Mossbauer spectroscopy information obtained

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. 

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... 

The miniaturized Mossbauer instruments have proven as part of the NASA Mars Exploration Rover 2003 mission that Mossbauer spectroscopy is a powerful tool for planetary exploration, including our planet Earth. For the advanced model of MIMOS II, the new detector technologies and electronic components increase sensitivity and performance significantly. In combination with the high-energy resolution of the SDD, it will be possible to perform XRF analysis in parallel to Mossbauer spectroscopy. In addition to the Fe-mineralogy, information on the sample s elemental composition will be obtained. 

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. 

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. 

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... 

Apart from fluorescence, several other methods may be used to obtain time-resolved information. In the case of proteins containing an iron atom, Mossbauer spectroscopy allows the determination, in the iron binding site, of not only root-mean-square shifts of atoms but also the times over which such shifts occur. Detailed investigations of myoglobin have yielded relaxation times on the order of 10 8 Proton NMR spectroscopy can be used to... 

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

As indicated in the previous discussion, Mossbauer spectroscopy provides information that when coupled with results using other structural techniques assists in determining the structure of the complex under analysis. The relationships between the various techniques are summarized in Table II. The Mossbauer chemical shift provides information about the 4 electron contribution to the bond between the metal and the ligands in a complex. Similar estimates can be obtained from the results of measurements on the fine structure in the x-ray absorption edge and nuclear magnetic resonance data. The number of unpaired electrons can be evaluated from magnetic susceptibility data, electron spin resonance, and the temperature coeflScient of the Mossbauer quadrupole splitting (Pr). 

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. 

These criteria for an ideal spectroscopic technique have been met, to a large extent, by Mossbauer spectroscopy. UHV conditions are not required since one utilizes y rays with energies in the keV range, and the technique lends itself easily to in situ studies. Furthermore, the technique is ideally suited for studies of small particle systems, and in several instances information about particle size can also be obtained. The unique feature of Mossbauer spectroscopy is the extremely high energy sensitivity of the technique. This allows detailed chemical, structural, and magnetic information to be obtained about atoms on the surface or in the bulk phase. 

There has been to some degree the belief that Mossbauer spectroscopy, although in principle an ideal technique for catalyst studies, for practical purposes can only be applied to problems in catalysis if the catalyst contains either iron or tin. Therefore, one of the main purposes of this review is to show how Mossbauer spectroscopy can be directly extended to many additional Mossbauer atoms or isotopes (such as antimony, europium, nickel, ruthenium, gold, and tungsten) and, perhaps more importantly, how the technique can be extended to obtain information about systems that do not contain a Mossbauer atom. ... 

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 above discussion exemplifies how a study of the different Mbssbauer parameters and their temperature dependences can give detailed information about the location of a non-Mossbauer isotope, lead, in its surrounding structure. It should perhaps for comparison be mentioned that the conventional technique of structure analysis, X-ray diffraction, did not enable the above information to be obtained, again showing the advantage of Mossbauer spectroscopy in the study of catalyst systems, which often may show X-ray amorphous features. 

---