Showing papers on "XANES published in 2003"
TL;DR: In this paper, the authors showed that only 0.6 Li per formula unit are actually inserted in the structure of small particles, while the remaining lithium (0.4) is engaged in irreversible reduction of surface groups, or capacitive behavior.
Abstract: The electrochemical reduction of hematite with various particle sizes by metallic lithium has been studied by means of X-ray diffraction (XRD) Mossbauer and extended X-ray absorption fine structure (EXAFS) spectroscopy. Previous in situ XRD analysis coupled with electrochemical data showed that lithium can be inserted in the nanosized sample up to 1 Li per Fe2O3 whereas bulk material undergoes an irreversible Li-driven transformation from an hexagonal anionic packing to a close cubic packed framework as soon as 0.03 Li is inserted in the corundum structure. The present data show that only 0.6 Li per formula unit are actually inserted in the structure of small particles. The remaining lithium (0.4) is engaged in irreversible reduction of surface groups, or capacitive behavior. Beyond the solid solution domains, both samples are multiphase, and consist of Li2Fe2O3, Fe0 clusters (10-15 A) and inserted -Fe2O3, which proportions are used to calculate the mean iron oxidation state in the electrode as the reaction proceeds. From these data, we found that electrolyte decomposition can occur at very different steps of the reduction depending on the texture of the active materials. In addition, during the reduction process, we evidenced a reaction of disproportionation (3Fe2+2Fe3+ + Fe0), an intense electrochemical grinding of the hematite particles and the formation of extremely fine metallic surface clusters. For the first time, the EXAFS/X-ray absorption near-edge structure signature of the divalent intermediate Li2Fe2O3 phase is obtained.
332 citations
TL;DR: X-ray absorption near-edge structure spectroscopy directly identified certain forms of soil P, while chemical fractionation provided indirect supporting data and gave insights on additional forms of P such as organic pools that were not accounted for by the XANES analyses.
Abstract: Knowledge of phosphorus (P) species in P-rich soils is useful for assessing P mobility and potential transfer to ground water and surface waters. Soil P was studied using synchrotron X-ray absorption near-edge structure (XANES) spectroscopy (a nondestructive chemical-speciation technique) and sequential chemical fractionation. The objective was to determine the chemical speciation of P in long-term-fertilized, P-rich soils differing in pH, clay, and organic matter contents. Samples of three slightly acidic (pH 5.5-6.2) and two slightly alkaline (pH 7.4-7.6) soils were collected from A or B horizons in two distinct agrosystems in the province of Quebec, Canada. The soils contained between 800 and 2100 mg total P kg(-1). Distinct XANES features for Ca-phosphate mineral standards and for standards of adsorbed phosphate made it possible to differentiate these forms of P in the soil samples. The XANES results indicated that phosphate adsorbed on Fe- or Al-oxide minerals was present in all soils, with a higher proportion in acidic than in slightly alkaline samples. Calcium phosphate also occurred in all soils, regardless of pH. In agreement with chemical fractionation results, XANES data showed that Ca-phosphates were the dominant P forms in one acidic (pH 5.5) and in the two slightly alkaline (pH 7.4-7.6) soil samples. X-ray absorption near-edge structure spectroscopy directly identified certain forms of soil P, while chemical fractionation provided indirect supporting data and gave insights on additional forms of P such as organic pools that were not accounted for by the XANES analyses.
309 citations
TL;DR: The potential of modern XANES theory is demonstrated by presenting first simulations of the dichroism in the XANes spectra of the PSII manganese complex.
Abstract: X-ray absorption spectroscopy (XAS) has become a prominent tool for the element-specific analysis of transition metals at the catalytic center of metalloenzymes. In the present study the information content of X-ray spectra with respect to the nuclear geometry and, in particular, to the electronic structure of the protein-bound metal ions is explored using the manganese complex of photosystem II (PSIII) as a model system. The EXAFS range carries direct information on the number and distances of ligands as well as on the chemical type of the ligand donor function. For first-sphere ligands and second-sphere metals (in multinuclear complexes), the determination of precise distances is mostly straightforward, whereas the determination of coordination numbers clearly requires more effort. The EXAFS section starts with an exemplifying discussion of a PSII spectrum data set with focus on the coordination number problem. Subsequently, the method of linear dichroism EXAFS spectroscopy is introduced and it is shown how the EXAFS data leads to an atomic resolution model for the tetra-manganese complex of PSII. In the XANES section the following aspects are considered: (1) Alternative approaches are evaluated for determination of the metal-oxidation state by comparison with a series of model compounds. (2) The interpretation of XANES spectra in terms of molecular orbitals (MOs) is approached by comparative multiple-scattering calculations and MO calculations. (3) The underlying reasons for the oxidation-state dependence of the XANES spectra are explored. Furthermore, the potential of modern XANES theory is demonstrated by presenting first simulations of the dichroism in the XANES spectra of the PSII manganese complex.
