Magnetite particles, Mossbauer

Figure 9.8 Evolution of Mossbauer spectra (300 K) for magnetite particles in the presence of fenric gel ( Fe(0H)37Fe304 =2.25) during ageing of the sus()ension. Reproduced by permission of Academic Press from [36]...

In general, as discussed earlier, the chemical properties of small particles may be different from the properties of the corresponding bulk samples. An investigation of this effect in oxidation reactions was made by Tops0e et al. (237) in the study of the room temperature oxidation of Fe304 particles 40 nm in size. The Mossbauer spectra of several partially oxidized samples and that of magnetite are shown in Fig. 37, in which it can be seen that oxidation is reflected in the ratio S of octahedral to tetrahedral spectral areas (see Section III, A, 2). Specifically, the value of S for stoichiometric magnetite... 

Flo. 37. Mossbauer spectra of small Fe304 particles exposed to air at room temperature and after C02/C0 treatment at 700 K. (a) and (b) are room temperature Mossbauer spectra of nonsupported magnetites exposed to air for 80 days (c) is sample (b) treated at 700 K with a C02/C0 mixture. Spectrum at room temperature. Zero velocity is with respect to a 57Co in copper source. Figure according to Tops0c el al. (237). 

In another study, the effect of silica incorporation into the Fe O lattice was studied (5,49,50) A 20% Fe O on silica catalyst was prepared using conventional techniques. Ir was found that while direct oxidation of the catalyst at 800 K produced the expected a -catalyst was previously reduced in CO/CO2 to produce magnetite, then subsequent oxidation resulted in the formation of y-Fe203 Figure 10 shows MOssbauer spectra of this catalyst after various thermal treatments. In these spectra, the central doublets were demonstrated to be a result of small iron oxide particles which were superparamagnetic at the conditions where the spectrum was recorded. The suppression of the y - oOg to a -Fe203 transition is characteristic of the substitution of roreign cations into the mag-... 

As is shown in Figure 3, Mossbauer spectra of t)Tpical airborne particles collected in Tokyo (urban area) and at Tokai (remote area) indicate the presence of paramagnetic Fe (iron in common silicate minerals), Fe , and magnetic components (hematite and magnetite)... 

Natural magnetite may also occur with a small particle morphology yielding a Mossbauer spectrum with asymmetrically shaped lines. Due to the increased overlap of the lines of both sextets in that case, it becomes difficult to determine the ratio R = S(Fe )/S(Fe ) accurately. 

For an industrial catalyst, energy dispersive X-ray analysis. X-ray powder diffraction, optical microscopy and Mossbauer spectroscopy show that part of the A1 and Ca atoms are dissolved in the magnetite lattice [15, 27, 28, 38-43]. The lattice constant of the magnetite phase of an industrial catalyst is 8.377 kX [15]. The lines in the X-ray powder diffractions diagram are broadened [15] the broadening is independent of particle size [15]. 

If ferric ions are precipitated at pH 8 in the presence of colloidal magnetite, a ferric gel forms immediately, Gel particles are physisorbed on the magnetite and are identified in Mossbauer spectroscopy by a quadrupolar doublet superimposed on two sextuplets of magnetite due to Fe + and respectively [36]. During ageing... 

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