides by first-principle simulations and XAS measurements

We show in this paper that in the presence of Zn ions a peculiar structural aggregation pattern of β-amyloid peptides in which metal ions are sequentially coordinated to either three or four histidines of nearby peptides is favored. To stabilize this configuration a deprotonated imidazole ring from one of the histidines forms a bridge connecting two adjacent Zn ions. Though present in zeolite imidazolate frameworks, remarkably in biological compounds this peculiar Zn-imidazolate-Zn topology is only found in enzymes belonging to the Cu,Zn-superoxide dismutase family in the form of an imidazolate bridging Cu and Zn. The results we present are obtained by combining X-ray absorption spectroscopy experimental data with detailed first-principle molecular dynamics simulations.

Bimetallic PtM (M = Pd, Ir) nanoparticle decorated multi-walled carbon nanotube enzyme-free, mediator-less amperometric sensor for H2O2

A new highly catalytic and intensely sensitive amperometric sensor based on PtM (where M=Pd, Ir) bimetallic nanoparticles (NPs) for the rapid and accurate estimation of hydrogen peroxide (H2O2) by electrooxidation in physiological conditions is reported. PtPd and PtIr NPs-decorated multiwalled carbon nanotube nanocatalysts (PtM/MWCNTs) were prepared by a modified Watanabe method, and were characterized by XRD, TEM, ICP, and XAS. The sensors were constructed by immobilizing PtM/MWCNTs nanocatalysts in a Nafion film on a glassy carbon electrode. Both PtPd/MWCNTs and PtIr/MWCNTs assemblies catalyzed the electrochemical oxidation of H2O2. Cyclic voltammetry characterization measurements revealed that both the PtM (M=Pd, Ir)/MWCNTs/GCE possessed similar electrochemical surface areas (∼0.55cm2), and electron transfer rate constants (∼1.23×10−3cms−1); however, the PtPd sensor showed a better performance in H2O2 sensing than did the PtIr counterpart. Explanations were sought from XAS measurements to explain the reasons for differences in sensor activity. When applied to the electrochemical detection of H2O2, the PtPd/MWCNTs/GC electrode exhibited a low detection limit of 1.2μM with a wide linear range of 2.5–125μM (R2=0.9996). A low working potential (0V (SCE)), fast amperometric response (<5s), and high sensitivity (414.8μAmM−1cm−2) were achieved at the PtPd/MWCNTs/GC electrode. In addition, the PtPd/MWCNTs nanocatalyst sensor electrode also exhibited excellent reproducibility and stability. Along with these attractive features, the sensor electrode also displayed very high specificity to H2O2 with complete elimination of interference from UA, AA, AAP and glucose.

ChemInform Abstract: Introducing a Magnetic Guest to a Tetrel‐Free Clathrate: Synthesis, Structure, and Properties of EuxBa8‐xCu16P30 (0 ≤ x ≤ 1.5).

AbstractThe title compounds are prepared by solid state reactions of the elements (1125 K, 360 h) and characterized by powder and single crystal XRD, XAS, electrical resistivity, specific heat, and magnetic susceptibility measurements.

Local Structure and Charge Distribution in Mixed Uranium–Americium Oxides: Effects of Oxygen Potential and Am Content

Partitioning and transmutation (P&T) of minor actinides (MA) is currently studied to reduce the nuclear waste inventory. In this context, the fabrication of MA bearing materials is of great interest to achieve an effective recycling of these highly radioactive elements. To ensure the in-pile behavior, nuclear oxide fuels have to respect several criteria including preservation of the fluorite structure and defined oxygen to metal ratio (O/M). In the case of Am bearing materials, such as U(1-y)Am(y)O(2±x) (y = 0.10, 0.15, 0.20), the O/M determination is quite challenging using conventional methods (TGA, XRD) because of the particular thermodynamic properties of Am. Despite the lack of experimental data in the U-Am-O system, thermodynamical models are currently developed to effectively assess the O/M ratio. In this work, the O/M ratios were calculated for various oxygen potentials using the cation molar fraction determined by XAS measurements. These results are an important addition to the experimental data available for the U-Am-O system. Moreover, XRD and XAS indicated that the fabrication of fluorite U(1-y)Am(y)O(2±x) solid solution was achieved for all Am content and oxygen potentials investigated. On the basis of the molar fraction, a description of the solid solution was proposed depending on the considered sintering conditions. Finally, the occurrence of an unexpected charge compensation mechanism was pointed out.

