Edge XANES Research Articles

Intermediate-range structure of tellurite glasses from near-edge absorption spectra

Simulations of the $\mathrm{Te}$ $K$ and ${L}_{3}$-edge XANES in an ${\mathrm{Nb}}_{2}{\mathrm{Te}}_{4}{\mathrm{O}}_{13}$ crystal are found to be in good agreement with experimental data for a large variety of tellurite crystals and glasses. The near-edge structure of the $\mathrm{Te}$ $K$ and ${L}_{3}$ edge is found to be determined by all the atom-position correlations within a cluster of about $5--8\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ in radius, rather than just the nearest neighbors. This suggests that the structure of tellurite glasses is closely related to those of crystals even in the range beyond nearest neighbors. Furthermore, it shows that XANES (and ELNES) technique can be used to probe intermediate-range structures around an absorbing atom.

Structural investigations of functionalized mesoporous silica-supported palladium catalyst for Heck and Suzuki coupling reactions

FSM-16 mesoporous silica-supported mercaptopropylsiloxane Pd(II) complex, Pd-SH-FSM, has been shown to act as an active, stabile, and recyclable heterogeneous catalyst for the Heck reaction of 4-bromoacetophenone with ethyl acrylate and for the Suzuki reaction of 4-bromoanisole with phenylboronic acid. The structure of the Pd species in Pd-SH-FSM before and after the reaction was well characterized by a combination of XRD, TEM, UV–vis, Pd K-edge XANES/EXAFS, and Pd L III -edge XANES, and the results were compared with those of a amorphous silica-supported mercaptopropylsiloxane Pd(II) complex (Pd-SH–SiO 2), unmodified FSM-16-supported Pd(OAc) 2 (Pd-FSM), and previously reported heterogeneous catalysts (Pd zeolite and Pd/C). The aggregation behavior of Pd species during the reaction greatly depends on the support. On Pd-SH-FSM after Heck and Suzuki reactions, Pd(II) species coordinated to the sulfur ligands were the main Pd species together with small Pd clusters as minor species, and the catalyst was reused without marked loss in the activity. On Pd-SH–SiO 2, relatively large numbers of the Pd(II) species were converted to Pd clusters after the Suzuki reaction. On ligandless Pd-FSM, most of the Pd was aggregated to form Pd metal particles. The activity order of the recycled catalysts (Pd-SH-FSM > Pd-SH–SiO 2 > Pd-FSM) indicates that the Pd(II) complexes on the sulfur ligands and possibly the small Pd clusters are more active than Pd metal particles. It is concluded that the sulfur ligands in the size-restricted mesopore are effective for preventing the aggregation of coordinated Pd complexes, and this results in high durability and recycling characteristic of the Pd-SH-FSM. The deactivation of the Pd complex via Pd aggregation was significant for the conventional Pd catalysts; [Pd(NH 3) 4] 2+ complexes in Pd zeolite and PdO nanoclusters on Pd/C were changed to less reactive metal particles or clusters after Heck and Suzuki reactions.

Pd–sepiolite catalyst for Suzuki coupling reaction in water: Structural and catalytic investigations

[Pd(NH 3) 4] 2+-exchanged sepiolite clay (Pd–sepiolite) has been applied to the catalytic Suzuki-type carbon–carbon coupling reactions of 4-bromophenol with phenylboronic acid or sodium tetraphenylborate in water. The Pd–sepiolite effectively catalyzed the reaction under mild reaction conditions (at room temperature in air). The Pd–sepiolite system exhibits higher yield than unsupported Pd(II) salts, [Pd(NH 3) 4]Cl 2-impregnated SiO 2 (Pd–SiO 2), and a commercially available Pd/C consisting of Pd metal particles. The structure of Pd species in the catalysts before and after the reaction was well characterized by a combination of XRD, TEM, UV-Vis, Pd K-edge XANES/EXAFS, and Pd L III -edge XANES. XAFS and TEM results confirmed the formation of metal particles after the reaction by unsupported Pd(II) salt and Pd–SiO 2. In contrast, for Pd–sepiolite the change in the structure of Pd species after the reaction was not significant; the highly dispersed Pd(II) complex, present before the reaction, was still the main Pd species together with the small Pd clusters (2–7 nm) as minor species. As a result of the high stability, Pd–sepiolite was reused without losing its activity. Significantly high turnover numbers (TON = 940,000) were also attained at reflux temperature. It is suggested that Pd metal precipitation during the reaction is inhibited by a strong electrostatic interaction of sepiolite with Pd(II) species.

