EXAFS Results Research Articles

Active Site Structure of Cu/ZSM-5: Computational Study

The coordination structures of Cu ionic species in ZSM-5 zeolite were investigated by molecular dynamics (MD) simulations and molecular orbital (MO) calculations. Al sites were first sought so as to reproduce the square-planar coordination of Cu(II), which is well characterized by the ESR spectra. When two Al's occupy T8 sites in the six-membered ring, the calculated coordination of Cu(II) agreed well with the experimental data. For this Al siting, Cu(II)−O−Cu(II) and two Cu(I)'s were respectively simulated, and the coordination of the former species was found to be consistent with the EXAFS results. Furthermore, from the MO calculation Cu−O−Cu was anticipated to be very reactive and thus might be the active site. The function of zeolite would therefore be the stabilization of this species. On the other hand, Cu dimer was found unstable, and the reaction mechanism based on it was supposed unlikely.

A structural study of gallium lanthanum sulphide glass bulk and thin films by x-ray absorption fine structure spectroscopy

Atomic-scale structure changes in gallium lanthanum sulphide bulk glass and ablation-deposited thin films have been studied by the x-ray absorption fine-structure (EXAFS) technique. EXAFS spectra have been recorded at the sulphur and gallium K edges, and the lanthanum edge, and this has allowed us to construct a detailed picture of the local structure in bulk glass and thin films. The EXAFS results indicate that there is chemical disorder in the structural network of the GLS thin films, although chemical ordering is predominant in bulk GLS glass. The existence of `wrong bonds', i.e. Ga - Ga and S - S bonds, in the structure has been discussed and correlated with optical absorption experiments undertaken on the same samples to provide a consistent picture of the local structure.

Differentiating Between Monodentate and Bidentate Carboxylate Ligands Coordinated to Uranyl Ions Using EXAFS

We have investigated the U L III -edge EXAFS of two carboxylato uranyl complexes of known structure to explore the possibilty of differentiating between monodentate and bidentate coordinated carboxylate ions. The interpretation of EXAFS results from uranyl complexes with carboxylate ions based solely on axial and equatorial bond lengths can be inconclusive; information from further-distant shells is often needed. Multiple scattering (MS) pathways along the O=U=O unit and in coordinating chelating or monodentate carboxylate groups can make substantial contributions to the EXAFS. Therefore, MS pathways were calculated using FEFF6 [1] and considered in the data analysis. EXAFS results are compared to XRD data reported in the literature.

Reversible Deformation of SiO2-Supported PtAu6Cluster Induced by the CO Adsorption by In Situ EXAFS

Structure and behavior of [(PPh 3 )Pt(AU(PPh 3 )) 6 ](1) supported on SiO 2 surface have been studied by in-situ EXAFS. The structure of the cluster 1 is maintained after deposition on SiO 2 at room temperature. Upon adsorption of CO little change in coordination number for Pt-Au is found in Pt L 3 -edge EXAFS though the coordination number of Au-Au slightly increases in Au L 3 -edge EXAFS and the peak corresponding to Pt(-Au-)P remarkably increases in the Fourier transformation of Pt L 3 -edge EXAFS. We have interpreted the increase in the intensity of the peak oorresponding to Pt(-Au-)P by the multiple scattering effect owing to the change of Pt-Au-P angle. The original EXAFS oscillations are regenerated for both Pt and Au L 3 -edges after the evacuation for 2 h at 353 K accompanied by the desorption of CO. From these EXAFS results and the analogy to the PtAu 6 chemistry in solution, the PtAu 6, framework transforms its structure to a more compact one by CO adsorption with the change in the angle of Pt-Au-P. Unlike the PtAu6 cluster in solution, CO reversibly desorbs from the cluster without collapse of the PtAu 6 framework.

In-Situ and Ex-Situ Fe K-Edge X-Ray Absorption Spectroscopic Studies for the Structural and Electronic Evolution of Strontium Ferrite upon Electrochemical Oxidation

A structural evolution from SrFeO 2.5 (brownmillerite) to SrFeO 3 (perovskite) upon electrochemical oxidation has been systematically studied by in-situ and ex-situ X-ray Diffraction (XRD) and X-ray absorption spectroscopy (XAS). According to the XRD analysis, the orthorhombic SrFeO 2.5 has been gradually converted to the cubic SrFeO 3 as the reaction proceeds. The mostly oxidized perovskite could be been obtained after 66 h, From the in-situ and ex-situ Fe K-edge XANES study, it has been found that the degree of oxidation goes to a maximum after 48 h and 30 h for the bulk and the surface, respectively. EXAFS results are also consistent with the above findings from the fact that no peak evolution could be observed after 48 h and 30 h for both ex-situ and in-situ studies, respectively.

