Showing papers on "XANES published in 1998"

Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure

Abstract: A self-consistent real-space multiple-scattering (RSMS) approach for calculations of x-ray-absorption near-edge structure (XANES) is presented and implemented in an ab initio code applicable to arbitrary aperiodic or periodic systems This approach yields a quantitative interpretation of XANES based on simultaneous, self-consistent-field (SCF) calculations of local electronic structure and x-ray absorption spectra, which include full multiple scattering from atoms within a small cluster and the contributions of high-order MS from scatterers outside that cluster In addition, the code includes a SCF estimate of the Fermi energy and an account of orbital occupancy and charge transfer We also present a qualitative, scattering-theoretic interpretation of XANES Sample applications are presented for cubic BN, ${\mathrm{UF}}_{6},$ Pu hydrates, and distorted ${\mathrm{PbTiO}}_{3}$ Limitations and various extensions are also discussed

Surface Structures and Stability of Arsenic(III) on Goethite: Spectroscopic Evidence for Inner-Sphere Complexes

Abstract: The adsorption and stability of arsenite [As(III)] on goethite (α-FeOOH) was investigated using a combination of standard batch techniques and X-ray absorption spectroscopy (XAS). The reactivity of As(III) with α-FeOOH at varying pH and As(III) concentration provided macroscopic evidence for strong complexation on the α-FeOOH surface. Extended X-ray absorption fine structure (EXAFS) spectroscopy gave an average As(III)−Fe interatomic distance of 3.378 ± 0.014 A, which is indicative of bidentate binuclear bridging As(III) complexes on the α-FeOOH surface and which is similar to other oxyanions which adsorb on α-FeOOH by an inner-sphere mechanism. X-ray absorption near-edge structure (XANES) analysis indicated that the As(III)−α-FeOOH surface complex is stable toward heterogeneous oxidation to As(V), as determined by the energy position of the X-ray absorption edge. The structural information from EXAFS was included in the description of As(III) adsorption on the α-FeOOH surface using a surface complexation...

Calculations of near-edge x-ray-absorption spectra of gas-phase and chemisorbed molecules by means of density-functional and transition-potential theory

Abstract: We explore the utility of density-functional theory ~DFT! in conjunction with the transition-potential ~TP! method to simulate x-ray-absorption spectra Calculations on a set of small carbon-containing molecules and chemisorbed species show that this provides a viable option for obtaining excitation energies and oscillator strengths close to the experimental accuracy of core-valence transitions Systematic variations in energy positions and intensities of the different spectra in the test series have been investigated, and comparison is made with respect to the static exchange-, self-consistent-field, and explicit electron-correlation methods The choice between standard exchange-correlation functionals is shown to be of little consequence for the valence resonant, here p*, parts of the x-ray-absorption spectra, while the long-range behavior of presently available functionals is found not to be completely satisfactory for Rydberg-like transitions Implementing a basis set augmentation technique, one finds that DFT methods still account well for most of the salient features in the near-edge x-ray-absorption spectra, save for the multielectron transitions in the near continuum, and for some loss of Rydberg structure For clusters modeling surface adsorbates, the DFT transition potential method reproduces well the spectral compression and intensity reduction for the valence level absorption compared to the free phase, provided fairly large clusters are taken into account While for near-edge x-ray-absorption fine-structure ~NEXAFS! spectra of free molecules the DFT-TP and Hartree-Fock/static exchange methods have complementary advantages, the DFT-TP method is clearly to be preferred when using clusters to simulate NEXAFS spectra of surface adsorbates @S0163-1829~98!00136-2#

Direct Synthesis and Characterization of Hydrophobic Aluminum-Free Ti−Beta Zeolite

Abstract: Incorporation of Ti into the framework of aluminium-free zeolite Beta has been achieved in F- medium and has produced hydrophobic selective oxidation catalysts. The Ti−Beta(F) materials have been characterized by X ray diffraction, infrared, Raman, ultraviolet, XANES, EXAFS, 29Si MAS NMR, and 1H→29Si CP MAS NMR spectroscopies, adsorption microcalorimetry, and catalytic testing. At near neutral pH the incorporation of Ti into the framework appears to present an upper limit of ca. 2.3 Ti/uc, beyond which anatase is detected in the calcined materials. However, at higher pH (ca. 11) larger amounts of Ti can be incorporated without anatase formation. After calcination, Ti incorporation in the framework is characterized by an increase in the unit cell volume, the appearance of one Raman band and three infrared bands in the region near 960 cm-1 and the presence of a strong absorption band in the 205−220 nm ultraviolet spectrum. By 29Si MAS NMR, 1H→29Si CP MAS NMR, and infrared spectroscopies it is concluded that...

