Showing papers on "Extended X-ray absorption fine structure published in 2002"

Parallel calculation of electron multiple scattering using Lanczos algorithms

Abstract: Real space multiple scattering calculations of the electronic density of states and x-ray spectra in solids typically scale as the cube of the system and basis set size, and hence are highly demanding computationally. For example, such x-ray absorption near edge structure (XANES) calculations typically require clusters of order ${N}_{R}$ atoms and s, p, and d states for convergence, with ${N}_{R}$ between about ${10}^{2}--{10}^{3};$ for this case about ${10}^{2}$ inversions of ${9N}_{R}\ifmmode\times\else\texttimes\fi{}{9N}_{R}$ matrices are needed, one for each energy point. We discuss here two ways to speed up these calculations: (1) message passing interface (MPI) parallel processing and (2) fast, Lanczos multiple scattering algorithms. Together these algorithms can reduce computation times typically by two orders of magnitude. These are both implemented in a generalization of the ab initio self-consistent FEFF8 code, which thus makes practical XANES calculations in complex systems with of order ${10}^{3}$ atoms. The Lanczos algorithm also yields a natural crossover between full and finite-order multiple scattering with increasing energy, thus differentiating the extended and near-edge regimes.

Formation of Ce1-xPdxO2-δ solid solution in combustion-synthesized Pd/CeO2 catalyst: XRD, XPS, and EXAFS investigation

Abstract: Pd/CeO2 (1 at. %) prepared by the solution-combustion method shows a higher catalytic activity for CO oxidation and NO reduction than Pd metal, PdO, and Pd dispersed over CeO2 by the conventional method. To understand the higher catalytic properties, the structure of 1 at. % Pd/CeO2 catalyst material has been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The diffraction lines corresponding to Pd or PdO are not observed in the high-resolution XRD pattern of 1 at. % Pd/CeO2. The structure of 1 at. % Pd/CeO2 could be refined for the composition of Ce0.99Pd0.01O1.90 in the fluorite structure with 5% oxide ion vacancy. Pd(3d) peaks in the XPS in I at. % Pd/CeO2 are shifted by 3 eV indicating that Pd is in a highly ionic +2 state. EXAFS studies show the average coordination number of 3 around Pd2+ ion in the first shell of 1 at. % Pd/CeO2 at a distance of 2.02 Angstrom, instead of 4 as in PdO. The second shell at 2.72 Angstrom is due to Pd-Pd correlation which is larger than 2.69 Angstrom in PdO. The third shell at 3.31 Angstrom having 7 coordination is absent either in Pd metal or PdO, which can be attributed to -Pd2+-Ce4+- correlation. Thus, 1 at. % Pd/CeO2 forms the Ce1-xPdxO2-delta type of solid solution having -Pd2+-O-2-Ce4+- kinds of linkages.

Experimental and Theoretical Studies on the Reaction of H2 with NiO: Role of O Vacancies and Mechanism for Oxide Reduction

Abstract: Reduction of an oxide in hydrogen is a method frequently employed in the preparation of active catalysts and electronic devices. Synchrotron-based time-resolved X-ray diffraction (XRD), X-ray absorption fine structure (NEXAFS/EXAFS), photoemission, and first-principles density-functional (DF) slab calculations were used to study the reaction of H2 with nickel oxide. In experiments with a NiO(100) crystal and NiO powders, oxide reduction is observed at atmospheric pressures and elevated temperatures (250−350 °C), but only after an induction period. The results of in situ time-resolved XRD and NEXAFS/EXAFS show a direct NiO→Ni transformation without accumulation of any intermediate phase. During the induction period, surface defect sites are created that provide a high efficiency for the dissociation of H2. A perfect NiO(100) surface, the most common face of nickel oxide, exhibits a negligible reactivity toward H2. The presence of O vacancies leads to an increase in the adsorption energy of H2 and substanti...

X-ray absorption near-edge structure calculations with the pseudopotentials: Application to the K edge in diamond and α -quartz

Abstract: We present a reciprocal-space pseudopotential scheme for calculating x-ray absorption near-edge structure (XANES) spectra. The scheme incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole interaction through large supercells (hundreds of atoms). The method is compared with recently developed Bethe-Salpeter approach. The method is applied to the carbon K edge in diamond and to the silicon and oxygen K edges in $\ensuremath{\alpha}$-quartz for which polarized XANES spectra were measured. Core-hole effects are investigated by varying the size of the supercell, thus leading to information similar to that obtained from cluster size analysis usually performed within multiple scattering calculations.

