Mossbauer spectroscopy limitations

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

The great advantage of Mossbauer spectroscopy is that it can be applied in situ. The major limitation of the technique is that it can only be applied to a couple of elements, among which iron and tin are the easiest to study. 

The recollless fraction, that Is, the relative number of events In which no exchange of momentum occurs between the nucleus and Its environment. Is determined primarily by the quantum mechanical and physical structure of the surrounding media. It Is thus not possible to observe a Mossbauer effect of an active nucleus In a liquid, such as an Ion or a molecule In solution. This represents a serious limitation to the study of certain phenomena It allows, however, the Investigation of films or adsorbed molecules on solid surfaces without Interference from other species In solution. This factor In conjunction with the low attenuation of Y-rays by thin layers of liquids, metals or other materials makes Mossbauer spectroscopy particularly attractive for situ studies of a variety of electrochemical systems. These advantages, however, have not apparently been fully realized, as evidenced by the relatively small number of reports In the literature (17). 

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

Because of the limited scope of this monograph, it is impossible to give a rigorous account of the work that has been accomplished in these fields. By the end of 2008, about 60,000 publications dealing with the use of Mossbauer spectroscopy had been documented in the literature. Excellent review articles on Mossbauer Spectroscopy Applied to Inorganic Chemistry (3 volumes) are given in [30, 34-35] in Chap. 1 and on Mossbauer Spectroscopy Applied to Magnetism and Materials Science (2 volumes) in [38, 40] in Chap. 1. 

The nuclear transitions are very sensitive to the local environment of the atom, and Mossbauer spectroscopy is a sensitive probe of the different environments an atom occupies in a solid material. By analyzing the chemical shifts and quadrupole splitting in Mossbauer spectra of samples containing Mossbauer-ac-tive nuclei, information on the state of oxidation and the local structure can be obtained. Only a few nuclei can be used for this purpose, so this method has limited but powerful applications. 

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

In this chapter, we will first describe what the Mossbauer effect is, then explain why it can only be observed in the solid state and in a limited number of elements. Next we discuss the so-called hyperfine interactions between the nucleus and its environment, which make the technique so informative. After a few remarks on spectral interpretation we go systematically through a number of examples which show what type of information Mossbauer spectroscopy yields about catalysts. 

In conclusion, Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the... 

A. G. Maddock, Mossbauer Spectroscopy Principles and Applications of the Techniques , Horwood Publishing Limited, 1997. 

The study of metals in biological systems requires techniques, some of them highly specific, some limited to certain aspects of the metal ion in question, some of more general applicability. Thus, Mossbauer spectroscopy in biological systems is restricted to iron-containing systems because the only element available with a Mossbauer nucleus is 57Fe. The EPR spectroscopic techniques will be of application only if the metal centre has an unpaired electron. In contrast, provided that crystals can be obtained, X-ray diffraction allows the determination of the 3-D structure of metalloproteins and their metal centres. 

Despite all the information that might be obtained using Mossbauer spectroscopy, some of its limitations naturally discouraged many chemists from using this new technique. Unfamiliarity with the basic principles, the fact that most of the early work was done only on iron and tin, and the lack of commercially available research quality equipment until 1965 were other reasons for the lack of interest. This symposium. The Mossbauer Effect and Its Application in Chemistry, was sponsored by Nuclear Science (formerly Nuclear Science Engineering Corp.), a division of International Chemical Nuclear Corp., with the hope that more chemists would learn how Mossbauer spectroscopy has been and can be used. 

Murad, E. (1990) Application of Fe Mossbauer spectroscopy to problems in clay minerals and soil science possibilities and limitations. Adv. Soil Sci. 12 125-157... 

Mossbauer spectroscopy is a selective tool for the quantitative analysis and spe-ciation of a very limited number of elements. It has been mainly used to study iron compounds—e.g., ceramics, as it gives valuable information about iron-bearing oxide and silicate minerals. This technique has been applied to the identification of the provenance of clay and used raw materials—the manufacturing method employed in pottery and, to a lesser extent, to the characterization of pigments and weathering crusts formed on stone monuments [23]. 

Using Mossbauer spectroscopy to monitor the formation of p-hematin under in vitro reaction conditions, Adams et al. have demonstrated that the reaction is a psuedo-zero-order process [109]. Such a process is consistent with a mechanism whereby a small concentration of heme is kept soluble via acetate, functioning as a phase-transfer catalyst, in a heme-saturated solution. In the rate limiting step, the soluble heme aggregates to P-hematin, which in turn grows until it precipitates from solution. There are clearly complicated heterogeneous reaction equilibria involved in the aqueous chemical formation of p-hematin. Consequently, it should be emphasized that the detailed mechanistic analysis of the complex solubilization of the species involved in the chemical synthesis... 

Because the greatest limitation for the general use of Mossbauer spectroscopy is that it cannot be observed for all elements, those criteria which restrict the application of Mossbauer spectroscopy to certain isotopes will be discussed in detail in Section II, A. Here, it will be shown which of these Mossbauer isotopes may be used to obtain chemical information, and how these isotopes may also be used to obtain in an indirect manner similar information about elements for which there exists no Mossbauer effect. 

The utility, and also the limitations, of Mossbauer spectroscopy in surface structure measurement can now be seen. While this determination using the Mossbauer effect alone may be possible, it is often difficult. The ultimate determination of surface structure and changes thereof, however, can be deduced through combined studies using the Mossbauer effect and other physical methods. 

Sn Mossbauer spectroscopy has been used to a limited extent to characterize some of the tin complexes described herein. A table of isomer shifts is provided here, but it is clear that much more data are required before any trends might emerge. 

The strength of Mossbauer spectroscopy is its ability to provide information about the environment of metal centres in large molecules, polymers and minerals, in both single- and multiphase specimens. The drawbacks of the technique are the limited number of elements to which it can be applied in practice, its insensitivity, its limitation to solid state studies, and the need for a suitable radioactive source. 

Mossbauer spectroscopy can indeed be a very useful technique for characterizing iron-containing clusters, but it will continue to be of limited use for other metals. 

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