Operando X-ray Absorption Fine Structure Spectroscopy and Its Applications to Catalysts

Synchrotron radiation micro-X-Ray Fluorescence (μ-XRF), X-ray photoelectron (XPS) and X-ray Absorption Fine Structure (XAFS) spectroscopies are applied for the study of paleontological findings. More specifically the costal plate of a gigantic terrestrial turtle Titanochelon bacharidisi and a fossilized coprolite of the cave spotted hyena Crocuta crocuta spelaea are studied. Ca L2,3-edge NEXAFS and Ca 2p XPS are applied for the identification and quantification of apatite and Ca containing minerals. XRF mapping and XAFS are employed for the study of the spatial distribution and speciation of the minerals related to the deposition environment.

The structural and electronic characterization of a material provides a basic understanding of its properties. Traditionally, diffraction techniques (XRD, neutron diffraction, LEED) are being used for most of the structural investigations and reliable structures can be determined for materials that exhibit a long-range structural order (like single crystals or polycrystalline material). X-ray Absorption Fine Structure (XAFS) spectroscopy is a powerful technique to characterize all forms of matter irrespective of their degree of crystallinity. EXAFS (Extended X-ray Absorption Fine Structure spectroscopy probes the local structure of a material. The local structure of highly disordered solids, amorphous materials and liquids can be unraveled with EXAFS. In addition, the chemical state and the electronic properties can be determined from the X-ray Absorption Near Edge Structure (XANES) which extends within 40 eV of the X-ray absorption edge. One of the major advantages of XAFS is its atomic selectivity which enables the investigation of the local structure of each different constituent of a sample. As shown by Fontaine (1993), the recent availability of high-brightness synchrotron radiation sources has resulted in a prosperous development XAFS spectroscopy.KeywordsPlatinum FoilEXAFS SpectrumEXAFS DataStereo ChemistryWhite Line IntensityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Local structures around transition-metal atoms in silicate glasses of the systems CaMgSi2O6 (Di)-CaMnSi2O6(CaMnPx), Di-CaCoSi2O6 (CaCoPx) and Di-CaNiSi2O6 (CaNiPx) were investigated with the XAFS (X-ray Absorption Fine Structure) spectroscopy, by making use of SR (Synchrotron Radiation). The obtained distances suggest that the preference to the octahedral coordinations is in the decreasing order Ni2+, Co2+ and Mn2+. This order can be explained in view of the difference between the crystal-field stabilization energies for octahedral and tetrahedral coordinations in the high-spin states. ELNES (Energy-Loss Near Edge Structure) spectra were also measured around the Mn L2,3 edges to determine the oxidation states of Mn ions in glasses of the system Di-CaMnPx. Chemical shifts of the L2,3 edge threshold-energy and the ratios of L3/L2 threshold peaks in all the examined glasses were similar to those in MnO but unlike those in MnO2. The observed tendency of chemical shifts in the XANES spectra of Mn K-edges was identical with that in ELNES. The shifts of Co K- and Ni K-edges in the XANES spectra of all the examined glasses were similar to those of Co-olivine and NiO, respectively. These facts suggest that Mn, Co and Ni ions in the glasses are in divalent states.

X-ray Photo Emission Electron Microscopy (X-PEEM) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy were applied to study the properties of amorphous hard carbon overcoats on disks and sliders, and the properties of the lubricant. The modification of lubricants after performing thermal desorption studies was measured by NEXAFS, and the results are compared to the thermal desorption data. The study of lubricant degradation in wear tracks is described. Sliders were investigated before and after wear test, and the modification of the slider coating as well as the transfer of lubricant to the slider was studied. The studies show that the lubricant is altered chemically during the wear. Fluorine is removed and carboxyl groups are formed.

Probing the structural role of Cr in stabilized tannery wastes with X-ray absorption fine structure spectroscopy

Thiophenic compounds are major constituents of fossil fuels and pose problems for fuel refinement. The quantification and speciation of these compounds is of great interest in different areas such as biology, fossil fuels studies, geology, and archaeology. Sulfur 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy has emerged as a qualitative and quantitative method for sulfur speciation. A firm understanding of the sulfur 1s NEXAFS spectra of organosulfur species is required for these analytical studies. To support this development, the sulfur 1s NEXAFS spectra of simple thiols and thioethers were previously examined, and are now extended to studies of thiophenic and aromatic thioether compounds, in the gas and condensed phases. High-resolution spectra have been further analyzed with the aid of Improved Virtual Orbital (IVO) and Δ(self-consistent field) ab initio calculations. Experimental sulfur 1s NEXAFS spectra show fine features predicted by calculation, and the combination of experiment and calculation has been used to improve the assignment of spectroscopic features important for the speciation and quantification of sulfur compounds. Systematic differences between gas and condensed phases are also explored; these differences suggest a significant role for conformational effects in the NEXAFS spectra of condensed species.

