Zeolite EXAFS analysis

Also for Co +-exchanged [56, 57] Y-type zeolites, EXAFS analysis showed that these cations exist as sixfold coordinated aquo complexes in the large... 

A cationic molybdenum sulfide cluster [Mo3S4(H20)9] " with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4Cl(H20)9p " with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization. 

Re-Pt interactions have also been evidenced by EXAFS analysis of catalysts prepared by impregnation of a solution of HRe(CO)s in zeolite-containing anionic... 

Figure 3 Plot of AU and the area ratio R = AVBc/AVBb as a function of the Pt-Pt coordination number N, as obtained from the EXAFS analysis. Also shown is the estimated dispersion, cluster diameter and number of atoms/particle as estimated assuming spherical clusters and FCC packing. Data are shown for the acidic supports (LTL[K/A1=0.63] zeolite and CI-AI2O3, SiOi) and for the basic supports (LTL [K/A1=1.25] zeolite and K-AI2O3, K-Si02).

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. 

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... 

Mo(CO)g and W(CO)g carbonyls on X and Y type zeolites has been reported and an EXAFS analysis of W(CO)g on Na5gY (with evidence of photo-oxidation products) has been reported... 

This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites. 

The changes in the CNs in the second TPO experiment are summarized in Figure 22.7 (2nd). The profile of the second TPO was different from that of the first TPO in that the oxidation of Pd clusters progressed without an increase in the CN (Pd—Pd). As a result, the removal of the Pd clusters from the zeolite framework and their oxidation took place simultaneously in the second TPO. The curve-fitting analysis of the EXAFS data obtained in the third TPR run is included in Figure 22.6 (3rd). The changes in the CNs were very similar to those of the second run. In other words, the formation of Pd4 clusters was observed after the disappearance of the... 

Structure Elucidation from Crystal Powders. For many practical materials, such as polymers and zeolite catalysts, it is impossible to synthesize large crystals. Therefore the structure has to be found from powders. Powder XRD (preferably using synchrotron radiation) and neutron diffraction are the most important techniques, but experiments using other analysis methods like High Resolution Electron Microscopy (HREM) and Electron Diffraction (ED), MAS-NMR and EXAFS can add valuable information (8). 

To explain this competition we hypothesized that Ti could play a role similar to that of A1 in the crystallization of zeolite Beta, that is, the creation of negative charges in the framework and thus its stabilization by interaction with the TEA+ templating cations. This hypothesis was also supported by the fact that the amount of TEA+ cations decomposing at T>620K in air (as determined by thermal analysis) was dependent on the total amount of A1 + Ti, rather than only on A1 (ref. 14). This hypothesis required the ability of zeolitic Ti to change its coordination number, something which obtained substantial support from XANES and EXAFS measurements (ref. 15). 

EXAFS has been used for the analysis of location or structure of zeolite-hosted CdS and CdSe clusters [73]. The persuasive power of this method suffers, however, from the lack of well-defined reference structures in model compounds needed for exact data analysis procedures,... 

The influence of the zeolite environment on the XPS BE of dispersed ions (vide supra) means that reference compoimds for this type of investigation are not easily available. This is not so much a problem for the starting material for which the highest oxidation state of the element is often plausible, but the identification of intermediate states, and sometimes of the final state of reduction, is not straightforward. As a first approximation, BE shifts known from bulk components (e.g., coordination compounds) are often used in the analysis of zeolite systems. Combination with bulk techniques sensitive to electronic structure can provide additional information notwithstanding possible differences between the conditions in the bulk crystallite and the surface layer. Thus, IR of adsorbed CO has been used to differentiate between Pt andPt(O) atoms in H-ZSM-5 [131], EXAFS was able to detect very small intra-zeolite Cu(0) clusters formed from Cu+ with almost identical XPS/XAES signature [108], Mossbauer spectroscopy suggested the presence of Fe in zeolites with doubtftil Fe 2p satellites [116], and ESR was employed to support the occurrence of Pd+ in the reduction of intra-zeolite Pd(II) [126,127]. 

Finally, in this section, there has been a substantial body of work on Pt L(III) edge studies of supported platinum catalysts. One relatively early paper describing an analysis of EXAFS data, without curve fitting, on a sample of zeolite Y was interpreted in... 

EXAFS is also well suited for the study of finely divided metal (or metal oxide or metal sulfide) clusters supported within the pore structure (see Chapter 6). These particles are readily observed by X-ray spectroscopy, even if they are disordered throughout the solid. Analysis can even determine the average particle size of such clusters, which is of vital importance in catalytic preparation. Typically, for example, platinum supported on zeolites (and other solid acids) is a highly effective catalyst in the reforming of hydrocarbons. 

Careful analysis of both powder X-ray dilfraction and EXAFS spectroscopic data located the cadmium sulfide as (CdS)4 cubes occupying the space within sodalite cages, with the Cd ions coordinated to framework oxygen atoms (Fig. 6.10). Furthermore, the clusters were observed to order between adjacent sodalite cages, to give superclusters or a superlattice structure. In subsequent work, a variety of compounds and elements have been prepared as well-defined clusters within zeolite frameworks, including metal oxides, selenides and phosphides, and these have been studied mainly with the view of determining the effects of cluster size on optical and electronic properties. 

The literature includes only a few examples of zeolite encaged clusters which have been characterized with EXAFS spectroscopy with a thorough analysis of the data. Examples of relatively thorough data analysis are those for clusters inferred to be triosmium carbonyls [53] in NaNj-treated NaY zeolite, [Ir4(CO)i2] [67] and [Irg(CO)i6] [5] in NaY zeolite, and [Ir6(CO)i5] [60] in NaX zeolite. 

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