XANES

About: XANES is a research topic. Over the lifetime, 7737 publications have been published within this topic receiving 188032 citations.

Characterization of iron speciation in urban and rural single particles using XANES spectroscopy and micro X-ray fluorescence measurements: investigating the relationship between speciation and fractional iron solubility

Abstract: . Soluble iron in fine atmospheric particles has been identified as a public health concern by participating in reactions that generate reactive oxygen species (ROS). The mineralogy and oxidation state (speciation) of iron have been shown to influence fractional iron solubility (soluble iron/total iron). In this study, iron speciation was determined in single particles at urban and rural sites in Georgia USA using synchrotron-based techniques, such as X-ray Absorption Near-Edge Structure (XANES) spectroscopy and microscopic X-ray fluorescence measurements. Soluble and total iron content (soluble + insoluble iron) of these samples was measured using spectrophotometry and synchrotron-based techniques, respectively. These bulk measurements were combined with synchrotron-based measurements to investigate the relationship between iron speciation and fractional iron solubility in ambient aerosols. XANES measurements indicate that iron in the single particles was present as a mixture of Fe(II) and Fe(III), with Fe(II) content generally between 5 and 35% (mean: ~25%). XANES and elemental analyses (e.g. elemental molar ratios of single particles based on microscopic X-ray fluorescence measurements) indicate that a majority (74%) of iron-containing particles are best characterized as Al-substituted Fe-oxides, with a Fe/Al molar ratio of 4.9. The next most abundant group of particles (12%) was Fe-aluminosilicates, with Si/Al molar ratio of 1.4. No correlation was found between fractional iron solubility (soluble iron/total iron) and the abundance of Al-substituted Fe-oxides and Fe-aluminosilicates present in single particles at any of the sites during different seasons, suggesting solubility largely depended on factors other than differences in major iron phases.

Relativistic XANES calculations of Pu hydrates

Abstract: A theoretical analysis of x-ray absorption near edge structure (XANES) at L{sub 3} and L{sub 1} edges for Pu hydrates with a formal oxidation state of Pu ranging from +3 to +6, using the {ital ab initio} multiple scattering code FEFF7, is presented. For each hydrate our calculations reproduce well the white line intensity and relative peak positions, which are the features commonly used for formal valence identification. In order to achieve such a degree of coincidence between theory and experiment, it was necessary to use a relativistic Dirac-Fock treatment of atomic densities and mixed Dirac-Fock{endash}local-density-approximation exchange-correlation potentials, not considered in previous actinide studies. We find that most of the white line intensity at L{sub 3} edges originates from scattering of the photoelectron, i.e., x-ray absorption fine structure. We also show that the white line shoulder peak, present in the case of plutonyl compounds (Pu{sup 5+} and Pu{sup 6+}), is due largely to constructive interference of scattering paths containing axial oxygens. Limitations of the FEFF7 code and ways to improve the quantitative agreement between XANES calculations and experimental spectra are also discussed. {copyright} {ital 1998} {ital The American Physical Society}

Architecture of Bimetallic PtxCo1−x Electrocatalysts for Oxygen Reduction Reaction As Investigated by X-ray Absorption Spectroscopy

Abstract: Carbon-supported PtxCo1−x alloy nanoparticles are prepared via a modified Watanabe’s process for oxygen reduction reaction (ORR). The relationship between the variations in alloying extent and Pt d-band vacancies in Pt−Co/C catalysts, which are tunable by changing the Pt and Co composition, are systematically studied. All of the catalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), and XAS. TEM images indicate that the dispersion of the metal nanoparticles on the carbon support is uniform. The XAS technique containing both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine spectroscopy (EXAFS) is utilized to extract the Pt d-band vacancies and alloying extent of Pt and Co in PtxCo1−x nanoparticles, respectively. Rotating disk electrode measurements of PtxCo1−x nanoparticle catalysts with various Pt:Co atomic compositions (3:1, 1:1, and 1:3) reveal that Pt1Co1/C nanocatalyst showed enhanced ORR activity. It i...

Angular dependence of core hole screening in LiCoO2 : A DFT+U calculation of the oxygen and cobalt K-edge x-ray absorption spectra

Abstract: Angular-dependent core hole screening effects have been found in the cobalt K-edge x-ray absorption spectrum of LiCoO2, using high-resolution data and parameter-free general gradient approximation plus U calculations The Co 1s core hole on the absorber causes strong local attraction The core hole screening on the cobalt nearest-neighbors induces a 2 eV shift in the density of states with respect to the on-site 1s-3d transitions, as detected in the Co K pre-edge spectrum Our density functional theory plus U calculations reveal that the off-site screening is different in the out-of-plane direction, where a 3 eV shift is visible in both calculations and experiment The detailed analysis of the inclusion of the core hole potential and the Hubbard parameter U shows that the core hole is essential for the off-site screening while U improves the description of the angular-dependent screening effects In the case of oxygen K edge, both the core hole potential and the Hubbard parameter improve the relative positions of the spectral features

Electronic structure of aromatic amino acids studied by soft x-ray spectroscopy.

Abstract: The electronic structure of phenylalanine, tyrosine, tryptophan, and 3-methylindole in the gas phase was investigated by x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C, N, and O K-edges. The XPS spectra have been calculated for the four principal conformers of each amino acid, and the spectra weighted by the Boltzmann population ratios calculated from published free energies. Instead of the single peaks expected from the stoichiometry of the compounds, the N 1s core level spectra of phenylalanine and tryptophan show features indicating that more than one conformer is present. The calculations reproduce the experimental features. The C and O 1s spectra do not show evident effects due to conformational isomerism. The calculations predict that such effects are small for carbon, and for oxygen it appears that only broadening occurs. The carbon K-edge NEXAFS spectra of these aromatic amino acids are similar to the published data of the corresponding molecules in the solid state, but show more structure due to the higher resolution in the present study. The N K-edge spectra of tryptophan and 3-methylindole differ from phenylalanine and tyrosine, as the first two both contain a nitrogen atom located in a pyrrole ring. The nitrogen K-edge NEXAFS spectra of aromatic amino acids do not show any measurable effects due to conformational isomerism, in contrast to the photoemission results. Calculations support this result and show that variations of the vertical excitation energies of different conformers are small, and cannot be resolved in the present experiment. The O NEXAFS spectra of these three aromatic compounds are very similar to other, simpler amino acids, which have been studied previously.