277 citations
TL;DR: In this article, a series of anorthite-diopside eutectic glasses containing 1 wt% 57 3+ /ΣFe ratios from X-ray absorption near edge structure spectra were investigated.
Abstract: Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57 3+ /ΣFe ratios from XANES spec- tra were investigated. The energy of the 1s ∅ 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s ∅ 4s and crest (1s ∅ 4p) transi- tions. The Fe 3+ /ΣFe ratios determined from linear combinations of end-member spectra (Fe 3+ /ΣFe ~0 and ~1) were found to deviate significantly from the Mossbauer values. This may indicate the sus- ceptibility of this method either to errors arising from the treatment of the background or to changes in Fe 2+ or Fe 3+ coordination with the Fe 3+ /ΣFe ratio. The general applicability of any XANES calibra- tion for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe 3+ /ΣFe values may be obtained from XANES spectra, with an accuracy com- parable to Mossbauer spectroscopy, by reference to empirical calibration curves derived from com- positionally similar standards.
273 citations
TL;DR: In this paper, nitrogen-doped ultrananocrystalline diamond (UNCD), Raman scattering, near edge xray absorption fine structure (NEXAFS), soft x-ray fluorescence (SXF), and secondary ion mass spectroscopy (SIMS) measurements were performed.
Abstract: The transport properties of diamond thin films are well known to be sensitive to the sp2/sp3-bonded carbon ratio, the presence of the grain boundaries and other defects, and to the presence of various impurities. In order to clarify the roles these factors play in the conduction mechanisms of nitrogen-doped ultrananocrystalline diamond (UNCD), Raman scattering, near edge x-ray absorption fine structure (NEXAFS), soft x-ray fluorescence (SXF), and secondary ion mass spectroscopy (SIMS) measurements were performed. Transmission electron microscopy analysis of nitrogen doped UNCD has previously indicated that the films are composed of crystalline diamond nano-grains with boundaries of amorphous carbon, and NEXAFS measurements reveal that the global amount of sp2-bonded carbon in these films increases slightly with nitrogen doping. The nitrogen content is quantified with high-resolution SIMS analysis, while NEXAFS and SXF indicates that the nitrogen exists primarily in tetrahedrally coordinated sites. These m...
243 citations
Abstract: We have investigated the local electronic and atomic structure of the LiMn0.5Ni0.5O2 electrode during the first charge and discharge process using in situ X-ray absorption spectroscopy (XAS) of the Mn and Ni K-edges. The Ni K-edge structure in the XANES spectrum shifts to higher energy during charge and shifts back reversibly during discharge in the higher voltage region of ∼4 V, whereas the Mn K-edge structure does not appear to exhibit a rigid edge shift. Further Li-ion intercalation during extended discharge in the 1-V plateau leads to the reduction of Mn4+ ions suggesting that the charge compensation in this region is achieved via the reduction of Mn4+ ions to Mn2+. Mn K-edge EXAFS results indicate that a small amount of Li is found in the Ni2+/Mn4+ layers. These Li ions in the transition metal layers are primarily present in the second coordination shell of Mn and not around Ni. Ni K-edge EXAFS fitting results suggest that the oxidation process upon Li deintercalation takes place in two steps: Ni2+ ...
214 citations
TL;DR: In this article, the effects of concentration on the first-shell structure of Ca2+ in aqueous solution were investigated using X-ray absorption fine structure (XAFS) spectroscopy.
