Showing papers on "Extended X-ray absorption fine structure published in 1990"
TL;DR: In this paper, the authors reported that there is no need to include a shell of oxygens at ca. 1.75 and 2.7 {angstrom}, and that there are one and possibly two shells of additional scatterers at longer distance (ca. 3.3 and 4.2 ) from the Mn.
Abstract: Manganese x-ray absorption spectra (XAS) are reported for the S{sub 1} state of highly purified, highly concentrated preparations of the oxygen-evolving complex (OEC) from photosystem II (PSII). Improvements in concentration (ca. 1.5 mM Mn) and detection efficiency (13-element solid-state detector array) have permitted a substantial improvement in data quality relative to previous solution XAS studies of PSII. Principal findings are that there is no need to include a shell of oxygens at ca. 1.75 {angstrom} in order to account for the Mn EXAFS, that there are 2-3 Mn-Mn distances of ca. 2.7 {angstrom}, and that there are one and possibly two shells of additional scatterers at longer distance (ca. 3.3 and 4.2 {angstrom}) from the Mn. Even with this higher quality data, it is not possible to use EXAFS to determine whether chloride is coordinated to the Mn. The structure consequences of these results are discussed in the context of proposed structural models. It is concluded that neither a cubane nor previously prepared butterfly type clusters can account for the observed features.
144 citations
TL;DR: In this article, the influence of small amounts of copper on the neutron-irradiation-induced embrittlement of reactor pressure vessel steels is investigated using fluorescence extended X-ray absorption spectroscopy.
Abstract: The influence of small amounts of copper on the neutron-irradiation-induced embrittlement of reactor pressure vessel steels is of considerable current interest. Previous work has shown that the embrittlement is associated with the formation of copper-rich precipitates, but uncertainties remain regarding their composition and form. This Letter reports the preliminary results from a structural investigation of such precipitates in solution-treated and thermally aged Fe–-Cu and Fe–-Cu–-Ni model alloys using fluorescence extended X-ray absorption spectroscopy. The most significant result is the first direct observation that small clusters in the peak hardness condition have a b.c.c. structure.
122 citations
TL;DR: In this paper, Ni(OH) 2 en phases α and β, a temperature ambiante and a temperature de l'azote liquide, a comparison of longueurs des liaisons.
Abstract: Etude de Ni(OH) 2 en phases α et β, a la temperature ambiante et a la temperature de l'azote liquide. Comparaison des longueurs des liaisons
118 citations
TL;DR: In this article, the evolution of the local structure around ferric ions has been followed through X-ray Absorption Spectroscopy (XAS) during the formation of hematite from ferric gels at 92°C.
118 citations
TL;DR: In this paper, the structure and composition of aqueous Pb(II) complexes sorbed onto γ-Al2O3, in situ, using X-ray absorption spectroscopy.
113 citations
TL;DR: In this paper, MgO-supported catalysts were prepared from [HRe(CO)5] and [H,Re3(CO),,] and characterized by extraction of surfaceOrganometallics, infrared and ultraviolet/visible spectroscopy, and extended X-ray absorption fine structure (EXAFS) capabilities.
Abstract: MgO-supported catalysts were prepared from [HRe(CO)5] and [H,Re3(CO),,] and characterized by extraction of surface
organometallics, infrared and ultraviolet/visible spectroscopy, and extended X-ray absorption fine structure (EXAFS)
spectroscopy. The EXAFS analysis and other data show that [H,Re,(CO),,] was initially deprotonated on the MgO surface,
giving a surface-bound anion with a structure comparable to that of the salt [Ph,As] [H2Re,(C0)12] and having a Re-Mg
distance of 2.39 8,. Heating of the supported cluster anion in helium to 225 OC led to oxidation and breakup of the cluster
framework, giving a mononuclear complex formulated as [Re(C0)3(0MgJ3(]w here the braces refer to groups terminating
the bulk oxide). The distances characterizing the bonding of the Re to the support are Re-O = 2.15 8, and Re-Mg = 2.80
8,. A Re-Re distance of 3.94 8, was observed, consistent with the decomposition of the cluster on the support to form ensembles
consisting of three of the Re subcarbonyls, for which a structural model is presented. Treatment of this sample in hydrogen
at 350 'C gave a Re species with oxygen neighbors at average distances of 1.94 and 2.45 8,. Heating of this sample to 500
OC in hydrogen led to reduction and conversion of most of the Re into metal crystallites. The several samples were tested
as catalysts, as described in a companion paper.