The synchrotron radiation (SR) X-ray absorption fine-structure spectroscopy (XAFS) technology was employed on Si Kedge absorption spectra for bulk 6H-SiC with different doping concentration. Their Fourier transform spectra were analyzed, which have shown a parabolic linear distribution of bond lengths. Through combined Raman and XAFS studies, the coincidental results could be obtained. In the Raman spectroscopy, the LO mode intensity becomes weaker and broader as the doping concentration increases. This indicates that the crystallinity is damaged by the heavy doping concentration. The Raman curves have been fitted by theoretical formulas and the accurate information of the intensity, peak position, and FWHM in each TO and LO modes have been obtained. By the EXAFS and the fitting program, the bond length of Si-C in 6H-SiC decreases as the doping concentration increases. It is believed to be caused by the nitrogen atoms substituting for carbon atoms in the SiC lattice. But further research work is needed to identify this. The little change in Si-Si bond length indicates the influence of doping is still under local structure, near the absorbed atoms.

The synchrotron radiation (SR) X-ray absorption fine-structure spectroscopy (XAFS) technology has been employed to obtained Zn K-edge absorption spectra for Cd1_1-xZn_xTe alloy with x = 0.03, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.00. Their Fourier transform spectra were analyzed, which have shown a bimodal distribution of bond lengths, suggesting distortion of the Te sub-lattice, so that the linear interpolation is true only in an approximate sense. Synthetic CdZnTe crystals can be used for the room temperature-based detection of gamma radiation. The radiation detection properties of CZT crystals vary widely. A common defect found in most high-quality CZT crystals is Te secondary phases, often located along grain boundaries. The secondary phases can be both large inclusions (>50 μm) and smaller precipitates (<50 μm). The Te secondary phases distributed throughout the crystal can cause changes to the detector leakage current, resulting in decreased radiation spectrometer performance. This set of Cd_1-xZn_xTe crystals were also measured by Raman scattering at room temperature. The two observed peaks at about 125 and 145 cm^-1 which can be assigned to Te A₁ and E phonon mode, respectively. The induced damage on the crystal surfaces by Raman laser has been discussed. It is suggested that in the case of highly Zn doping CdZnTe crystals, the ZnTe bond were broken by laser exposing and then free Te atoms are migrating to these heated areas which cause Te precipitate. Further, the results of the soft X-ray measurements have been also presented and this part of the experimental data needs to do more penetrating analysis in the future.

Technetium carbonates complexes produced by chemical, electrochemical and radiolytic methods have been studied by UV-Visible, X-ray Absorption Fine Structure (XAFS) and Density Functional Theory methods. The (NH4)2TcCl6 salt was dissolved in 2 M KHCO3. The resulting purple solution was analyzed by XAFS and UV-Visible spectroscopy. The UV-Visible spectra exhibits a band centered at 515 nm. The XAFS results were consistent with the presence of polymeric species containing the [Tc2(μ−O)2]4+ core coordinated to carbonate ligand. Concerning the electrochemical methods, the pertechnetate anion was electrochemically reduced in concentrated carbonate solution [(CO3 2−)=5 M and (HCO3 −)=0.5 M]. For the radiolytic reduction, the speciation of Tc under Helium ions particle beam and γ radiation was examined by UV-Visible and XAFS spectroscopy in high concentrated carbonate media. In concentrated carbonate solutions, pertechnetate as Tc(VII), was not reduced under irradiation due to the formation of carbonate radical which is a strong oxidant. Then, the solution proposed was the addition of formate to the solution which can scavenge hydroxyl radical 10 times faster than carbonate and prevent re-oxidation of reduced technetium. The XANES and EXAFS spectroscopies, approved by theoretical methods, revealed that the final product of the radiolytic reduction of pertechnetate is in the +IV oxidation state. The final structure of the reduced product by He2+ radiolysis was the same as electrochemical reduction. From this complex determination and evolution vs. the dose, this study is reporting the solubility of the Tc(IV) complex.

Immobilization of Pb in vitrified and devitrified industrial wastes: Evaluation of structural stability using XAFS spectroscopies

