EXAFS Observations

To elucidate the mechanism for the formation of Pt nanowires in the mesoporous chatmels we have made TEM and EXAFS observations of the H2PtCl,5/HMM-l (5wt% Pt) sample with methanol/water vapor under the exposure of UV-vis light by changing the irradiation time. Figure 15.1 la-e shows snap-shots of TEM images of the samples after irradiation for 4—48h [32, 34]. After 4h, small Pt... 

The state of tin in Pt/Sn/alumina catalysts was investigated bv Li and Shia (25) via Mossbauer spectroscopy (i/9Sn enriched isotopes) and XPS. The former technique indicated the presence of Sn+, Sn+2 and Sn, in proportions that depended on the method of preparation, but in all cases the Sn+4 component dominated. These conclusions were confirmed by the XPS experiments. Additional TPR tests on the reduced catalyst and on samples exposed to air showed that reoxidation of Pt/Sn/alumina reduced preparations was rather slow, confirming our EXAFS observations. The presence of zero valent tin in similar preparations, using the acetone complexation procedure, was recently confirmed by Li, Stencel and Davis (12) in an extended XPS investigation. For reduced samples, with a Pt Sn ratio 1 5, these authors estimated that approximately 68% of the tin was in the metallic state. However, they observed that exposure of the sample to air for 10 minutes entirely eliminated the XPS detectable Sn°. Their data also indicated that upon reduction, chlorine migrated from the surface to the alumina. Thus, XPS which measures surface composition indicates a higher sensitivity to oxidation than was demonstrated by our EXAFS experiments, which is a bulk diagnostic. 

Extended x-ray absorption fine stmcture measurements (EXAFS) have been performed to iavestigate the short-range stmcture of TbFe films (46). It is observed that there is an excess number of Fe—Fe and Tb—Tb pairs ia the plane of the amorphous film and an excess number of Tb—Fe pairs perpendicular to film. The iacrease of K with the substrate temperature for samples prepared by evaporation is explained by a rearrangement of local absorbed atom configurations duting the growth of the film (surface-iaduced textuting) (47). 

Fine structure extending several hundred eV in kinetic energy below a CEELS peak, analogous to EXAFS, have been observed in REELS. Bond lengths of adsorbed species can be determined from Surface Electron Energy-Loss Fine Structure (SEELFS) using a modified EXAFS formalism. 

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

EXAFS is observed as a modulating change in the absorption coefficient caused by the ejected electron wave back-scattering from the surrounding atoms, resulting in interference between ejected and back-scattered waves. It is defined as ... 

Since the fine structure observed is only associated with the particular absorption edge being studied, and the energy of the absorption edge is dependent on the element and its oxidation state, EXAFS examines the local structure around one particular element, and in some cases, an element in a given oxidation state. A fuller picture can therefore be obtained by studying more than one absorbing element in the sample. 

In pyridinium chloride ionic liquids and in l,2-dimethyl-3-hexylimida2olium chloride ([HMMIMjCl), where the C(2) position is protected by a methyl group, only [PdClJ was observed, whereas in [HMIMjCl, the EXAFS showed the formation of a bis-carbene complex. In the presence of triphenylphosphine, Pd-P coordination was observed in all ionic liquids except where the carbene complex was formed. During the Heck reaction, the formation of palladium was found to be quicker than in the absence of reagents. Overall, the EXAFS showed the presence of small palladium clusters of approximately 1 nm diameter formed in solution. 

Fig. 2. Schematic drawing of the synthetic [FeeSeCLie] cluster [from (S)]. Such a cluster will contain not only the usual Fe-Fe and Fe-S distances of 2.7 and 2.3 A, respectively, but also a long Fe-Fe distance of 3.7 A. This long distance is not observed in EXAFS measurements (6).

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). 

It was shown by EXAFS that the structure and dispersion of the Mo sulfide species in MoSx/NaY were unaltered by a prolonged sulfidation of 20-h at 673 Kina stream of H,S/Hj. This fact indicates that highly dispersed Mo sulfide species are thermally stable. On the other hand, with MoSx/AljOj prepared by using Mo(CO) [12], a considerable agglomeration of highly dispersed Mo sulfide species was observed at a shorter treatment time. It is considered that highly dispersed Mo sulfide clusters are thermally stabilized in zeolite cavities. 

The IR spectra in Fig.7 indicate the preferential adsorption of NO on the Co sites. It may be conjectured that the Mo sulfide species are physically covered by the Co sulfide species or that Co-Mo mixed sulfide species are formed and the chemical natures of the Co and Mo sulfides are mutually modified. The Mo K-edge EXAFS spectra were measured to examine the formation of mixed sulfide species between Co and Mo sulfides. The Fourier transforms are presented in Fig.8 for MoSx/NaY and CoSx-MoSx/NaY. The structural parameters derived from EXAFS analysis are summarized in Table 1. The structure and dispersion of the Mo sulfides in MoSx/NaY are discussed above. With the Co-Mo binary sulfide catalyst, the Mo-Co bondings are clearly observed at 0.283 nm in addition to the Mo-S and Mo-Mo bondings. The Mo-Co distance is close to that reported by Bouwens et al. [7] for a CoMoS phase supported on activated carbon. Detailed analysis of the EXAFS results for CoSx-MoSx/NaY will be presented elsewhere. It is concluded that the Co-Mo mixed sulfides possessing Co-S-Mo chemical bondings are formed in CoSx-MoSx/NaY. 

That observed for TS-1 is not peculiar for TS-1 only and can be observed on other titanosilicates like Ti-MSA, a mesoporous amorphous material that has Ti(Vl) centers exposed on the surface of the pores [124,125]. In this case, easier experiments could be performed by Prestipino et al. [50] as the peroxo/hydroperoxo complexes can be formed by dosing f-butyl hydroperoxide (which does not enter the 10-membered rings of TS-1). The XANES spectrum of Ti-MSA in vacuum is typical for almost r -like Ti(IV) centers (the intensity of the Ai T2 pre-edge peak being only 0.69, as compared with 0.91 for TS-1). Upon dosing the t-butyl hydroperoxide in decane solution on Ti-MSA, a spectrum similar to that obtained on TS-1 contacted with anhydrous H2O2 is observed on both XANES and EXAFS regions [50]. When the... 

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]. 

The longer metal-oxygen distances of about 2.6 A observed by EXAFS spectroscopy for these and related supported metal clusters suggest weak interactions between the metal and surface oxygen atoms these EXAFS contributions are not determined with as much confidence as those characterized by the shorter distances, and the interactions are not well understood. 

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