EXAFS 1 Technique

We now describe the procedures used by us and oth- to estimate the morphology of the PtM clusters from the obtained four coordination numbers. Hwang et utilize a technique quite similar to ours described below to determine the experimental morphology (i.e., trial and error until an optimal model is reached), but define some useful parameters as follows Jr = NptR,/(Nptpt + NptRu) 200 and Jru = Nrur/CNruRu + Nr r) 200 for PtRu 1 1 catalysts. Then (Jr,Jru) equals (0,0) for a completely separated stracture, (100,100) for a fully alloyed structure, and 

Stoupin et al. assume a core stracture surrounded by a more amorphous shell rich in Ru, and therefore determine a parameter Sru = (n - (1-D)/xru)/D, where D is the dispersion (fraction of atoms at the surface) determined assuming spherical particles and based on the estimated particle size from the EXAFS (or XRD) results, and n is the nominal molar ratio (R Ru, perhaps determined from XRF). Here Sr represents the relative composition of the snrface (i.e., it is the fraction of Ru atoms relative to the total on the surface), and xr is an estimate of the relative composition of the core using in situ EXAFS results. Their final results suggest that the surface of a PtRu ETEK cluster is rich in Ru, and is heavily oxidized, similar to Hwang s and our conclusions. 

The optimal models for three catalysts, the PtRu ETEK, the PtRu Watanabe, and the PtsRu Watanabe, are shown in Fig. 10, using our teehnique. However, their uniqueness ean be legitimately questioned. That is why we inelude Fig. 11 to illustrate just 

Eigure 11. Various models for the PtRu E-TEK catalyst used to arrive at optimum morphology.Shaded areas represent 5% deviation from experimental EXAES fits. Eor clarity, lower error bar encompasses both Ru-Ru and Pt-Ru (Ru-Pt and PtRu forced to be equal in the EXAES fit). 

In summaiy, although several procedures for estimating cluster morphology are noted above, the basic premise is as follows  

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The EXAFS technique is used primarily for investigations of disordered materials and amorphous solids. Figure 8.35(b) shows how interference occurs between the wave associated with a photoelectron generated on atom A and the waves scattered by nearest-neighbour atoms B in a crystalline material. 

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

A surface sensitive version of the EXAFS technique has been attempted ten years ago, and has proven to be successful in a large variety of surface chemisorption and interface formation problems. In the following we recall very briefly what makes SEXAFS different from EXAFS and what is the specific information that can be withdrawn from the SEXAFS data, and address the problems of metal-metal interface formation, and metal-semiconductor interface formation with detailed examples. 

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. 

An alcohol reduction method has been applied to the synthesis of polymer-stabilized bimetallic nanoparticles. They have been prepared by simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoparticles can be prepared by refluxing the alcohol-water (1 1 v/v) mixed solution of palladium(II) chloride and hexachloro-platinic(IV) acid in the presence of poly(/V-vinyl-2-pyrrolidone) (PVP) at about 90-95°C for 1 h (Scheme 9.1.5) (25). The resulting brownish colloidal dispersions are stable and neither precipitate nor flocculate over a period of several years. Pd/ Pt bimetallic nanoparticles thus obtained have a so-called core/shell structure, which is proved by an EXAFS technique (described in Section 9.1.3.3). 

EXAFS studies120 have shown that the Mo02+ core is readily identifiable in a variety of known compounds. These have been used120 to calibrate the EXAFS technique which was subsequently used to identify this core in sulfite oxidase and in desulfo xanthine oxidase.82... 

In principle, EXAFS information may be obtained for most or all of the elements in a catalyst. Thus, for multicomponent samples, the characterization of local surroundings for all (or almost all) the elements may be obtained. However, we stress that the radial distribution function cannot be transformed into a unique three-dimensional structure. Therefore, the EXAFS technique is not ideal for providing such information and the data representing materials consisting of several different phases may often be too difficult to analyze meaningfully. 

The EXAFS technique has been especially useful for metalloproteins. It has often provided the first clues as to the identity of atoms (O, N, S) surrounding a metal atom and either covalently bonded to it or coordinated with it (Chapter 16). Interpretations are often difficult, and a common approach is to try to simulate the observed spectrum by calculation from a proposed structure.118 Tautomerism in crystalline Schiff bases (see Eq. 23-24) has been studied by nearedge X-ray absorption fine structure (NEXAFS) employing soft X-rays.119... 

Ferritin has also been compared with iron-dextran by the EXAFS technique.1108 The apoferritin controls the deposition of the core. Reconstitution of ferritin under a range of conditions always gives the same structure, which is not the case in the absence.of apoferritin. There are metal-binding sites on the protein shell. There is evidence for the binding of iron to apoferritin, probably by carboxyl groups, but there is little detailed information on these sites.1098 On the other hand, other metal ions inhibit the formation of ferritin and may do this by binding at or close to the iron sites. Of most significance appear to be results on Tb3+, Zn2+ and V02+,... 

