Showing papers on "Extended X-ray absorption fine structure published in 1999"

Inorganic electronic structure and spectroscopy

Abstract: Preface. Contributors, Volume I. Contents, Volume II. Contributors, Volume II. 1. Ligand Field Theory and the Properties of Transition Metal Complexes (A. Lever & E. Solomon). 2. Electron Paramagnetic Resonance Spectroscopy (A. Bencini & D. Gatteschi). 3. Mossbauer Spectroscopy (P. Gutlich & J. Ensling). 4. Polarized Absorption Spectroscopy (M. Hitchman & M. Riley). 5. Luminescence Spectroscopy (T. Brunold & H. Gudel). 6. Laser Spectroscopy (E. Krausz & H. Riesen). 7. IR, Raman, and Resonance Raman Spectroscopy (R. Czernuszewicz & T. Spiro). 8. Photoelectron Spectra of Inorganic and Organometallic Molecules in the Gas Phase using Synchrotron Radiation (G. Bancroft & Y. Hu). 9. X-Ray Absorption Spectroscopy and EXAFS Analysis: The Multiple-Scattering Method and Applications in Inorganic and Bioinorganic Chemistry (H. Zhang, et al.). 10. Electronic Structure Calculations on Transition Metal Complexes: Ab-Initio and Approximate Models (C. Martin & M. Zerner). 11. Electronic Structure Calculations: Density Functional Methods with Applications to Transition Metal Complexes (J. Noodleman & D. Case). Index.

Structure of Hydrous Ruthenium Oxides: Implications for Charge Storage

Abstract: Hydrous ruthenium oxide (RuO2·xH2O or RuOxHy) is a mixed electron−proton conductor with a specific capacitance as high as 720 F/g/proton, making it a candidate material for energy storage. The correlation between the structure and properties of RuO2·xH2O materials is not well understood due to their amorphous nature and compositional variability. In this study, ruthenium oxides with the compositions RuO2·2.32H2O, RuO2·0.29H2O, and anhydrous RuO2 are characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) and extended X-ray fine structure (EXAFS) analyses. XANES cannot be used to distinguish between Ru(III) and Ru(IV) in the hydrous oxides, but the EXAFS analyses show large differences in the short-range structures of the materials. Whereas anhydrous RuO2 has the rutile structure comprising chains of RuO6 octahedra linked in three dimensions, the structure of RuO2·0.29H2O is rutile-like at the RuO6 core, but less connected and progress...

Estimation of mean size and shape of small metal particles by EXAFS

Abstract: The possibility to estimate the mean size and shape of metal particles from their average coordination number, which is typically obtained from EXAFS (extended X-ray absorption fine structure) analysis, is evaluated. While the number of neighbors in the first and second coordination shell were found to be not significantly dependent on the particle shape, and thus can be used to estimate the particle size, a substantial influence of the particle shape was found to be present for the coordination numbers of higher neighboring shells. Consequently, a method using the coordination number of the first or second shell (N1 or N2) to estimate the average particle size and that of a higher shell (N3 or N4) to estimate the particle shape is presented.

The Use of XAFS to Distinguish between Inner- and Outer-Sphere Lead Adsorption Complexes on Montmorillonite.

Abstract: Adsorption mechanisms of Pb on montmorillonite were investigated by conducting equilibrium and X-ray absorption fine structure (XAFS) spectroscopy studies. Data from the batch equilibrium studies indicate that Pb could be adsorbing via two mechanisms, depending on ionic strength. At low ionic strength (I = 0.006 M) Pb adsorption is pH-independent: 97% of the available Pb was removed from solution at pH 4.42 and 100% at pH 8.0. This behavior is consistent with an outer-sphere complexation mechanism. At high ionic strength (I = 0.1 M) Pb adsorption is pH-dependent, suggesting inner-sphere complexation as the adsorption mechanism: 43% of the available Pb was removed from the solution at pH 4.11 and 98.9% at pH 7.83. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy results reveal that in the sample equilibrated at I = 0.006 M, pH 4.48-6.40 the local atomic structure (LAS) surrounding the adsorbed Pb is similar to the LAS surrounding Pb(2+) (aq), confirming that the adsorption mechanism is outer-sphere complexation. In the system equilibrated at I = 0.1 M, pH 6.77 the XANES and EXAFS results show that the LAS surrounding the adsorbed Pb atom is similar to the LAS surrounding reference compounds in which Pb is forming covalent bonds (Pb(4)(OH)(4+)(4) (aq) and a sample of gamma-Al(2)O(3) with Pb adsorbed via inner-sphere complexation). These similarities indicate that Pb is forming inner-sphere complexes on the montmorillonite at this ionic strength and pH. In samples equilibrated at I = 0.006 M, pH 6.77 and I = 0.1 M, pH 6.31 the XAFS results suggest that Pb is forming both inner- and outer-sphere adsorption complexes. This observation could not be distinguished by making macroscopic observations only. Thus, the results of this study reveal important information on Pb sorption behavior on clays and also provides insights into the use of XAFS to determine sorption mechanisms. Copyright 1999 Academic Press.