Local structure and the phase transitions of BaTiO3

Abstract: We have measured and analyzed the Extended X-ray Absorption Fine Structure (EXAFS) of BaTiO3 at the barium k edge and the X-ray Absorption Near Edge Structure (XANES) at the titanium K edge. Our structural data show that the sequence of phase transitions in this material as the temperature increases is explained by a disordering of domains wherein the local structural environment remains approximately rhombohedrally distorted at all temperatures around both metal sites. As the temperature is raised, the long range correlations between these local distortions change, resulting in the observed sequence of phase transitions. Our measurements confirm the model of eight-site disorder used to explain the phase diagram of BaTiO3. We show that EXAFS and XANES are sensitive probes of both the magnitude and direction of the local structural distortions which accompany ferroelectricity and therefore are sensitive probes of the microscopic mechanism of ferroelectricity.

Structural and Electronic Properties of Sol−Gel Titanium Oxides Studied by X-ray Absorption Spectroscopy

Abstract: X-ray absorption spectroscopy (XAS) has been used to study the local Ti environment in titania xerogel samples containing nanoparticles of different sizes. The xerogels were prepared by hydrolysis of titanium isopropoxide followed by peptization with HNO3 and size control was achieved through calcination in air at different temperatures. An amorphous precipitate obtained by hydrolysis of titanium isopropoxide prior to peptization with HNO3 has also been studied. The X-ray absorption near edge structure (XANES) of the precipitate possesses a characteristic preedge that is dominated by a transition designated as A2 at 4970.7 eV, and assigned to five coordinate Ti. This A2 component is detected in all of the studied xerogels. Its intensity decreases as the surface-to-volume ratio of the titania particles decreases, suggesting that it is associated with surface layers of the anatase particles. Other changes observed in the XANES of the xerogels as a function of particle size include broadening of 1s → np tran...

Adsorption and oxidation of arsenite on goethite

Abstract: Arsenic toxicity, mobility, and bioavailability in soil-water systems are highly dependent on its oxidation states and chemical species. In this study, both indirect (FTIR) and direct (X-ray Absorption Near Edge Structure (XANES) spectroscopic techniques were applied to investigate the adsorption

Pre-edge fine structure of the 3d atom K x-ray absorption spectra and quantitative atomic structure determinations for ferroelectric perovskite structure crystals

Abstract: A complete interpretation is proposed for the pre-edge fine structure (PEFS) of the x-ray Ti K-absorption spectra for perovskite structure crystals. The interpretation is based on the results of numerous calculations performed by a modified full multiple scattering method which provides the theoretical spectra for the 3d transition metal oxides in fair agreement with experiment. It is shown that the three main peaks in the PEFS have quite different origin. The first long-wave side peak A is caused mainly by quadrupole transitions. The middle peak B is caused by the p-d mixture effect and the high intensity of it is considered to be a qualitative spectroscopic indication of ferroelectricity in the perovskite structure crystal. A simple formula is obtained which expresses the area under peak B through the lattice constants and mean-square displacement of the absorbing Ti atom from the instantaneous centre of the coordination polyhedron. The peak B area averaged over thermal atomic vibrations is determined by the three-particle atomic distribution function. The short-wave side peak C is caused by the Ti 1s electron transition to the unoccupied 3d states of the neighbouring transition metal atoms. We show that an additional peak on the short-wave side of peak C occurs if there are 4d atoms (for instance Zr atoms in the vicinity of the absorbing Ti atom in the (PZT) solid solution) within the oxygen atom octahedrons surrounding the absorbing 3d atom. The area under peak is directly determined by the average number of 4d atoms in the vicinity of the absorbing Ti one.

Structural characterization and thermal stability of MoS2 intercalation compounds

Abstract: The intercalation of MoS2 with Co and Fe complexes has been accomplished via exfoliation of LixMoS2 by reaction with water, followed by flocculation in the presence of cationic guest species. The resulting self-assembled compounds have been characterized by X-ray absorption spectroscopy, electron diffraction, powder X-ray diffraction, and magnetic measurements. The 2H-type (undistorted) MoS2 undergoes a structural distortion to 1T-type MoS2 during the Li intercalation, which is partially maintained upon exfoliation and flocculation. A structural model of 1T-MoS2 is proposed on the basis of X-ray absorption spectroscopy and electron diffraction, in which the unit cell has P3 symmetry with lattice parameters a = 6.52(5) A, c = 6.14 A. The Mo atoms are shifted approximately 0.5 A from their positions in 2H-MoS2 to form trigonal clusters in 1T-MoS2, with a concomitant change in coordination geometry from trigonal prismatic to distorted octahedral. Layers of Co(OH)2 were intercalated between the distorted and/...