Multisite Adsorption of Cadmium on Goethite

Recently a new general ion adsorption model has been developed for ion binding to mineral surfaces (Hiemstra and van Riemsdijk, 1996). The model uses the Pauling concept of charge distribution (CD) and is an extension of the multi-site complexation (MUSIC) approach. In the CD–MUSIC model the charge of an adsorbing ion that forms an inner sphere complex is distributed over its ligands, which are present in two different electrostatic planes. In this paper we have applied the CD–MUSIC model to the adsorption of metal cations, using an extended data set for cadmium adsorbing on goethite. The adsorption of cadmium and the cadmium–proton exchange ratio were measured as function of metal ion concentration, pH, and ionic strength. The data could be described well, taking into account the surface heterogeneity resulting from the presence of two different crystal planes (the dominant 110 face and the minor 021 face). The surface species used in the model are consistent with recent EXAFS data. In accordance with the EXAFS results, high-affinity complexes at the 021 face were used in the model.

The relationship between glass structure and poling-induced optical second harmonic intensity for ZnO–TeO2 glasses

Local ordering around Te and Zn atoms of ZnO–TeO2 glass systems is investigated by Te K and Zn K EXAFS using synchrotron radiation. Tellurium K EXAFS results suggest the coordination states of tellurium atoms are changed from TeO4 trigonal bipyramids to TeO311 polyhedra or TeO3 trigonal pyramids with increasing of ZnO. Zinc K EXAFS results indicate that the Zn atom has the second oxygen shell which locates at the distance of 0.222–0.228 nm. The second harmonic intensity of the poled glass samples was also measured. The compositional dependence of the second harmonic intensity is explained in terms of the structural change of ZnO–TeO2 glass with the addition of ZnO.

Characterization of Electroactive Cs Ion-Exchange Materials using XAS

ABSTRACTVarious ion exchange materials have been proposed for the removal of Cs from high level waste streams produced during the reprocessing of fuel rods. Cs can be released from loaded traditional exchange resins by elution and then the resin can be reused. However large quantities of secondary wastes are generated. Another class of “single use” exchangers is directly incorporated in the loaded state into a solid waste form (e.g. borosilicate glass logs). A third alternative is electroactive ion-exchange materials, where the uptake and elution of Cs are controlled by an applied potential. This approach has several advantages over traditional reusable ion-exchange resins including much reduced secondary waste, higher Cs selectivity, and higher durability.XAS experiments were conducted at the Fe K-edge and Cs Lm-edge on a series of electrochemically produced nickel ferrocyanide films to determine the effects of deposition conditions and subsequent alkali exchange on structural and chemical aspects of the films. The deposition conditions include methods described in the literature and PNNL proprietary procedures. Although the performance and the durability of the films do vary with processing conditions, Fe K-edge EXAFS results indicate that all deposition conditions result in the. formation of the cubic phase. Initial results from Cs Lm-edge EXAFS analysis suggest that the Cs ion is present as a hydrated species.

Nanocrystalline copper doped zinc oxide gas sensors

Abstract Samples of nanocrystalline Cu doped ZnO have been prepared and characterised by several techniques including chemical analysis, X-ray powder diffraction and X-Ray absorption spectra (EXAFS). The particle sizes are typically less than 100 A on the largest diameter. The diffraction results show that lightly doped samples, less than 10 mole per cent Cu, are single phase with the ZnO crystal structure. The EXAFS results indicate that although the Zn2+ ions occupy normal cation sites the Cu2+ ions are in highly disordered sites, suggesting that there is possible surface aggregation. Gas sensing experiments show a good response to low ppm concentrations of toluene vapour.

Nanocrystalline nickel doped zinc oxide gas sensors

Samples of nanocrystalline Ni doped ZnO have been prepared and characterised by several techniques including chemical analysis, X-ray powder diffraction and X-ray absorption spectra (EXAFS). The particle sizes are typically less than 100Åon the largest diameter. The diffraction results show that lightly doped samples are single phase within the ZnO crystal structure. The EXAFS results indicate that the Ni2+ ions occupy some of the sites which would normally be occupied by Zn2+. Gas sensing experiments show a good response to low ppm concentrations of toluene vapour and a change in semiconductor type behaviour with temperature.

The state of platinum in a Pt/SO 42−/ZrO 2 super acid catalyst

The state of platinum in a Pt/SO 4 2−/ZrO 2 super acid catalyst has been investigated by means of Pt L-edge XAFS. The XANES showed that Pt is electron deficient after the activation by hydrogen treatment, while EXAFS results indicated the presence of PtO and PtPt pairs as in PtO 2 and Pt metal, respectively. The reported contradictory results that Pt is metallic by XRD analysis and Pt is in an oxidized state by XPS are explained by the present work; an oxidized platinum particle with a metallic core is present in the activated catalyst.