Abstract: X-ray absorption fine structure (XAFS) spectroscopy was used to probe the effects of concentration on the first-shell structure of Ca2+ in aqueous solution. Measurements were carried out under ambient conditions using a bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne. The Ca K-edge EXAFS spectrum for 6 m CaCl2 yielded no evidence for the formation of significant numbers of Ca2+−Cl- contact ion pairs even at such high concentration, a result confirmed by comparison with the data for a dilute (0.2 m) reference solution of the perchlorate. A mean coordination number of 7.2 ± 1.2 water molecules and an average Ca−O distance of 2.437 ± 0.010 A were determined for 6 m CaCl2, and these parameters are also consistent with earlier EXAFS measurements on dilute Ca2+ solutions. Comparison of the pre-edge and near-edge (XANES) spectrum against those for various references, including the crystalline hydrates, provided further confirmation of the lack of change in the Ca2+ first-shell structur...
201 citations
TL;DR: In this paper, the structural and electronic properties of CeO2 and Ce1-xZrxO2 nanoparticles were investigated using time-resolved X-ray diffraction, Xray absorption near-edge spectroscopy (XANES) and X-Ray absorption nearedge spectrograms.
Abstract: In this article the structural and electronic properties of CeO2 and Ce1-xZrxO2 nanoparticles are investigated using time-resolved X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES)...
192 citations
TL;DR: In this paper, the photocatalytic reactivity of transition metal oxides and ions in their efficiency and selectivity were found to depend strongly on their local structures, which are controlled by the unique and restricted framework structures of zeolites.
Abstract: Transition metal oxides (Ti, V, Mo, Cr) incorporated within the framework of zeolites as well as transition metal ions (Cu+, Ag+, Pr3+) exchanged within the zeolite cavities were found to exhibit high and unique photocatalytic activities for various reactions such as the decomposition of NOx (NO, N2O) into N2 and O2 or the reduction of CO2 with H2O to produce CH4 and CH3OH. Various in situ spectroscopic investigations of these catalytic systems using photoluminescence, X-ray absorption fine structure (XAFS) (X-ray absorption near edge structure (XANES) and Fourier transform of EXAFS (FT-EXAFS)), electron spin resonance (ESR), FT-IR, etc. revealed that the photo-excited states of these transition metal oxides or ions play a vital role in these photocatalytic reactions. The photocatalytic reactivity of these oxides and ions in their efficiency and selectivity were found to depend strongly on their local structures, which are controlled by the unique and restricted framework structures of zeolites.
173 citations
TL;DR: 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 and of its cryogenic photoproduct Mb*CO.
Abstract: 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
169 citations
TL;DR: Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions.
Abstract: Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions. A unique, previously not observed, pre-edge feature is detected in zeolites H-Mordenite and steamed and unsteamed H-Beta at temperatures above 675 K. Spectra simulations using the full multiple scattering code Feff8 identify the unique pre-edge feature as three-coordinate aluminum. The amount of three-fold coordinated aluminum is a function of temperature and pretreatment of a zeolite: a steamed zeolite Beta contains more three-coordinate aluminum than an unsteamed sample. No clear differences between zeolites H-Mordenite and H-Beta were observed. Octahedrally coordinated aluminum forms in zeolites H-Mordenite and H-Beta at room temperature in a stream of wet helium. This octahedrally coordinated aluminum is unstable at temperatures higher than 395 K, where it quantitatively reverts to the tetrahedral coordination.
TL;DR: The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions as mentioned in this paper.
Abstract: The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl2/γ-Al2O3 systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu+N2, Cu+(CO)3, Cu+(NH3)(CO) and Cu+(NO)2) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.
TL;DR: In this article, X-ray absorption fine structure (XAFS) was used to analyze the solid electrolyte interface (SEI) layer on the graphite anode and the LiCoO2 cathode in a lithium-ion battery.
TL;DR: Measurements on mass loss and the decrease in the C=O bond density as measured by oxygen-edge XANES (NEXAFS) spectroscopy in thin films of poly(methylmethacrylate) (PMMA), studied in a vacuum, are reported.
Abstract: Radiation damage sets a fundamental limit for studies with ionizing radiation; cryo-methods are known to ease these limits. Here, measurements on mass loss and the decrease in the C=O bond density as measured by oxygen-edge XANES (NEXAFS) spectroscopy in thin films of poly(methylmethacrylate) (PMMA), studied in a vacuum, are reported. While cryo-methods allow more than 95% of the mass to remain at doses up to 10^7 Gy, there is little difference in C=O bond density versus dose between 298 K and 113 K sample temperatures. At both temperatures the critical dose for bond breaking is ∼15 × 106 Gy.