Electron population analysis by full-potential x-ray absorption simulations

Abstract: We present the first successful attempt at calculating cluster full-potential x-ray absorption near-edge structure (XANES) spectra, based on the finite difference method. By fitting XANES simulations onto experimental spectra we are able to perform electron population analysis. The method is tested in the case of Ti $K$-edge absorption spectrum in ${\mathrm{TiO}}_{2}$, where the amount of charge transfer between Ti and O atoms and of the screening charge on the photoabsorber is obtained taking into account both dipolar and quadrupolar transitions.

Adsorption of Bi-Isonicotinic Acid on Rutile TiO2 (110)

Abstract: Bi-isonicotinic acid ~2,28-bipyridine–4,48-dicarboxylic acid! is the ligand of several organometallic dyes, used in photoelectrochemical applications. Therefore the atomic scale understanding of the bonding of this molecule to rutile TiO2(110) should give insight into the crucial dye–surface interaction. High resolution x-ray photoelectron spectroscopy ~XPS!, near edge x-ray absorption fine structure ~NEXAFS!, and periodic intermediate neglect of differential overlap ~INDO! calculations were carried out on submonolayer bi-isonicotinic acid rutile TiO2(110). Data from multilayers is also presented to support the submonolayer results. For a multilayer, XPS shows that the carboxyl groups remain in the ~pristine! protonated form, and NEXAFS show that the molecular plane is tilted by 57° with respect to the surface normal. For the submonolayer, the molecule bonds to the rutile TiO2(110) surface via both deprotonated carboxyl groups, with a tilt angle of 25°, and additionally an azimuthal orientation of 44° with respect to the @001# crystallographic direction. The adsorbant system was also investigated by quantum mechanical calculations using a periodic INDO model. The most stable theoretical adsorption geometry involves a twist around the molecular axis, such that the pyridine rings are tilted in opposite directions. Both oxygen atoms of each carboxyl group are bonded to five-fold coordinated Ti atoms ~2M-bidentate!, in excellent agreement with the experimental results.

Carbon Support Effects on Bimetallic Pt−Ru Nanoparticles Formed from Molecular Precursors

Abstract: We describe the preparation, structural characterization, and support interactions experienced by two different compositions of Pt−Ru nanoparticles supported on several carbons (carbon black, fullerene soot, and desulfurized carbon black). The bimetallic nanoparticles, obtained by reduction of the neutral molecular precursors PtRu5C(CO)16 and Pt2Ru4(CO)18 (the latter of which lacks a central “stabilizing” carbide core) at elevated temperatures in a hydrogen atmosphere, show a structural homology, exhibiting exceptionally narrow size and compositional distributions. A detailed structural picture of the nanoparticles has been deduced on the basis of in-situ extended X-ray absorption fine structure (EXAFS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray analysis (EDX), and X-ray absorption near edge structure (XANES). These techniques reveal that the bimetallic nanoparticles have Pt/Ru compositions of 1:5 and 2:4, respectively, and average diameters lying between 1.0 and 1.5 nm. Th...