Structural Investigation of Combustion Synthesized Cu/CeO2 Catalysts by EXAFS and Other Physical Techniques: Formation of a Ce1-xCuxO2-δ Solid Solution

Abstract: The structure and chemical environment of Cu in Cu/CeO2 catalysts synthesized by the solution combustion method have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and extended X-ray fine structure (EXAFS) spectroscopy. High-resolution XRD studies of 3 and 5 atom % Cu/CeO2 do not show CuO lines in their respective patterns. The structure could be refined for the composition Ce1-xCuxO2-δ (x = 0.03 and 0.05; δ ∼ 0.13 and 0.16) in the fluorite structure with 5−8% oxide ion vacancy. High-resolution TEM did not show CuO particles in 5 atom % Cu/CeO2. EPR as well as XPS studies confirm the presence of Cu2+ species in the CeO2 matrix. Redox potentials of Cu species in the CeO2 matrix are lower than those in CuO. EXAFS investigations of these catalysts show an average coordination number of 3 around the Cu2+ ion in the first shell at a distance of 1.96 A, indi...

Cesium adsorption on clay minerals: an EXAFS spectroscopic investigation.

Abstract: Cesium adsorption on the clay minerals vermiculite and montmorillonite is described as a function of surface coverage using extended X-ray adsorption fine structure spectroscopy (EXAFS). Cesium (Cs) possessed a variable coordination environment consisting of Cs-O distances between 3.2 and 4.3 A; however, disorder typical of the Cs coordination environments prevented the resolution of all oxygen shells. On the basis of the influence of Cs loading and exchangeability on this structural arrangement, we could recognize both inner-sphere and outer-sphere adsorption complexes. The shorter Cs-O bond distance belongs to outer-sphere complexes typical of hydrated ions. In inner-sphere complexes, partially or fully dehydrated Cs coordinates directly to siloxane groups of the clay minerals forming longer Cs-O bonds. The inner-sphere adsorption complexes may have occurred within the interlayer or at frayed edge sites and were less extractable than the outer-sphere complexed Cs. Both coordination number ratios and linear combination fitting of EXAFS spectra were useful in estimating the fractions of inner-sphere and outer-sphere adsorption complexes. Our results show that X-ray absorption spectroscopy (XAS), and particularly EXAFS, is a valuable technique for exploring the type of Cs binding in environmental samples.

XPS, XANES and EXAFS investigations of CuO/ZnO/Al2O3/ZrO2 mixed oxide catalysts

Abstract: Systematic X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (AES), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies were undertaken to investigate the electronic structure, chemical states and local geometry of the active species in the CuO/ZnO/Al2O3/ZrO2 multicomponent mixed oxide catalysts employed in the oxidative steam reforming of methanol (OSRM) reaction for H2 production. The core level XPS and AES indicated the existence of CuO and ZnO-like species. Two kinds of zirconium species, one similar to that of ZrO2 and another with relatively higher electron density were noticed from the Zr 3d core level XPS of Zr- containing catalysts. The valence band (VB) XPS studies revealed that for Zr-containing catalysts, the Cu 3d anti-bonding orbital splits from the main VB and shifts toward lower binding energy (BE). The surface Cu/(Al + Zr) ratios were found to be close to those in the bulk while segregation of Zn at the surface was evidenced in all samples. The XANES and EXAFS results also indicated the existence of CuO and ZnO-like species, whose local environments are modified with respect to the chemical composition. The EXAFS study of the Zr-containing catalysts indicated the existence of a “Cu–O–Zr” bonding with a Cu–Zr distance in the range 3.5 to 3.9 A. The results indicated the existence of a Cu–Zr synergistic interaction in these catalysts which improved the catalytic performance in the OSRM reaction

Hydrogen and higher shell contributions in Zn2+, Ni2+, and Co2+ aqueous solutions: an X-ray absorption fine structure and molecular dynamics study.

Abstract: A detailed investigation of the hydration structure of Zn2+, Ni2+, and Co2+ in water solutions has been carried out combining X-ray absorption fine structure (EXAFS) spectroscopy and Molecular Dynamics (MD) simulations. The first quantitative analysis of EXAFS from hydrogen atoms in 3d transition metal ions in aqueous solutions has been carried out and the ion−hydrogen interactions have been found to provide a detectable contribution to the EXAFS spectra. An accurate determination of the structural parameters associated with the first hydration shell has been performed and compared with previous experimental results. No evidence of significant contributions from the second hydration shell to the EXAFS signal has been found for these solutions, while the inclusion of the hydrogen signal has been found to be important in performing a quantitative analysis of the experimental data. The high-frequency contribution present in the EXAFS spectra has been found to be due to multiple scattering (MS) effects inside...

Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation

Abstract: The oxidation state and local geometry of tellurium in MoVTeNbO catalysts used in the ammoxidation or oxidation of propane were characterized by X-ray absorption, Mossbauer, and X-ray photoelectron (XPS) spectroscopies. The results obtained by Mossbauer, and XPS spectroscopies showed that the catalysts contained Te(IV) in the bulk and mainly Te(VI) at the surface. X-ray absorption fine structure (EXAFS) measurements allowed to determine that the tellurite entities corresponded to TeO4E trigonal bipyramid in the hexagonal phase and to TeO3E somewhat distorted trigonal pyramid in the orthorombic one. The completely determined environment appeared to correspond in both phases to crystallographic sites in hexagonal channels. These results allowed to determine the stoichiometries of the two phases which are TeM3O10 for the hexagonal phase and Te2M20O57 for the orthorombic phase (M=Mo, V, Nb). 125Te Mossbauer isomer shift and pre-peak surface of the X-ray absorption spectra of the Te LIII-edge have been correlated and contributions of the Te(5s) and Te(2p3/2) to the structure have been analyzed.

Simultaneous Presence of Cationic and Reduced Gold in Functioning MgO-Supported CO Oxidation Catalysts: Evidence from X-ray Absorption Spectroscopy

Abstract: Gold clusters on the surface of MgO powder (calcined at 673 K) were prepared from adsorbed [Au(CH3)2(acac), where acac is C5H7O2] and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES). One sample initially contained gold predominantly in the form of clusters approximated as Au6 on the basis of the EXAFS data showing first- and second-shell Au−Au coordination numbers of 4.0 ± 0.4 and 1.0 ± 0.1, respectively. The other sample initially contained larger clusters, with an average diameter of about 30 A (containing about 100 atoms each, on average), as shown by the EXAFS first- and second-shell Au−Au coordination numbers of 9.4 ± 0.9 and 3.5 ± 0.4, respectively. The samples, in each of the three gases CO, O2, and He and in the presence of CO + O2 during CO oxidation catalysis, were investigated by EXAFS spectroscopy and XANES in a cell that was also a flow reactor. Data obtained during steady-state CO oxidation indicate the presen...

Surface relaxation in liquid water and methanol studied by x-ray absorption spectroscopy

Abstract: X-ray absorption spectroscopy is a powerful probe of local electronic structure in disordered media. By employing extended x-ray absorption fine structure spectroscopy of liquid microjets, the intermolecular O–O distance has been observed to undergo a 5.9% expansion at the liquid water interface, in contrast to liquid methanol for which there is a 4.6% surface contraction. Despite the similar properties of liquid water and methanol (e.g., abnormal heats of vaporization, boiling points, dipole moments, etc.), this result implies dramatic differences in the surface hydrogen bond structure, which is evidenced by the difference in surface tension of these liquids. This result is consistent with surface vibrational spectroscopy, which indicates both stronger hydrogen bonding and polar ordering at the methanol surface as a consequence of “hydrophobic packing” of the methyl group.

Environmental and biological applications of extended x-ray absorption fine structure (exafs) and x-ray absorption near edge structure (xanes) spectroscopies

Abstract: XAS (X-ray absorption spectroscopy) has proven to be a powerful technique in several fields including the biological and environmental sciences. It has enabled scientists to analyze samples that could not be analyzed using classical techniques such as XRD (X-ray diffraction). In addition, it allows for the direct determination of elemental oxidation states, where the use of other methods is time consuming, cumbersome, and can lead to false results. XAS experimentation has allowed the direct determination of the local coordination environments in many different samples. This review article briefly explains some of the concepts related to two of the most important techniques of XAS, which are EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure). An explanation of some of the parameters used to obtain XAS data and data reduction from raw spectroscopic data is discussed. Finally, some applied examples are given that has been obtained though the use of XAS ...

EXAFS and neutron diffraction study of the Heusler alloy Co 2 MnSi

Abstract: Spintronic devices, devices that rely upon the spin of the conduction electron to control device properties, are of increasing scientific and commercial interest. These devices commonly are heterostructures with ferromagnetic metals used as spin injectors. The performance of these heterostructured devices can be optimized by using a 100% spin-polarized ferromagnet. Recent band-structure calculations predict that the Heusler alloy ${\mathrm{Co}}_{2}\mathrm{MnSi}$ should be a 100% spin-polarized ferromagnet. However, band-structure calculations further suggest that chemical disorder in the ${\mathrm{Co}}_{2}\mathrm{MnSi}$ lattice should lower the degree of spin polarization. In this work, we present neutron diffraction and extended x-ray-absorption fine-structure measurements (EXAFS) on bulk ${\mathrm{Co}}_{2}\mathrm{MnSi},$ which reveal extensive chemical disordering even in pure-phase samples and discuss the limitations of EXAFS in measuring this sort of disorder.

Local structure and luminescence of nanocrystalline Y2O3:Eu

Abstract: Nanosized Y2O3:Eu was synthesized by using a solution–combustion method. The samples were characterized by x-ray diffraction, extended x-ray absorption fine structure (EXAFS), and photoluminescence spectroscopy. Our EXAFS result showed that the local environments surrounding both Y and Eu atoms had obviously changed when the particle size was smaller than 10 nm. The coordination number and the bond length of Y (Eu)–O shell increased and the disorder became larger. This change also produced an emission peak and the charge-transfer band showed an unusual redshift as the particle size decreased. The luminescence properties can be explained by use of the EXAFS results.

