Showing papers on "Extended X-ray absorption fine structure published in 1991"

Extended X-ray absorption fine structure determination of the structure of cobalt in carbon-supported Co and Co-Mo sulfide hydrodesulfurization catalysts

Abstract: The structure of the cobalt present in carbon-supported Co and Co-Mo sulfide catalysts was studied by means of X-ray absorption spectroscopy at the Co K-edge and by X-ray photoelectron spectroscopy (XPS). Thiophene hydrodesulfurization activities were used to measure the catalytic properties of these catalysts. By comparison of the EXAFS and XANES spectra of the catalysts with those of c09sS and Cos2 model compounds, it was concluded that all Co atoms in a catalyst prepared with nitrilotriacetic acid as complexing agent were in the "Co-Mo-S" state, while the Co atoms in a conventionally prepared catalyst were partly present in a CO$8-like structure and partly in a "Co-Mo-S" structure. The Co atoms in the To-Mc-S" state have a distorted 5- to 6-fold sulfur coordination, and on the average, every Co atom is in contact with two Mo atoms at a distance of 2.80 A. On the basis of these data, the most likely position for the Co atoms is in front of the square sulfur faces of the MoS6 trigonal prisms along the edges of the MoS, crystallites with two additional sulfur atoms or H2S molecules attached. The Co atoms in the sulfided Co/C catalyst have Co-S and Co-Co coordinations as in c09sg, although the sulfur coordination number is higher.

Remarkable Spreading Behavior of Molybdena on Silica Catalysts. An In Situ EXAFS-Raman Study.

Abstract: In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

X-ray absorption spectroscopy study of the titania- and alumina-supported tungsten oxide system

Abstract: Tungsten oxide supported on titania is an important material for the selective catalytic reduction of nitrogen oxides NO{sub x}. A structure analysis by means of X-ray absorption spectroscopy (XANES and EXAFS) is reported for materials prepared by spreading of WO{sub 3} in physical mixtures with the support oxide as well as by impregnation from aqueous solution. A comparison is also made with alumina-supported materials. Analysis of the XANES region at the W L{sub 1} and W L{sub 3} edges indicated that tungsten is hexavalent and anchored to the surface as WO{sub 5} and WO{sub 4} units, the relative proportion of which increases with loading. When water is absorbed, pseudooctahedrally coordinated species are formed in both cases. The analysis of the EXAFS provides additional support for the existence of these structures, which contain oxo groups W{double bond}O and W-O-W bridges. A tentative structure model is proposed, in which islands of surface tungstate species are formed by branched chains of WO{sub 5} units. The chains are assumed to be terminated by WO{sub 4} units, the WO{sub 5}/WO{sub 4} ratio thus increasing with chain length or island size, which obviously increases with loading.

Noncubic symmetry in garnet structures studied using extended x-ray-absorption fine-structure spectra

Abstract: The noncubic symmetry appearing in garnets that can be represented by A3B5O12 is investigated. Extended x-ray-absorption fine-structure (EXAFS) spectra about Fe, Ga, and Y K-edge and Gd and Er L(III)-edge absorptions of Y3Fe5O12, Gd3Ga5O12, Y3Ga5O12, Y3Al5O12, and Er3Al5O12, respectively, are measured and analyzed. It is first confirmed from the results that about 10% of the A atoms exchange their sites with about 5% of the B atoms. This means, in garnet structure, 10% of the A atoms enter the (a) sites, which should be occupied by B atoms according to the crystallographic data; at the same time, those 5% of B atoms replaced by A go into the (c) sites. It is estimated that about two A and two B atoms exchange their sites with each other in a unit cell. This site exchange can be interpreted as an intrinsic characteristic of the garnet structure. Combining EXAFS results with other experimental ones on noncubic symmetry in garnets, a heuristic structural model is proposed to explain the noncubic symmetry in garnet structure tentatively, in which the atomic positions of atoms at (a) sites in four-body diagonals are shifted by small displacements and two B atoms at (a) sites in the [111] axis are assumed to be replaced by A atoms in a unit cell. Calculations of the x-ray-diffraction relative intensity are carried out with this model. The calculated results on the relations of intensities between {222} diffraction peaks, which are forbidden in cubic symmetry, are in agreement with the experimental ones for Y3Fe5O12 and Gd3Ga5O12. It is recognized that the site exchange between A and B atoms and the small displacement of atomic positions are mainly responsible for the noncubic symmetry in garnets. This noncubic symmetry can be described in R3BAR; i.e., the lowering of space-group symmetry from Ia3BARd into R3BAR occurs for the garnet structure.

