Zeolite Mossbauer data

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. 

In the present study Fe(Pc)(Py)2 encaged into Y-zeolite is studied under catalytic conditions of oxidation of hydroquinone to benzoquinone in acetic acid media. Most of the measurements were performed in frozen slurries. The changes of characteristic Mossbauer data are correlated with proposed steps of the removal of pyridine ligands. From the comparison of data obtained at different temperatures the probability of the Mossbauer effect is also considered for identification of various iron species. Finally, the results are correlated with the catalytic properties, as well. 

It was shown (Ovanesyan et al., 2000) that iron complexes formed during the thermal treatment of FeZSM-5 zeolite perform single-turnover cycles of methane oxidation to methanol at ambient conditions when nitrous oxide is used as a source of oxygen. The long-living active intermediate is capable of transferring an accepted O atom into a C-H bond of methane to produce methanol at 100% selectivity. On the basis of joint Mossbauer and catalytic data, the structure and composition of iron active centers are suggested. 

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. 

A detailed study of iron-promoted rhodium clusters was conducted by Schii-nemann et al. [235] using TPR, FTIR, TEM and Mossbauer spectroscopy. NaY was exchanged with Fe ions in FeS04 solutions, then rhodium was exchanged from [Rh(NH3)5Cl]Cl2 solutions. The co-exchanged zeolite was calcined from room temperature to 500 °C with a ramp of 0.5 °C min. After reduction at 500°C most of the iron remained in ionic form, but bimetallic clusters with a low Fe content were also formed. Treatment in NaOH of the reduced zeolite followed by calcination and reduction maximized the Fe content that attained ca. 50% according to ferromagnetic resonance data. The Rh-Fe clusters were disrupted in a CO atmosphere with formation of rhodium and iron carbonyls. 

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