Mossbauer effect spectroscopy

L. Bowen In Sb Mossbauer Spectroscopy, Mossbauer Effect Data Index (Plenum, New York, 1973). 

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special Interest because of their high catalytic activity for dloxygen reduction. The Interaction of the adsorbed macrocycles with the substrate and their orientation are of Importance In understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytlc layers Include visible reflectance spectroscopy surface enhanced and resonant Raman and Mossbauer effect spectroscopy. This paper Is focused principally on the cobalt and Iron phthalocyanlnes on silver and carbon electrode substrates. 

Mossbauer effect spectroscopy, MES, Is based on the ability of certain nuclei to undergo recoilless emission and absorption ofY rays (16). The energy and multiplicity of the ground and excited states of a given nucleus are modified by the chemical environment. It Is thus most often necessary to compensate for the differences In... 

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). 

Up to the present time, the Mossbauer effect has been observed with nearly 100 nuclear transitions in about 80 nuclides distributed over 43 elements (cf. Fig. 1.1). Of course, as with many other spectroscopic methods, not all of these transitions are suitable for actual studies, for reasons which we shall discuss later. Nearly 20 elements have proved to be suitable for practical applications. It is the purpose of the present book to deal only with Mossbauer active transition elements (Fe, Ni, Zn, Tc, Ru, Hf, Ta, W, (Re), Os, Ir, Pt, Au, Hg). A great deal of space will be devoted to the spectroscopy of Fe, which is by far the most extensively used Mossbauer nuclide of all. We will not discuss the many thousands of reports on Fe... 

In order to understand the Mossbauer effect and the importance of recoUless emission and absorption, one has to consider a few factors that are mainly related to the fact that the quantum energy of the y-radiation used for Mossbauer spectroscopy (Eo K, 10-100 keV) is much higher than the typical energies encountered, for instance, in optical spectroscopy (1-10 eV). Although the absolute widths of the... 

The previous chapters are exclusively devoted to the measurements and interpretation of Fe spectra of various iron-containing systems. Iron is, by far, the most extensively explored element in the field of chemistry compared with all other Mdssbauer-active elements because the Mossbauer effect of Fe is very easy to observe and the spectra are, in general, well resolved and they reflect important information about bonding and structural properties. Besides iron, there are a good number of other transition metals suitable for Mossbauer spectroscopy which is, however, less extensively studied because of technical and/or spectral resolution problems. In recent years, many of these difficulties have been overcome, and we shall see in the following sections a good deal of successful Mossbauer spectroscopy that has been performed on compounds of... 

The first comprehensive review article on Zn Mossbauer spectroscopy, covering extensively the theory of hyperfine interactions, describing the spectrometer and cryogenic systems and reviewing the Zn Mossbauer effect studies of the early stage appeared in 1983 [64]. 

Zn Mossbauer effect measurements are technically difficult to perform, and so far only few laboratories, mainly those in Munich [68-71] and St. Petersburg (see Sect. 7.2.2.4) have published significant contributions on the characterisation of zinc containing compounds and metallic materials with Zn Mossbauer spectroscopy. Selected work is briefly discussed in the following sections. 

Kistner et al. [109] were the first to observe the Mossbauer effect in Ru. Kistner also reported the Mossbauer spectroscopy study of ruthenium compounds and alloys [110]. 

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 Mossbauer effect as a spectroscopic method probes transitions within an atom s nucleus and therefore requires a nucleus with low-lying excited states. The effect has been observed for 43 elements. For applications in bioinorganic chemistry, the 57Fe nucleus has the greatest relevance and the focus will be exclusively on this nucleus here. Mossbauer spectroscopy requires (a) the emission of y rays from... 

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. 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

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... 

Table I shows the various Mossbauer nuclides—i.e., the nuclides where the Mossbauer eflFect has actually been seen. Not all of these are as easy to exploit as the Fe and 9Sn cases referred to above. However, with improved techniques a number of these should prove accessible to the chemist. Representative elements of almost all parts of the periodic table are tractable by these techniques. It seems clear, however, that the methods of Mossbauer spectroscopy are no longer technique-oriented but that this field is becoming a problem-oriented discipline. In other words, the Mossbauer effect is now used successfully in many cases not only to demonstrate the effect or to corroborate physical evidence obtained by other means—NMR, or infrared, or kinetic studies— but also to solve new chemical problems.

Until the discovery of the Mossbauer effect, the possibility of directly observing nuclear y-ray transitions between individual nuclear magnetic substates seemed remote because of the small energy differences involved however, the extremely high energy resolution of Mossbauer spectroscopy has made it possible to resolve these transitions directly in some isotopes, and it is this feature that is so valuable for investigating... 

Ca2pe205. Geller et al, (15) and Gonser et al. (19) have recently used the Mossbauer effect to study Ga2Fc20r>. This study is an example of how the Mossbauer effect in conjunction with crystal chemical arguments can be used to work out the magnetic structure of a material without using neutron spectroscopy. 

Ti ossbauer spectroscopy is the term now used to describe a new ana-lytical technique which has developed using y-ray nuclear resonance fluorescence or the Mossbauer effect. For most of the time since Rudolf Mossbauer s discovery in 1958 it was the physicist who utilized this new tool. Starting approximately in 1962 some chemists realized the potential of this new technique. Since then they have applied Mossbauer spectroscopy to the study of chemical bonding, crystal structure, electron density, ionic states, and magnetic properties as well as other properties. It is now considered a complimentary tool to other accepted spectroscopic techniques such as NMR, NQR, and ESR. 

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. 

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

Thanks to the extensive literature on Aujj and the related smaller gold cluster compounds, plus some new results and reanalysis of older results to be presented here, it is now possible to paint a fairly consistent physical picture of the AU55 cluster system. To this end, the results of several microscopic techniques, such as Extended X-ray Absorption Fine Structure (EXAFS) [39,40,41], Mossbauer Effect Spectroscopy (MES) [24, 25, 42,43,44,45,46], Secondary Ion Mass Spectrometry (SIMS) [35, 36], Photoemission Spectroscopy (XPS and UPS) [47,48,49], nuclear magnetic resonance (NMR) [29, 50, 51], and electron spin resonance (ESR) [17, 52, 53, 54] will be combined with the results of several macroscopic techniques, such as Specific Heat (Cv) [25, 54, 55, 56,49], Differential Scanning Calorimetry (DSC) [57], Thermo-gravimetric Analysis (TGA) [58], UV-visible absorption spectroscopy [40, 57,17, 59, 60], AC and DC Electrical Conductivity [29,61,62, 63,30] and Magnetic Susceptibility [64, 53]. This is the first metal cluster system that has been subjected to such a comprehensive examination. 

In systems containing a number of physically inequivalent sites, Mossbauer effect spectroscopy (MES) can often allow the determination of the properties of the individual sites. This also proved to be the case here [24],... 

Mossbauer Effect Spectroscopy without Sampling Application to Art and Archaeology... 

Tt is well known that iron is a common constituent of works of art and of artifacts of archaeological interest. This is undoubtedly because of its occurrence in most rocks of the earths crust and in a variety of colored compounds. The chemistry of iron compounds is fairly complex, and the form in which it is eventually found in an object can provide information on the source and the technique of manufacture of that object. Mossbauer effect spectroscopy (MES) is particularly suitable for studying iron and its compounds. Already there are two published reports 1,2) of MES applied to the study of ancient pottery. However, those studies, like most MES work, required sample taking and preparation. We have explored the possibility of nondestructive MES and now report on practicality of this method. Since there are many good descrip-... 

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