Structure and properties of molybdenum oxide nitrides as model systems for selective oxidation catalysts

Molybdenum oxide nitride (denoted as Mo(O,N)3) was obtained by ammonolysis of α-MoO3 with gaseous ammonia. Electronic and geometric structure, reducibility, and conductivity of Mo(O,N)3 were investigated by XRD, XAS, UV-Vis spectroscopy, and impedance measurements. Catalytic performance in selective propene oxidation was determined by online mass spectrometry und gas chromatography. Upon incorporation of nitrogen, Mo(O,N)3 maintained the characteristic layer structure of α-MoO3. XRD analysis showed an increased structural disorder in the layers while nitrogen is removed from the lattice of Mo(O,N)3 at temperatures above ~600 K. Compared to regular α-MoO3, Mo(O,N)3 exhibited a higher electronic and ionic conductivity and an onset of reduction in propene at lower temperatures. Surprisingly, α-MoO3 and Mo(O,N)3 exhibited no detectable differences in onset temperatures of propene oxidation and catalytic selectivity or activity. Apparently, the increased reducibility, oxygen mobility, and conductivity of Mo(O,N)3 compared to α-MoO3 had no effect on the catalytic behavior of the two catalysts. The results presented confirm the suitability of molybdenum oxide nitrides as model systems for studying bulk contributions to selective oxidation.

Zinc modulates copper coordination mode in prion protein octa-repeat subdomains

In this work we present and analyse XAS measurements carried out on various portions of Prion-protein tetra-octa-repeat peptides in complexes with Cu(II) ions, both in the presence and in the absence of Zn(II). Because of the ability of the XAS technique to provide detailed local structural information, we are able to demonstrate that Zn acts by directly interacting with the peptide, in this way competing with Cu for binding with histidine. This finding suggests that metal binding competition can be important in the more general context of metal homeostasis.

Iron speciation in urban dust

An improved understanding of anthropogenic impacts on ocean fertility requires knowledge of anthropogenic dust mineralogy and associated Fe speciation as a critical step toward developing Fe solubility models constrained by mineralogical composition. This study explored the utility of micro-focused X-ray absorption spectroscopy ({mu}-XAS) in characterizing the speciation of Fe in urban dust samples. A micro-focused beam of 10 x 7 {mu}m made possible the measurement of the Fe K edge XAS spectra of individual dust particles in the PM5.6 size fraction collected in Newark, New Jersey, USA. Spectral analysis indicated the presence of mixtures of Fe-containing minerals within individual dust particles; we observed significant magnetite content along with other Fe(III)-(hydr)oxide minerals which could not be conclusively identified. Our data indicate that detailed quantitative determination of Fe speciation requires extended energy scans to constrain the types and relative abundance of Fe species present. We observe heterogeneity in Fe speciation at the dust particle level, which underscores the importance of analyzing a statistically adequate number of particles within each dust sample. Where possible, {mu}-XAS measurements should be complemented with additional characterization techniques such as {mu}-XRD and bulk XAS to obtain a comprehensive picture of the Fe speciation in dust materials. X-ray microprobesmore » should be used to complement bulk methods used to determine particle composition, methods that fail to record particle heterogeneity. Keywords - Urban dust; Iron; Speciation; Micro-focused X-ray absorption spectroscopy.« less

An in situ X-ray spectroscopic study of Mo6+ speciation in supercritical aqueous solutions

In situ XRF and Mo K-edge XAS measurements were made on the ID20-B beam line at the APS on MoO3 in 1 M H2O2 aqueous solution, at temperatures between 400 and 600 °C. The samples were analyzed using a modified Bassett-type hydrothermal diamond anvil cell. Our XRF measurements show that MoO3 is highly soluble in the supercritical H2O2 aqueous fluid. Analysis of XAS spectra shows that the Mo6+ ion exhibits consistent speciation in the H2O2 aqueous solution at temperatures ranging from 400 to 600 °C.