Sensitivity of Extended X-Ray-Absorption Fine Structure to Thermal Expansion

Abstract: The sensitivity of extended x-ray-absorption fine structure (EXAFS) to thermal expansion has been studied by temperature-dependent measurements on germanium. The first cumulant does not reproduce the thermal expansion owing to vibrations normal to the bond. The perpendicular relative displacement $〈\ensuremath{\Delta}{u}_{\ensuremath{\perp}}^{2}〉$ has been for the first time experimentally obtained; the ratio $〈\ensuremath{\Delta}{u}_{\ensuremath{\perp}}^{2}〉/〈\ensuremath{\Delta}{u}_{\ensuremath{\Vert}}^{2}〉$ is in agreement with vibrational model calculations. Low-temperature quantum effects on the 3rd cumulant have been for the first time observed. The possibility of measuring thermal expansion from the 3rd cumulant is demonstrated, provided that quantum effects are taken into account.

Near-edge x-ray absorption fine structure and Raman characterization of amorphous and nanostructured carbon films

Abstract: Amorphous and nanostructured carbon films were grown by using two different techniques: ion sputtering and cluster beam deposition. The films were studied by near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. Depending on the precursors, atoms, or clusters, the films are characterized by a different sp2/sp3 ratio which influences the mechanical and the electronic properties. Due to the sensitivities of NEXAFS (local order) and Raman (medium-range order), we have characterized and compared the structure of the films over different length scales. The complementarity of NEXAFS and Raman techniques for the characterization of disordered forms of carbon is here presented and discussed. We also present an original method of NEXAFS spectra calibration allowing a better determination of peak positions.

Synthesis and Characterization of Hydrotalcites Containing Ni(II) and Fe(III) and Their Calcination Products

Abstract: A layered double hydroxide with the hydrotalcite-like structure containing Ni(II) and Fe(III) cations in the brucite-like layers with the formula [Ni0.7Fe0.3(OH)2](CO3)0.15·0.94 H2O has been prepared by coprecipitation. On hydrothermal treatment, a coproduct, identified as a NiFe2O4 spinel, is formed. Both samples, as well as the solids obtained from them by calcination at 450 °C (where only mixed oxides are present) and 750 °C (where crystallization of well defined phases has taken place), have been characterized by powder X-ray diffraction (PXRD), X-ray absorption (XAS) and FT-IR spectroscopies, thermal analysis (differential and thermogravimetric), temperature-programmed reduction, and specific surface area assessment. XAS results show that in both calcined samples all Ni(II) cations are in octahedral holes, while Fe(III) cations are equally distributed between octahedral and tetrahedral holes. For the sample calcined at 750 °C, coordination parameters at the first and second shells of Ni(II) and Fe(II...

New Analysis of Oxidation State and Coordination Environment of Copper Ion-Exchanged in ZSM-5 Zeolite

Abstract: The changes of structure and electronic state of copper ion species during the heat treatment of copper ion-exchanged ZSM-5 zeolite (CuZSM-5) as well as the interaction with CO molecules have been investigated by using various spectroscopic techniques such as infrared (IR) and emission spectroscopy (ES), electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) consisting of a XANES (X-ray absorption near edge structure) and an EXAFS (extended X-ray absorption fine structure) and through the measurements of heat of adsorption and adsorption isotherm. About 70% of the divalent copper ions (Cu2+) exchanged in CuZSM-5 were found to be reduced to the monovalent copper ions (Cu+) during the heat treatment at 873 K in vacuo, the latter species having a linear or a planar coordination structure with a coordination number of 2 or 3 with respect to the nearest-neighboring oxygen atoms at a distance of 1.98 A. It was found from the ES data that the Cu+ species strongly interact with CO molecules at ...

Core-hole effects in x-ray-absorption spectra of fullerenes

Abstract: Near-edge x-ray-absorption fine-structure spectra of two fullerenes, ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70},$ have been simulated by means of density-functional theory techniques using ground-state Kohn-Sham orbitals, transition state potentials, or full core-hole potentials. The very good experimental agreement obtained when the full core hole explicitly is taken into account gives an indication of significant excitonic effects for the x-ray-absorption spectra of fullerenes.

