Zeolite distances from EXAFS

EXAFS experiments of the Rh CO)2 complex in DAY zeolite [234] provided an excellent opportunity to compare the interatomic distances derived from EXAFS spectra with the calculated structural data. Good agreement between the EXAFS parameters and the optimized structure of the Rh (CO)2 complex on a zeolite four-ring support the hypothesis that the system Rh (CO)2/DAY is correctly modeled by the calculated T4 stmcture (Table 4.4). Also the computed C-0 vibrational frequencies of this system are close to those from IR spectra [234,236,237], with deviations below 12 cm (Table 4.4). 

Most EXAFS data show that the lattice parameter of a small particle is contracted as compared with the bulk metal. For example, Renouprez and co-workers125,135 observe for Pt encased in Y-zeolite a Pt-Pt distance contracted from 0.277 to 0.270 nm. Similarly, Hamilton et al.123 observe Cu-Cu distances varying from 0.233 for very small clusters, to 0.250 for... 

What emerges from this detailed EXAFS analysis is, first, that the tin is indeed substituted into the zeolitic framework. In many cases this kind of information is all that can be obtained from such an analysis—a first-shell fit in which the bond lengths and coordination numbers are consistent with a framework species versus a nonframework one. Flowever, in this example it was possible to analyze higher-shell data, up to a distance of 5 A, and thereby to determine the site in the zeolite framework where the tin is substituted. It is believed that the unique selectivity of this catalyst in Baeyer-Villiger oxidation reactions is a consequence of the occupation of specific crystallographically well-defined sites by tin in the framework of the zeolite in a spatially uniform manner. 

The alcohols used were 1-propanol and tert-butanol (with and without deuteration of the hydroxyl group) from Aldrich . The preparation of WZ (W content 6 wt%) is described elsewhere [2]. In brief, the catalyst is obtained by aqueous incipient wetness impregnation of hydrous zirconia with a solution containing the appropriate amount of ammonium metatungstate. The sample is subsequently calcined at 1096 K for one hour under flowing air. EXAFS measurements carried out at the National Synchrotron Light Source [7] showed that the WZ sample has an average W-O coordination number and distance of 4.7 and 1.69 A respectively. The surface area of the calcined material is 36 m /g. XRD reveals that a small amount of monoclinic zirconia was formed upon calcination. The Y-zeolite catalyst was... 

As can be seen, two bands at 102 and 73 cm" arise. However, a reliable interpretation of this spectrum is quite difficult. The key problem in interpreting far-infrared spectra of silicon-rich zeolites such as ZSM-5 is connected with the fact that, due to the low cation concentration, structural information about cation sites are so far rather scarce. Under the mentioned conditions it would certainly be a substantial progress if the vibrational assignment in the far-infrared region could be assisted by other suitable cation-sensitive techniques which provide additional information. One way, as chosen in Ref. [363], is to start from X-ray absorption spectroscopy (XAS) giving access to the local environment of the cations and their coordination spheres. For the dehydrated Ba-ZSM-5 sample a six-fold oxygen coordination at a distance of 2.75 A was obtained for Ba " ions by EXAFS analysis of the XAS spectrum. In a second step, positions in the unit cell of ZSM-5, which fulfill these criteria, were searched by computer simulation... 

EXAFS metal carbonyl clusters met clusters bimetallic clusters semiconductor clusters structures of metal, metal carbonyl, and semiconductor clusters interaction of these clusters with zeolite framework Structures are derived from coordination number and distance of metal-metal, metal-support, and metal-adsorbate contributions useful for characterization of metal-support interface interpretation of EXAFS data ould be based on the spectra of well-diar-acterized standards of molecular analogues of the supported spe-des that have been diaracterized by X-ray crystallography. 

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