112 citations
TL;DR: In situ transmission x-ray absorption spectroscopy was used to investigate changes in the structure of as it was cycled as a nonsintered nickel oxide electrode in concentrated alkali.
Abstract: In situ transmission x‐ray absorption spectroscopy was used to investigate changes in the structure of as it was cycled as a nonsintered nickel oxide electrode in concentrated alkali. Results were obtained for dry as‐prepared electrodes and electrodes that were charged or discharged. Changes were observed in the (0001) plane of the as a function of cycling. In the charged material there was a contraction in the Ni‐O and Ni‐Ni distances. The EXAFS results and the XANES spectra strongly indicate that the Ni‐O coordination is symmetrical in , whereas the coordination is asymmetric in the charged material.
99 citations
TL;DR: The subtle differences observed between the S states are best explained by an increase in the spread of Mn-Mn distances occurring during the S2----S3 state transition, which suggests the presence of two inequivalent di-mu-oxo-bridged binuclear structures in the S3 state.
Abstract: O2-evolving photosystem II (PSII) membranes from spinach have been cryogenically stabilized in the S3 state of the oxygen-evolving complex. The cryogenic trapping of the S3 state was achieved using a double-turnover illumination of dark-adapted PSII preparations maintained at 240 K. A double turnover of PSII was accomplished using the high-potential acceptor, Q400, which is the high-spin iron of the iron-quinone acceptor complex. EPR spectroscopy was the principal tool establishing the S-state composition and defining the electron-transfer events associated with a double turnover of PSII. The inflection point energy of the Mn X-ray absorption K-edge of PSII preparations poised in the S3 state is the same as for those poised in the S2 state. This is surprising in light of the loss of the multiline EPR signal upon advancing to the S3 state. This indicates that the oxidative equivalent stored within the oxygen-evolving complex (OEC) during this transition resides on another intermediate donor which must be very close to the manganese complex. An analysis of the Mn extended X-ray absorption fine structure (EXAFS) of PSII preparations poised in the S2 and S3 states indicates that a small structural rearrangement occurs during this photoinduced transition. A detailed comparison of the Mn EXAFS of these two S states with the EXAFS of four multinuclear mu-oxo-bridged manganese compounds indicates that the photosynthetic manganese site most probably consists of a pair of binuclear di-mu-oxo-bridged manganese structures. However, we cannot rule out, on the basis of the EXAFS analysis alone, a complex containing a mononuclear center and a linear trinuclear complex. The subtle differences observed between the S states are best explained by an increase in the spread of Mn-Mn distances occurring during the S2----S3 state transition. This increased disorder in the manganese distances suggests the presence of two inequivalent di-mu-oxo-bridged binuclear structures in the S3 state.
93 citations
TL;DR: In this article, a new carbonyl cluster is suggested to be an isomer of (A), which is transformed into Rh{sub 6}(CO){sub 12}({mu}{sub 3}-CO-sub 4} at temperatures higher than 423 K.
Abstract: NaY zeolite entrapped Rh{sub 6}(CO){sub 16} and the product of its oxidation followed by reduction, (Rh{sub 6}){sub red}/NaY, have been structurally characterized by EXAFS and FTIR spectroscopies and CO/H{sub 2} chemisorption Rh{sub 6}(CO){sub 12}({mu}{sub 3}-CO){sub 4}, (A), in NaY supercages has been synthesized with minor formation of Rh{sup +}(CO){sub 2}, (B), by carbonylation of Rh{sup 3+}/NaY at 373-423 K The coordination of Rh atoms (coordination number and bond length of Rh-Rh and Rh-CO bonds) and the C-O stretching modes of (A) are similar to those of crystalline Rh{sub 6}(CO){sub 16} The new carbonyl cluster is suggested to be an isomer of (A), Rh{sub 6}(CO){sub 12}({mu}{sub 2}-CO){sub 4}, which is transformed into Rh{sub 6}(CO){sub 12}({mu}{sub 3}-CO){sub 4} at temperatures higher than 423 K
91 citations
TL;DR: In this paper, the unoccupied molecular orbitals of metal-free phthalocyanine (H2PC), nickel-phthalocyanin (NiPC), and copper-phthalocalyanine(CuPC) adsorbed on single crystal surfaces have been studied by inverse photoemission spectroscopy (IPES) and near-edge x-ray absorption spectroscopic (NEXAFS) experiments.
