Zeolites EXAFS functions

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... 

The release of zeolite protons due to the cutting of the original anchors has been independently detected by FTIR. It was found that the intensity of the supercage OH band between 3640 and 3650 cm increases on admission of dry CO to the reduced sample, in agreement with Eq. (7) and Fig. 8 (752). The effect of CO on the migration of monoatomically dispersed Pd in NaHY is clearly demonstrated by the EXAFS functions shown in Fig. 9 and their Fourier transforms shown in Fig. 10. 

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]...

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).

Fig. 1. Model of rhodium dicarbonyl complex on dealuminated Y zeolite, as determined by IR and EXAFS spectroscopies and density functional theory. The Rh atom, near the upper center of the figure, has two CO ligands bonded to it, pointing upward, and two oxygen atoms of the zeolite lattice below. An Al atom is located between these two oxygen atoms. The dangling atoms of the cluster model of the zeolite are capped by hydrogen atoms for the calculation (Goellner, Gates, etal., 2000).

Comparison of Structural Parameters Determined by Density Functional Theory and by EXAFS Spectroscopy Characterizing Samples Made from Chemisorption of [Rh(CO)2(acac)] on Dealuminated Y Zeolite (Goellner, Gates et al., 2000) ... 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

An interesting study of platinum dispersion in NaY zeolite as a function of platinum concentration (2,5, and 10 wt.% Pt) was reported by Ryoo et al. [174]. The platinum clusters were prepared by conventional ion exchange with Pt(NH3), calcination at 583 K and reduction at 673 K. The Pt-Pt coordination numbers measured by EXAFS were constant in the three samples (5.0,5.9 and 5.1, respectively), and the cluster nuclearity measured by Xe adsorption isotherms was also constant (ca. 50 atoms). Similar results were also obtained with... 

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