White light emission of Eu3+-based hybrid xerogels

Abstract: The luminescence spectra and extended x-ray-absorption fine-structure (EXAFS) measurements of a series of ${\mathrm{Eu}}^{3+}$-based organic/inorganic xerogels were reported and related to the local coordination of the lanthanide cations. The hybrid matrix of these organically modified silicates, classed as $U(2000)$ ureasils, is a siliceous network to which short organic chains containing oxyethylene units are covalently grafted by means of urea bridges. The luminescent centers were incorporated as europium triflate, ${\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_{3}{\mathrm{SO}}_{3}{)}_{3},$ and europium bromide, ${\mathrm{EuBr}}_{3},$ with concentrations $200g~ng~20$ and $n=80,$ 40, and 30, respectively---where $n$ is the number of ether oxygens in the polymer chains per ${\mathrm{Eu}}^{3+}$ cation. EXAFS measurements were carried out in some of the ${U(2000)}_{n}{\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_{3}{\mathrm{SO}}_{3}{)}_{3}$ xerogels ($n=200,$ 80, 60, and 40). The obtained coordination numbers $N$ ranging from 12.8, $n=200,$ to 9.7, $n=40,$ whereas the average ${\mathrm{Eu}}^{3+}$ first neighbors distance $R$ is 2.48--2.49 \AA{}. The emission spectra of these multiwavelength phosphors superpose a broad green-blue band to a series of yellow-red narrow ${}^{5}{\stackrel{\ensuremath{\rightarrow}}{{D}_{0}}}^{7}{F}_{0--4}$ ${\mathrm{Eu}}^{3+}$ lines and to the eye the hybrids appeared to be white, even at room temperature. The ability to tune the emission of the xerogels to colors across the chromaticity diagram is achieved by changing the excitation wavelength and the amount of salt incorporated in the hybrid host. The local environment of ${\mathrm{Eu}}^{3+}$ is described as a continuous distribution of closely similar low-symmetry network sites. The cations are coordinated by the carbonyl groups of the urea moieties, water molecules, and, for ${U(2000)}_{n}{\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_{3}{\mathrm{SO}}_{3}{)}_{3},$ by the ${\mathrm{SO}}_{3}$ end groups of the triflate anions. No spectral evidences have been found for the coordination by the ether oxygens of the polyether chains. A mean radius for the first coordination shell of ${\mathrm{Eu}}^{3+}$ is calculated on the basis of the emission energy assignments. The results obtained for ${U(2000)}_{n}{\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_{3}{\mathrm{SO}}_{3}{)}_{3},$ 2.4 \AA{} for $90g~ng~40$ and 2.6 and 2.5 \AA{} for $n=30$ and 20, respectively, are in good agreement with the values calculated from EXAFS measurements. The energy of the intraconfigurational charge-transfer transitions, the redshift of the ${}^{5}{\stackrel{\ensuremath{\rightarrow}}{{D}_{0}}}^{7}{F}_{0}$ line, with respect to the value calculated for gaseous ${\mathrm{Eu}}^{3+},$ and the hypersensitive ratio between the ${}^{5}{\stackrel{\ensuremath{\rightarrow}}{{D}_{0}}}^{7}{F}_{2}$ and ${}^{5}{\stackrel{\ensuremath{\rightarrow}}{{D}_{0}}}^{7}{F}_{1}$ transitions, point out a rather low covalency nature of the ${\mathrm{Eu}}^{3+}$ first coordination shell in these xerogels, comparing to the case of analogous polymer electrolytes modified by europium bromide.

Cavity-enhanced absorption spectroscopy of molecular oxygen

Abstract: A high-finesse optical cavity was employed to perform highly sensitive spectroscopy of molecular oxygen at wavelengths near 763 nm. An equivalent absorption length of ∼1 km was obtained by a 26-cm-long optical cavity with a finesse of 6000. An extended cavity diode laser was frequency locked to the cavity, and pure absorption profiles were recovered by monitoring of the cavity transmission during continuous scans of the cavity resonance through O2 rotational lines, allowing a detailed investigation of the line shapes. Phase modulation of the laser at a frequency equal to the cavity free-spectral-range frequency was employed for detection of weak absorption signals inside the cavity. A minimum detectable absorption coefficient of 6.9×10-11 cm-1 Hz-1/2 was measured. Finally, a test of the symmetrization postulate in 16O nuclei was demonstrated.