Atom-resolved electronic spectra for alq3 from theory and experiment

Abstract: The electronic structure of Alq3 is investigated using density functional theory-based calculations, photoemission and near-edge x-ray absorption fine structure. The distinct features of the observed spectra are understood in terms of contributions from the different atoms and molecular orbitals. Fingerprints of the molecular bonding and of the individual atoms are identified. These results are meant to be a reference for the monitoring of chemical processes that Alq3 may undergo during fabrication or degradation of light-emitting devices, and for the understanding of the effects of ligand or metal substitution.

Aluminium X-ray absorption Near Edge Structure in model compounds and Earth’s surface minerals

Abstract: Aluminium K-edge X-ray absorption near edge spectra (XANES) of a suite of silicate and oxides minerals consist of electronic excitations occurring in the edge region, and multiple scattering resonances at higher energies. The main XANES feature for four-fold Al is at around 2 eV lower energy than the main XANES feature for six-fold Al. This provides a useful probe for coordination numbers in clay minerals, gels, glasses or material with unknown Al-coordination number. Six-fold aluminium yields a large variety of XANES features which can be correlated with octahedral point symmetry, number of aluminium sites and distribution of Al-O distances. These three parameters may act together, and the quantitative interpretation of XANES spectra is difficult. For a low point symmetry (1), variations are mainly related to the number of Al sites and distribution of Al-O distances: pyrophyllite, one Al site, is clearly distinguished from kaolinite and gibbsite presenting two Al sites. For a given number of Al-site (1), variations are controlled by changes in point symmetry, the number of XANES features being increased as point symmetry decreases. For a given point symmetry (1) and a given number of Al site (1), variations are related to second nearest neighbours (gibbsite versus kaolinite). The amplitude of the XANES feature at about 1566 eV is a useful probe for the assessment of AlIV/Altotal ratios in 2/1 phyllosilicates. Al-K XANES has been performed on synthetic Al-bearing goethites which cannot be studied by 27Al NMR. At low Al content, Al-K XANES is very different from that of α-AlOOH but at the highest level, XANES spectrum tends to that of diaspore. Al-K XAS is thus a promising tool for the structural study of poorly ordered materials such as clay minerals and natural alumino-silicate gels together with Al-subsituted Fe-oxyhydroxides.

Electronic Properties and Phase Transformations in CoMoO4 and NiMoO4: XANES and Time-Resolved Synchrotron XRD Studies

Abstract: The thermal stability of a series of cobalt and nickel molybdates (AMoO4·nH2O, α-AMoO4, and β-AMoO4; A = Co or Ni) was examined using synchrotron-based time-resolved X-ray powder diffraction (XRD). The results of X-ray absorption near-edge spectroscopy (XANES) indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the α-phases to tetrahedral in the β-phases and hydrates. Upon heating of AMoO4·nH2O, evolution of gaseous water was seen at two different temperature ranges: 100−200 °C for reversibly bound H2O; 200−400 °C for H2O from the crystal structure. The results of time-resolved XRD show a direct transformation of the hydrates into the β-AMoO4 compounds (following a kinetics of first order) without any intermediate phase. This is probably facilitated by the similarities that AMoO4·nH2O and β-AMoO4 have in their structural and electronic properties. The XRD experiments show that the α-AMoO4 → β-AMoO4 transitions occur at m...

Deconvolution Analysis of Ga K-Edge XANES for Quantification of Gallium Coordinations in Oxide Environments

Abstract: A quantitative analysis of gallium species (GaO4 tetrahedra and GaO6 octahedra) by means of X-ray absorption spectroscopy (XANES and EXAFS) at the Ga K-edge was performed on a series of gallium oxides with various phases and SiO2-supported ones. XANES spectra of gallium oxides showed two distinguishable peaks that were assigned to tetrahedral and octahedral gallium oxide species on the basis of a comparison with those of reference compounds. A deconvolution analysis of XANES spectra was carried out on the assumption that the absorption due to each species is expressed by a set of Gaussian and arctangent functions, and the tetrahedral/octahedral ratios were quantitatively estimated from the ratio of the peak areas. By the EXAFS curve-fitting analysis, the tetrahedral/octahedral ratios were also estimated, and they were in good agreement with those obtained by XANES analysis. It was concluded that the Ga atoms with a different coordination symmetry (tetrahedral or octahedral) can be estimated quantitatively...