Structure of Iron-Substituted ZSM-5

The siting of Fe and of charge-compensating protons in the zeolite (Fe,Si)-ZSM-5 is studied using defect energy minimization techniques. The small energy difference between various T sites suggests that there will be a distribution of the Fe over a number of substitution sites. The OH group has a significantly lower energy in sites adjacent to the Fe. The calculated geometries compare well with EXAFS results. The minimum-energy location of the TPA(+) template is also calculated. The result differs from those proposed on the basis of previous EXAFS experiments. Our new model gives an equally satisfactory refinement of the EXAFS data.

EXAFS and Raman studies of dysprosium doped sodium sulfates

Results of Raman, conductivity and EXAFS studies of dysprosium doped sodium sulfates at concentrations of 0.5% and 3% of Dy3+ are presented. The Raman studies show that the lower dopant level (0.5% mole of Dy3+) produces the phase III structure whereas the higher doping (3% mole of Dy3+) results in a change to the phase I form. The conductivity increases with dopant level. The EXAFS results, which probe the immediate local environment of the dopant ion, show that the nearest neighbor shells for the two structures are almost identical and only subtle differences can be seen in the second nearest neighbor shells. This is consistent with the concept that it is the number rather than nature of the dopant ion that influences the phase stabilization process. The EXAFS studies also showed the absence of dysprosium clustering. These studies confirm the complementary nature of the Raman and EXAFS techniques, the former probing the host structure and the latter addressing the local dopant ion environment.

Copper speciation in sulfidic solutions at low sulfur activity: Further evidence for cluster complexes?

The solubility of two a s 0 -buffering assemblages in the Cu-S system have been studied: chalcocite-djurleite (Cc-Dj) and anilite-covellite (An-Cv). Ion activity products, [Cu +]HS −] 1 2 [H +] − 1 2 (25°C, I = 0) at equilibrium, derived from solubility measurements in penicillamine solutions, are 10 −17.01 ± 0.05 (Cc-Dj) and 10 −17.14 ± 0.10 (An-Cv), from which ΔG° f = −82.11 kJ/mol for Cc and −74.77 kJ/mol for An. In the An-Cv assemblage, a Cu 2 S = 0.55 (2 σ = 0.2) vs. 1.00 in the Cc-containing assemblage. The difference in a Cu 2 S between the two assemblages is used in a novel way to estimate stoichiometry of Cu-HS complexes. The solubility of both assemblages (0.7-0.01 M NaHS, pH 7–12.5, 25°C) can be fit with a model incorporating the same two chemical species, one containing an odd number of Cu atoms (Cu(HS) 2− 3, CU 3S 4H 2− 3, or a higher multimer) and the other containing an even number of Cu atoms (Cu 2S(HS) 2 2−, Cu 4S 4H 2 2−, etc.). The trimer-tetramer model fits the combined data for the two assemblages distinctly better than the monomer-dimer model, but this result is very sensitive to uncertainty in a Cu 2 S. Along with EXAFS results, the weight of the evidence favors small cluster complexes (2–5 Cu atoms), but is inconclusive at the present level of resolution. Multimers can be rationalized because condensation of metal-centered monomers to clusters provides a means for soft acid/base elements to maintain favored coordination geometries at low ligand to metal ratios. Based on the fitting methods developed here, previous covellite solubility data from this laboratory are reinterpreted in terms of Cu 2S 2(HS) 3 3−, Cu 2S 3)(S 4) 2−, and Cu(S 9)S 10) 3−; the last of these could also be represented by the trimer, Cu 3(S 7) 3 3−, which is homologous with a known complex. With the measured equilibrium constants, the speciation of Cu in the sulfidic zone of the Black Sea is calculated. Covellite is the stable Cu-S mineral, but the sulfidic water column is vastly supersaturated with respect to it. Most of the sulfidic water column is modestly (2.5–5.5 times) supersaturated with respect to Cc, hinting that this mineral metastably controls ΣCu. The slight supersaturation suggests that Cc occurs as 10–100 nm particles.

Structural investigation and dielectric properties of Na-mordentte-encapsulated selenium

Abstract A crystal chemical study has shown that both small pore and large pore Na-mordenite exposed to Se vapour incorporate this element into their framework. The maximum amounts of selenium inserted are equal to 8.3 and 23.5 wt % respectively. EXAFS results have allowed to conclude that Se-Se bond lengths are 0.04 Å shorter in Na mordenite with respect to the value in the hexagonal selenium. Dielectric results are compatible with incorporation of selenium chains within the pores of the mordenites.