Multiple-Scattering EXAFS Analysis of Tetraalkylammonium Manganese Oxide Colloids

Abstract: X-ray absorption spectroscopy at the Mn K edge was employed to elucidate the structure of colloidal tetraalkylammonium (TAA) manganese oxides in sols and gels obtained by different preparation and heat treatment procedures. Two series of colloidal TAA MnOx prepared with tetrapropylammonium (TPA) and tetraethylammonium (TEA) cations were studied. Several manganese oxides, birnessite, and feitknechtite were also measured and served as model compounds for structural refinements. Near edge structure (XANES) analysis revealed different average valences of the colloidal systems. As synthesized and heat-treated, TAA colloids exhibited an average valence of 3.6−3.7, whereas gelled TAA colloids showed a lower average valence of ∼3.5. Extended absorption fine structure (EXAFS) analysis was carried out to distances of ∼6.0 A around the central Mn atom using theoretical backscattering phases and amplitudes calculated from the ab initio FEFF code. All multiple-scattering (MS) paths with a weight of 2% and more with re...

Heterogeneity of Framework Ti(IV) in Ti-Silicalite as Revealed by the Adsorption of NH3. Combined Calorimetric and Spectroscopic Study †

Abstract: The room-temperature adsorption of NH{sub 3} on Ti-silicalite (TS-1) was studied by means of IR and XANES-EXAFS spectroscopies and by microcalorimetry. The propensity of framework tetrahedral Ti(IV) heteroatoms to expand their coordination shell upon adsorption of NH{sub 3} was monitored by the perturbation induced on the Ti-sensitive framework stretching band (960 cm{sup {minus}1}) and on the preedge peak at 4967 eV in XANES spectra. In both cases a modification of the local geometry of titanium sites upon adsorption of NH{sub 3} as an additional ligand was deduced. The amount of NH{sub 3} specifically adsorbed on the Ti(IV) sites was estimated by comparing volumetric data obtained for TS-1 with those obtained for a Ti-free silicalite taken as reference material. At p{sub NH{sub 3}} = 50 Torr, the number of NH{sub 3} molecules adsorbed per Ti atom was found to be close to 2, indicating that virtually all Ti atoms are involved in the interaction and have completed their 6-fold coordination shell. The molar heats of adsorption on the Ti(IV) sites were estimated following the same procedure. It was found that the molar heat of NH{sub 3} adsorption on titanium is higher (q{sub m} = 95--83 kJ/mol) than that measured for themore » silicalite matrix (q{sub m} = 66--58 kJ/mol). The interaction with ammonia was found to be essentially reversible at room temperature but for a significant amount of adsorbed species irreversibly held on titanium sites. The zero-coverage heat of adsorption was quite high (q {approx} 200 kJ/mol) not only for the first run of adsorption involving both irreversible and reversible interaction but also for the second run involving only the reversible component (q {approx} 130 kJ/mol). The evolution of the heat of adsorption with coverage was typical of heterogeneous surfaces; this is due not only to the presence of sites active toward ammonia on the silica matrix but also to the heterogeneous distribution of Ti(IV) sites, suggesting that a considerable number of framework sites (among the 12 available in the orthorhombic MFI framework) are occupied in a nearly equidistributed way. EXAFS measurements allowed the elongation of the Ti-O bond upon interaction with NH{sub 3} (0.05 {+-} 0.03 {angstrom}), the Ti-N distance (1.93 {+-} 0.03 {angstrom}), and the average number of adsorbed NH{sub 3} molecules per Ti site (1.9 {+-} 0.3) to be estimated. The results were in good agreement with the volumetric-calorimetric data. The effect of the pretreatment of TS-1 samples with ammonium acetate aqueous solution was also investigated by comparing the spectroscopic and the quantitative calorimetric results for both untreated and treated TS-1 samples.« less

The structure of U6+ sorption complexes on vermiculite and hydrobiotite

Abstract: The sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ionexchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized L1- and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ionexchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

Contribution of shape resonance and Pt–H EXAFS in the Pt L2,3 X-ray absorption edges of supported Pt particles: Application and consequences for catalyst characterization