X-ray absorption and Raman characterizations of TeO 2–Ga 2O 3 glasses

The thermodynamic properties of transparent glasses prepared in the TeO 2–Ga 2O 3 system were investigated by differential scanning calorimetry. The change of the thermal parameters as a function of the chemical composition is discussed. Raman and both Te L III and Ga K edge X-ray absorption spectroscopies at room temperature were used to examine the short range order. Analyses of the spectra suggest that the addition of Ga 2O 3 content to the TeO 2 glass matrix induces the transformation of trigonal bipyramids (TeO 4E, E=lone electronic pair 5s 2 of Te) to trigonal pyramids (TeO 3E) with formation of Te–O–Ga bridging bonds. Furthermore, Ga K edge XANES and EXAFS studies show that Ga atoms exhibit both tetrahedral (GaO 4) and octahedral (GaO 6) environments.

XAFS study on the local structure of Ti in amorphous mesoporous titania

We applied XAFS spectroscopies to investigate the local structure of Ti in mesoporous titania synthesized with a primary amine template, which has an extremely large surface area (>1200 m2 g−1) and an amorphous nature, because the atomic structure was expected to differ in many points from that in all the other known titanium oxides. The coexistence of 5- and 6-fold coordinations of Ti is demonstrated by deconvoluting the pre-edge peaks of the Ti K edge XANES. Their populations are evaluated as 39% and 61%, respectively. The number of 5-fold Ti is decreased by a calcination at 673 K, accompanied by a considerable decrease of the BET surface area. The anharmonic pair distribution function is used for the analysis of EXAFS from the Ti–O bond in order to calculate reliable structural parameters. Template extraction considerably increases the Debye–Waller factor, suggesting that the micelles filling the pores suppress large vibrations of the Ti–O bond in mesoporous titania.

Manganese, iron and sulfur K edge XAFS of promoted sulfated zirconia catalysts

Promoted sulfated zirconia samples were prepared by the incipient wetness technique to produce isomerization catalysts which were active for the conversion of n-butane to isobutane at 338K (up to 10% conversion of 1% n-butane, 1atm., 0.25 h(-1) WHSV). The local structure of Fe and Mn in promoted sulfated zirconia was investigated using fluorescence yield XAS. Spectra were taken of calcined samples, activated samples, and samples after reaction with n-butane (maximum activity and deactivated). Factor analysis reveals that the Mn K edge XANES can be described by a linear combination of the spectra of two separate components, and that the ratio of these components changes with activation of the catalyst, and during use in the n-butane isomerization reaction. The change in ratio of the Mn species during activation and reaction results in a reduction of the average Mn valence from 2.4 to 2.2. The Fe K edge XANES was not similarly affected by activation and reaction with n-butane.

Pt L-edge XANES as a probe of Pt clusters

The sensitivity of Pt L-edge XANES to local geometric and electronic structure in various Pt(n) clusters is investigated using the ab initio self-consistent FEFF8 code. Calculations based on FEFF8 are found to be in good agreement with experiment. For pure Pt clusters the XANES can distinguish between 2- and 3-dimensional clusters. Self-consistency is important in determining the variation of XANES with cluster size. The effect of a support is also studied. In Pt-Cl(x) clusters the presence of a Cl-Pt bond leads to a "hybridization peak," i.e., a peak in the Cl d-density of states (d-DOS) mixed with Pt d-states. For Pt-H clusters hydrogen addition is well correlated with the growth of a broad shoulder on the white line. This change is attributed largely to AXAFS, i.e., to a corresponding change in the atomic background absorption.

Electronic Structure of LaCuO3: X-Ray Absorption Fine Structure Theoretical Analysis

Oxygen K-edge and copper L 2,3 -edge X-ray absorption spectra of LaCuO 3 have been analyzed theoretically on the basis of a full multiple scattering formalism in the real space. Combined oxygen K-edge and Cu L 3 -edge XANES is shown to be a useful tool for the study of the electronic structure of LaCuO 3 . The full multiple scattering approach is found to be successful to analyze experimental oxygen K-edge XANES in LaCuO 3 , while the single scattering approximation is not valid in this case. It is found that unoccupied oxygen p-states interact with Cu 3d ones in a specific resonant manner.

Radiation-induced degradation of polydiene sulfones as obtained by sulfur K-edge XANES.

Radiation-induced degradation of polydiene sulfones as obtained by sulfur K-edge XANES.