Diels-Alder reactions of butadienes with the Si(100)-2×1 surface as a dienophile: Vibrational spectroscopy, thermal desorption and near edge x-ray absorption fine structure studies

Abstract: The mechanism and intermediates of a Diels–Alder-type cycloaddition reaction between dienes and the silicon dimers of a Si(100)-2×1 surface, which was theoretically predicted by Doren and Konecny, have been investigated. The reactions of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene were studied using multiple internal reflection infrared spectroscopy, thermal desorption spectrometry, and near edge x-ray absorption fine structure (NEXAFS) measurements. The results show that the compounds physisorb on Si(100)-2×1 at cryogenic temperature. Infrared studies of the room temperature adsorption of both dienes indicate that reaction leads to the formation of stable, chemisorbed Diels–Alder adducts. By NEXAFS measurements on 2,3-dimethyl-1,3-butadiene, we determine that the angle between the π orbitals of the reaction product and the Si(100)-2×1 surface is near 40°. Upon heating, the chemisorbed butadienes primarily decompose to form adsorbed carbon and hydrogen at the surface. Hydrogenation of chemisorbed butadienes by atomic hydrogen was also investigated, and conversion from the Diels–Alder geometry to [2+2] bonding is observed. This effect is attributed to cleavage of the Si–Si dimer bond upon atomic hydrogen exposure. Temperature dependent studies suggest the presence of an activation barrier for chemisorption at low temperature. The parallels between our experimental results and the theoretical studies of these reactions are discussed.

XANES, EXAFS and FTIR characterization of copper-exchanged mordenite

Abstract: Over-exchanged CuI-mordenite has been prepared by gas phase reaction between the protonic form of the zeolite and CuICl No evidence was found for the presence of CuII ions or copper species in microaggregates EXAFS analysis revealed that CuI ions in the outgassed zeolite are surrounded by 25 ± 035 oxygen atoms of the zeolite framework, the average CuI–O distance being 196 (±002) A Both X-ray absorption and FTIR spectroscopy showed that the zeolite adsorbs CO at room temperature with formation of CuI(CO)n adducts (n = 1, 2) At liquid nitrogen temperature, tricarbonyl species were also formed The IR spectra of adsorbed CO showed broad and complex bands, which is consistent with the fact that the zeolite contains different types of cation sites accessible to carbon monoxide This was further demonstrated by comparison with corresponding IR and EXAFS spectra of CO adsorbed on CuI-ZSM-5

Effect of Ag‐promotion on Pd catalysts by XANES

Abstract: Pd L3 near‐edge absorption measurements (XANES) were performed on four commercial acetylene hydrogenation catalyst samples, with and without the Ag promoter. The Pd L3 edge XANES spectra showed that the Ag‐promoted catalysts have relatively weaker absorption peaks and they follow the same order as the relative commercial performances of the four catalysts studied in terms of selectivity in ethylene purification, which indicates that there are increases in the Pd d‐band electron densities due to the addition of Ag. These results provide a reasonable explanation for the observed improvement in selectivity of the Ag‐promoted acetylene hydrogenation catalysts. The Ag L3 XANES spectra of the supported Pd–Ag catalysts indicate the absence of a white‐line feature which seems to suggest that the charge transferred from Ag to Pd may not be the d‐type.

Electronic Structures and Chemical Bonding of Fluorinated Fullerenes Studied by NEXAFS, UPS, and Vacuum-UV Absorption Spectroscopies

Abstract: The chemical bonding and the electronic structures of C60Fx and C70Fx were investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and UV photoemission spectroscopy (UPS), which are useful methods for examining the unoccupied and the occupied states, respectively. With these results and XPS measurements, we derived the electronic energy diagram of C60Fx and discussed the change of the electronic structure from that of C60 by fluorination. The energies of the LUMO and the Fermi level of solid C60F48 were estimated to be −5.0 and −5.4 eV below the vacuum level, indicating that highly doped C60Fx is a strong electron acceptor. The electronic absorption spectra of C60Fx solutions deep into the vacuum-ultraviolet region were also measured, and the isomerism of C60Fx was discussed by comparing the observed results with theoretical simulations.