Abstract: The unoccupied molecular orbitals of metal‐free phthalocyanine (H2PC), nickel‐phthalocyanine (NiPC), and copper‐phthalocyanine (CuPC) adsorbed on single crystal surfaces have been studied by inverse photoemission spectroscopy (IPES) and near‐edge x‐ray absorption spectroscopy (NEXAFS). The features observed in the experimental data are assigned by reference to molecular orbital calculations. Carbon and nitrogen 1s NEXAFS measurements of NiPC adsorbed on Cu(110) surface indicates that the phthalocyanine molecular planes are parallel to the surface. X‐ray absorption data recorded at the Ni LIII and LII edges show that the unoccupied Ni 3d level is located in the plane of the phthalocyanine rings.
89 citations
TL;DR: The EXAFS Debye-Waller factor shows that the clusters have 40% less thermal vibration than does the bulk, and a theory for this effect is presented, based on the change of bulk modulus with volume, the measured contraction of the clusters, and the softening effect of the surface modes.
Abstract: We performed optical absorption and temperature-dependent extended x-ray-absorption fine-structure (EXAFS) measurements on 55-atom gold clusters made by the Schmid process. The results are consistent with a fcc cuboctahedron structure, but with first-neighbor distances contracted by 0.075 \AA{}. The electronic properties, as detected by optical and x-ray absorption, are as expected for a small chunk of gold. The EXAFS Debye-Waller factor shows that the clusters have 40% less thermal vibration than does the bulk. We present a theory for this effect, based on the change of bulk modulus with volume, the measured contraction of the clusters, and the softening effect of the surface modes.
TL;DR: It is shown that two structurally distinct types of zinc sites are available in the enzyme, including the eight-zinc native enzyme and the enzyme with the vacant sites of (b) occupied by four lead ions.
Abstract: The zinc coordination in 5-aminolevulinate dehydratase was investigated by extended x-ray absorption fine structure (EXAFS) associated with the zinc K-edge. The enzyme binds 8 mol of zinc/mol of octameric protein, but only four zinc ions seem sufficient for full activity. The authors have undertaken a study on four forms of the enzyme: (a) the eight-zinc native enzyme; (b) the enzyme with only the four zinc sites necessary for full activation occupied; (c) the enzyme with the vacant sites of (b) occupied by four lead ions; (d) the product complex between (b) and porphobilinogen. They have shown that two structurally distinct types of zinc sites are available in the enzyme. The site necessary for activity has an average zinc environment best described by two/three histidines and one/zero oxygen from a group such as tyrosine or a solvent molecule at 2.06 {plus minus} 0.02 {angstrom}, one tyrosine or aspartate at 1.91 {plus minus} 0.03 {angstrom}, and one cysteine sulfur at 2.32 {plus minus} 0.03 {angstrom} with a total coordination of five ligands. The unoccupied site in (b) is dominated by a single contribution of four cysteinyl sulfur atoms at 2.28 {plus minus} 0.02 {angstrom}. Spectra from samples (c) and (d) show onlymore » small changes from that of (b), reflecting a slight rearrangement of the ligands around the zinc atom.« less
TL;DR: In this paper, the structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied by means of Mo K-edge EXAFS.
Abstract: The structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied
by means of Mo K-edge EXAFS. In situ EXAFS measurements were carried out at liquid N2 temperature on freshly sulfided
(at 673 K) catalysts. Due to the high signal-to-noise ratio of the experimental data, detailed information about the structural
parameters of the active phase could be obtained. The catalysts probably consist of small MoS2-like particles with an auerage
local ordered structure of5-6 Mo atoms (Mo/C) and 7-8 Mo atoms (Co-Mo/C). Comparison with literature data shows
that the MoS2 dispersion in these carbon-supported catalysts is probably as good as in their alumina-supported counterparts.
