EXAFS Refinement Research Articles

Effect of SnO2/SeO2 on Au nano-particles doped silicate glasses: a structural study using XAS and EXAFS refinements

Au LIII edge X-ray absorption spectroscopic measurements were carried out over a series of Au nanoparticles embedded glasses with SnO2 and SeO2. The results obtained from EXAFS and XANES confirms the typical characteristics and structure of gold nanoparticles in these glasses. Raman analysis was performed to understand the structure of the glass. XANES data reveal that these glasses show Au0 and Au+1 oxidation state depending on reducing agent. EXAFS refinements were performed using ARTEMIS software to calculate bond lengths, R-factor and Debye–Waller factor on these glasses.

Species fine structure of transition metal Cu(II) in aqueous chloride-bearing solutions: Insights from X-ray absorption spectroscopy and ab initio XANES calculations

Knowledge of the structure and speciation of aqueous Cu(II)-chloride complexes is important for understanding Cu(II) behavior in the deep removal technology of Cu impurity from nickel electrolysis anolytes containing chloride. In this paper, X-ray absorption spectroscopic measurements are reported for dissolved copper in lithium chloride (up to ~ 16 mol kg−1) solutions at room temperature. The speciation and structure of corresponding solutions has been probed by performing a combined ab initio XANES theoretical and experimental analysis. The EXAFS spectrum was analyzed as well within this approach. Our XAS data and ab initio XANES calculations favored the five-coordinated [Cu(H2O)5]2+ with square pyramidal configuration over the four-(square planar and tetrahedron, [Cu(H2O)4]2+) and six-coordinated (octahedron, [Cu(H2O)6]2+) structure in dilute Cl− solution (~ 0.55 mol kg−1). This is also supported by the EXAFS refinement with the [CuO5] model having the lowest statistical error. In the highest Cl− concentration solution, the results of both methods show that tetrahedral [CuCl4]2– complexes are predominant. Upon manually adjusting its geometric parameter to the distortion degree α of ~ 18° and average CuCl bond distance of 2.25 Å of a squashed tetrahedral model, not only does the calculated XANES spectrum well reproduce the experimental spectrum, but also the statistical error in the EXAFS refinements is lowest.

Speciation of nickel (II) chloride complexes in hydrothermal fluids: In situ XAS study

In situ XAS data of Ni(II) chloride solutions with various salinities (0–7.68m) were collected to investigate the stoichiometry and geometry of Ni(II) chloride complexes from room temperature up to 369°C at 400bar, and to 434°C at 600bar. Increasing temperature and/or salinity results in a change in the coordination of the Ni(II) chlorocomplexes from octahedral to (distorted) tetrahedral. Octahedral species predominate within the whole salinity range at room temperature and up to ~200°C, and tetrahedral species become significant beyond this temperature. At 369°C and 400bar, octahedral species remain in equilibrium with tetrahedral species, and the number of chloride ligands in the first coordination shell depends on the Cl:Ni molar ratio. EXAFS refinements and ab initio XANES calculations show that the highest order chlorocomplex present over the investigated pressure, temperature and composition ranges is the distorted tetrahedral complex [NiCl3(H2O)]−, which predominates in the highest Cl concentration (7.68m) solution at 434°C and 600bar. A quantitative thermodynamic analysis of the XANES and EXAFS data reveals that the XAS data are consistent with the speciation models derived from recent UV-Vis spectrophotometric measurements (25°C–250°C, 100bar; Liu et al., 2012b) and the high temperature, high pressure solubility experiments of Lin and Popp (1984) and Fahlquist and Popp (1989), if the NiCl2(aq) species exists in both octahedral ([NiCl2(H2O)4](aq)) and tetrahedral ([NiCl2(H2O)2](aq)) forms, with the ratio of octahedral to tetrahedral decreasing at high temperature (>200°C). The new XAS data show that the octahedral to tetrahedral transition in Ni(II) chloride complexes occurs at higher temperature and/or salinity than the corresponding Co(II) complexes (Liu et al., 2011).

X-ray-excited optical luminescence and X-ray absorption fine-structures studies of CdWO4scintillator

X-ray-excited optical luminescence (XEOL) emission and excitation spectra as well as the EXAFS signal of CdWO(4) were measured in the energy region of the Cd and W absorption edges. From EXAFS refinement, structural parameters such as number of atoms, distance from the absorbing atom and width of coordination shells in the W neighborhood were determined. The role of W-O interactions on the intrinsic luminescence of CdWO(4) is discussed. The efficiencies of conversion, transfer and emission processes involved in the scintillation mechanism showed to be high when self-trapped excitons are formed locally by direct excitation of W ions. Annihilation of these excitons provides the characteristic scintillation of CdWO(4), a broad band emission with maximum at 500 nm. The presence of two energetically different O positions in the lattice gives rise to the composite structure of the luminescence band, and no influence of extrinsic defects was noticed. A mismatch between the X-ray absorption coefficient and the zero-order luminescence curves corroborates that the direct excitation of Cd ions induces secondary electronic excitations not very effective in transferring energy to the luminescent group, WO(6).