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}

Effect of Heat Treatment on the Redox Properties of Iron Porphyrins Adsorbed on High Area Carbon in Acid Electrolytes: An in Situ Fe K-Edge X-ray Absorption Near-Edge Structure Study

Abstract: Electronic aspects of an iron porphyrin, 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (FeTMPPCl) molecularly dispersed on a high area carbon black (Black Pearls 2000, BP) and then heat-treated (or thermally activated) at 800 °C in a flowing inert atmosphere, were investigated in situ in 1.0 M H3PO4 by Fe K-edge X-ray near-edge structure (XANES). Profound differences were observed between the ex situ (dry) XANES of electrodes incorporating FeTMPPCl/BP before and after heat treatment and, likewise, in the case of the thermally treated FeTMPPCl/BP before and after immersion of the electrodes in the electrolyte. Monotonic shifts in the absorption edge toward higher energies were observed for heat-treated FeTMPPCl/BP as the potential was increased over a range of over 1 V. The overall magnitude of the shift, ca. 2.5 eV, was virtually the same as that obtained with non-heat-treated (or intact) FeTMPPCl/BP, which occurred in a much narrower potential range, ca. 0.2 V. In contrast to t...

XANES studies of the reduction behavior of (Ce1-yZry)O2 and Rh/(Ce1-yZry)O2

Abstract: Using X-ray absorption near-edge spectroscopy (XANES) at the Ce LIII edge, we have measured the extent of reduction of Rh-loaded and Rh-free, mixed Ce-Zr oxides under hydrogen as a function of temperature. The high surface area, mixed oxides were synthesized by sol-gel techniques and hypercritical drying. Using a simple spectrum subtraction method, the degree of reduction has been measured and compared with previous results for CeO2 and (Ce0.5Zr0.5)O2. Addition of Zr lowers the temperature of reduction and increases the extent of Ce reduction. Rh catalyzes the reduction process at low temperatures but does not substantially affect the extent of reduction achieved at high temperature. A synergism between Rh and Zr is found which leads to very high reducibility in the range of 400–600 K.

In situ examination of uranium contaminated soil particles by micro-x-ray absorption and micro-fluorescence spectroscopies

Abstract: Two complimentary spectroscopic techniques, X-ray absorption and fluorescence spectroscopy have been conducted at spatial scales of 1 to 25 μm on uranium contaminated soil sediments collected from two former nuclear materials processing facilities of the DOE: Fernald, OH and Savannah River Site, SC. A method of imbedding particles in a non-reactive Si polymer was developed such that individual particles could be examined before and after extraction with a wide range of chemicals typically used in sequential extraction techniques and others proposed forex situ chemical intervention technologies. Using both the micro-X-ray fluorescence (XRF) and micro-X-ray Absorption Near Edge Structure (XANES) techniques, both elemental and oxidation state distribution maps were generated on individual particles before and following chemical extraction. XANES can determine the relative proportion of U(VI) and U(IV) in phases comprising individual particles before and after extraction and showed that greater than 85% of the uranium existed as hexavalent U(VI). Fluorescence spectra of contaminated particles containing mainly U(VI) revealed populations of uranyl hydroxide phases and demonstrated the relative efficacy and specificity of each extraction method. Correlation of XAS and fluorescence data at micron scales provides information of U oxidation state as well as chemical form in heterogeneous samples.

Near edge x-ray absorption fine structure study of benzene adsorbed on metal surfaces: Comparison to benzene cluster complexes

Abstract: Near edge x-ray absorption fine structure (NEXAFS) spectroscopy has been used to study benzene adsorbed on several close-packed metal surfaces [Au(111), Rh(111), Pt(111), and Ru(0001)] under the same experimental conditions. The NEXAFS spectra reveal significant differences with regard to the shapes and positions of resonances and the dichroism observed for the resonance intensities. These differences correlate directly with the binding energy and can be attributed to differences in the electronic coupling of the benzene molecule to the substrate and to adsorption in- duced changes in the molecular structure of the benzene molecule. A comparison to the NEXAFS spectra recorded for the benzene metal cluster complexes [Ru(η6–C6H6)(CH3CN)3]2+ and Ru3(CO)9(μ3:η2:η2:η2–C6H6) indicates that the μ3-arene complex, where the benzene ring is bonded to three metal atoms, can be regarded as a reasonable cluster analog for the adsorption of benzene on the Ru(0001) transition metal surface.

Analysis of the x-ray absorption spectra of linear saturated hydrocarbons using the Xα scattered-wave method

Abstract: The Xα scattered-wave (Xα-SW) method was used to calculate the C K x-ray absorption (NEXAFS) spectrum of propane which previously was used as a model system for larger saturated hydrocarbons. The absorption resonances below the ionization threshold are analyzed in detail and characterized with respect to their valence and/or Rydberg-character. The shape resonances above the ionization threshold were investigated by calculating the eigenchannel wave functions involved in the formation of the resonances. The applicability of the results to the spectra of long alkane chains is studied, in particular for the determination of molecular orientation from their polarization dependence.