A detailed EXAFS data analysis shows the presence of carbon neighbors next to the Mo atoms in both catalysts with a Mc-C
coordination distance of 2.1 * 0.1 A. This short distance could imply an intimate interaction between the active phase and
the carbon support, which may explain the high active-phase dispersion. From the value of 2.1 8, it can be inferred that
the Mo-C coordination is restricted to the exposed Mo atoms and that a support carbon atom takes the place of a sulfur
vacancy. The high-quality EXAFS spectra show evidence for the existence of cobalt neighbors next to the Mo atoms in
the Co-Mo/C catalyst, with a Mo-Co coordination distance of 2.8 f 0.1 A.
TL;DR: It is concluded that, in the presence of hydroxylamine, illumination causes a reduction of the OEC, resulting in a state resembling S0, and a significant structural rearrangement occurs between the S0* and S1 states.
Abstract: Hydroxylamine at low concentrations causes a two-flash delay in the first maximum flash yield of oxygen evolved from spinach photosystem II (PSII) subchloroplast membranes that have been excited by a series of saturating flashes of light. Untreated PSII membrane preparations exhibit a multiline EPR signal assigned to a manganese cluster and associated with the S2 state when illuminated at 195 K, or at 273 K in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). We used the extent of suppression of the multiline EPR signal observed in samples illuminated at 195 K to determine the fraction of PSII reaction centers set back to a hydroxylamine-induced S0-like state, which we designate S0*. The manganese K-edge X-ray absorption edges for dark-adapted PSII preparations with or without hydroxylamine are virtually identical. This indicates that, despite its high binding affinity to the oxygen-evolving complex (OEC) in the dark, hydroxylamine does not reduce chemically the manganese cluster within the OEC in the dark. After a single turnover of PSII, a shift to lower energy is observed in the inflection of the Mn K-edge of the manganese cluster. We conclude that, in the presence of hydroxylamine, illumination causes a reduction of the OEC, resulting in a state resembling S0. This lower Mn K-edge energy of S0*, relative to the edge of S1, implies the storage and stabilization of an oxidative equivalent within the manganese cluster during the S0----S1 state transition. An analysis of the extended X-ray absorption fine structure (EXAFS) of the S0* state indicates that a significant structural rearrangement occurs between the S0* and S1 states. The X-ray absorption edge position and the structure of the manganese cluster in the S0* state are indicative of a heterogeneous mixture of formal valences of manganese including one Mn(II) which is not present in the S1 state.
TL;DR: In this article, the effect of oxygen incorporation within sputter-deposited MoS 2 solid lubricant films was studied using extended X-ray absorption fine structure (EXAFS), Xray photoelectron spectroscopy (XPS) and X-Ray diffraction (XRD).
Abstract: The morphology and crystallite structure/orientation of sputter-deposited MoS 2 solid lubricant films have been shown to be strongly affected by oxygen-containing species in the sputtering chamber. We have studied the effect of oxygen incorporation within these films using extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Films produced at The Aerospace Corporation, the National Centre of Tribology (U.K.), Hohman Plating and Manufacturing, and NASA Lewis Research Center were studied to determine common characteristics of different MoS 2 films produced by sputter deposition. In agreement with previous results, XPS of the films indicated a bulk anion:cation ratio ( i.e. (S+O):Mo) of about 2, and XRD revealed that the films consisted completely of MoS 2 -like phases that were compressed in the ( h k 0) directions as compared with pure MoS 2 . This “edge plane compression” appeared to correlate with oxygen content. EXAFS confirmed the absence of MoO 2 in the films and indicated that the films actually consist of two similar MoS 2 -like phases. Correlation of the EXAFS results with those obtained from XPS and XRD indicated that the two phases are MoS 2 and an MoS 2- x O x substitutional solution. The solid solution may be described as MoS 2 with oxygen atoms that have substituted for sulfur atoms in the MoS 2 crystal lattice. Relative peak height changes in the EXAFS radial distribution curves indicated that increasing amounts of oxygen cause a reduction in the size of crystallites in the films. The presence of the MoS 2- x O x phase may explain the superior tribological performance of MoS 2 films in some applications, which has been shown to correlate with oxygen impurities in the films.
TL;DR: In this article, the adsorption of ammonium heptamolybdate (AHM) on alumina has been studied by using FTIR and Raman spectroscopies, TPR, and EXAFS.
