EXAFS metallic behavior

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

According to EXAFS experiments, the number of oxygen atoms relative to metal atoms in the air-oxidized state is much higher in fine crystallites than in large crystals, so that their behavior approaches that of oxides. 

Jacobs, G., Ji, Y., Davis, B.H., Cronauer, D., Kropf, A.J., and Marshall, C.L. 2007. Fischer-Tropsch synthesis Temperature programmed EXAFS/XANES investigation of the influence of support type, cobalt loading, and noble metal promoter addition to the reduction behavior of cobalt oxide particles. Appl. Catal. A Gen. 333 177-91. 

The accessibility of new techniques such as EXAFS brings researchers a powerful tool for unambiguous determination of the true core metallic framework of such systems. Thus, the relationship between the parent carbonyl precursor, the support and the final metal-supported particles has been studied at the structural atomic level in some cases. This can allow differentiation of the catalytic behavior of supported metal particles with bulk-like properties from that of supported metal clusters, opening the way to understanding the mechanism of metal-catalyzed reactions and extending the concept of sensitive or insensitive structure reactions from metal aggregates to clusters. 

A significant step to the combination of our knowledge about the static structure in liquids and their kinetic behavior has recently been made by application of an in-laboratory stopped flow EXAFS experiment [32]. This is an EXAFS spectrometer operated in the dispersive mode and a stopped-flow unit positioned along the x-ray path. Since results of very short time measurements can be accumulated in this way, with the appropriate selection of the system structural studies of reaction intermediates can be determined, which has not been possible before. Results are reported [33] on a partial structural change around the Cu(II) ion of a reaction intermediate at the formation of a Cu(II) porphyrin complex in the metal substitution reaction of the Hg(II) porhyrin complex with the Cu( o ion in an aqueous acetate buffer solution. The measurements showed that the Cu-N distance in the reaction intermediate are elongated by about 0.04A in comparison with the final product. 

Asakura K, Bando KK, Iwasawa Y, Arakawa H, Isobe K (1990) Metal-assisted hydroformyla-tion on a SiO -attached Rh dimer. In situ EXAFS and FT-IR observations of the dynamic behaviors of the dimer site. J Am Chem Soc 112 9096... 

The final task in the evaluation of SCM for predicting sorption behavior was to assess its ability to predict sorption for a weakly sorbing (outer-sphere) divalent metal ion. Because previous studies reporting EXAFS results for Sr(II) sorption to aluminum oxides were not available in the literature prior to our modeling efforts, Sr(II) XAS data were collected as part of this work. The Sr(II) XAS data were collected for a strontium nitrate solution and the samples shown in Table 7-7. 

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