Double salt problems are of practical importance in the chemical industry. However, only limited discussion has appeared concerning an elucidation of the mechanism of formation, because reliable information on the structure of ions in solution is not available. The development of solution X-ray diffraction and EXAFS techniques can throw new light on the problem to elucidate the mechanism of formation of double salts. As a case study we take a series of double salts M C1-MgCl2,nH20, where M1 denotes an alkali metal or an ammonium ion. 

As an example, Fig. 5.6 depicts a typical diffraction spectrum. It is evident that long range order does not exist in our chalcogenide samples. However, the broad difffactrogram peak centered at 20 = 42.5° has the characteristic of a nanodivided ruthenium metal [22]. This points out that the active center in this chalcogenide materials is essentially of metallic nature. The material, either in powder or colloidal form, was analyzed by the EXAFS technique [11]. The local range order of this technique allowed for some structural determination of our samples. Thus, for example, the co-ordination distances for ruthenium-selenium and ruthenium-ruthenium are R(RU-se) = 2.43 A y R(ru.rU) = 2.64 A, respectively. The metal-metal co-ordination distance is of the same order of magnitude as that of well known cluster based materials such as the Chevrel phase [35, 37], cf. Fig. 5.2b. This testifies that the used chemical route leads to the formation of cluster-like materials. 

In one of the first papers on the application of XAFS spectroscopy to catalysis (Lytle et al., 1974) is the statement "... these results demonstrate that the EXAFS technique can be a powerful tool for studying catalysis in order to determine the precise structural relationship between catalytically active sites and the surrounding atoms." It is exactly this precise structural relationship that is the critical kind of information needed if true structure-property relationships are to be developed. 

In cases where there is a low concentration of cation of interest, if the cations are highly disordered in the zeolite framework, or if good crystalline samples are unavailable, atom specific or environment-specific spectroscopic probes may be preferable to determine local structures about the cation in the zeolite. NMR (4), IR ( 5, 6) ESR (7-10), optical (9,10), MSssbauer effect (11-15), and x-ray absorption studies (2,16,17,18) have been used to determine cation microenvironments. In particular, it has been shown that EXAFS (Extended X-ray Absorption Fine Structure) of the cation can often be used to give direct structure information about cation environments in zeolites, but EXAFS techniques, while giving radial distances and relative coordination numbers, are insensitive to site symmetry and cannot, in general, give both coordination numbers and relative site populations. Clearly it is desirable to use complementary spectroscopic techniques to fully elucidate the microenvironments in dilute, polycrystalline zeolite systems. 

The EXAFS technique but using radiation reflected at a low angle from the surface and hence giving data for the surface atomic layers (see EXAFS)... 

Extended X-ray absorption fine structure (EXAFS) is a useful technique to provide the local stmeture of solutions. It is able to determine the r and N values at relatively dilute concentrations and in organic solvents as well. The solvation structures of Br in various solvents as determined by EXAFS is discussed in the present paper. Moreover, the EXAFS technique has been applied to the aqueous solution surface. At the surface the solvation... 

Structure of Br may not be the same as that of the bulk. Some of the molecular dynamics calculations predict that halide anions in water tend to float on the surface of clusters consisting of water molecules rather than within water. This effect may cause a dissimilar solvation structure to that of the bulk. In addition, if the anion is segregated at the surface by surfactants such as large alkylammonium cations, the anion density at the surface should be high and its environment differ from the bulk. This is a preliminary report of the first experimental study of the solution surface by the EXAFS technique. This technique provides us information on the gas/liquid interface, the structure of Langmuir films, and the effect of the interface on chemical reactions. 

In the present paper XANES and EXAFS techniques were applied to characterize zinc species with respect to their coordination in zinc substituted MFI type zeolite (H-[Zn]MFI) and zinc exchanged H-ZSM-S (ZnH-[Al]MFI). Octahedral coordination of zinc at cationic positions in hydrated ZnH-[Al]MFI was determined. In H-[Zn]MFI, zinc should be surrounded by four lattice oxygen and two other species in a further distance. Average Zn-0 distances mcrease in the order H-[Zn]MFI < ZnO < ZnH-[Al]MFI. Upon heating to 775 K a change of the anc coordination due to dehydration can be clearly observed on ZnH-[Al]MFI but not on H-[Zn]MFI. 

Ferritin has also been compared with iron-dextran by the EXAFS technique." ... 

Sinfelt has greatly contributed to the catalyses of bimetallic nanoparticles [18]. His group has thoroughly studied inorganic oxide-supported bimetallic nanoparticles for catalyses and analyzed their microstructures by an EXAFS technique [19-22]. Nuzzo and co-workers have also studied the structural characterization of carbon-supported Pt/Ru bimetallic nanoparticles by using physical techniques, such as EXAFS, XANES, STEM, and EDX [23-25]. These supported bimetallic nanoparticles have already been used as effective catalysts for the hydrogenation of olefins and carbon-skeleton rearrangement of hydrocarbons. The alloy structure can be carefully examined to understand their catalytic properties. Catalysis of supported nanoparticles has been studied for many years and is practically important but is not considered further here. 

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