Local environment and valence state of iron in microinclusions in fibrous diamonds: X-ray absorption and Mössbauer data

Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Congolese placers were investigated using X-ray absorption and Mössbauer spectroscopies. It is shown that the diamonds could be divided into two main groups differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe 2+ and Fe 3+, most likely with Fe 2+ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: The presence of metallic Fe and Fe 3O 4 is inferred from XAS measurements. Spatially resolved XANES and Mössbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of f O 2 .

High‐pressure High‐temperature Synthesis, Magnetic Properties and X‐ray Absorption Spectroscopy of Phases RE3Sn7 and hp‐RESn3–x (RE = Tb, Ho, Er)

AbstractThe high‐temperature high‐pressure syntheses of binary phases hp‐RESn3–x (RE = Tb, Ho, Er) was reinvestigated showing good agreement with previously determined stability fields concerning pressure and temperature conditions. Phases hp‐RESn3–x adopt a cubic Cu3Au arrangement (a = 466.52(6) pm for hp‐TbSn3–x; a = 465.44(8) pm for hp‐HoSn3–x and a = 464.98(9) pm for hp‐ErSn3–x). Superstructure reflections observed in experiments with synchrotron radiation indicate an ordered arrangement for HoSn3–x prepared at 8.4(8) GPa and 1270 K (space group Pm$\bar3$m). hp‐HoSn3–x transforms exothermally at 460 K and ambient pressure into the low‐pressure modification HoSn3–x indicating that the ordered Cu3Au‐type arrangement is a metastable high‐pressure phase., A second set of phases is identified as compounds RE3Sn7 (RE = Ho, Er) being isotypic to the ambient pressure modifications Gd3Sn7 and Tb3Sn7 (space group Cmmm; lattice parameters a = 434.95(7) pm, b = 2615.6(6) pm, c = 442.09(7) pm for Ho3Sn7 and a = 434.29(5) pm, b = 2607.2(4) pm, c = 441.49(5) pm) for Er3Sn7). The crystal structure of compounds RE3Sn7 is interpreted as an intergrowth of AlB2‐, CaF2‐ and AuCu3‐like segments. Analysis of the chemical bonding in Er3Sn7 by means of the electron localizability approach reveals two‐center Sn–Sn bonding in the AlB2‐like segment and multi‐center bonding in the AuCu3‐ and CaF2‐like motifs., The electrical resistivity of ρ ≈︁ 31 μΩ cm at 300 K reveals that hp‐HoSn3–x is a metallic conductor. The phases hp‐RESn3–x and RE3Sn7 (RE = Ho, Er) show paramagnetic Curie–Weiss behavior of the rare earth metal ions Ho3+ and Er3+, respectively. Ho3Sn7 and Er3Sn7 order antiferromagnetically at 10.0 K and 8.2 K, respectively. The high‐pressure phases RESn3–x show no magnetic order above 2 K. XAS measurements on the compounds Tb3Sn7 and hp‐TbSn3–x indicate a 4f 8 configuration evidencing the oxidation state +3 for terbium.

Local environment and valence state of iron in microinclusions in fibrous diamonds: X-ray Absorption and Mössbauer data

Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Zairean placers were investigated using X-ray Absorption and Mössbauer spectroscopies. It is shown that the diamonds could be divided into two main groups, differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe^2+^ and Fe^3+^, most likely with Fe^2+^ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: the presence of metallic Fe and Fe~3~O~4~ is inferred from XAS measurements. Spatially resolved XANES and Mössbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of fO~2~.

