Local EXAFS investigations

HDS catalysts have been characterized extensively with a wide variety of tools, and several extensive reviews of the subject have been presented (85,88-91). Substantial effort has been aimed at relating catalytic activity and selectivity to microscopic properties such as catalyst composition, electronic structure, and geometric structure. EXAFS investigations of working catalysts have provided information about the composition, average local coordination, and interatomic distances of atoms in the catalyst clusters. It has been concluded that the active phase under operating conditions is MoS2-like particles with a dimension of 10—20 A (92-94). 

The EXAFS investigations of the apical oxygen were discussed in sect. 4.4. Here we discuss the local structure of the planes in conjunction with the displacive martensitic transformation at the onset of the overdoped phase. 

Kubozono Y et al (1994) An EXAFS investigation of local-structure around Rb in aqueous-solution. Z Naturforsch A 49 727-729... 

Ruthenium and copper are not miscible hence, homogeneous alloy particles will not be formed in supported Ru-Cu catalysts. As copper has a smaller surface free energy than ruthenium, we expect that if the two metals are present in one particle, copper will be at the surface and ruthenium in the interior (see also Appendix 1). This is indeed what chemisorption experiments and catalytic tests suggest [40], EXAFS, being a probe for local structure, is of particular interest here because it investigates the environment of both Ru and Cu in the catalysts. 

The local structure around the lanthanide ions with differing redox potentials Eu(III)/(II) (-0.35 V vs. NHE), Yb(III)/(II) (-1.05 V), and Sm(III)/(II) (-1.55 V) in Ti02 particles was investigated by EXAFS. The photocatalytic reaction and EXAFS studies were also carried out for a calcined Yb(III) ion adsorbed-Ti02 catalyst [157], The photocatalytic activity of these lanthanides toward methyl blue photodecomposition was very similar, suggesting that adsorbed lanthanide ions on TiOz particles scarcely assist the high-photocatalytic activity of TiOz catalyst. However, the way in which the catalysts were activated was of high importance. The photocatalytic activity of the calcined Yb/TiOz... 

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 measurements (EXAFS) have been performed to investigate the short-range structure of TbFe films (46). It is observed that there is an excess number of Fe—Fe and Tb—Tb pairs in the plane of the amorphous film and an excess number of Tb—Fe pairs perpendicular to film. The increase of K with the substrate temperature for samples prepared by evaporation is explained by a rearrangement of local absorbed atom configurations during the growth of the film (surface-induced texturing) (47). 

As mentioned earlier, the information provided by EXAFS is limited to local effects and only the atomic identity of the immediate neighbours of the atom under investigation can be determined. Also, as can be seen from the examples quoted above, it cannot discriminate between oxygen and nitrogen atoms. This is understandable in view of their adjacent positions in the Periodic Table, but somewhat restrictive when they are both common ligating atoms of transition... 

Even when this target is reached, it must be kept in mind that XRD can, by the very nature of its basic physics, fall short of describing the structure of a catalyst at all length scales. It misses out on variations of the local structure that are better addressed by EXAFS spectroscopy (Clausen et al., 1993, 1998). Furthermore, XRD is insensitive to the texture and microstructure with dimensions larger than about 10 nm, which are better investigated by electron microscopy or gas adsorption techniques—and the surface structure of a working polycrystalline catalyst is in most cases inaccessible by XRD. 

XAFS spectroscopy provides element-specific information about the local chemistry and physical structure of the element under investigation. XANES provides information about the chemical state of the element, including the oxidation state, and sometimes the local geometry (via selection rules), and EXAFS provides quantitative information about the... 

Sankar and co-workers have used X-ray absorption fine structure (EXAFS) measurements with a synchrotron source to investigate the local environment around the titanium-centred active site in TS-1, during the epoxidation of... 

EXAFS has recently become more extensively used in the study of vanadium in complexes and in biology to investigate the nature of the local environment around the vanadium atom. ... 

These recent investigations demonstrate the utility of x-ray analysis techniques in the study of ionomers, and this utility is certain to continue undiminished. The growing availability of synchrotron x-ray beamlines will certainly provide a major impetus in the development and use of these techniques. In the area of EXAFS, we are currently studying the effect of anion type on structure and physical properties. Since the acid or of commonly-used bound ions, such as sulfonate, phosphonate, or quaternary amine, varies widely, it is the local coordination structure will also vary To date, such experiments have only been performed on... 

A iKitable exception is the recent, application of EXAFS to plasticized Zn sulfonated polystyrene ionomers (73-711. Thi.s technique was used to investigate the effect of various diluents on the local struct.ure of the Zn cation. Plasticizers termed, "non-coordinatirig" (because they do not complex metal cations) - in particular, toluene, acetonitrile, and DOP - were shown to have a minimal influence on the cation s local structure. At most, these cause a slight increase in disorder, but they do not. alter the coordination structure around the cation. These diluents clearly partition themselves into the nonpolar regions of the ionomer. 

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