Cells Mossbauer

The in situ Mossbauer experiments were conducted with 90% - Fe enriched 9 1 Ni/Fe oxyhydroxide films which were deposited in the fashion described above onto a gold on Melinex support(12) in a conventional electrochemical cell. Prior to their transfer into the in situ Mossbauer cell, the electrodes were cycled twice between 0 and 0.6 V vs. Hg/HgO,OH" in 1 M KOH. Two such films were used in the actual Mossbauer measurements in order to reduce the counting time. The in situ Mossbauer cell involved in these experiments was previously described. 

The Fe(C0)5, (C Hj Fe and mixed metal zeolites were reduced in hydrogen in our in-situ Mossbauer cell between temperatures of 300°C and 500°C for various times, 4 hours to 25 hours, depending on the form of iron introduced into the zeolites and depending on the particular zeolite. 

FIGURE 21. In situ Mossbauer cell for measurements involving a fuel cell type electrode operating as an oxygen cathode. 

The magnetic susceptibilities (x) or magnetization (a = x H) for the systems described here were measured by the In-sltu Faraday method described by Mulay [1]. The Mossbauer Spectra were obtained with an Austin Associates spectrometer with a 57co/Rh matrix source (50 mCi) and Nuclear data electronics. In-sltu measurements were made, using a cell similar to that described by Delgass et al. [7]. 

The Mossbauer spectroscopy cell used in this investigation has been described in detail elsewhere (9). The Mossbauer spectroscopy instruments and the fitting routine used are also described elsewhere (9). All isomer shifts are reported relative to metallic iron at room temperature. 

Seven representative Mossbauer spectra are presented. The five spectra of Figure 1 are a sequence recorded following various treatments of sample 1, the fully hydroxylated sample. Spectrum 1A was recorded at liquid nitrogen temperature after Fe(C0)5 hac been admitted to the sample cell. This is essentially a spectrum of frozen Fe(C0)5. Spectrum IB was recorded at room temperature after the sample had been heated to 350 K for 10 hours. This spectrum consists of two components a zero-valent iron species (34%) and an Fe + species (66%). The numbers in parentheses represent the fraction of the total spectral area produced by the... 

The change in the Mossbauer spectrum of an antiferromagnetic material produced by an external field is considerably less dramatic than for a ferromagnetic material. Since there is no net moment per unit cell for Hext to interact with, as a first approximation in a randomly oriented antiferromagnet, Hext will increase H in about half the atoms and decrease H in the others, depending on the orientation of an individual atomic spin. The net result is to broaden the lines of the Mossbauer... 

Typical X-ray diffraction patterns of the Fe oxides are shown in Figure 7.16. They provide the three parameters, namely line (angle) position, width and intensity from which the nature of the oxide, its quantity (in a mixture), its unit cell parameters and its crystallinity (crystal size and order) can be deduced. The crystal structure of unknown compounds may also be determined. XRD is still the most reliable way to identify a particular oxide because it is based on the long range order of the atoms, whereas most other methods (e.g. Mossbauer spectroscopy, EXAFS) characterize the atoms and their immediate (short range) environment. 

Fig. 17.3 Magnetic hyperfine field (left) and width of the outer lines of the sextets (right) obtained from Mossbauer spectra of ferritins, isolated from human spleen, limpet hemolymph and bacterial cells Pseudomonas aeruginosa) as a function of temperature (Webb St.Pierre, 1989 with permission).

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