Zeolites EXAFS-XANES

The maximum content of titanium in Ti-beta zeolite appears to be higher than in the other materials. A value of x = 0.038 has been reported without formation of extra-framework Ti02. From the characterization of Ti-beta zeolite by XANES and EXAFS, it has been concluded that Tilv in the calcined material is tetrahedrally coordinated, isolated from other TiIV ions, and surrounded by OSi groups. In the presence of H20, Tilv increases its coordination and very likely undergoes hydrolysis of the Ti—O—Si bonds forming TiOH and SiOH groups (Blasco et al., 1993). 

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

Prior to characterization encapsulation must be ensured and clusters formed outside the cavities must be ruled out. Only then can characterization be reliably carried out. A battery of techniques is available for this purpose, such as C,Xe and metal NMR, EXAFS/XANES, XPS, IR and UV-VIS spectroscopy, electron microscopy, ESR, XRD, etc. Among these methods electronic spectroscopy plays an important role. The UV-VIS spectra reflect changes in the oxidation state of the metal as well as structural changes forced by incarceration and so serve as a valuable tool for the ascertainment of intrazeolite complexation. Although vibrational spectroscopy is most frequently applied, sometimes using the IR spectra as fingerprints for identification, it is inadequate to predict the exact structure of the clusters as these spectra maybe different from those in solution or in the sofid state due to interaction with the zeolite matrix. In any case, reliable characterization requires the combined application of complementary analytical methods. 

Recently X-ray photoelectron methods such as EXAFS, XANES and DEXAFS have been applied together with IR spectroscopy to zeolites, giving information on the oxidation state of the cations, the coordination sphere and its dimensions as well as the changes of both during reactions in time-resolved experiments [113,139,140]. 

Some preparations of iron exchanged into zeolite H-MFI by vapor-phase FeCL are known to be active and selective catalysts for the reduction of NO, with hydrocarbons or ammonia in the presence of excess oxygen and water vapor (45,46). The active centers in Fe/MFI are assumed to be binuclear, oxygen-bridged iron complexes, as follows from H2-TPR, CO-TPR, and ESR data (45,47) and EXAFS and XANES results (48,49). These complexes are structurally similar to the binuclear iron centers in methane monooxygenase enzymes that are employed by methanotrophic bacteria in utilization of methane as their primary energy source (50). It is believed that molecular oxygen reacts with these centers to form peroxide as the initial step in this chemistry (50). 

Ni(C0)4 in dealuminated NaY (Si/Al > 400) shows one band at 2046 cm , similar to that of tetrahedrally coordinated Ni(CO)4 in THF solution. No change of the Ni oxidation state and no loss of CO ligands after adsorption of Ni(CO)4 in alkali zeolite Y are detected with XANES and EXAFS spectroscopies. However, the appearance of four IR bands, which shift when the Ni(CO>4 loading or the alkali cations are varied, indicates an interaction of the type -OC—Ni, where = Na or Li. A reactive Ni(CO)3 in-... 

Fig. 54 shows the XANES and EXAFS spectra of the Cu(l)ZSM-5 zeolite after addition of CO (180). The addition of CO causes a dramatic decrease in the intensity of the band attributed to the Is 4p transition in the XANES of the Cu(I)ZSM-5. In the EXAFS spectra of these catalysts, the Cu-O peak became smaller and shifted to a longer atomic distance (i.e., 1.5 —> 1.8 A), and new peaks attributed to the C atom (Cu-C) and O atom (Cu-C-O) of the adsorbed CO molecule appear due to the addition of CO onto the catalyst. These results indicate that CO molecules adsorb on the isolated Cu ions strongly enough to distort the Cu coordination geometry (180). 

Figure 68 shows the XANES and EXAFS spectra of (a) Cu(ll)ZSM-5 and (b) Cu(l)ZSM-5 catalysis prepared by the evacuation of the former at 973 K. These spectra exhibit four kinds of bands due to transitions ls-3d (A), Is 4pz (Is 4p7T ) (B), Is 4px,y(ls 4ptr ) (C), and multiple scattering (D) (179, 180). The Cu(ll)ZSM-5 sample dried at 373 K shows a well-separated weak preedge band due to the Is 3d transition(A) and an intense band due to the Is 4p transition. The band due to the Is -> 4pz transition (B) can be observed as a shoulder of the band due to the Is - 4px,y transition (C) accompanied by their shake-up bands (B and C ). The presence of a band due to the Is 3d transition (A), which is forbidden by the selection rule in the case of perfect octahedral symmetry, and shake-up bands (B and C ) indicates that the Cu(II)zeolite sample contains predominantly Cu(ll) ions with slightly distorted symmetries. These results coincide with results of EPR experiments (shape, g-tensors, and A factors) which indicate the presence of distorted hydrated Cu ions in the Cu(II)zeolite sample. 

Fig. 68. XANES (a-d) and EXAFS spectra (a -d ) of Cu(II)ZSM-5 (a, a ), Cu(I)ZSM-5 (b, b ), Cu(I)Y zeolite (c, c ), and Cu(I)mordenite catalysts (d, d ). Cu(I)zeolite catalysts were prepared by evacuation of the original Cu(II)zeolite samples at 973 K [reproduced with permission from Yamashita et at (fW)].

State of zinc in MFI type zeolites characterized by XANES and EXAFS... 

In the present paper XANES and EXAFS techniques were applied to characterize zinc species with respect to their coordination in zinc substituted MFI type zeolite (H-[Zn]MFI) and zinc exchanged H-ZSM-S (ZnH-[Al]MFI). Octahedral coordination of zinc at cationic positions in hydrated ZnH-[Al]MFI was determined. In H-[Zn]MFI, zinc should be surrounded by four lattice oxygen and two other species in a further distance. Average Zn-0 distances mcrease in the order H-[Zn]MFI < ZnO < ZnH-[Al]MFI. Upon heating to 775 K a change of the anc coordination due to dehydration can be clearly observed on ZnH-[Al]MFI but not on H-[Zn]MFI. 

In the present study, highly dispersed titanium oxides included within the zeolite cavities (Ti-oxide/Y-zeolite) and framework (Ti-MCM-41,-48) were prepared using an ion-exchange method and hydrothermal synthesis to be used as photocatalysts for the reduction of CO2 with H2O at 328 K. The characterization of these catalysts by means of in situ photoluminescence, diffuse reflectance absorption, XAFS (XANES and FT-EXAFS), and ESR measurements have been carried out and special attention has been focused on the relationship between the structure of the titanium oxide species and the reaction selectivity in the photocatalytic reduction of CO2 with H2O to form CH3OH. 

Micro SXRF (Synchrotron-based micro-X-ray fluorescence) spectroscopy is an excellent tool to study elemental distributions on microscopic level. This method helps to understand the role of particular minerals in sorption properties of complex mineral assemblies. Iron and managanese oxide phases were detected in polished thin sections of natural zeolitic tuff by SXRF. This section was contacted with synthetic ground water containing Pu(V), and Pu was found to be predominantly associated with manganese oxide, but not with hematite [41]. Micro SXRF can be combined with micro-XANES, and micro-EXAFS to determine the oxidation state and coordination environment of Pu adsorbed at different regions of the tuff [42]. 

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