Zeolites Mossbauer spectroscopy

XL30). Mossbauer spectroscopy (KFKIj was applied to follow the state of Fe species in the zeolites. Carbon monoxide and ammonia adsorption (monitored with FTIR) (EQUINOX 55) was used to determine the nature, concentration and acid strength of the active sites in the Fe-TON zeolites. 

Keywords Fe-exchanged zeolite A, 57Fe Mossbauer spectroscopy, hydrothermal crystallization... 

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. 

Photochemical activation (15) and thermal activation (11,16, 17) of iron carbonyl complexes In various zeolites have been reported. Part of our study Is to use Mossbauer spectroscopy to Investigate the behavior of Fe(C0)5 on several zeolites when activated photochemically and thermally. Another part of our study Is to Investigate the novel preparation method of Scherzer and Fort (18) that Introduces iron Into (in their study) zeolite NH Y as an anionic complex. Finally, we will report the preparation of ferrocene sublimed onto zeolite ZSM-5. The photochemical and thermal activation of these systems will be reported as well as preliminary results of the photochemical isomerization of olefins by Fe(C0)5 zeolites and the thermal activation of Fischer-Tropsch catalytic systems. It also should be noted here that our Mossbauer studies involve an in-situ pretreatment cell which can be heated to 500°C under various gaseous atmospheres. 

The complementarity and interplay of the results of EXAFS and Mossbauer spectroscopies provide means by which crystal chemistry of certain cations in zeolites may be successfully studied. While neither technique is completely adequate as a "stand-alone tool for such studies, the combination of the two can be used to map the chemical nature, environment and location of cations where this information would otherwise be inaccessible. 

Components of fluidized cracking catalysts (FCC), such as an aluminosilicate gel and a rare-earth (RE) exchanged zeolite Y, have been contaminated with vanadyl naphthenate and the V thus deposited passivated with organotin complexes. Luminescence, electron paramagnetic resonance (EPR) and Mossbauer spectroscopy have been used to monitor V-support interactions. Luminescence results have indicated that the naphthenate decomposes during calcination in air with generation of (V 0)+i ions. After steam-aging, V Og and REVO- formation occurred. In the presence of Sn, Tormation Of vanadium-tin oxide species enhance the zeolite stability in the presence of V-contaminants. 

Thus, x-ray powder diffraction, electron paramagnetic resonance, luminescence and Mossbauer data suggest that a complex of Sn, V0+ and oxygen forms that leads to the passivation of vanadium when deposited on the zeolite, and on zeolite/gel mixtures. This complex may be a compound like VpSnO, or similar higher molecular weight species. Evidence of Sn/V all oy formation has not been found from Mossbauer spectroscopy. 

This review is concerned primarily with optical spectroscopic methods for characterising zeolites and molecules adsorbed in zeolites. The electromagnetic spectrum spans the range from radiofrequencies to X-radiation. Spectroscopic techniques included in this range are, in order of increasing frequency, NMR, EPR, infrared, UV-VIS and Raman, XPS, XAS and Mossbauer spectroscopies. 

A particular method for studying complexes of iron is Mossbauer spectroscopy [7]. For instance, a- and p modifications of Fe(Pc) were studied by this method [8], and the dioxygen derivatives, as well. It was found that the oxygen molecule interacts with the complex, and (p-Oxo)-bis-phthalocyaninato-iron(lll) is formed [9]. The stability of the complex depends strongly on the solvents, e.g. in presence of strong N-bases the Fe ll) form is restored at room temperature, even in presence of oxygen [10]. Further, the Y-zeolite encaged Fe(Pc) was also studied, namely stabilization of the pyridine complex, Fe(Pc)(Py)2, as well as the effects of preparation conditions and presence of various counterions in the framework (Na, K, Rb) on the formation of Fe(Pc)(Py)2 are reported [11-13]. 

Figure 8.20 shows a transmission electron microscopy (TEM) image of iron oxide nanoparticles generated in FeZSM-5 zeolite after calcinations and steam treatment. Dark spots in this image correspond to homogeneously dispersed iron oxide nanoaggregates of 1-2 nm (Domenech et al., 2002d,e). Changes in iron species can be monitored by Te Mossbauer spectroscopy, as illustrated in Figure 8.21. This figure shows spectra taken at 300 K in the air for FeZSM-5 (a) as-synthesized, (b) calcined, and...

In the present communication, we have reported the synthesis and characterisation of Fe substituted NaA zeolite using a variety of techniques Fe Mossbauer spectroscopy and MAS NMR spectroscopy. It is... 

From Mossbauer spectroscopy, the environment of Fe2+ ions in zeolites has been deduced (34). In summary, research in recent years on zeolites has been concerned with those materials of specific commercial importance. There seems to be a direct relation between the level of scientific interest and the area of major application. This, of course, may be prompted in part by the availability of materials since some zeolite minerals are rare and difficult to obtain. Many zeolites and many potentially interesting aspects of zeolite chemistry and zeolite properties have been neglected so far. 

For analysis of the state of iron were employed EPR, FTTR, and Mossbauer spectroscopies. For structural interpretation of these results the concept of divalent transition metal cation siting was used as recently established for pentasil ring zeolites in a wide range of metal concentrations and Si/Al compositions. With help of UV-Vis and FTIR this approach evidenced three zeolite coordination of divalent cations in similar six-membered rings of framework local structures. Three cationic fiiamework sites were thus suggested, denoted as a, p and y. (For details see [7-11]). 

CsHg/NOx molar ratio of consumed propane and reduced NO was between 0.5 and 1.0 for all three iron zeolites up to this temperature. Moreover, the seleetivily of the SCR reaction over Fe-FER and Fe-BEA was nearly stable up to 450 C, while steeply decreasing with temperature for Fe-MFI. Thus, at 450 °C nearly 7 molecules of CaHg are formally consumed for 1 molecule of converted NOx over Fe-MFI, and stiU only about 1.5 molecule of CaHg for Fe-FER. That is obviously due to unselective oxidation of the propane at higher temperatures, connected to the undesirable activity of the iron oxide species evidenced in Fe-MFI catalyst by EPR and Mossbauer spectroscopy. 

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]. 

A series of ultrastable zeolite Y samples containing Fe(III) ions were prepared by solid-state ion exchange of the ammonium forms of zeolite Y under deep-bed conditions at various temperatures [19]. The properties of these materials were compared with those of FeY samples prepared by conventional solid-state ion exchange using Mossbauer spectroscopy. 

Zeolites, NaX and NaY, partially exchanged with 5 wL-% Fe were reduced with sodium vapour produced by the thermal decomposition of sodium azide. The metalhc iron particles produced had dimensions of a few nanometers, as revealed by Mossbauer spectroscopy and various other techniques [20]. 

The coordination properties of Eu cations ion-exchanged into zeolites A,X and Y have been studied by Eu-151 Mossbauer spectroscopy [21 ]. In another study using Eu-151 Mossbauer spectroscopy [22] it was demonstrated that Eu cations within the supercages of zeolite Y are partially reduced to Eu by hydrogen. 

Iron in the framework of zeolite beta has been thoroughly characterised using RT and 4.2 K Mossbauer spectroscopy. An external appUed field (4.13 T) was also applied at 4.2 K. The IS of 0.22 mm s at RT and 0.32 mm s" at 4.2 K and the narrow, sharp hyperfine sextet at 4.2 K imder the applied field demonstrated that most of the Fe + ions were present as tetrahedral iron in the framework [34]. 

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