Abstract: The adsorption of ammonium heptamolybdate (AHM) on {gamma}-Al{sub 2}O{sub 3} and {alpha}-AlOOH has been studied by using FTIR and Raman spectroscopies, TPR, and EXAFS. It is shown that molybdate adsorption on alumina cannot be explained in terms of electrostatic binding to protonated surface hydroxyls but can be rationalized by assuming (i) a reaction between the molybdate and basic surface OH groups, leading to the decomposition of the adsorbing molecule/ion, and (ii) physisorption on coordinatively unsaturated (cus) Al{sup 3+} sites. In the AHM/{alpha}-AlOOH case, surface precipitation, presumably of (NH{sub 4}){sub 2}Mo{sub 3}O{sub 10}, takes place at surprisingly low initial AHM concentrations. The presence of (divalent) cations can influence the shape of the AHM/{gamma}-Al{sub 2}O{sub 3} isotherm but does not affect the ultimate loadings that can be achieved through processes (i) and (ii).
TL;DR: In this article, the attached Rh dimers were found to be more active and selective than a conventionally prepared Rh/SiO{sub 2} catalyst, and the structure change of the Rh dimer sites in each reaction step for catalytic hydroformylation was followed by means of in-situ FT-IR and in situ EXAFS techniques.
Abstract: trans-(Rh(C{sub 5}Me{sub 5})(CH{sub 3})){sub 2}({mu}-CH{sub 2}){sub 2} was supported on the SiO{sub 2} surface by the reaction with OH groups of SiO{sub 2} at 313 K, followed by evacuation at 373 K. In this attaching reaction, the Rh dimer complexes were bound to the SiO{sub 2} surface through Rh-O(surface) bonds, losing one CH{sub 3} ligand and one C{sub 5}Me{sub 5} ligand per Rh dimer. The attached Rh dimers were found to be more active and selective than a conventionally prepared Rh/SiO{sub 2} catalyst. The structure change of the Rh dimer sites in each reaction step for catalytic hydroformylation was followed by means of in-situ FT-IR and in situ EXAFS techniques. The Rh-Rh bond in the attached Rh dimers (Rh-Rh = 0.262 nm) was cleaved by CO adsorption to form monomer pairs (Rh(C{sub 2}H{sub 5})(CO){sub 2}(O-Si) + Rh(C{sub 5}Me{sub 5})(O-Si)). Heating the monomer pairs to 423 K under vacuum resulted in CO insertion, with new peaks exhibited at 1,710 and 1,394 cm{sup {minus}1} due to the acyl ligand.
TL;DR: In this article, an extended x-ray absorption fine structure (EXAFS) and IR studies were performed on Ru catalysts supported on {gamma}-Al{sub 2}O{sub 3}, MgO, SiO, and TiO{Sub 2} to elucidate the mechanism of the CO adsorption-induced disruption of metal clusters.
Abstract: EXAFS (extended x-ray absorption fine structure) and IR studies were performed on Ru catalysts supported on {gamma}-Al{sub 2}O{sub 3}, MgO, SiO{sub 2}, and TiO{sub 2} to elucidate the mechanism of the CO adsorption-induced disruption of metal clusters. EXAFS results show that after reduction, Ru atoms exist on all supports as small metal clusters, but the particle sizes and metal-support interactions vary with the support. CO adsorption onto Ru/{gamma}-Al{sub 2}O{sub 3} and Ru/MgO led to the disruption of Ru-Ru bonds. By comparing with IR spectroscopic observations it is concluded that new species like O-Ru-CO and O-Ru-(CO){sub 2} on {gamma}-Al{sub 2}O{sub 3} and O{sub n}-Ru-CO and (O{sub n}-Ru){sub 2}-CO (n = 3,4) on MgO are formed after CO admission. On the other hand, no evidence of disruption of Ru clusters by CO adsorption was obtained in Ru/SiO{sub 2} and Ru/TiO{sub 2}. The IR study showed, contrary to previously proposed models, that no H{sub 2} evolution takes place during the CO adsorption-induced disruption. A new reaction scheme that is consistent with their observations is presented.
TL;DR: In this article, the ionic configuration of NiCo 2 O 4 may be represented by the formula: Co x 2+ Co 1- x 3+ [ Ni 1-x 2+ Ni x III Co 1 III ] O 4 2-x ∼ 0.8
TL;DR: In-situ X-ray absorption spectra of an electrochemically deposited monolayer and bilayer of copper on a gold (100) surface immersed in an electrolytic solution and under potential control were obtained by using fluorescence detection under grazing incidence, the X-rays being polarized parallel to the plane of the gold surface as discussed by the authors.