Local structure around Er in silica and sodium silicate glasses

Abstract: Extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) measurements have been performed on three different Er-doped glasses: silica and sodium silicate doped with Er in the molten phase, and silica doped by MeV ion implantation. All samples show a luminescent transition centered around a wavelength of λ = 1.54 μ m, corresponding to an intra-4f transition of Er 3+ . EXAFS data show that the Er ions have about six O first neighbors, at a distance of 2.26 A in all systems. Both silica glasses show a second-neighbor ErSi shell at 3.11 A which is not seen in the sodium silicate glass. This difference is reflected in a difference in the PL spectra. The ErO shell in the silica glass shows 0.01 A 2 more disorder than that in the Na-loaded material. These findings are rationalized in terms of the network-modifying effect of Na. Both EXAFS and PL show no significant differences between silica samples prepared in the molten phase or by implantation. At an Er concentration of ∼1 at.%, no direct ErEr bonds are observed.

A study of cation environment and movement during dehydration and reduction of nickel-exchanged zeolite Y by x-ray absorption and diffraction

Abstract: Using synchrotron radiation, we have monitored, by a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy and high-resolution powder X-ray diffraction (XRD), (1) the location of Ni 2+ ions in the hydrated form of nickel ion exchanged Y zeolite (Si/Al=2.25) at room temperature, (2) the environment and evidence of movement of Ni 2+ ions upon heating the zeolite to 300°C in vacuo, and (3) the changes in local environment of the nickel brought about by subsequent reduction in hydrogen

X-ray Absorption Studies of Ceria with Trivalent Dopants

Abstract: The local structural environments of Sc3+ and Gd3+ in ceria were studied for the first time by using EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) spectroscopy. Our results indicate that Sc3+ in 10-mol%-Scz03-doped ceria has a 7-fold-coordinated Sc-ordered structure with two different Sc-0 distances (2.13 and 2.26 A). An octahedrally coordinated Sc-0 cluster is present in 5 mol% sczo3-5 mol% Gd203-Ce02, because of the strong scavenging effect of Sc3+ ions on oxygen vacancies created by Gd3+. The Gd3+ in this sample is largely unassociated. ScZO3 (5 mol%) and GdzO3 (5 or 10 mol%) form random solid solutions with CeOz. These results are consistent with the current concept of defect structures in this structural family. [Key words: X-ray, absorption, ceria, dopants, defects.]

A XANES and EXAFS investigation of titanium silicalite

Abstract: XANES and EXAFS investigations of titanium silicalite-1, a microporous material with the structural features of zeolite ZSM-5, show that a large part of the titanium is coordinated octahedrally and probably occupies framework positions, a local environment that provides an explanation for the extraordinary caltalytic properties of this material.

In situ extended x‐ray absorption fine structure spectroscopy of thin‐film nickel hydroxide electrodes

Abstract: A convenient in situ electrochemical cell has been developed which permits x‐ray absorption measurements on thin‐film electrodes under electrochemical polarization Extended x‐ray absorption fine structure spectra from highly disordered α‐Ni(OH)2 films, before and after polarization, provided quantitative results on the lattice contraction accompanying oxidation Upon oxidation of α‐Ni(OH)2, the Ni‐O distance contracted from 205 to 186 A, and the Ni‐Ni distance contracted from 309 to 282 A The observed 10% contraction in the brucite plane is consistent with the 367 nickel valence in a K(NiO2)3 phase The present technique extends x‐ray absorption spectroscopy to the study of a variety of unstable materials not amenable to ex situ techniques

Cationic trirhenium rafts on gamma-Al2O3 : characterization by X-ray absorption spectroscopy

