Zeolites EXAFS

XPS and IR indicate that Pd is electron deficient in acid zeolites, and has an electron excess in alkaline zeolites EXAFS gives some evidence of this in PtHL.104 Pt particles outside the channels are not significantly modified by the zeolite basicity, and hence give unselective aromatisation, like Y zeolite.174... 

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. 

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. 

In the present study, we synthesized in zeolite cavities Co-Mo binary sulfide clusters by using Co and Mo carbonyls and characterized the clusters by extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and high resolution electron microscopy (HREM). The mechanism of catalytic synergy generation in HDS is discussed. 

Table 1 Structural Parameter as Derived from the Mo K-Edge EXAFS for Mo and Co-Mo Sulfide Catalysts Encaged in a NaY Zeolite...

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. 

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]. 

Calculations on the basis of density functional theory have been used to check the structure of Rh clusters supported on zeolite X the results indicate a slightly twisted prism, nearly matching the octahedron inferred on the basis of the EXAFS data [28,29]. (These clusters were inferred not to be bare but to have hydride ligands, as described later). 

Fig. 3 Ir4 cluster supported at the six-ring of zeolite NaX as represented by density functional theory samples were characterized by Extended X-ray absorption fine structure (EXAFS) spectroscopy and other techniques [32]...

In the carbonylation of MeOH in the presence of Rh-exchanged zeolites, the Rhm ions are reduced to Rh1 ions, which lead to Rh-dicarbonyl and Rh-carbonyl-acetyl complexes.29-32 IrY and RhY zeolites catalyze the carbonylation of MeOH in the presence of a Mel promoter. The kinetics have been determined and IR spectra suggested that with the Ir catalyst the ratedetermining step was the addition of MeOH to the active species followed by migration of a Me coordinated to Ir. With the Rh catalyst, oxidative addition of Mel was the rate-determining step.33 A series of EXAFS measurements was made to determine the structural basis for... 

CO and particle size, 39 159 CO on zeolite-encaged metal, 39 158 EXAFS functions, 39 155-157 Pd particle location and size, 39 155-158... 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

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... 

The initial interaction of Ru3(CO)i2 with the surface of several oxides has been extensively studied by EXAFS and IR spectroscopy [70,100]. Representative studies of materials prepared from Ru3(CO)i2 and zeolites ]101, 102] or MCM-41 related supports [97] have also been reported. Ru/Si02 has also been prepared by the sol-gel method, using Ru3(CO)i2 and tetramethoxysilane as precursors [98] spheroidal metal particles homogeneously dispersed into the sihca matrix with a size range from 1 to 4nm were obtained after decarbonylation at 573 K under He, followed by H 2-treatment. 

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. 

High resolution electron microscopy (HREM) is also used extensively for structural examination of zeolites, particularly for intergrowths and faults. EXAFS has been used to determine the local coordination geometry of the exchangeable cations and how this changes on reaction or dehydration. 

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