Multiedge refinement of extended x-ray-absorption fine structure of manganese zinc ferrite nanoparticles

Abstract: The structure of nanoparticle manganese zinc ferrites synthesized by a reverse micellar method is determined by analysis of the extended x-ray-absorption fine structure in combination with other techniques. Both empirical and theoretical standards are employed; manganese, zinc, and iron edges are refined simultaneously. It is determined that samples synthesized under similar conditions sometimes exhibit a markedly different distribution of cations between the available sites in the spinel structure; this in turn causes significant differences in the magnetic properties of the samples. In addition, it is found that the mean-square displacements for manganese-oxygen bonds are consistently higher than for zinc-oxygen bonds, perhaps due to the presence of manganese ions of more than one valence.

In Situ Iridium LIII-Edge X-ray Absorption and Surface Enhanced Raman Spectroscopy of Electrodeposited Iridium Oxide Films in Aqueous Electrolytes

Abstract: Structural and electronic aspects of IrO2 films prepared by electrodeposition on Au substrates were investigated by in situ LIII-edge X-ray absorption and surface enhanced Raman spectroscopies in both acid and alkaline aqueous solutions. Linear correlations were found between the extent of oxidation of Ir3+ in the films determined from a statistical fit of the white line, which includes contributions from each of the sites differing by a single electron, and from coulometric analysis of the voltammetric curves. Analysis of the extended X-ray absorption fine structure (EXAFS) yielded Ir−O bond lengths decreasing in the sequence 2.02, 1.97, and 1.93 A for Ir3+, Ir4+, and Ir5+ sites, respectively. Whereas SERS provided evidence for the presence of crystalline IrO2 in the highly hydrated films, the lack of intense shells in the Fourier transform of the EXAFS function beyond the nearest oxygen neighbors indicates that the films do not display long-range order.

Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy

Abstract: The crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.

X-ray absorption spectroscopy of the cubic and hexagonal polytypes of zinc sulfide

Abstract: We investigate the sensitivity of x-ray absorption spectroscopy to cubic-hexagonal polytypism in which nearest-neighbor positions are virtually unchanged. Experimental spectra and multiple-scattering calculations are reported at the major absorption edges in the zinc-blende and wurtzite modifications of ZnS. We demonstrate that d-like unoccupied bands are preferentially affected, as only L-edge absorption is sensitive to this structural transition. The results allow near-edge x-ray absorption spectroscopy to be evaluated as a detection method for crystal structure modifications in nano-scale systems.

Combined XANES and EXAFS analysis of Co 2 + , Ni 2 + , and Zn 2 + aqueous solutions

Abstract: A combined x-ray-absorption near edge structure (XANES) and extended x-ray-absorption fine-structure (EXAFS) quantitative analysis of ${\mathrm{Co}}^{2+},$ ${\mathrm{Ni}}^{2+},$ and ${\mathrm{Zn}}^{2+}$ in water solutions based on fitting procedures of both low- and high-energy ranges has been carried out. The hydrogen contribution has been accounted for in both analyses and the effect of its inclusion on the structural parameters has been highlighted. The structural results obtained from the XANES and EXAFS analyses are in good agreement, confirming the validity of the application of the multiple-scattering theory in the low-energy range of the x-ray-absorption spectra. A systematic shortening of the ion-water first shell distance of about $0.03 \mathrm{\AA{}}$ is obtained from the XANES analyses, as compared to the EXAFS one. The origin of this systematic effect has been deeply investigated and it has been found to be due to the low-energy behavior of the real part of the Hedin-Lundqvist potential used in the calculations.

Structure of Pt-Co/Al2O3 and Pt-Co/NaY bimetallic catalysts: characterization by in situ EXAFS, TPR, XPS and by activity in Co (carbon monoxide) hydrogenation

Abstract: Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt-Co/NaY and Pt-Co/Al2O3 bimetallic catalysts. The EXAFS experiments were carried out at the Pt LIII and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H2 results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt-Co bimetallic clusters, in which only Pt-Co bonds exist (no Pt-Pt bonds). TPR and XPS results indicating a reduction of Co2+ ions in Pt-Co/NaY to a greater extent than in Pt-Co/Al2O3 give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt-Co bimetallic system.

Characterization of hydrogen bond acceptor molecules at the water surface using near-edge x-ray absorption fine-structure spectroscopy and density functional theory

Abstract: We present a combined experimental/computational study of the near-edge x-ray absorption fine structure of the liquid water surface which indicates that molecules with acceptor-only hydrogen bonding configurations constitute an important and previously unidentified component of the liquid/vapour interface.