Abstract: Rhenium surface species, derived by treating [H,Re,(CO),,] adsorbed on y-A1203 in hydrogen at 400 'C, formed extremely small surface grouping of rhenium atoms having an average Re-Re coordination number of 2, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Results of X-ray photoelectron spectroscopy and threshold resonance experiments show that the rhenium was cationic, with oxidation states of about +4 to +6. Infrared spectroscopy was used to follow the decomposition of surface species derived from [H3Re3(C0)12a]d sorbed on y-A1203,t he data indicating that Re subcarbonyls such as [Re(CO)3(O-AI)2(HO-Al)] were formed as intermediates. On the basis of the Re-O and Re-Re coordination parameters determined by EXAFS spectroscopy, a structural model of cationic trirhenium rafts on the y-A1203 surface is suggested. This structure is characterized by a Re-Re distance of 2.67 A, which is greater than the distances characterizing the quadruple and triple Re bonds (2.3 A) in Re complexes but shorter than the distance in bulk metallic Re (2.74 A). The Re-Re distance suggests an intermediate bond order, roughly 1.5. The trirhenium rafts are suggested to be tethered to surface oxygens with a Re-0 distance of 2.05 A indicated by the EXAFS data. A longer Re-O distance, 2.58 A, is also indicated by the EXAFS data, and this is suggested to be indicative of interactions of the rhenium with surface hydroxyl groups. Another y-Al2O3-supported Re sample was prepared by a conventional method with aqueous NH4ReOr as the precursor, with treatment in hydrogen at 450 OC. The resulting surface species were highly nonuniform, consisting of surface rhenium oxide species and metallic rhenium particles.

Synthesis, characterization, and catalytic properties of silica-attached one-atomic-layer niobium oxide catalysts

Abstract: One-atomic-layer niobium oxide catalysts were prepared on SiO 2 surface by using two-stage attaching reactions between Nb(OC 2 H 5 ) 5 and silanols and wered characterized by X-ray diffraction (XRD), electron probe microanalysis (EPMA), X-ray fluorescence (XRF), and extended X-ray absorption fine structure (EXAFS)

Time-resolved UV absorption of polyimide

Abstract: The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm2 excitation and recovers with a fast time constant ≈34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS 1 and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S 1) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.

Sol-gel route to silicon suboxides. Preparation and characterization of silicon sesquioxide

Abstract: The title compound was prepared by sol-gel method from Si 2 Cl 6 . NMR, XANES and EXAFS experiments showed silicon sesquioxide as an individual phase in which each silicon atom is bonded to 3 oxygen atoms and 1 silicon atom. Thus, the 29 Si chemical shift was found at −71 ppm (broad peak), and the XANES spectrum exhibited a characteristic absorption edge at 1840.5 eV. The thermal behavior of SiO 1.5 was studied. No separation of phases Si and SiO 2 could be observed by heating due to the cleavage of the SiSi bonds by adsorbed H 2 O.

Orientation and absolute coverage of furan and 2,5‐dihydrofuran on Ag(110) determined by near edge x‐ray absorption fine structure and x‐ray photoelectron spectroscopy

Abstract: The orientation, absolute coverage and core‐electron binding energies of furan (C4H4O) and 2,5‐dihydrofuran (C4H6O) on Ag(110) have been measured using near edge x‐ray absorption fine structure and x‐ray photoelectron spectroscopy. Both furan and 2,5‐dihydrofuran (2,5‐DHF) are tilted 22±7° from the plane of the surface for both submonolayer and monolayer coverages. The saturation monolayer coverages on the Ag(110) surface of 0.45±0.07 ML for furan and 0.41±0.06 ML for 2,5‐DHF are consistent with expectations based on van der Waals radii. The C(1s) binding energies for a monolayer of furan on Ag(110) are 284.8 and 286.2 eV, while the C(1s) binding energies for 2,5‐DHF are 285.1 and 286.6 eV. The O(1s) binding energies are 532.8 and 533.8 eV for furan and 2,5‐DHF monolayers, respectively. The onset of excitations from C(1s) levels to the continuum lies well above the Fermi level due to the weak bonding interactions with the surface.

Structure and optical absorption of liyv2o5 thin films

Abstract: Films of lithiated vanadium pentoxide (LiyV2O5) are of interest for ion storage in electrochromic “smart windows” and in lithium battery technology. In this paper we study optical absorption of LiyV2O5 films in the 0.3 < λ < 2.5 μm wavelength range, encompassing visible light and solar radiation, in relation to the micro-structure.

Ge K-Edge Extended X-Ray Absorption Fine Structure Study of the Local Structure of Amorphous GeTe and the Crystallization

Abstract: The local structure and crystallization of amorphous GeTe (a-GeTe) were examined by means of Ge K-edge EXAFS. In a-GeTe, both Ge-Ge and Ge-Te bonds were observed to exist in nearest neighbors of Ge. The average coordination number around Ge is 3.7, which is close to the tetrahedral structure. A random covalent network (RCN) model seems to be suitable for the local Structure. After a-GeTe crystallizes at 129°C, the Ge-Ge bond disappears and the Ge-Te bond length increases considerably. As temperature rises, in a-GeTe the Debye-Waller factor of the Ge-Te bond increases greatly, while that of the Ge-Ge bond increases only slightly. At the crystallization, it is found that the fluctuation of the Ge-Te bond length plays a major role in the change of the local structure and bonding state around Ge.