TL;DR: The local environment of the Ni atoms in the Ni-Mo-S phase has been elucidated by Ni K-edge EXAFS and XANES measurements of carbon-supported sulfided NiMo catalysts.
Abstract: The local environment of the Ni atoms in the Ni-Mo-S phase has been elucidated by Ni K-edge EXAFS and XANES measurements of carbon-supported sulfided Ni-Mo catalysts. The results show that the Ni atoms have a low sulfur coordination number (less than six). This is contrary to recent reports in which an octahedral-like sulfur coordination have been suggested. It is proposed that the Ni atoms in Ni-Mo-S are located at the\((10\bar 10)\) edges of the MoS2 structure in square and tetragonal pyramidal type sites. These two types of sites can easily interconvert during sulfur addition/extraction and may as such be involved in the catalytic cycle.
TL;DR: In this paper, the authors investigated the local atomic environments around chromium atoms in 15Ni-15Cr austenitic steels with separate or combined addition of carbon, nitrogen and silicon.
Abstract: Local atomic environments around chromium atoms in 15Ni-15Cr austenitic steels with separate or combined addition of carbon, nitrogen and silicon are investigated by means of XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) measurements. A peak is observed on the rising slope of the chromium K-edge absorption spectrum. The peak is most distinct for the steels without alloying of carbon, nitrogen, or silicon. The peak is weaker in the steels with combined addition of silicon with carbon or nitrogen, and single addition of silicon than with separate addition of carbon and nitrogen. Height of the first peak in the Fourier transforms of EXAFS spectra of the solution-treated state lowers with addition of interstitial elements: carbon or nitrogen. These results suggest that interstitial atoms gather around chromium atoms to form a kind of Interstitial-Substitutional (I-S) complexes such as a shortrange ordered structure. Silicon probably promotes formation of the I-S complexes around chromium atoms. In the steels alloyed with carbon or nitrogen, the height of the first peak is elevated by cyclic or uniaxial tensile deformation probably because of the reduction in density of the I-S complexes. Plastic deformation produces no significant modification in the XANES spectra. The I-S complexes, which may form in Cr-Ni and Cr-Mn austenitic steels containing both or one of carbon and nitrogen, are considered to play an important role in solid solution hardening in the solution-treated state and also in work softening in the low cycle fatigue test owing to their breakaway during plastic deformation.
TL;DR: The π-donor type bond orientation of ethylene and propylene on clean and atomic oxygen covered Ag(110) surfaces has been determined with near-edge x-ray absorption fine-structure (NEXAFS) measurements as mentioned in this paper.
Abstract: The π bond orientation of ethylene and propylene on the clean and atomic oxygen covered Ag(110) surfaces has been determined with near‐edge x‐ray absorption fine‐structure (NEXAFS) measurements. The π system of ethylene is tilted 20±5° and 18±5° from the surface normal for ethylene on the clean and atomic oxygen covered surfaces, respectively. The tilt angle of the π system is 20±5° on the clean surface and 31±5° on the atomic oxygen covered surface. On both the clean and atomic oxygen covered surface, the carbon–carbon double bond in ethylene and propylene appears to form a π‐donor type bond with the Ag(110) surface.
TL;DR: In this paper, the authors characterized the presence of highly dispersed platinum in BaKL zeolite and showed that the average platinum cluster in the BaKl material consists of 5 or 6 atoms, consistent with the results of all three techniques.
Abstract: Platinum supported on BaKL zeolite was characterized by Transmission Electron Microscopy (TEM), hydrogen chemisorption, and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The results of all three techniques indicate the presence of highly dispersed platinum in the zeolite pores. There is no evidence of platinum outside the zeolite pores. The EXAFS data determine a Pt-Pt coordination number of 3.7, suggesting that the average platinum cluster in the zeolite consists of 5 or 6 atoms, consistent with the TEM and chemisorption data. The EXAFS data also provide evidence of the platinum-zeolite interface, indicated by Pt-O contributions at 2.14 and 2.70 A, and a Pt-Ba contribution at 3.8 A. The Pt/BaKL zeolite is one of the most highly dispersed supported platinum samples and one of the most structurally uniform supported metal catalysts.
