Extended X-ray Absorption Fine-Structure Spectroscopy EXAFS

Intermediates were also observed in the synthesis of a neutral cluster, Ir4(CO)i2, from Ir(CO)2(acac) in the cages of zeohte NaY these were characterized by IR and extended X-ray absorption fine structure (EXAFS) spectroscopies, the latter being a technique ideally suited to investigation of small, highly dispersed species present in small amoimts in sohds. The spectra indicated dimeric intermediates, possibly Ir2(CO)8 [ 16], when the reaction was carried out in the near absence of water in the zeohte in contrast, the reaction in the dehydrated zeolite was faster, and no evidence of intermediates was observed [16]. 

Fig. 3 Ir4 cluster supported at the six-ring of zeolite NaX as represented by density functional theory samples were characterized by Extended X-ray absorption fine structure (EXAFS) spectroscopy and other techniques [32]...

Determination of the structures of Grignard reagents continues to be of interest, and reviews on this subject have appeared.126,127 Most of the structure authentications are done on crystalline materials, although solution studies performed with extended X-ray absorption fine structure (EXAFS) spectroscopy are also available. The Grignard compounds MeMgBr and EtMgBr in BunzO were studied at room temperature and —85 °C with EXAFS. At both temperatures, dimers are observed (Mg-(/r-Br) = 2.5 A Mg-O = 2.0 A).128... 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

In an effort to more fully elucidate the structure and reactivity of metal oxide pillared clays, we have been investigating the structure-reactivity properties of chromia-pillared derivatives (17). In the following sections, we provide an example of the structure-catalytic reactivity properties of chromia-pillared montmorillonites. Also, we report our initial efforts to structurally characterize the intercalated chromia aggregates by Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. Unlike previously reported metal oxide pillared clays, chromia-pillared clay exhibits strong K-edge absorption and fine structure suitable for determination of metal-oxygen bond distances in the pillars. 

The similarities in catalytic reactivity between Cr3 53-montmorillonite and chromia supported on alumina suggest that the structure of the intercalated chromia particles may resemble the structure of the bulk oxide. In order to obtain structural information for the chromia aggregates in pillared clays, we have initiated structural studies of these materials. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is being recognized as an effective tool for determining the local structure of a variety of materials. The basic principles and utility of this technique have been discussed elsewhere (18.). ... 

The spectroscopic techniques described in this section include IR, Raman, and UV-visible spectroscopy, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Techniques based on particle scattering, transitions in the nucleus, and radioisotope techniques that produce radiation that is a measure of the chemical environment are described in Sections IV.B and C. Some of these techniques, such as IR and UV-visible spectroscopy, have been applied to studies of catalysts for more than 30 years, whereas others, such as EXAFS, are relatively new to catalytic studies. 

Clearly there are many methods that can be used to probe a catalyst s structure under reaction conditions, as documented in other chapters in this series (Advances in Catalysis, Volumes 50 and 51, and this volume). In this chapter we focus on X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, sometimes simply referred to as X-ray absorption fine structure (XAFS) spectroscopy. In this review the term XAFS will be used generically, but EXAFS and XANES will be used when the information is specifically related to the extended or near edge structure, respectively. 

Particle diameter Inductively coupled plasma-optical emission spectrometry (ICP-OES) Fourier transform infrared spectrometry Mass spectrometry X-ray fluorescence Extended X-ray absorption fine structure (EXAFS) spectroscopy X-ray absorption near edge (XANES) spectroscopy Static and dynamic laser light scattering Scanning probe technologies... 

Spectroscopy. - In the following paragraphs we discuss how various spectroscopy tools have been used to illuminate the rich surface chemistry of supported polynuclear metal complexes. These methods include infrared (both diffuse reflectance and transmission), ultraviolet and visible (UV-Vis), x-ray photoelectron spectroscopy (XPS), and x-ray absorption, near edge spectroscopy s (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy s. 

