Mossbauer spectroscopy experiments

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

Figure 1. Typical geometries for Mossbauer spectroscopy experiments (a) transmission (b) scattering...

Mossbauer Spectroscopy Experiments. The Mossbauer spectrum of the as received iron foil is presented in Figure 8A and shows the six-peaked pattern characteristic of the pure metal. Figure 8B is the spectrum recorded after treatment with steam at 800°C and clearly demonstrates the dramatic change in the nature of the specimen. Although the six-peak spectral pattern of metallic iron still remains, it is clear that it is now no longer the major component the majority of iron has been converted to another phase. 

The Gonversion Electron Mossbauer Spectroscopy experiments are very useful, particularly in nondestructive testing and study of surfaces and thin layers because low-energy conversion electrons in matter have a limited range (typically 100 nm for the nucleus Fe), the method is especially useful for the study of thin layers. Apart from the transmission spectroscopy, the... 

The typical Mossbauer spectroscopy experiment described above is depicted in Figure 1.3. Resonant absorption occurs when the energy of the gamma ray just matches the nuclear transition energy for a Mossbauer... 

Experimental determinations of for fine particles show some inconsistencies. This is not surprising, as detailed studies on ultrathin films have shown that changes may depend on several parameters. Generally, is lowered with respect to the saturation magnetization value for the corresponding bulk material. The decrease should be mainly due to spin canting at the surface, that is, disorientations of spins directions, as evidenced by Mossbauer spectroscopy experiments. " A decrease of the magnetic moment per spin at the surface is also possible for metallic particles. However, for the present it is difficult to predict such effects because of the lack of systematic studies on the different types of particles. 

A lack of spin alignment has been observed by Mossbauer spectroscopy experiments in applied fields up to 100 kOe for several nanometer ferrimagnetic oxides, for instance, with... 

Figure 5.1 Schematic illustration of recording the resonance absorption line in a Mossbauer spectroscopy experiment and relative transmission of gamma quanta as a function of Doppler velocity. 2000 lOP Publishing Ltd.

Most Mossbauer spectroscopy experiments are conducted either in the transmission mode, in which a source of well-defined characteristics is used to examine the spectral properties of an unknown absorber, or in the emission mode, in which the source of radiation becomes the sample under investigation and a known or standard absorber is employed to determine the transition energy differences. In both cases, any of a number of y-ray detectors is utilized to record the amount of radiation transmitted through the absorber. 

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

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. 

Until then, the purification of the Fepr protein had been a laborous job as a 240-L batch yielded only as little as 5 mg of protein. With the overexpression clones of the Fepr proteins, the range of proteinconsuming studies such as Mossbauer spectroscopy, EXAFS, and, last but not least, crystallization experiments was greatly extended. Thus, several groups set off to systematically investigate the spectroscopic properties of both Fepr proteins, poised at all four (proposed) redox states. 

The third prominent interaction in iron Mossbauer spectroscopy is the magnetic hyperfine interaction of the Fe nucleus with a local magnetic field. As explained in detail in Chap. 4, it can be probed by performing the Mossbauer experiment in the presence of an applied external magnetic field. 

The application of Mossbauer spectroscopy in chemistry requires a prior knowledge of the nuclear states and transitions involved. In this section, we shall describe the determination of nuclear parameters by means of Mossbauer experiments with Os nuclei. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

The results from Mossbauer spectroscopy in applied magnetic fields clearly prove that the spin transition in the dinuclear compounds under study proceeds via [HS-HS][HS-LS][LS-LS]. Simultaneous spin transition in both metal centres of the [HS-HS] pairs converting the dinuclear pairs directly to [LS-LS] pairs can apparently be excluded, at least in the present systems. This is quite surprising in view of the fact that the present dinuclear complexes are centrosymmetric (in other words the two metal centres have identical surroundings, and should therefore experience the same ligand field strength and, consequently, thermal spin transition should occur simultaneously in both centres). 

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... 

The present method is still in its early stage of application. Both ex situ and in situ type measurements are applicable to a variety of mineral/aqueous solution interfaces. For example, the mechanism of selective adsorption of cobaltous ions on manganese minerals can be studied by this method. In addition to the two Mossbauer source nuclides described in the present article, there are a number of other nuclides which can be studied. We have recently started a series of experiments using Gd-151 which is a source nuclide of Eu-151 Mossbauer spectroscopy. Development of theory on surface magnetism, especially one including relaxation is desirable. Such a theory would facilitate the interpretation of the experimental results. 

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