Hexanuclear iridium clusters supported on magnesium oxide

Abstract: [Ir4(C0)12o] n the surface of MgO reacts to give [HIr4(CO)ll]-,w hich upon treatment with co at 200 OC gives [Ir6(C0)15]2- on the surface in high yield. Treatment of the latter with a solution of [PPN][CI] leads to extraction of [Ir6(C0)15]2b-y cation metathesis. The surface-bound [Ir6(CO)15]2w- as characterized by extended X-ray absorption fine structure (EXAFS) sk :e icnt rcolsocsoep ayg,r weeimthe tnhte w diatht at hined piucabtliinshge adn c rayvsetraalgloeg Irra-plrh icc odoartad ifnoart itohnis naunmiobne rin o tfh 4e. 1so alindd s taant ea.v Terhaeg er eIarc-Itrio ndsi sotfa nthcee iorfi d2iu.7m7 carbonyls on the basic MgO surface closely parallel those of iridium carbonyl anions in basic solutions. Treatment of the MgO-supported [h6(CO)IS]2- in He followed by H2 at 300 OC gives decarbonylated surface species shown by EXAFS spectroscopy to have iridium cluster frames substantially unchanged from those of the carbonyl anion precursor. The decarbonylated iridium clusters are among the most nearly uniform supported metals.

How is radiation energy absorption different between the condensed phase and the gas phase

Abstract: (1991). How is radiation energy absorption different between the condensed phase and the gas phase? Radiation Effects and Defects in Solids: Vol. 117, No. 1-3, pp. 143-162.

The temperature dependence of collision‐induced absorption by oxygen near 6 μm

Abstract: Coefficients for oxygen absorption in the infrared induced by collisions with O2 and N2 are reported over the range 1400-1800/cm and 225-356 K. These coefficients are used to calculate the absorption for O2 in air as a function of temperature and wavenumber, and comparisons are made with previous determinations. In addition, structured absorption features superimposed on the broad collision-induced absorption band, which were observed at all temperatures studied, are interpreted in terms of the presence of (O2)2 and O2-N2 van der Waals molecules.

Fluorescence x‐ray absorption fine structure measurements using a synchrotron radiation x‐ray microprobe

Abstract: X‐ray absorption fine structure (XAFS) measurements in regions less than 20 μm in diameter were realized using an x‐ray microprobe employing the fluorescence detection method. To realize an energy tunable intense small x‐ray beam, an ellipsoidal mirror was used as the synchrotron radiation focusing element combined with a double‐crystal monochromator. Distortion of a XAFS spectrum due to the self‐absorption effect is discussed in detail. The degree of the distortion was experimentally evaluated from the measurement of the x‐ray fluorescence intensity as a function of takeoff angle, and the distortion was significantly reduced with the small takeoff angle detection geometry. Utilizing this technique, reliable XAFS spectra were obtained from a rock sample containing several minerals.

XAS study of lanthanum coordination environments in glasses of the system K2OSiO2La2O3

Abstract: The La L1 and L3 XANES and L3 EXAFS have been investigated for the series of glasses 10K2O50SiO2x La2O3 (x = 1, 5, 10) and (10 − x)K2O40SiO2−(x/3)La2O3 (x = 7.5, 5, 2.5) and model compounds La2O3, LaAlO3, LaPO4, La2NiO4, La2CuO4 and La(OH)3. An edge resonance at ∼ 25 eV above the L1 edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La2O3 concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 A ( ± 0.03 A ) for LaO. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.

Study of the lattice sites of Ti and Ni impurities in LiNbO3 single crystals, by means of X-ray absorption spectroscopy

Abstract: The structural environment of Ti and Ni ions incorporated into LiNbO3 single crystals during the growing process has been studied by using X-ray absorption spectroscopies: extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopies. It has been found that for the impurity concentration used (1% of TiO2 and 0.1% NiO in the melt) both ions, Ti4+ and Ni2+, are incorporated at the Li site of the LiNbO3 lattice.