Thermal effects in highly dispersed iron catalysts

The M6ssbauer spectra of three Fe/SiO2 catalysts with 5 wt% iron content show the presence of several Fe species and display different magnetic behaviours when the precursors are subjected to various thermal treatments. Based on the M6ssbauer parameters and CO chemisorption measurements, the average crystal sizes of the catalysts are estimated and discussed in connection with the thermal pretreatment severity and magnetic properties of the samples.

ing parameters in table I. Fig. 2 shows the M6ssbauer spectra of the catalysts and table 2 displays the corresponding parameters. The relative number of Fe ° atoms in each catalyst was calculated assuming equalf factors for all species and their total number was obtained in combination with atomic absorption measurements. The average volumetric-superficial diameters were calculated following the same methodology as in ref. [1] and their values were: 44 + 6 A, 43 + 6 A and 20 + 4 A for cv, c8 and c344 respectively.

Discussion
M6ssbauer spectra of the precursors ( fig. 1) show the presence of two peaks in the central region. These signals were fitted with two doublets with approximately equal isomer shifts, 6, and different quadrupole splittings, A, and were assigned to ct-Fe203 [2,3]. The p344 sample shows also a magnetically split component which corresponds to 0t-Fe203. Its origin might be due to the growth of a-Fe203 crystallites by sintering. The presence of a new magnetic anisotropy barrier caused by an interaction between crystallites coming close to each other because of the prolonged thermal treatment was also considered. However, rough estimates of anisotropy barriers point towards the first explanation since the calculated magnetocrystalline constant is much higher than the other anisotropies. The fitting of the central part of the spectra of the catalysts pose the additional problem that the signal originated in atoms embedded in crystallites of different sizes are overlapped. The deconvolution of these spectra would require measurements in different external conditions, i.e., at several temperatures, with applied external magnetic fields, etc., in order to assess the relative contributions of the various Fe species to the total signal. With the present data the hyperfine parameter distributions were simulated with doublets of asymmetric components and large linewidths characteristic of spectra originated in particles of different sizes.
The spectra display also the presence of paramagnetic Fe 2+ compounds. The spectra had to be fitted with one quadrupole doublet (as in ref. [6]) and one singlet for c8 and c344 and only with one singlet for cv similarly to Niemantsverdriet [5].
Finally, the presence of a third doublet characteristic of Fe 3+ is observed only in c8 and c344. This suggests that when the severity of the thermal treatment increased, Fe 3+ ions were able to migrate inside the support to sites where it is more difficult to undergo reduction. The fitting of c344 spectrum was particularly difficult because there is a great fraction of Fe 3+ in the support with significant 6 and Zl distributions. Therefore, the fitting was performed by fzxing 6 of the Fe°(sp) species assuming that their 6 distribution resulted only in a larger F. Measurements at different temperatures are currently in progress.
The Fe ° dvA values obtained from chemisorption of CO and M6ssbauer spectroscopy for cv and c8 are equal within errors. However, their M6ssbauer spectra are quite different. Considering that in metal supported systems usually there exists a distribution of crystallite sizes, the observed behaviour would be indicative of the growth of the bigger crystallites and the decrease of the smaller ones without a change of the distribution mean value. Table 2 MSssbauer parameters of reduced catalysts.