Until recently x-ray absorption fine structure (XAFS) measurements in the 1-2 keV region remained a challenging experimental task. This was primarily due to the lack of an adequate monochromator crystal that possessed both the required x-ray properties (large d-spacing, high resolution and reflectivity) and materials properties (ultra-high vacuum (UHV) capability, damage resistance in a synchrotron radiation beam, absence of constituent element absorption edges and stability, both thermal and mechanical). Traditionally, XAFS spectra in this photon energy range have been measured in a piece-wise fashion using a combination of monochromator crystals. Very recently, we have an experimental breakthrough in XAFS spectroscopy in this soft x-ray region. This energy region is of great importance for materials and basic research since the K-edges of Na (1070 eV), Mg (1303 eV), Al (1557 eV) and Si (1839 eV), the L-edges of some 4p elements from Ga to Sr and the M-edges of the rare-earth elements fall within this energy window of the electromagnetic spectrum. YB{sub 66}, a complex binary semiconducting yttrium boride having a cubic crystal structure with a lattice constant of 23.44 {angstrom} has been singled out as a candidate monochromator material for synchrotron radiation in the 1-2 keV region. There is no intrinsic absorption by the constituent elements in this region, which can adequately be dispersed by the (400) reflection having a 2d value of 11.76 {angstrom}. In terms of vacuum compatibility, resistance to radiation damage, thermal and mechanical stability, YB{sub 66} satisfies all the material requirements for use as a monochromator in a synchrotron beam. In the past few years, LLNL in collaboration with a number of other research institutes has pioneered the development of this unique man-made crystal for use as soft x-ray monochromator with synchrotron light sources for materials science studies. 23 refs., 4 figs.

Direct speciation of chromium in coal combustion by-products by X-ray absorption fine-structure spectroscopy

Chapter 7 - Nanolayer Analysis by X-Ray Absorption Fine Structure Spectroscopy

Isotopic exchange (IE) of trace metals is an established method for characterizing metal reactivity in soils, but it is still unclear which metal species are isotopically exchangeable. In this study, we used IE to quantify ‘labile’ zinc (Zn) in 51 contaminated soils that were previously studied by Zn K‐edge X‐ray absorption fine structure (XAFS) spectroscopy and sequential extraction (SE). All soils had been contaminated by runoff water from 17‐ to 74‐year‐old galvanized power‐line towers. They covered a wide range in pH (4.0–7.7), organic carbon (0.9–10.2%), clay (3.8–45.1%) and Zn concentrations (251–30 090 mg kg −1 ). Isotopic exchange was also performed on selected Zn minerals used as references for linear combination fitting of XAFS spectra. The isotopically exchangeable fraction (% E ) of Zn generally decreased with increasing pH, but small % E values were also observed for acidic soils with a large fraction of Zn in hydroxy‐interlayered minerals (Zn‐HIM). The fraction of Zn identified by XAFS spectroscopy as (tetrahedrally and octahedrally coordinated) ‘sorbed Zn’ agreed reasonably well with the isotopically exchangeable fraction but was in many cases larger than the % E , indicating that some ‘sorbed Zn’ may be isotopically non‐exchangeable, such as Zn sorbed in micropores of Fe oxyhydroxides. Zinc identified by XAFS spectroscopy as Zn precipitates (Zn phyllosilicates, Zn‐layered double hydroxide (Zn‐LDH) or hydrozincite) or as Zn‐HIM was largely isotopically non‐exchangeable (‘non‐labile’). Comparison between IE and extraction results suggested that the isotopically exchangeable Zn was mainly extracted in the first two fractions of the SE. However, non‐labile Zn was also extracted in these first two fractions for some soils, including a hydrozincite‐containing soil. Despite the presence of Zn‐LDH and/or Zn phyllosilicates in almost all soils, the Zn concentrations in solution and labile Zn increased with increasing soil total Zn at a given pH, which contradicts the concept of precipitation control by a single phase. Solution Zn was well predicted from the labile Zn following a sorption model.

The new ligand bis(1H-pyrazol-1-yl)acetyl-3,5-dimethyladamantane-1-amide (LMem) was synthesized by conjugating the drug memantine with the bifunctional species bis(pyrazol-1-yl)acetic acid and used as supporting ligand of copper(II) and copper(I) complexes 1-7. In the synthesis of the CuI complexes, the lipophilic triphenylphosphine (PPh3) and hydrophilic 1,3,5-triaza-7-phosphaadamantane (PTA) were selected as co-ligands, in order to stabilize copper in +1 oxidation state and to confer different solubility properties to the corresponding metal complexes. The electronic and molecular structures of CuI and CuII coordination compounds were investigated by high resolution Synchrotron Radiation-induced X-ray Photoelectron Spectroscopy (SR-XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The local structure around the copper ion sites was studied combining Density Functional Theory (DFT) modelling and X-ray Absorption Fine Structure (XAFS) spectroscopy, in both X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structures (EXAFS) regions. X-ray diffraction (XRD) studies were carried out on suitable crystals to describe the molecular structure and the intermolecular contacts of the LMem ligand. Among all Cu complexes tested, compounds 4 and 5 exhibited potent antiproliferative and cytotoxic effects in U87, T98, and U251 glioma cell lines. These effects were associated with increased reactive oxygen species (ROS) production and mitochondrial dysfunction, as evidenced by mitochondrial depolarization and altered intracellular distribution. Furthermore, the cytotoxic activity of these compounds was shown to be Cu-dependent, as it was effectively inhibited by the Cu chelator tetrathiomolybdate, confirming the essential role of copper in their mechanism of action.