K-edge XANES Spectra Research Articles

XANES investigations and catalytic properties of coprecipitated AlF3/MgF2 phases

Stepwise replacement of aluminium by magnesium in β-AlF3 phases leads to considerable alterations in the structure and surface properties of the calcination products of co-precipitates. This is accompanied by significant changes in the catalytic activity for heterogeneous chlorine/fluorine exchange reactions. The maximum catalytic activity is reached between 10 and 20 atom% magnesium doping. Due to the decrease of the strength of Lewis acid sites with further increasing the Mg content, the catalytic activity is more and more diminished. The XRD patterns indicate a loss of crystallinity for β-AlF3 with increasing Mg content. From Mg K-edge XANES and Al K-edge XANES spectra of Mg added β-AlF3 phases one can conclude a partial incorporation of Mg into the β-AlF3 lattice. Furthermore, a partial electron transfer from magnesium to the β-AlF3 lattice resulting in an increased electron deficiency (increased Lewis acidity) at low Mg concentrations can be stated, a result which correlates well with the increased catalytic activity observed for co-precipitated phases with lower Mg contents. The insertion of Mg into the β-AlF3 lattice is also supported by XPS studies on the same samples which show an increase of the modified Auger parameter for magnesium of about 0.4 eV, which indicates a change of the chemical environment of the ionized magnesium atom.

XANES spectroscopic study of the electronic structure of Ti in KTiOPO 4 and some of its isomorphous compounds

Ti K-edge XANES spectra of KTiOPO 4, an important material for second harmonic generation (SHG) and several of its isomorphs including MTiOPO 4 ( M = Na, Ag) and KTi 1− x Sn x OPO 4 ( x = 0.1, 0.4, 0.5, 0.6) have been recorded. The pre-edge regions of these spectra exhibit characteristic features that are assigned to electronic transitions from the Ti 1 s to the unoccupied Ti 3 d and 4 p levels. The intensities of these pre-edge features are very sensitive to slight changes in the asymmetry of the octahedral Ti coordination sphere. Thus, in the series KTi 1− x Sn x OPO 4 the successive substitution of Ti by Sn causes a substantial decrease of the Ti coordination asymmetry. This decrease is correlated with a loss in SHG intensity.

Determination of Mg Site Geometry in Pyroxenes by Full Multiple Scattering Calculations

We present a comparison between experiment and Full Multiple Scattering calculations at the magnesium K edge for two pyroxenes: diopside where magnesium atoms are in Ml site and enstatite where magnesium atoms are in two distinct sites M1 and M2. The purpose of this work is to determine how the number of octahedral sites on the one hand and the site distortion on the other hand affect the magnesium Kedge XANES spectra.

A Study Using Sulfur K-Edge XAS of Bitumens, Asphaltenes, Maltenes and their Oxidation Products by Comparison with Model Compounds

High resolution Sulfur K-edge XANES spectra have been recorded at the ESRF on test samples of bitumen, asphaltene and maltene fractions and various model compounds. All these fractions were found to have a low content of sulfidic species as opposed to thiophenic species and it is therefore not surprising that thermally oxidized samples (RTFOT) had also a low content (1%) of sulfoxides. This is indeed consistent with the absence of any detectable S=O signature in the EXAFS spectra which were recorded over more than 1300 eV using the undulator gap scan technique.

Intracrystalline and Electronic Structures of Copper(II) Complexes Stabilized in Two-Dimensional Aluminosilicate

X-ray absorption spectroscopic studies at the Cu K-edge have been performed for [Cu(en)2]2+, [Cu(cyclam)]2+, and their intercalated forms of two-dimensional layer silicate to examine how the structural and electronic modifications influence the stabilization of copper complexes in the charged interlayer space. According to the EXAFS analysis, copper complex ions in the layer silicate are stabilized at the center of the siloxane ring, and negatively charged layers are likely to act as axial counteranions. It is spectroscopically confirmed that the covalent bonding character between copper and ligand is enhanced by the intercalation into the silicate layers, depending upon the kind of ligands. There is a good linear relationship between the shoulder peak position of Cu K-edge XANES spectra and d−d transition band position of UV−visible spectra, which means that the former reflects the ligand-to-metal charge transfer effect.

In-Situ XAFS Investigation of K-Promoted Co Catalysts

In situ XAFS spectroscopy studies have been conducted at reaction conditions on potassium-promoted cobalt catalysts supported on SiO2 and Al2O3, and on similar samples without potassium. Measurements were conducted at temperature up to 200°C under both reducing (flowing H2) and simulated Fischer-Tropsch (F-T) reaction (flowing CO/H2) conditions. K and Co K-edge XANES spectra indicate that K interacts with both the Co phase and the SiO2 support, and that K promotion inhibits the reduction of the Co/SiO2 catalyst in H2 at 200°C. However, under CO hydrogenation conditions in the presence of water vapor, the differences in degree of reducibility are less since the unpromoted catalyst becomes more oxidized. For the K-promoted Co/Al2O3 catalyst, Co K-edge XANES and EXAFS spectra show that Co is initially present as a mixture of Co3O4 and CoO. Under H2 at 200°C, these oxides are reduced to a mixture of CoO and Co metal. During reaction at 200°C in H2/CO, the Co phases are reconverted to a mixture of Co3O4 and CoO. Without K, the Co in Co/Al2O3 is reduced almost completely to Co metal under H2 at 200°C and remains in that form even during reaction at 200°C in H2/CO. Only a minor trace of CoO is observed.