Structure and ionic conductivity of rapidly quenched AgI–Ag 2WO 4 superionic conductor glasses

Silver ion conducting glasses composed of (AgI) x –(Ag 2WO 4) 1− x ( x=0.80–0.55) were prepared by a rapid melt quenching method. Their physical properties were investigated by X-ray, DSC, impedance and Ag-109 NMR measurements. The glass transition temperatures were in the range from 110°C to 187°C. The glasses show unusually high ionic conductivities of 2.5 × 10 −3 S/cm at x=0.55 and 2.3 × 10 −1 S/cm at x=0.80, which varied exponentially with increasing AgI content. Ag-109 NMR spectra also support the high mobility of silver ions in the glasses. Structures of the glasses were investigated by FTIR and W-L 1 edge XANES. From these results it is concluded that the tungsten ions are octahedrally coordinated by oxygen ions as in crystalline Ag 2WO 4 and Ag 26I 18W 4O 16, which is quite different from the structure of AgI–Ag 2MoO 4 glasses.

Electrocatalysis of CO Tolerance in H2/CO Mixtures by Carbon Supported PtMo in a Proton Exchange Membrane Fuel Cell

We report 2 to 3 fold enhancement in activity for CO oxidation in PEM fuel cell by highly dispersed carbon supported PtMo alloy nano-crystals (PtMo/C) as compared to the current state of the art PtRu/C at 85°C with 100 ppm CO in H 2 . The half-cell polarization characteristics show a lower susceptibility to changes as a function of temperature (below 100°C), CO concentration (5-100 ppm) and alloy composition (1:1 to 1:5, Mo:Pt) in contrast to Pt/C and PtRu/C. Cyclic voltammograms show both Pt and Mo features and a redox couple involving Mo (at ∼0.5 V). CO stripping voltammograms show two peaks, the first at low overpotentials (∼0.05 V) followed by a second initiated at ∼ 0.5 V, the second peak is akin to oxidation of CO on Pt/C. Preliminary analysis of the Mo K edge XANES confirmed the presence of Mo as an oxide at potentials as low as 0.0 V and the redox couple at ∼ 0.5 V.

C60FeC60Complexes in Fe Intercalated Fullerite Studied by X-Ray Absorption

The aim of Fe K edge XANES and EXAFS studies is to reveal the local structure around Fe ions in C60Fex intercalated fullerites. The measurements are performed in transmission mode at LN2 temperature for C60 Fey powder samples and other materials: ferrocene Fe(C5H5)2, Fe metal foil and α-Fe2 O3. The main result is that the XAFS spectra of the C60Fe2 intercalate is almost identical to that of ferrocene and very different from α-Fe and α-Fe2O3. The results support opinion that in the C60 Fey intercalates the ferrocene-like bonds in complexes C60 FeC60 are formed. Additionally, the XAFS spectra showed that neither remarkable amount of Fe metallic clusters nor oxidized iron particles are present in the C 60Fex sample.

High Resolution Cu L3,2-Edge XANES Studies of Cu-Au Alloys

High resolution Cu L 3,2 -edge XANES have been obtained for Cu metal and a series of Cu-Au alloys (CU 3 Au, CuAu and CuAu 3 ). It was found that the Cu L 3 -edge XANES for all the samples under investigation exhibit the characteristic three-peak resonances predicted by theory. The CU L 3 -edge XANES shows some interesting systematics which can be correlated to charge redistribution in Cu-Au alloys.

M3,2-Edge X-Ray Absorption Near Edge Structure of 5d Metals

We report M 3,2 edge XANES measurements for a series of 5d metals, Hf, Ta, W, Re, Ir, Pt and Au using a total electron yield technique. It was found that the M 3,2 XANES for most 5d metals exhibit intense resonances at the threshold (whitelines). The area under the whiteline correlates with the unoccupied densities of d states of the metal. The M 3,2 XANES features and their systematics are comparable to their L 3,2 counterparts. These results clearly suggest that M 3,2 edge XANES can be used as an alternative probe to the conventional L 3,2 -edge XANES for the unoccupied densities of d states of the 5d metals and their compounds.

XAFS Study of Fe Intercalated Fullerite

Fe K edge XANES and EXAFS studies were performed to reveal the local structure around Fe ions in C 60 Fe 2 fullerites. The measurements were carried out in transmission mode at LN2 temperature for the monoclinic and fcc C 60 Fe 2 fullerite powder samples and other materials: ferrocene Fe(C 5 H 5 ) 2 , Fe metal foil and α-Fe 2 O 3 . The XAFS spectra of the monoclinic C 60 Fe 2 fullerite is almost identical to that of ferrocen. The XAFS spectra of the fcc C 60 Fe 2 fullerite have shown that Fe neutral atoms, Fe 2+ and Fe 3+ ions appear in this fullerite due to annealing at temperature 850 K. The EXAFS study has proved the presence of small Fe clusters in this form of fullerite.