Abstract: The electronic and geometric effects induced by hydrogen chemisorption on small platinum particles supported on high surface-area saponite clay and zeolite LTL were studied by near edge X-ray absorption fine structure (XAFS) spectroscopy. A new subtraction procedure was developed to separate the electronic from geometric effects. A significant Pt–H extended X-ray absorption fine structure (EXAFS) scattering (structural effect) was found for energy values between 0 and 20 eV. In addition, the Pt–H antibonding state (electronic effect) was found to produce a shape-resonance and was isolated from the near edge of the L3 X-ray absorption spectrum. Moreover, for Pt/LTL the shape and energy of the shape-resonance was found to strongly depend on the acidity/alkalinity of the support material, implying a direct influence of the support on the electronic properties of the platinum particles. The results of the study of the resonance state and the Pt–H EXAFS scattering demonstrate the potential of these techniques for characterization of hydrogen chemisorption, metal-promoter, and metal-support effects in catalysis research.

Comparison of the Manganese Cluster in Oxygen-Evolving Photosystem II with Distorted Cubane Manganese Compounds through X-ray Absorption Spectroscopy

Abstract: X-ray absorption spectroscopy has been employed to assess the degree of similarity between the oxygen-evolving complex (OEC) in photosystem II (PS II) and a family of synthetic manganese complexes containing the distorted cubane [Mn(4)O(3)X] core (X = benzoate, acetate, methoxide, hydroxide, azide, fluoride, chloride, or bromide). These [Mn(4)(m(3)-O)(3)(m(3)-X)] cubanes possess C(3)(v)() symmetry except for the X = benzoate species, which is slightly more distorted with only C(s)() symmetry. In addition, Mn(4)O(3)Cl complexes containing three or six terminal Cl ligands at three of the Mn were included in this study. The Mn K-edge X-ray absorption near edge structure (XANES) from the oxygen-ligated complexes begin to resemble general features of the PS II (S(1) state) spectrum, although the second derivatives are distinct from those in PS II. The extended X-ray absorption fine structure (EXAFS) of these Mn compounds also displays superficial resemblance to that of PS II, but major differences emerge on closer examination of the phases and amplitudes. The most obvious distinction is the smaller magnitude of the Fourier transform (FT) of the PS II EXAFS compared to the FTs from the distorted cubanes. Curve fitting of the Mn EXAFS spectra verifies the known core structures of the Mn cubanes, and shows that the number of the crucial 2.7 and 3.3 A Mn-Mn distances differs from that observed in the OEC. The EXAFS method detects small changes in the core structures as X is varied in this series, and serves to exclude the distorted cubane of C(3)(v)() symmetry as a topological model for the Mn catalytic cluster of the OEC. Instead, the method shows that even more distortion of the cubane framework, altering the ratio of the Mn-Mn distances, is required to resemble the Mn cluster in PS II.

Pre-edge structure analysis of Ti K-edge polarized X-ray absorption spectra in TiO2 by full-potential XANES calculations.

Abstract: The pre-edge region of Ti N-edge polarized XANES spectra in TiO2-mtile is investigated by full-potential calculations based on the finite-difference method. Both dipolar and quadrupolar transitions are considered. The use of"non muffin-tin" potential allows a clear interpretation of the pre-edge features. The results are consistent with Full-potential LAPW band structure calculations, and are also compared with multiple-scattering calculations.

Structure and catalytic combustion activity of atomically dispersed Pt species at MgO surface

Abstract: We have investigated the structure of Pt(A)/MgO and PtMo 6 /MgO catalysts which were prepared from Pt(C 5 H 7 O 2 ) 2 and [PtMo 6 O 24 ] 8− followed by calcination, respectively, and their catalytic activities for the propane + O 2 reaction. The Pt(A)/MgO and PtMo 6 /MgO catalysts with atomically dispersed Pt ions were found to be as active as metallic Pt particles supported on MgO for propane combustion. A distorted spinel-like structure around Pt ion embedded at the MgO surface was proposed for the active Pt species by means of EXAFS. The atomically dispersed Pt structure was reduced with propane at 623 K to form Pt 6 clusters with Pt–O bonding to the MgO support. The Pt cluster chemically attached to the MgO surface was selective for the propane dehydrogenation. Restructuring of the Pt clusters to the atomically dispersed Pt structure was observed by oxidation with O 2 at 623 K.