TL;DR: In this paper, a review of data obtained by X-ray absorption fine structure (XAFS) spectroscopy on the speciation of mercury captured on a variety of sorbent materials from simulated combustion flue gases at low temperatures (<200 °C).
TL;DR: The use of X-ray absorption near edge spectroscopy (XANES) is described to provide information about the relative proportions of platinum(II) and platinum(IV) complexes by analyzing the XANES edge height.
Abstract: Here we describe the use of X-ray absorption near edge spectroscopy (XANES) to provide information about the relative proportions of platinum(II) and platinum(IV) complexes by analyzing the XANES edge height. The intracellular reduction of platinum(IV) complexes in cancer cells has been observed directly, and the proportion of reduction after 2 h was found to correlate with the reduction potentials of the complexes.
TL;DR: In this paper, the analysis of the K edge of 3D or 4d transition metals when nanometer-scale metallic clusters are considered is considered. But it is not sufficient to consider only the electronic state of the metal of interest to perform a linear combination analysis.
Abstract: We consider the analysis of the K edge of 3d or 4d transition metals when nanometer-scale metallic clusters are considered. From a practical point of view, numerical simulation of the XANES part of the K absorption spectrum of most elements of the periodic table can be performed through full multiple scattering calculations. Then, on the basis of a linear combination of the XANES spectra of reference compounds, the presence of the different phases present inside the materials can be quantified. Here, we show that for nanometer scale metallic clusters, it is not sufficient to consider only the electronic state of the metal of interest to perform a linear combination analysis. In the case of these peculiar materials, special attention has to be paid to different structural parameters, for example, the size and morphology of the cluster, the interatomic distance (taking into account contraction/dilatation processes), and the presence of heterometallic bonds (in the case of bimetallic clusters). Moreover, this approach is not specific to the metallic state. As a conclusion, the quantitative measurement of the structural parameters coming from EXAFS analysis constitutes an invaluable starting point for the FEFF-PCA simulation. The fact that major results coming from the emergence of dynamical studies, namely, Quick-EXAFS or energy dispersive EXAFS, are now obtained will lead to significant breakthroughs in the understanding of the genesis/reactivity of nanometer-scale entities.
TL;DR: In this paper, the structural and electronic properties of mixed-metal oxides were studied using first-principles density functional (DF) calculations and XRD results and DF calculations showed that doping with calcium induces relatively minor variations (<0.05 A) in the cell dimensions of ceria.
Abstract: Synchrotron-based time-resolved x-ray diffraction (TR-XRD), x-ray absorption near edge spectroscopy (XANES), Raman spectroscopy (RS), and first-principles density functional (DF) calculations were used to study the structural and electronic properties of Ce–Ca mixed-metal oxides. The XRD results and DF calculations show that doping with calcium induces relatively minor variations (<0.05 A) in the cell dimensions of ceria. However, the presence of Ca leads to slightly distorted tetragonal structures, a substantial strain in the lattice of the oxide and a tendency to form O vacancies in an ideal Ce1−xCaxO2 solid solution. The two latter effects can be a consequence of the large number of oxygen neighbors that Ca is forced to have in Ce1−xCaxO2 and differences in the electronic charges of calcium and cerium cations. The Ce1−xCaxO2−x systems are not fully ionic. Cation charges derived from the DF calculations indicate that these systems obey the Barr model for charge redistribution in mixed-metal oxides. The Ca atoms in Ce1−xCaxO2−x are more electropositive than the cations in CaO, while the Ce cations of Ce1−xCaxO2−x are less electropositive than those of CeO2. These trends are consistent with XANES measurements at the Ca K- and Ce LIII-edges. The cation charge redistributions should be taken into consideration when explaining or predicting the chemical and catalytic properties of Ce1−xCaxO2−x. Ca induces structural and electronic perturbations on ceria quite different from those found after doping with Zr. The behavior of Ce1−xCaxO2−x illustrates the drastic effects that doping with an electropositive element can have on the properties of ceria.
TL;DR: In this paper, Sulfur K-edge X-ray absorption spectroscopy (XANES) was used to identify S oxidation states and assess the impact of land use changes on the amount, form, and distribution of organic S in particle-size separates and their humic substance extracts.