Angular Dependent XAFS Studies of a MoSi2Single Crystal

We report angular dependent X-ray absorption measurements at the Mo L 3,2 edge and the Si K-edge for a MoSi 2 single crystal. The crystal was oriented so that an azimuthal rotation of the crystal about the surface normal (direction of the incident photons) would align the c-axis of the crystal anywhere between parallel and perpendicular to the polarization of the photons. It was found that while the Mo L 3,2 edge XANES shows little angular dependence, the Si K-edge XANES shows a very strong polarization dependence of which the intensity exhibits a two-fold symmetry. This angular dependence of the Si K-edge XANES is attributed to the high densities of unoccupied states localized perpendicular to the Si-Si bond.

Si Core Level XANES of Organometallic Compounds Containing Si-Ge Bonds: Experimental and Theoretical Observations

Si K- and L 2,3 -edge XANES for Si(GeMe 3 ) 4 and Ge(SiMe 3 ) 4 are studied with the aid of MS-Xα calculation. Good agreement is found between the experiment and the theory. The lowest energy features are assigned to the transitions to mostly unoccupied orbitals with significant Si-Ge contribution.

Study of the Local Structure of Molybdenum−Magnesium Binary Oxides by Means of Mo L3-Edge XANES and UV−Vis Spectroscopy

Mo L3-edge XANES and diffuse reflectance UV−vis spectra have been recorded for structural studies of Mo−Mg binary oxides which catalyze metathesis reactions of olefins after pretreatment with H2 at 773 K. The XANES of reference compounds revealed that the local symmetry around the Mo ion as well as the valence state affects the spectral features. From the XANES of Mo−Mg binary oxides in oxidized/reduced states, it is concluded that MoO4 tetrahedra are the main component in the near-surface region for samples of x (Mo/(Mo + Mg)) ≤ 0.7 in the oxidized state. For samples in the reduced state, the tetrahedral species remain, but reduced Mo ions including MoO2 species are also formed, while the MoO2 phase is formed in the bulk phase at x = 0.5−0.8. In the latter samples, Mo ions are easily reduced by treatment with H2. The active species for metathesis reactions relates to formation of the MoO2 phase not only in the bulk but also in the near-surface region.

Investigation of the Structure of Microporous Ti-Si Mixed Oxides by X-ray, UV Reflectance, FT-Raman, and FT-IR Spectroscopies

A series of high surface area, microporous, Ti-Si mixed oxides having a wide range of elemental compositions was prepared by a sol-gel method. Since the materials were noncrystalline, X-ray absorption spectroscopy at the Ti K edge was used to probe the first-shell coordination environment around Ti. Incorporation of Si into the mixed oxide shortened the average Ti-O bond distance derived from EXAFS and disrupted the normal octahedral coordination of pure titania. The shortest Ti-O bond distance was 1.82 [angstrom] and was attributed to Ti primarily residing in tetrahedral sites, which was also indicated by a significant pre-edge peak present in Ti K edge XANES. Shifts in the UV absorption edges for the mixed oxides were attributed to titania domain sizes decreasing with increasing Si content in accord with the well-known quantum size effect. The smallest domain size was estimated as less than 1 nm for a sample with a Ti:Si atomic ratio of 1:8. Lattice vibrations in the materials were probed with FT-Raman and FT-IR absorption spectroscopies, and the results were consistent with a shortening of the Ti-O bond and the formation of Ti-O-Si linkages in the mixed oxides. In situ experiments showed that adsorbed water profoundly affects the localmore » environment around surface Ti atoms. 44 refs., 11 figs., 3 tabs.« less

X-Ray Absorption Near Edge Structure Analysis of HgMnSe, HgFeSe and HgTeSe

Experimental studies of the Se K X-ray absorption edges have been carried out in HgTe0.5Se0.5, Hg0.8 Mn0 .2 Se and Hg0.9Fe0.1Se. A comparison is made for Hg0.5Mn0.5Se and Hg0,5 Fe0,5 Se theoretical results obtained by the self-consistent, semi-relativistic LMTO method within the LD approximation. Small differences of Se K edge XANES have been shown for HgTeSe and HgSe in contrary to evident differences for HgMnSe and HgFeSe. This behaviour of the absorption Se K edges is related to significant, hybridized contribution of the 3d and 4s Mn or Fe unoccupied states. The maximal contributions of the hybridized bands of the hypothetic zinc-blende MnSe or FeSe were estimated using the virtual crystal model. These maximal contributions are situated at about 6 eV and 17 eV in zinc-blende MnSe and at -0.8 eV and 5.5 eV in zinc-blende FeSe. For HgFeSe apart from the evidence of the strongly localized resonant donor (empty for low Fe concentrations) the complementary hybridized contribution of Fe 3d empty states also situated around the conduction band minimum should be taken into consideration.