Local environment and valence state of iron in microinclusions in fibrous diamonds: X-ray Absorption and Mössbauer data

AbstractIron valence state and local environment in a set of fibrous diamonds from Brazilian and Zairean placers were investigated using X-ray Absorption and Mössbauer spectroscopies. It is shown that the diamonds could be divided into two main groups, differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe^2+^ and Fe^3+^, most likely with Fe^2+^ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: the presence of metallic Fe and Fe~3~O~4~ is inferred from XAS measurements. Spatially resolved XANES and Mössbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of fO~2~.

In Situ GIXAFS and HERFD-XAS Studies of a Pt-modified Rh(111) Electrode

The oxygen reduction reaction at Pt electrodes has a high overpotential which reduces the efficiency of fuel cells. The origin of the overpotential has been seen in the formation of stable Pt-O species at high potentials. Aiming at an unambiguous characterization of such species, we use in situ grazing incidence x-ray absorption spectroscopy at the Pt L3 edge to study the geometric and electronic structure of ultrathin Pt layers on M(111) substrates. We present Pt L3-edge grazing incidence x-ray absorption fine structure (GIXAFS) and high energy resolution fluorescence detection (HERFD) XAS measurements of Pt/Rh(111) in 0.01 M HClO4 solution. In the HERFD-XAS experiment, we used a multi-crystal analyzer to suppress the core hole lifetime broadening. The HERFD spectra reveal additional spectral features of the near-edge region which can be important for an accurate structure model of the Pt/electrolyte interface.

Coupling CP-MD Simulations and X-ray Absorption Spectroscopy: Exploring the Structure of Oxaliplatin in Aqueous Solution

A combined experimental-theoretical approach applying X-ray absorption spectroscopy and ab initio molecular dynamics (CP-MD) simulations is used to get insight into the structural determination of oxaliplatin, a third-generation anticancer drug of the cisplatin family, in aqueous solution. Experimental Pt L(III)-edge EXAFS and XANES spectra of oxaliplatin in water are compared with theoretical XAS spectra. The latter are obtained as statistically averaged spectra computed for a set of selected snapshots extracted from the MD trajectory of ethyldiamineoxalatoplatinum(II) (EDO-Pt) in liquid water. This compound is a simplified structure of oxaliplatin, where the outer part of the cyclohexane ring contained in the cyclohexanediamine ligand of oxaliplatin has been removed. We show that EDO-Pt is an appropriate model to simulate the spectroscopical properties of oxaliplatin given that the cyclohexane ring does not generate particular features in neither the EXAFS nor the XANES spectra. The computation of average EXAFS spectra using structures from the MD simulation in which atoms are selected according to different cutoff radii around the Pt center allows the assignment of spectral features to particular structural motifs, both in k and R-spaces. The outer oxygen atoms of the oxalate ligand (R(Pt-O(II)) = 3.97 +/- 0.03 A) are responsible for a well-defined hump at around 6.5 A(-1) in the k(2)-weighted EXAFS spectrum. The conventional EXAFS analysis data procedure is reexamined by its application to the simulated average EXAFS spectra. The structural parameters resulting from the fit may then be compared with those obtained from the simulation, providing an estimation of the methodological error associated with the global fitting procedure. A thorough discussion on the synergy between the experimental and theoretical XAS approaches is presented, and evidence for the detection of a slight hydration structure around the Pt complex is shown, leading to the suggestion of a new challenge to experimental XAS measurements.

Lead extraction from waste funnel cathode-ray tubes glasses by reaction with silicon carbide and titanium nitride

As a possibility to clean waste CRT glass, treatment of lead-containing glass with a reducing agent, SiC or TiN, leads to a porous material containing metallic lead, Pb(0), located on the surface of the pore, and unreduced lead, Pb(II). The influences of reducing agent content, of the time, and at last of the temperature on lead reduction were analysed. Our investigations have pointed out significant differences as a function of the used reducing agent. CRT glass heat treated with SiC lead to less Pb(0), compared to TiN as shown by X-ray diffraction, and differential scanning calorimetry (DSC). It has been also evidenced that lead reduction occurs on randomized zones inside the sample leading to macroscopic lead beads inside glassy samples. XPS and XAS measurements were also carried out to investigate the local structure of lead and have evidenced a change of role of lead inside the glassy framework in function of the used conditions.