The Effect of the State of Pd on Methane Combustion in Pd-Doped LaFeO3

Pd-based LaFeO3 perovskite-type catalysts have been prepared by the amorphous citrate method with the aim to expose Pd at the surface of LaFeO3 (Pd/LaFeO3) and to force Pd to form a solid solution with LaFeO3 (LaFe0.95Pd0.05O3). The catalysts have been tested for methane combustion in the temperature range of 200−900 °C. The state of Pd in the various samples has been accurately characterized using XANES, EXAFS, XPS, H2-TPR, and DRIFTS. The shape of the white line of the Pd K edge XANES spectrum of LaFe0.95Pd0.05O3 indicates that palladium most probably exhibits distorted octahedral coordination. Hence, Pd is fully incorporated in the perovskite structure, substituting Fe, which is also confirmed by EXAFS refinement. On the contrary, the XANES spectrum of Pd/LaFeO3 corresponds more to Pd in square planar coordination, suggesting that Pd is mainly distributed on the surface of LaFeO3, most probably in the form of PdO. The different oxidation states and local coordination of Pd in LaFe0.95Pd0.05O3 (most likely Pd3+ in distorted octahedral coordination) and Pd/LaFeO3 (predominantly Pd2+ in square planar coordination) reflect the distinct availability of Pd for reaction and coincide with the different catalytic performances of the catalysts. Pd/LaFeO3 exhibits high performance for the oxidation of methane (T50% = 460 °C), which correlates with the high concentration of Pd species on the oxide surface. On the contrary, when Pd is incorporated within LaFeO3, the corresponding catalyst shows poor performance similar to that of LaFeO3. Reaction at 900 °C causes serious changes in the structure of the catalysts, which were interpreted in light of the initial structure, the “self-regenerative property” of perovskite-type oxides, and the PdO ↔ Pd equilibrium.

An X-ray absorption study of gold coordination compounds: EXAFS refinements and double electron excitation background

X-ray absorption spectra at the Au L edges of Au(PPh3)Cl, Au(PPh3)2Cl and KAuCl4.2H2O have been collected as a function of temperature. Data analysis of the extended X-ray absorption fine structure was carried out with the GNXAS package performing an appropriate treatment of the multiple-scattering effects involving several ligand atoms. The accuracy of the current analysis procedures for the Au L edges is assessed and the sensitivity to coordination numbers and distances is established. Evidence for the presence of a (2p4f) double-electron excitation in the atomic Au background is obtained for the first time. The results presented in this paper are of general importance for improving data analysis at the Au L edges and in particular for improving the EXAFS results in Au cluster compounds.

EXAFS study of the coordination environment of aluminum in a series of silica-rich glasses and selected minerals within the Na 2OAl 2O 3SiO 2 system

Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na 2OAl 2O 3SiO 2 minerals and glasses. The purpose of this study is to determine if EXAFS can accurately characterize these known Al environments in terms of AlO distances and coordination numbers, and to characterize the unknown Al environment in the glasses. Al XANES spectra were collected for corundum, sillimanite, albite, jadeite and nepheline, as well as jadeite glass, nepheline glass and a 75 mol% silica glass series having R ( = Al/Na) ranging from 0.28 to 1.61. Al EXAFS spectra were collected for corundum, sillimanite, jadeite and albite, as well as jadeite glass, albite glass and a 75 mol% silica glass with an R of 1.61. Al XANES spectra for our model compounds have two edge maxima at 1568 and 1572 eV for minerals having octahedral Al, while one edge maximum is present at 1566 eV for minerals with tetrahedral Al. The glasses all have a single edge maximum near 1566 eV, indicating tetrahedral-like Al sites in the glasses. The results of our Al EXAFS refinements of the model compounds are in agreement to within ±0.03 Å for AlO distances and to within ±23% for coordination numbers of recent crystal structure refinements in the literature. The EXAFS results for the glasses clearly indicate Al in tetrahedral sites, yielding AlO distances near 1.77 Å, well within the range of average AlO distances for aluminate tetrahedra of aluminosilicate minerals. Our XANES and EXAFS results indicate tetrahedral Al throughout our 75 mol% silica glass series, which may be consistent with the existence of tricluster units occurring in the Al-rich glasses in the sodium aluminosilicate system.