X‐ray absorption spectroscopy study of the Ti environment in MgH2 supported Ziegler catalysts

The EXAFS results indicate that the MgH 2 -supported Ziegler catalyst reveals a different type of active center fixation than that of the conventional MgCl 2 -supported catalysts. Furthermore the Ti species is not supported onto the MgH 2 surface simply through physisorption, but rather fixed through chemical bonding

HTiCl(THF) ∼0.5] x - a highly reactive titanium hydride and an active species in the McMurry reaction

The complex [TiCl 3(THF) 3] reacts with catalytically-prepared solid magnesium hydride (MgH 2★) or dissolved magnesium hydride (MgH 2′) with evolution of hydrogen to give a highly reactive titanium hydride, [HTiCl(THF) ∼0.5] x ( 1). The well-known low valent titanium species, obtained by reduction of TiCl 3 with LiAlH 4, utilized in the McMurry reaction, has been shown to be 1. An X-ray absorption spectroscopy study (EXAFS) of 1 reveals that the Ti absorber is surrounded by O (from THF) and Cl atoms plus two types of Ti neighbors. Possible structural models for 1 compatible with the EXAFS results are proposed. Complex 1 is an active reagent for the coupling reaction of benzophenone to give tetraphenylethene. During the reaction hydrogen is liberated and the inorganic side product has been shown to be titanium(III) oxychloride; thus the McMurry reaction employing 1 as the reagent can be described by Ph 2CO+ 1 THF or→ toluen 0.5 PPh 2CCPh 2 + 0.5H 2↑ + [TiOCl(THF) x ]. On the basis of these results, a new interpretation of the mechanism for the McMurry reaction is presented. Complex 1 also undergoes a number of carbenoid type reactions, which may proceed via a “titanium carbenoid” intermediate 9a–c.

Ligand Isomerism and Stacking in Square Planar Platinum(II) Complexes. 2. Evidence for Mixed-Isomerism in Pt[dmbp(CSN)2

Three linkage isomers of Pt[dmbp(CSN) 2 ], where dmbp=4,4'-dimethylbipyridine, have been prepared and examined by FT-IR, 13 C solution and 15 N MAS NMR, and Pt-L 3 EXAFS spectroscopies, in order to elucidate the bonding properties of these square planar complexes. The spectral data show that these complexes can exist as the kinetically favored -(SCN) 2 form I, the-(SCN)(NCS) form II, obtained by heating I as the neat solid, and the thermodynamically favored -(NCS) 2 form III, obtained by recrystallization of I from aprotic solvents. Evidence for a Pt-Pt interaction in solid III is inferred from the Pt-L 3 EXAFS results

Genesis and Growth of Pt Particles on MgO: Characterization of X-ray Absorption Spectroscopy and Infrared Spectroscopy

Samples prepared by the reaction of [Pt(C[sub 3]H[sub 5])[sub 2]] with MgO powder were treated in H[sub 2] at temperatures in the range 25-150[degrees]C and characterized by X-ray absorption spectroscopy and infrared spectroscopy. As the temperature of treatment in H[sub 2] was increased, the infrared spectra indicated the disappearance of bands attributed to the allyl ligands, and the sample became gray, the color characteristic of a supported metal. Adsorption of CO was characterized by a single infrared peak in the v[sub CO] region at 2070 cm[sup [minus]1], consistent with that observed for CO chemisorbed on metallic Pt. The Fourier transforms of the EXAFS functions measured near the Pt L[sub III] absorption edge for each sample confirm the presence of metallic Pt, showing peaks at approximately 2.7, 3.8, 4.7, and 5.6 A, consistent with the first, second, third, and fourth nearest neighbors in bulk fcc Pt. The EXAFS results indicate that the particle size increased from approximately 8 A after treatment in H[sub 2] at room temperature, to about 9 A after a similar treatment at 50[degree]C, to about 10 A after treatment at 150[degree]C. The metal-support interface was characterized by two metal-support oxygen contributions, one at 2.18 A and onemore » in the range 2.7-2.9 A. The total Pt-O coordination number decreased monotonically as the treatment temperature in H[sub 2] increased. This result confirms that the fraction of Pt atoms in contact with the support decreased as the particle size increased. 34 refs., 9 figs., 4 tabs.« less

Comparison of the cation environment in polymer electrolytes based on poly(ethylene oxide) and transition-metal bromides

The local structure of polymer electrolyte systems based on poly(ethylene oxide), PEO, containing MBr2(M = Co, Ni, Cu and Zn) salts has been studied by UV–VIS spectroscopy and EXAFS. The results indicate the presence both of anionic complexes MBr42–(and perhaps MBr3O–) for Co, Ni and Cu systems and the six-coordinate cationic species required for charge compensation, namely MO62+ and MO42+(and perhaps MO5Br+ and MO3Br+). In contrast to these transition-metal salts, for ZnBr2 the formation of a neutral salt–PEO complex, ZnBr2O2, is favoured. In the case of CoBr2–PEO and NiBr2–PEO systems, much better fits for EXAFS results are obtained when two oxygen shells are considered. This supports the view that the oxygen from the PEO backbone coordinates with these metal ions forming structural units appoaching octachedral and tetrahedral geometry.