Information from Mossbauer spectroscopy

In the expression for the isomer shift, the term [c — nuclear constant, which has been determined either by direct nuclear measurement or by the measurement of the isomer shift for compounds with known electronic structures (57). Ideally then, a measure of 8 provides a determination of 0S(O) 2, the latter related to the electronic structure as expressed, for example, by the occupation numbers of the various orbitals, e.g., 3d74s1 for metallic iron (Fig. 7) (52). In this manner, it is often possible to identify the oxidation state of the Mossbauer atom and to deduce information concerning the bonding of this atom to its surroundings. In some cases, different electronic structures may have similar values of 0S(O) 2 (the low-spin Fe2+ and Fe3+ pair is an example), and electronic structure information from Mossbauer spectroscopy is most... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

Two examples of Mossbauer studies show the detailed information that can be obtained from Mossbauer spectroscopy. 

Carl W. Seidel During this discussion I hope that the panel and the audience will comment on the future of Mdssbauer spectroscopy in chemistry, both as an investigative tool for different elements and as to the type of information that is or may be available from Mossbauer spectroscopy. We will begin with questions on Dr. Herberts paper. 

The third parameter obtained from Mossbauer spectroscopy is the isomer shift (IS). When expressed relative to the pure metals it provides information on the charge transfer and redistribution between s- and d-electrons upon compound formation (Miedema and van der Woude 1980). In ternary intermetallics the interpretation of IS is rather complicated (De Vries et al. 1985). For this reason we will leave IS out of consideration in the discussion of the experimental results. 

The application of Mossbauer spectroscopy in diverse fields of qualitative and quantitative analysis is based on the ease with which hyperfine interactions can be observed. The information obtained from Mossbauer spectroscopy may be correlated with other methods by which HI can be examined such as NMR, EPR, ENDOR, PAC (perturbed angular correlations), nuclear orientation and neutron scattering. However, Mossbauer spectroscopy often proves to be experimentally simpler, more illustrative and an efficient method for studying applied problems. Mossbauer nuclei are ideal spies supplying information on both the microscopic and macroscopic properties of solids. 

Mosshauer effect The resonance fluorescence by y-radiation of an atomic nucleus, returning from an excited state to the ground state. The resonance energy is characteristic of the chemical environment of the nucleus and Mossbauer spectroscopy may be used to yield information about this chemical environment. Used particularly in the study of Fe. Sn and Sb compounds. 

Four different material probes were used to characterize the shock-treated and shock-synthesized products. Of these, magnetization provided the most sensitive measure of yield, while x-ray diffraction provided the most explicit structural data. Mossbauer spectroscopy provided direct critical atomic level data, whereas transmission electron microscopy provided key information on shock-modified, but unreacted reactant mixtures. The results of determinations of product yield and identification of product are summarized in Fig. 8.2. What is shown in the figure is the location of pressure, mean-bulk temperature locations at which synthesis experiments were carried out. Beside each point are the measures of product yield as determined from the three probes. The yields vary from 1% to 75 % depending on the shock conditions. From a structural point of view a surprising result is that the product composition is apparently not changed with various shock conditions. The same product is apparently obtained under all conditions only the yield is changed. 

From the applications of Ni Mossbauer spectroscopy in solid-state research, it is clear that (1) information from isomer shift studies is generally not very reliable because of the smallness of the observed isomer shifts and the necessity of SOD shift corrections which turn out to be difficult, and (2) useful information about magnetic properties and site symmetry is obtained from spectra reflecting magnetic and/or quadrupolar interactions. 

Chemical information from hafnium Mossbauer spectroscopy can primarily be deduced from the quadrupole-splitting parameter. In Table 7.4, we have listed the quadrupole coupling constants eQV for some hafnium compounds. Schafer et al. 

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

Information on the chemical state of iridium on going from the molecular precursors, and its adsorption on the surface of the support can be obtained by Ir Mossbauer spectroscopy. It allows to estimate the composition of the Ir-containing alloys that are possibly formed during the activation treatment of supported bimetallic systems. The main results obtained in the application of Ir Mossbauer spectroscopy to characterize two Ir-containing bimetallic supported nanoparticles, i.e., Pt-Ir on amorphous silica and Fe-Ir on magnesia are presented and discussed... 

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