TL;DR: In this paper, the authors characterized at high temperature a uniform heterogeneous catalyst for the oligomerization of hydrocarbons (a nickel-exchanged zeolite of initial composition Na59Al59Si133O384·xH2O treated with an aqueous solution of NiCl2 so as to yield a homogeneous distribution of Ni, with Si/Ni = 7) by recording the extended X-ray absorption fine structure (EXAFS) above the Ni edge and also its high-resolution diffraction pattern (at λ= 1.5486 A
Abstract: We have characterized at high temperature a model uniform heterogeneous catalyst for the oligomerization of hydrocarbons (a nickel-exchanged zeolite of initial composition Na59Al59Si133O384·xH2O treated with an aqueous solution of NiCl2 so as to yield a homogeneous distribution of Ni, with Si/Ni = 7) by recording the extended X-ray absorption fine structure (EXAFS) above the Ni edge and also its high-resolution diffraction pattern (at λ= 1.5486 A). We have obtained unique insights into the microenvironment of the Ni2+ ions in the as-prepared and the dehydrated as well as the reduced state of the catalyst; in particular, values of atomic coordinates, site-occupancy and bond lengths, have been obtained. High-resolution X-ray diffraction, using a specially constructed environmental cell, has enabled the precise location of sorbed Xe atoms in a sodium-exchanged zeolite X adsorbent (1.5 atoms of Xe per unit cell) under a pressure of 1.75 bar at room temperature to be determined from a Rietveld refinement of the powder diffraction pattern. Direct proof of the role of the strongly polarizing Na+ ion located at the SΠ site in firmly binding the Xe to the inner wall of the supercage is obtained.X-ray absorption near-edge structures (XANES) as well as parallel EXAFS studies above the Al, Si and Ga edges on a variety of other zeolites are also reported. Combined (Ga edge) XANES and EXAFS studies show that gallosilicate networks akin to those present in crystalline (gallo) zeolite X and Y are already formed in the precursor gel and are also present in the aqueous solution in contact with the colloidal-amorphous material, which together constitute the gel. We also report one of the first applications of Al EXAFS for the semi-quantitative study of the dealumination of faujasitic zeolites.
TL;DR: The relationship between strain and carrier concentration has been found to be approximately linear and can be described by the presence of a size and an electronic effect, as recently proposed in the literature.
Abstract: Laser‐annealed and further thermally annealed arsenic implanted silicon specimens have been investigated in a range of doses from 1×1016 to 5×1016 As/cm2, with different experimental techniques: electrical measurements, transmission electron microscopy (TEM), double‐crystal x‐ray diffractometry (DCD), and extended x‐ray absorption fine structure analysis (EXAFS). On the as laser‐annealed samples, in the whole range of doses examined, a lattice contraction of the doped layer has been evidenced by DCD, whereas, on the same specimens, EXAFS measurements have shown the presence of a local expansion around substitutional As atoms. The relationship between strain and carrier concentration has been found to be approximately linear and can be described by the presence of a size and an electronic effect, as recently proposed in the literature. The former effect represents the atomic size contribution, while the latter is the strain induced by the variation of the conduction‐band minima due to the doping. After a s...
TL;DR: In this paper, the authors show that all the Ru atoms in the catalyst are in small metal clusters and that the oxygen atoms at the cluster-support interface have a significant contribution to the EXAFS signal.
Abstract: Highly dispersed Ru/Al{sub 2}O{sub 3} catalysts were prepared from Ru{sub 3}(CO){sub 12}. EXAFS (extended X-ray absorption fine structure) studies show that all the Ru atoms in the catalyst are in small metal clusters. The Ru clusters are so small that the oxygen atoms at the cluster-support interface have a significant contribution to the EXAFS signal. Adsorption of CO completely changes the structure of this small metal cluster as is evidenced from EXAFS oscillation; Ru-Ru bonds are disrupted and Al-O-Ru-(CO){sub n} (n = 1-2) is formed. When the temperature is raised, Ru atoms agglomerate and small metal clusters are recovered. Infrared spectroscopic observation supports the conclusions obtained from EXAFS.
TL;DR: The anion sublattice is shown to suffer the largest distortion, but the cation sub lattice also exhibits some relaxation, and the nearest-neighbor bond lengths at both room temperature and low temperature are found to be constant.