Abstract: Sulfur K-edge X-ray absorption spectroscopy (XANES) was used to identify S oxidation states and assess the impact of land use changes on the amount, form, and distribution of organic S in particle-size separates and their humic substance extracts. Soil samples (0-10 cm) were collected from natural forest, tea plantations, and cultivated fields at Wushwush and from natural forest, Cupressus plantations and cultivated fields at Munesa sites in Ethiopia. Sulfur XANES spectra measured directly from the size separates could not be quantitatively analyzed due to high background noise. However, qualitative comparison of spectra from size separates and their humic extracts were very similar and thus provides a characteristic fingerprint of S in mineral soils. X-ray absorption near-edge spectroscopy showed the presence of most reduced (sulfides, disulfides, thiols, and thiophenes), intermediate (sulfoxides and sulfonates) and highly oxidized S (ester-SO 4 -S) forms. Sulfur in intermediate oxidation states was dominant (39-50%; where 66-96% of it being sulfonate S) in humic extracts from clay, while highly oxidized S dominated (40-56%) the silt spectra. Concentrations of C-bonded and ester-SO 4 -S extracted by the HI fractionation did not correlate with those from XANES (ester-SO 4 -S revealed by XANES vs. HI-fractionation, r = 0.23; P plantations > cultivated fields. In contrast, highly oxidized S increased in the order: natural forests < plantations < cultivated fields at both sites. Our results indicated that C-bonded S (most reduced and intermediate S) may represent the more labile forms of organic S compounds compared with ester-SO 4 -S. Therefore, S K-edge XANES has a significant potential to evaluate the influence of anthropogenic changes on the nature and distribution of S and to follow its dynamics in terrestrial ecosystems.
TL;DR: LiNi0.65Co0.25Mn0.1O2 sintered at 850 °C has shown the best characteristics in terms of initial capacity (198 mA h g−1) and capacity retention (92%) as mentioned in this paper.
Abstract: LiNi1−x−yCoxMnyO2 cathode materials were successfully synthesized by a sol–gel method. X-Ray diffraction patterns show that these materials have a typical layered structure with Rm space group. The effect of Mn content and sintering temperature on the surface morphology has been carefully examined by scanning electron microscopy. Among the substituted materials synthesized, LiNi0.65Co0.25Mn0.1O2 sintered at 850 °C has shown the best characteristics in terms of initial capacity (198 mA h g−1) and capacity retention (92%). However, excessive Mn substitution (y > 0.3) leads to a significant decrease in both initial capacity and capacity retention. Preliminary X-ray absorption near edge structure (XANES) results for LiNi1−x−yCoxMnyO2 cathode materials revealed that the oxidation state of Ni is decreased when the amount of Mn substituted for Ni is increased.
TL;DR: In this paper, X-ray photoelectron spectroscopy (XPS), Raman spectro-computed tomography (RSA), and Xray absorption near edge structure (XANES) was used to characterize the surface TaO5/TaO6 molecular structures of supported tantalum oxide catalysts.
Abstract: Supported tantalum oxide catalysts on Al2O3, TiO2, ZrO2, and SiO2 supports were prepared by the incipient wetness impregnation method. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray absorption near edge structure (XANES) under hydrated and dehydrated conditions. The Al2O3-, TiO2-, and ZrO2-supported tantalum oxide catalysts possess similar surface TaOx molecular structures, consisting primarily of polymerized surface TaO5/TaO6 species at high surface coverage. The surface Ta atom density at monolayer coverage are also similar and were found to be 4.5, 6.6, and 6.3 Ta atoms/nm2 on the Al2O3, TiO2, and ZrO2 supports, respectively. The SiO2-supported tantalum oxide catalyst is very different and consists of isolated TaO4 species with a much lower maximum surface Ta density, 0.7 Ta atoms/nm2, due to the lower concentration and reactivity of the silica hydroxyls. The catalytic properties of the surface TaOx species were chemically probed with the metha...
TL;DR: In this article, the X-ray absorption near-edge spectroscopy (XANES) analysis of sorption complexes has the advantages of high sensitivity and relative ease and speed of data collection (because of the short k-space range) for characterization of environmentally significant surface complexes and precipitates at very low surface coverages.
TL;DR: In this paper, simultaneous calculations of both K-edge x-ray-absorption near-edge structure (XANES) and ground-state electronic structure of $3d$ transition-metal oxides are presented.