Atomic-layer-resolved bandgap structure of an ultrathin oxynitride-silicon film epitaxially grown on6H-SiC(0001)

Electronic structures of a silicon-oxynitride (SiON) layer ($\ensuremath{\sim}0.6\text{ }\text{nm}$ in thickness) epitaxially grown on $6H\text{-SiC}(0001)$ were investigated on atomic-layer scale using soft x-ray absorption spectroscopy and x-ray emission spectroscopy (XAS and XES) and first-principles calculations. The SiON layer has a hetero-double-layered structure: an interfacial silicon nitride layer and a silicon oxide overlayer. The element-specific XAS and XES measurements revealed layer-resolved energy-band profiles. Measured gap sizes are $6.3\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{eV}$ at the nitride layer and $8.3\ifmmode\pm\else\textpm\fi{}0.8\text{ }\text{eV}$ at the oxide layer. The nitride and oxide layers have almost the same energy of conduction-band minimum (CBM) being $\ensuremath{\sim}3\text{ }\text{eV}$ higher than CBM of the SiC substrate. The energy-band profiles of the SiON layer are qualitatively reproduced by the calculations. The calculations show that broadening of bandgap of the substrate occurs only at an interfacial SiC bilayer.

Cobalt Oxalate Nanoribbons as Negative-Electrode Material for Lithium-Ion Batteries

Orthorhombic cobalt oxalate dihydrate has been prepared in the form of nanoribbons by a reverse micelles method. The crystallographic structure of the resulting solid differs from the monoclinic massive product. A careful dehydration of the nanocrystals leads to anhydrous cobalt oxalate in which the nanoribbon-shaped particles are preserved and Co2+ ions are located in a centrosymmetric environment. CoC2O4 is used for the first time as high-capacity lithium storage materials with improved rate performance. The anhydrous solids react with lithium, leading to metallic cobalt and lithium oxalate, as shown by XAS and FTIR measurements. The new electrode material displays reversible capacities close to 900 mA·h·g−1 between 0 and 2 V versus lithium by a novel reaction mechanism which involves cobalt reduction−reoxidation.

SURFACE SEGREGATION STUDIES OF SOFC CATHODES: COMBINING SOFT X-RAYS AND ELECTROCHEMICAL IMPEDENCE SPECTROSCOPY

AbstractA system to grow heteroepitaxial thin-films of solid oxide fuel cell (SOFC) cathodes on single crystal substrates was developed. The cathode composition investigated was 20% strontium-doped lanthanum manganite (LSM) grown by pulsed laser deposition (PLD) on single crystal (111) yttria-stabilized zirconia (YSZ) substrates. By combining electrochemical impedance spectroscopy (EIS) with x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy XAS measurements, we conclude that electrically driven cation migration away from the two-phase gas-cathode interface results in improved electrochemical performance. Our results provide support to the premise that the removal of surface passivating phases containing Sr2+ and Mn2+, which readily form at elevated temperatures even in O2 atmospheric pressures, is responsible for the improved cathodic performance upon application of a bias.

Electronic and Local Structures of La1-x SrxCoO3-δ Studied by In-Situ Micro XAS Measurements

Electronic and local structures of the perovskite-type La0.6Sr0.4CoO3-d, which is a promising cathode material for solid oxide fuel cells, were investigated by means of in-situ micro x-ray absorption spectroscopy measurements under working conditions of solid oxide fuel cells, 1073 K of temperature and 10-105 Pa of oxygen partial pressure. It was found that variations of electronic and local structures of La0.6Sr0.4CoO3-d depending on variations of ambient oxygen partial pressure were clearly detected and could be associated with formation/disappearance of oxygen vacancies in the oxide. It was shown that in-situ x-ray absorption spectroscopy measurements could be a useful tool to examine electronic and local structures of high temperature materials in use.