Abstract: Bond lengths, Debye-Waller factors, and site occupancy in the diluted magnetic semiconductor ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se have been measured using extended x-ray-absorption fine structure. The nearest-neighbor bond lengths at both room temperature and low temperature (77 K) are found to be constant as a function of alloy composition within the experimental uncertainty of 0.01 \AA{}. Because the average cation-cation distance changes with Mn content, these results necessarily imply distortion of the tetrahedral bond angles. The anion sublattice is shown to suffer the largest distortion, but the cation sublattice also exhibits some relaxation. The repercussions of these results are discussed, in terms of the amount of cation and anion sublattice distortion at low temperature and its connection to the superexchange mechanism occurring between the ${\mathrm{Mn}}^{2+}$ ions and mediated by the intervening anion in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se.
TL;DR: An EXAFS analysis method that is accurate and generally applicable and can be implemented on a fast microcomputer within a reasonable amount of computation time is presented and a set of parameters which seems to account for the limitations in the employed theory are discussed.
Abstract: We present an EXAFS analysis method that is accurate and generally applicable and can be implemented on a fast microcomputer within a reasonable amount of computation time. The analysis is based on the comparison between a calculated EXAFS spectrum of a parametrized model with the experimental spectrum and the refinement of the model parameters. The calculated EXAFS spectrum is evaluated taking into account the curved-wave nature of the photoelectron and multiple scattering up to and including third-order collinear scattering. Such an expansion treats accurately all multiple-scattering contributions that contribute to the Fourier transform of the EXAFS spectrum up to the fourth-neighbor distance. The comparison between theory and experiment is thus limited to this distance. In this calculation we schematically express the contributions of the various scattering configurations \ensuremath{\eta} in the form ${\mathrm{\ensuremath{\chi}}}_{0}^{\mathrm{\ensuremath{\eta}}}$=${\mathit{F}}_{\mathrm{\ensuremath{\eta}}}$(k)sin[${\mathit{kL}}_{\mathrm{\ensuremath{\eta}}}$+${\mathrm{\ensuremath{\Theta}}}_{\mathrm{\ensuremath{\eta}}}$(k)] and expand to lowest order the amplitude ${\mathit{F}}_{\mathrm{\ensuremath{\eta}}}$ and the phase ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{\ensuremath{\eta}}}$ functions in terms of all structural and potential parameters. This expansion is quite accurate and significantly speeds up the computation. We discuss a set of parameters which seems to account for the limitations in the employed theory, providing a very good fit between theory and experiment. The analysis method and the parameters have been implemented successfully on a number of systems. We present here the analysis of the copper EXAFS spectrum. The experimental data contain 20 independent experimental points; yet we obtain a very good fit with only six parameters. The results also show the importance of both double and triple collinear scattering as well as the importance of noncollinear double scattering.
01 Sep 1990
TL;DR: In this paper, the design and calibration of a proposed in situ spectral absorption meter is evaluated using a laboratory prototype, which includes a silver coated (second-surface) glass tube, a tungsten light source (stabilized by means of optical feedback), a monochromator, and a solid state detector.
Abstract: The design and calibration of a proposed in situ spectral absorption meter is evaluated using a laboratory prototype. The design includes a silver coated (second-surface) glass tube, a tungsten light source (stabilized by means of optical feedback), a monochromator, and a solid state detector. The device measures the absorption coefficient plus a portion of the volume scattering function. Theoretical analyses and laboratory experiments which explore the magnitude and variation of the errors due to scattering and internal reflections are described. Similar analyses are performed on the Cary 1 18 Spectrophotometer to allow cross calibration. Algorithms to yield the abscrption coefficient and the zenith-sun diffuse attenuation coefficient are presented and evaluated. Simultaneous measurement of the beam attenuation or backscattering coefficient allows use of algoriThms with much narrower error bands. The various methods of obtaining absorption and diffuse attenuation values are compared. Procedures for using reverse osmosis filtration to produce a clean water calibration standard are described. An absorption spectrum for pure water is obtained. Development of the absorption meter is proceeding along two lines: 1) a two-wavelength side-by-side LED is being fabricated to allow an in situ chlorophyll a absorption meter to be constructed, and 2) scientific projects using a shipboard or laboratory flow.-through pumping system are being planned.