Iron oxide, Mossbauer spectra

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 6.2 gives an example where XRD readily reveals the phases in an Fe-MnO Fischer-Tropsch catalyst [7], The pattern at the top is that of an MnO reference sample. The diffraction pattern of the reduced Fe-MnO catalyst shows a peak at an angle 29 of 57°, corresponding to metallic iron, and two peaks which are slightly shifted and broadened in comparison with the ones obtained from the bulk MnO reference. The Mossbauer spectrum of the reduced catalyst contains evidence for the presence of Fe2+ ions in a mixed (Fe,Mn)0 oxide [7], and thus it appears justified to attribute the distortion of the XRD peaks to the incorporation of Fe into the MnO lattice. Small particle size is another possible reason why diffraction lines can be broad, as we discuss below. 

We also examined the effects of various heat treatments in both reducing and oxidizing atmospheres. An indochinite reduced for 12 hours in a hydrogen atmosphere at 950 °C. yielded the Mossbauer spectrum displayed in Figure 5. The curve shown is the result of the computer fit the data points are not given, but the closeness of fit is about the same as in the other spectra. The spectrum clearly shows that a certain amount of the original tektite remains the two broad lines at —0.11 and 1.83 mm./sec. match the original tektite lines closely. There is also clear evidence of the presence of metallic iron since the six narrow... 

In the most oxidized state (circa OmV), EPR spectroscopy shows a signal (g = 2.02) characteristic of [3Ee-4S] clusters in the +1 state. In agreement, the low temperature (4.2 K), Mossbauer spectrum shows the correspondent typical magnetic component. However, this component only accounts for approximately 27 per cent of the total iron. The remaining 73 per cent, invisible to EPR spectroscopy, can be assigned to two diamagnetic [4Ee-4S] clusters. 

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

Mossbauer spectrum, the positions and separations of which depend on the oxidation state, electronic configuration, magnetic ordering, coordination number and symmetry of the iron atoms in a mineral structure. Computed peak areas enable Fe3+/Fe2+ ratios and site populations of Fe2+ ions in crystal structures to be readily determined for several minerals without interference from coexisting Mn, Cr and other transition elements. 

As shown by the Mossbauer spectrum (see Figure 9.12), where are reported the position of the lines for iron and all the possible iron oxides, that is, FeO, Fe203, and Fe304 and was shown that the only oxide present was Fe304 [121],... 

Hobson and Campbell reported that the Mossbauer spectrum of a sample of iron oxide on silica gel, which has been calcined at 500°C for 16 hr, showed (65) a small chemical isomer shift of 0.29 mm see -1 and an unusually large quadrupole splitting of 1.87 mm sec-1. This splitting was noted to be larger than 1.60 mm sec-1 calculated by Flinn et al. (17) for a ferric ion in an octahedral environment with 1 oxygen atom missing but less than 2.26 mm sec -1 calculated for the... 

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