Zeolite Mossbauer spectra

The Mossbauer spectrum of ferrous Y-zeolite is somewhat similar to that of the reduced silica gel samples (103). The spectrum consists of two overlapping and partially resolved doublets with the inner doublet, 3 = 0.89 mm sec-1 and A = 0.62 mm sec-1, being attributed to the ferrous ion on the surface. In both the Y-zeolite and the reduced iron oxide on silica samples, the inner doublets representing surface ferrous states are the first to be affected by adsorption of polar molecules, but in the case of Y-zeolite the addition of excess amounts of water or ammonia causes the disappearance of the spectrum, and this has been interpreted in terms of "solvation of the ferrous ions by absorbate causing weakening of the bonding to the crystalline lattice. It is also possible that the spectrum is a composite representing a multiplicity of parameters. 

The ferrocene-exchanged ZSM-5 zeolite shows a Mossbauer spectrum that when analyzed gives a 40% ferrocinium ion signal with an isomer shift of 0.53 mm/second. It is likely that water in the zeolite is being reduced as the ferrocene is oxidized. Reduction of this sample in hydrogen causes a complete loss of both the ferrocinium ion and the ferrocene Mossbauer features and a new appearance of alpha iron(0) metal and a new quadrupole split doublet of 0.4 mm/second and a quadrupole splitting of... 

The data for mixed metal zeolites as first prepared by Scherzer and Fort (18) shown in Tables I and XI are quite extensive. The reported isomer shifts and quadrupole splittings are for the iron atoms in the anionic state. Each of these unreduced samples show Mossbauer spectra that are in close agreement with literature values of the corresponding iron coordination complexes. Typical examples of unreduced and reduced samples are shown in Figures 3 and 4. We note here that preparations 16 through 22 are new and are developments of our laboratory and that 9 through 15 are preparations based on the work of Scherzer and Fort (18). Samples 16 and 17 show that this method can be extended to other zeolites like ZSM-5. If no transition metal cation is used in the synthesis, no Mossbauer spectrum for the corresponding anion is observed. Therefore, the nature of the cation is critical and complexation of the anion to a cation is necessary for anion inclusion. Certain transition metal cations (Ru + for instance) do not seem to bind the anion. 

Figure 7 Mossbauer spectrum of nanometer-sized metallic iron particles in zeolite NaX obtained at T = 4.2K before (A) and after (B) subtraction of two Fe(ll) contributions with Si = 0.75 mm s and AEqi = 0.8 mm s (5% relative contribution) and S2 = 0.85 mm s and AEq2 = 1.85mms (6% relative contribution). The analysis of the magnetic hyperfine field distribution identifies metallic a-Fe-nanoparticles. (From Schiinemann, Winkler, Butzlaff and Trautwein. With kind permission from Springer Science Business Media)...

Figure 3. Mossbauer spectrum for Iron leaded (1 weight %) NaX Zeolite.

The system FeCl2/NH4-Y turned out to be a very interesting example of the application of Mossbauer spectroscopy in the field of solid-state ion exchange in zeolites [149,150]. The spectra (cf. Fig. 44) were decomposed (vide infra cf., as an example. Fig. 45), and the oxidation states and coordination of incorporated iron were deduced on the basis of assignments reported earlier [ 154]. Results are presented in Table 10. When the above mixture was investigated as prepared, the Mossbauer spectrum provided evidence that 57 % of the iron was oxidized to the trivalent state (cf. Table 10, RI = 57). From the Mossbauer parameters it was concluded that the Fe(III) species were almost perfectly octahedrally coordinated. The remaining non-oxidized iron occurred as three different species, viz., (i) partially dehydrated Fe(II) chloride (21%) (ii) also partially dehydrated. 

An iron-exchanged mordenite was also studied by Meisel et al. (182), who incorporated Fe3+ into the zeolite structure. Upon calcination at temperatures greater than 500 K the appearance of Fe2+ was noted in the Mdss-bauer spectrum, and for calcination temperatures higher than 770 K the formation of a-Fe203 was observed to take place inside the mordenite. For the iron- mordenite system, it can now be seen that the Mossbauer effect provides information about the chemical state, symmetry, interaction strength with the support, and location on the support of the resonant iron ions. This information enhances the understanding of the catalytic activity of this and other zeolites (178). 

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. 

For Fe, Ti + and V4+ substitutions for silicon in ZSM-5 zeolite, ESR, Mdssbauer and UV-vis techniques were observed to be very informative. Fe " is a paramagnetic d ion and Ti +and V4+ paramagnetic d ions. In the former case (Fe ) in high spin state (S=5/2) one expects a fine structure in the ESR spectrum. Three kinds of Fe ion environments could be identified by UV-vis, ESR and Mossbauer spectroscopies as described in ref. 11. They are schematized below ... 

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