The generation of a first-order difference by the use of anomalous scattering methods (either X-ray or neutron) offers the advantage that only a single sample is required. However, little use has been made of this possibility so far. On the other hand, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate, in a limited way, gMo( ) and the existence of inner-sphere complexing 69). 

A selective tool to study the local structure around an impurity is the Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Despite this fact, only the work of Emura et al. on Cu -doped NaCl [149] and NaBr [150] has, to our knowledge, employed this technique in the field of off-centre impurities in insulating materials. In contrast with magnetic resonance techniques, EXAES can also be used to explore non-paramagnetic impurities provided they can absorb X-rays, a fact that excludes tight elements tike Li or C. By contrast, EPR is better to work with low impurity concentrations below about 50 ppm and also when different oxidation states of the same element (for instance, Fe + and Fe" " [151]) are simultaneously present in a given sample. 

Using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Waychunas et al. (1993) determined the geometry of As(V) adsorbed on ferrihydrite and goethite and confirmed the formation of a binudear, and to a lesser extent, monodentate inner-sphere surface complex. Manning et al. (1998) used EXAFS to show that As(III) also forms inner-sphere surface complexes on goethite. (Note, a thorough review of the molecular-scale structure and chemistry of As adsorbed by crystalline and x-ray amorphous metal oxides is presented by Foster (this volume). 

V NMR spectroscopy is a particularly useful method for solution studies of vanadium(V) complexes. Several correlations of the shielding in vanadium(V) complexes have been reported and have been used as empirical guides to the coordination number of the vanadium.57-64 Application of these correlations should, however, be used cautiously. 170 NMR spectroscopy65 and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used for the characterization of solution complexes.66 Methods including potentiometry, X-ray diffraction, and UV-visible and IR spectroscopic techniques are also widely used in the characterization of the properties of vanadium complexes. Other methods for the characterization of low-level (sub-microgram) vanadium complexes have been described 67,68... 

The intent of this chapter is not to survey noninvasive surface spectroscopy but to illustrate briefly how it is applied to resolve the Stummian issue of whether inner-sphere surface complexes form. For this purpose, the application of electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) spectroscopies to elucidate metal cation speciation and the use of extended X-ray absorption fine structure (EXAFS) spectroscopy to detect surface anion species will be described. Emphasis will be on the interpretation of spectra. Sample preparation and instrumentation details were reviewed in recent volumes edited by Hawthorne (55) and Perry (27). Because the constant capacitance model was developed in the context of adsorption by hydrous oxides, these... 

The structure of bimessite was refined on the basis of electron and X-ray diffraction data (32-36). It contains layers of edge-shared MnOe octahedra separated by about 7.2 A. One out of six octahedral sites is unoccupied, and vacancies are balanced by counterions like Na+, K+, or Mg2+. Extended X-ray absorption fine structure (EXAFS) spectroscopy (37) showed that vemadite can be viewed as a three-dimensional mosaic of single and multiple octahedral chains with a variable length of between 1 and n octahedra. These chains are joined by shared comers. 

Another valuable tool is extended x-ray absorption fine structure (EXAFS) spectroscopy. This absorption technique makes use of the modulation in absorption coefficient above an element absorption edge, due to backscatterlng of the ejected photoelectron by neighboring atoms. By suitable data treatment, it is possible to determine the elemental type, number, and distance of the atoms coordinated to the absorbing cation. Thus, EXAFS probes the atomic-scale structure in the material, while SAXS and ASAXS probe the microdomain-scale structure. By using both scattering and absorption techniques, a full picture of ionomer morphology may be developed. 

Two questions remain, however why is the modulus enhancement, attributed primarily to these Interlocking loops, greater for the Ca and Sr lonomers than for the Ni, Zn, and Cd telechelics, and why is the stress-hardening behavior exhibited only by the telechelics neutralized with Ca and Ni Small-angle x-ray scattering (SAXS) and extended x-ray absorption fine structure (EXAFS) spectroscopy were employed to address these questions. 

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