Abstract: Simultaneous calculations of both K-edge x-ray-absorption near-edge structure (XANES) and ground-state electronic structure of $3d$ transition-metal oxides are presented. The calculations are based on a self-consistent one-electron real-space Green's-function approach, with many-body effects incorporated in terms of final-state potentials and a complex energy-dependent self-energy. The results are found to be in semiquantitative agreement with experiment at the metal K edges, except at the edge itself where a leading edge peak is found to be systematically low in intensity. A scattering theoretic interpretation is presented, which correlates the structure in the XANES with projected electronic density of states. This interpretation illustrates the crossover from a molecular orbital to a continuum resonance description of excited states. The importance of the core-hole potential in these calculations is also discussed.
TL;DR: In this article, high resolution measurements of near edge X-ray absorption fine structure spectra (NEXAFS) at the carbon and oxygen K edges of the series of related organic molecules are reported.
Abstract: We report high resolution measurements of near edge X-ray absorption fine structure spectra (NEXAFS) at the carbon and oxygen K edges of the series of related organic molecules acetaldehyde, acetone, formic acid, methanol, dimethyl ether, and paraldehyde. The spectra are compared with previous measurements of these gases and formaldehyde and with theoretical calculations. Much more fine structure is resolved than previously, particularly at the carbon edge. The results are in good overall agreement with the theoretical predictions of Plashkevych et al. (Chem. Phys. 2000, 260, 11) that the methyl group spectrum is relatively independent of the functional groups to which it is bonded, as are the carbon and oxygen carbonyl group spectra. On the other hand, oxygen atoms in hydroxyl and ether groups are strongly influenced by neighboring atoms. All of the carbon edge spectra investigated show fine structure, and only in the case of acetone do the density of states and number of vibrational degrees of freedom b...
TL;DR: In this paper, the authors present a detailed analysis of molecular actinide cations using both XANES and EXAFS tools, showing that the LIII edge, located in the hard X-ray region, provides a useful fingerprint of the cation polyhedron.
Abstract: Early actinide elements (from U to Am) have the ability to form linear trans-dioxo complexes with formal valence of the actinide being equal to (V) or (VI). For instance, the ubiquity of the uranyl cation in uranium aqueous chemistry is the basis for its very important industrial and environmental concerns. As a result, the physical chemistry of the actinyl moieties has been the subject of constantly growing investigations. Among all the spectroscopic probes, X-ray absorption spectroscopy is a particularly useful element and energy-selective technique. This article reviews the investigation of molecular actinyl complexes using both XANES and EXAFS tools. The absorption edge features have long been used to characterize the frontier orbitals of the absorbing atom. In the case of actinide cations, the LIII edge, located in the hard X-ray region, provides a useful fingerprint of the cation polyhedron. Tentatively, simple molecular orbital considerations together with full multiple scattering simulation codes have provided significant interpretations of the edge features. Various examples involving a distortion of the actinyl coordination sphere, starting from the aqueous species are given. Complementary structural data can be obtained in the EXAFS region of the absorption spectrum. In the literature, such molecular systems have been well documented from uranyl aqueous chemistry to neptunyl or plutonyl coordination complexes with oxygen donor ligands. Furthermore, complexation mechanisms upon absorption onto mineral surfaces have been increasingly investigated over the past few years. Overall, contribution of the XAS technique to a better understanding of the actinide bonding is demonstrated from various examples of the literature and the authors’ data. Importance of the simulation codes in order to better describe the absorption features is also strongly underlined. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
TL;DR: In this article, the interaction of sulfur-based oil additives on steel was examined using X-ray absorption near edge structure (XANES) spectroscopy, and it was found that the chemical nature of these films was strongly dependent on the operating environment for the additives.
