Extended X-ray absorption fine structure EXAFS technique

Ru/Cu (22) and Pt/Ir (23) on silica. They analyzed the detailed structure of these samples by an extended x-ray absorption fine structure (EXAFS) technique, showing an alloy structure for the nanoparticles with a diameter of 1-3 nm, associated with their special properties. 

The platinxm-platinum distance in the three compounds isolated so far is 2.95 A. Both Lippard (22) and Mansy (23) have suggested that a similar reaction could occur with guanine, in the manner shown in Figure 9. It seemed that it should be possible to detect such a reaction with bases in DNA by looking for the Pt-Pt distance of 2.95 A with extended x-ray absorption fine structure (EXAFS) techniques. We have performed such experiments on the Cornell High Energy Synchrotron Source. 

Characterize new electrocatalysts by high-resolution transmission electron microscopy (HRTEM), in-situ fourier transform infrared (FTIR) spectroscopy and extended x-ray absorption fine structure (EXAFS) techniques. 

XRD techniqnes are used to obtain information on the crystal stmcture [1,2,11,333, 421,422,424,425], The in sitn stndy of an electrode is also possible, i.e., following the changes as a fimction of potential. The X-ray absorption near edge stractirre (XANES) and extended X-ray absorption fine structure (EXAFS) techniques are also applied to study noncrystalline materials. 

There are several attractive features of the Extended X-ray Absorption Fine Structure (EXAFS) technique which make it a powerful structural tool notably (i) it is extremely fast, (ii) both sample preparation and data collection are relatively easy, without the requirement of single crystals, (iii) being sensitive to short-range order in atomic arrangements, it can focus on the local environment of specific absorbing atoms, and (iv) the technique is useful for a wide variety of materials such as amorphous solids, liquids, solutions, gases, polymers, and surfaces. 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

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]...

In general, several spectroscopic techniques have been applied to the study of NO, removal. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are currently used to determine the surface composition of the catalysts, with the aim to identify the cationic active sites, as well as their coordinative environment. 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

X-ray imaging, selenium in, 22 101. See also Extended X-ray absorption fine structure (EXAFS) analysis X-ray imaging techniques, 16 504 X-ray imaging tests, 26 440 X-ray-induced X-ray emission, 24 109 X-ray instruments, 26 411... 

Inner electrons are usually excited by X-rays. Atoms give characteristic X-ray absorption and emission spectra, due to a variety of ionization and possible inter-shell transitions. Two relevant refined X-ray absorption techniques, that use synchrotron radiation, are the so-called Absorption Edge Fine Structure (AEFS) and Extended X-ray Absorption Fine Structure (EXAFS). These techniques are very useful in the investigation of local structures in solids. On the other hand, X-Ray Fluorescence (XRF) is an important analytical technique. 

Extended X-ray absorption fine structure (EXAFS) A technique for observing the local structure around a metal centre, using X-rays from a synchrotron source. The atom of interest absorbs photons at a characteristic wavelength and the emitted electrons, undergoing constructive or destructive interference as they are scattered by the surrounding atoms, modulate the absorption spectrum. The modulation frequency corresponds directly to the distance of the surrounding atoms while the amplitude is related to the type and number of atoms. In particular, bond lengths and coordination numbers may be derived. 

Thanks to the extensive literature on Aujj and the related smaller gold cluster compounds, plus some new results and reanalysis of older results to be presented here, it is now possible to paint a fairly consistent physical picture of the AU55 cluster system. To this end, the results of several microscopic techniques, such as Extended X-ray Absorption Fine Structure (EXAFS) [39,40,41], Mossbauer Effect Spectroscopy (MES) [24, 25, 42,43,44,45,46], Secondary Ion Mass Spectrometry (SIMS) [35, 36], Photoemission Spectroscopy (XPS and UPS) [47,48,49], nuclear magnetic resonance (NMR) [29, 50, 51], and electron spin resonance (ESR) [17, 52, 53, 54] will be combined with the results of several macroscopic techniques, such as Specific Heat (Cv) [25, 54, 55, 56,49], Differential Scanning Calorimetry (DSC) [57], Thermo-gravimetric Analysis (TGA) [58], UV-visible absorption spectroscopy [40, 57,17, 59, 60], AC and DC Electrical Conductivity [29,61,62, 63,30] and Magnetic Susceptibility [64, 53]. This is the first metal cluster system that has been subjected to such a comprehensive examination. 

Traditionally, X-ray absorption edge measurements have been used to determine oxidation states of metals in complex materials. The extended X-ray absorption fine structure (EXAFS), on the other hand, provides structural information such as bond distances and coordination numbers even with powdered samples, crystalline or amorphous, the fine structure essentially resulting from short-range order around the absorbing atom. The technique is also useful for studying solid surfaces (SEXAFS). The observation of fine structure beyond the K-absorption edges of materials dates back to... 

Very small bimetallic particles on carrier are often X-ray diffraction amorphous and it is not easy to gain any information on their composition and structure. In spite of these difficulties, very important information on the texture of bimetallic particles has been obtained by extended X-ray absorption fine structure (EXAFS) (25-28). Results obtained by Sinfelt et al. (25,26) demonstrated that a very detailed picture can be obtained of, e.g., Ru-Cu catalysts by using this advanced technique, which may shortly appear to be the most important for the study of multicomponent catalysts. 

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