X-ray absorption study of Ti-bearing silicate glasses

Ti K-edge XANES spectra have been collected on a series of Ti-bearing silicate glasses with metasilicate and tetrasilicate compositions. The intensity of the preedge feature in these spectra has been found to change with glass composition and varies from 29 to 58% (normalized intensity) suggesting a variation in structural environent around the absorbing atom. The pre-edge peak intensity increases for the alkali titanium tetrasilicate glasses from 35% to 58% in the order Li < Na < K < Rb, Cs whereas for the metasilicate compositions there is a maximum for the K-bearing glass. The pre-edge peak intensity remains constant for the alkaline earth titanium metasilicate glasses, Ca and Sr (34%) but increases slightly for Ba (41%). As the intensity of this feature is inversely correlated with coordination number, a comparison of the pre-edge intensity data for the investigated glasses with those of materials of known coordination number leads us to establish a regression equation and to infer that the average coordination number of Ti in these glasses ranges from 4.8 to 5.8. Large alkali cations appear to stabilize a relatively low average coordination number for Ti in silicate melts. The Ti structural environment results appear also to vary as a function of SiO2 content within the K2O-TiO2-SiO2 system. A number of physical properties of the melts from which these glasses were quenched and of other Ti-bearing silicate melts, have been determined in recent years. Clear evidence of a variable coordination number of Ti, consistent with the interpretation of the present XANES data is available from density measurements. These and other property determinations are compared with the present spectroscopic observations in an attempt to relate structure and properties in these melts which contain a major component with variable coordination number.

Sulphur speciation in bitumens and asphaltenes by X-ray absorption fine structure spectroscopy

The S L-edge and K-edge XANES spectra were collected on bitumen and asphaltene samples as well as model compounds in order to determine the feasibility of monitoring transformations of organic sulphur in bitumen during genesis, processing, and upgrading. Using the spectra of model compounds as fingerprints, it was possible to speciate the sulphur forms in bitumen and asphaltene. The reduced form of sulphur such as thiophenes are the predominant form of sulphur in the bitumen and asphaltene samples analyzed. No significant amounts of oxidized sulphur forms could be detected. Spectra collected using total electron yield and fluorescence yield were identical, confirming that there is no preferential partitioning at the bitumen/air interface, or oxidation during sample handling. Also important was the finding that bitumen could be analyzed at moderately high vacuum (10 −8 torr). Thus XANES represents a rapid, nondestructive mean of characterizing specific classes of compounds in complex matrices, such as bitumen.

X-ray-absorption studies of cation ordering and valence in the T*-phase La2-x-yRxSryCuO4 (R=Sm, Eu, Gd, Tb, and Dy).

X-ray-absorption measurements have been made at the lanthanide L edges, Sr and Cu K edges in the ${\mathit{T}}^{\mathrm{*}}$-phase ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}\mathrm{\ensuremath{-}}\mathit{y}}$${\mathit{R}}_{\mathit{x}}$${\mathrm{Sr}}_{\mathit{y}}$${\mathrm{CuO}}_{4}$ compounds with R=Sm, Eu, Gd, Tb, and Dy. The La and rare-earth x-ray-absorption near-edge (XANES) data and the extended x-ray-absorption fine-structure data both clearly indicate that the R and La ions selectively enter the M' site and M site, respectively. The degree of this site ordering is nearly perfect and is similar for the Sm, Eu, Gd, Tb, and Dy members of this family. The ${\mathit{L}}_{3}$ XANES data show that the La, Sm, Eu, Gd, and Dy are trivalent in these compounds, but about 20% of the Tb ions are tetravalent in the Tb material. The Cu K-edge XANES spectra are consistent with a divalent state for copper, and are essentially identical for the as-prepared nonsuperconducting samples and the high-pressure oxygenated superconducting counterparts. This indirectly suggests that the hole carrier in the ${\mathit{T}}^{\mathrm{*}}$-phase superconductors is largely associated with oxygen atoms as in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$. Based on the results of cation-ordering, cation-valence, and oxygen-content measurements, the superconducting behavior of Sm, Em, and Gd ${\mathit{T}}^{\mathrm{*}}$ phases and the lack of superconductivity in the Tb and Dy compounds are discussed.

Full Multiple-Scattering Approach to Na K-Edge XANES of NaCl–KCl Mixed Crystal

In this work we studied the Na K-edge XANES spectra of pure NaCl crystal and NaCl–KCl mixed crystal measured by Murata et al. to clarify the geometric and electronic structure around X-ray absorbing atoms by use of the short-range order full multiple scattering calculation. The changes observed in the experimental XANES for the increase of KCl concentration are qualitatively explained by assuming that in a very small scale Nail and KCl grains are to be in contact with each other. We also found that the Na–Cl distance in the boundary region remains nearly the same as in NaCl crystal. The net charge on Na ion in pure NaCl crystal is also estimated and found to be slightly smaller than that in NaF crystal.