Abstract: This study examines the interaction of sulfur-based oil additives on steel. Sulfurized isobutylene, dialkyldithiocarbamate and sulfurized esters were the additives investigated in this report. For the first time, X-ray absorption near edge structure (XANES) spectroscopy has provided detailed insight into the chemistry of both the thermo-oxidative and tribochemical films generated from these additives. It was found that the chemical nature of these films was strongly dependent on the operating environment for the additives. The XANES revealed that thermally, all three S additives reacted very similarly with steel to form a film mainly comprised of iron sulfate at temperatures as low as 100 °C. The ample supply of diffused oxygen from the base oil along with oxide naturally present on the substrate allowed for the complete oxidation of the S from the decomposed additive to iron sulfate. Tribochemical films were comprised of different forms of sulfur than observed for the thermo-oxidative films. The moderate AW conditions yielded a mixture of both oxidized and reduced forms of sulfur, with pyrite, FeS2, being the major constituent. Rubbing between the steel pin and the substrate partially depleted the oxide layer present, allowing the additive to interact intimately with the fresh substrate, yielding FeS2. Under extreme-pressure conditions, complete removal of the oxide layer occurred with a drastic increase in the interfacial temperatures between the pin and v-block, allowing for the complete thermal decomposition of FeS2 to FeS to occur. AFM imaging of the AW films revealed the presence of tiny smooth domains randomly oriented, which were completely different from the pad-like structures observed for AW films generated from a typical ashless thiophosphate additive. The inability of the sulfur-based additives to form large pad-like structures, which can ultimately support the load, resulted in poor AW protection to the metal.
TL;DR: Hard x-ray absorption spectroscopy is combined with scanning microtomography to reconstruct full near-edge spectra of an elemental species at each location on an arbitrary virtual section through a sample as mentioned in this paper.
Abstract: Hard x-ray absorption spectroscopy is combined with scanning microtomography to reconstruct full near-edge spectra of an elemental species at each location on an arbitrary virtual section through a sample. These spectra reveal the local concentrations of different chemical compounds of the absorbing element inside the sample and give insight into the oxidation state, the local atomic structure, and the local projected free density of states. The method is implemented by combining a quick scanning monochromator and data acquisition system with a scanning microprobe setup based on refractive x-ray lenses.
TL;DR: In this paper, X-ray absorption near edge structure (XANES) has been used to characterize the chemistry of tribochemical wear pads generated from a paraffinic base oil with a zinc-dialkyl-dithiophosphate additive on steel surfaces and high-velocity oxygen fuel (HVOF) thermal spray coatings.
Abstract: X-ray Absorption Near Edge Structure (XANES) has been used to characterize the chemistry of tribochemical wear pads generated from a paraffinic base oil with a zinc-dialkyl-dithiophosphate additive on steel surfaces and high-velocity oxygen fuel (HVOF) thermal spray coatings. The phosphorus K- and L- edge XANES spectra show that the tribofilms formed on steel and the HVOF coatings have the same chemical nature. Also, mechanical properties of these tribofilms were examined by nanoindentation techniques using both Hysitron and Interfacial Force Microscopy (IFM) instruments. The elastic moduli extracted from indentation force-displacement (f-d) curves have demonstrated that tribofilms on steel and HVOF coatings have similar surface topography and mechanical properties.
TL;DR: It is shown that dichroism can be observed in electron energy loss spectrometry (EELS) in the transmission electron microscope (TEM) and a new experimental setup for dichroic measurements in the TEM is proposed.
TL;DR: High-spatial-resolution XANES maps for the different S species across adjacent calcite prisms confirm their distinctive distributions in the molluscan shells and illustrate the presence of narrow, submicron transverse growth features.
Abstract: The microstructure and composition, including chemical speciation of sulphur (S), of two mollusc shells were investigated using a combination of scanning electron microscopy, X-ray absorption near-edge structure spectroscopy (XANES) and electron probe microanalysis (EPMA). The shell of Pinna is composed of monocrystalline, and Pinctada, of polycrystalline, calcite prisms separated by organic-rich walls. Sulphur speciation information from XANES spectra using a scanning X-ray microscope showed that the protein S content of the interprismatic walls is higher than the SO4 content, whereas the reverse is true for the intraprismatic structures. High-spatial-resolution XANES maps for the different S species across adjacent calcite prisms confirm their distinctive distributions in the molluscan shells and illustrate the presence of narrow, submicron transverse growth features. On a larger scale, a series of wider growth zones, incorporating the submicron zones, are aligned parallel to each other and cross-cut many calcite prisms. EPMA element maps for magnesium (Mg) and S demonstrate that these growth increments are compositionally zoned, comprising alternating layers of high mineral (Mg-rich) and high organic (S-rich) components. Additionally, these maps confirm that the organic interprismatic walls have lower Mg and higher S than the intraprismatic structures.