Mossbauer spectroscopies

Mossbauer spectrum, the positions and separations of which depend on the oxidation state, electronic configuration, magnetic ordering, coordination number and symmetry of the iron atoms in a mineral structure. Computed peak areas enable Fe3+/Fe2+ ratios and site populations of Fe2+ ions in crystal structures to be readily determined for several minerals without interference from coexisting Mn, Cr and other transition elements. 

The Mossbauer parameters of dppf complexes are collected in Table 1-4. Consistent with other ferrocenylphosphines [75], dppf shows a reduced quadrupole splitting compared with that of ferrocene (2.33 vs. 2.42 mm s ). On complexation, the isomer 

Other spectroscopic techniques that have been used in the study of dppf complexes are XPS, UPS [43] and UV/VIS spectroscopy. XPS provides useful complementary information on the differentiation of pendant and coordinated phosphines [42, 80]. Caution needs to be exercised, however, as P(2p) core binding energy is not very sensitive towards changes in the ligand coordination mode or the oxidation state of the metal [42]. The electronic absorption spectra of dppf and MCl2(dppf-P,P ) complexes (M = Co, Ni, Pd, Pt, Zn, Cd, Hg) exhibit an easily detected band at 405—465 nm, attributed to a transition in the ferrocenyl moiety [43]. The presence of any iron-to-metal bonding is usually indicated by an intense a - ct transition [21], as evident in the electronic spectrum of [Pd(dppf-P, P )(PPh3)][BF4]2, 3 [40]. 

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 Mossbauer effect, discovered by Rudolf L. Mossbauer in 1957, can in short be described as the recoil-free emission and resonant absorption of gamma radiation by nuclei. In the case of iron, the source consists of Co, which decays with a half-life of 270 days to an excited state of Fe (natural abundance in iron 2%). The latter, in turn, decays rapidly to the first excited state of this isotope. The final decay generates a 14.4 keV photon and a very narrow natural linewidth of the order of nano eV. 

The isomer shift, d, arises from the Coulomb interaction between the positively charged nucleus and the negatively charged s-electrons, and is thus a measure for the s-electron density at the nucleus, yielding useful information on the oxidation state of the iron in the absorber. An example of a single line spectrum is fee iron, as in stainless steel or in many alloys with noble metals. 

If the nucleus feels both a magnetic field and an electric field gradient, and the electric quadrupole interaction is small, then the excited levels shift further and make the sextet asymmetrical, as observed in the spectrum of Fe203. 

The Mossbauer effect can only be detected in the solid state because the absorption and emission events must occur without energy losses due to recoil effects. The fraction of the absorption and emission events without exchange of recoil energy is called the recoilless fraction, f. It depends on temperature and on the energy of the lattice vibrations /is high for a rigid lattice, but low for surface atoms. 

Mossbauer absorption spectroscopy (MAS) Mossbauer emission spectroscopy (MES) 

Mossbauer spectroscopy is a nuclear technique, but why should such a technique be useful for the study of catalysts The answer is simple the nucleus, being at the heart of the atom, feels precisely what the state of the atom is. Mossbauer spectroscopy analyzes the energy levels of the nucleus with extremely high accuracy, and in this way it reveals for example what the oxidation state of the atom is, or how large the magnetic field is at the nucleus. In this way, we can determine in straightforward manner the compound to which the atom belongs. 

The great advantage of Mossbauer spectroscopy for catalyst research is that it uses y-radiation of high penetrating power, such that the technique can be applied in situ. An economic advantage is that the technique is relatively inexpensive, with equipment costs being about a factor of ten less than for electron microscopy or photoelectron spectroscopy. 

The experimental aspects of Mossbauer spectroscopy are well developed and are documented in considerable detail elsewhere (e.g. Gonser, 1975 Greenwood Gibb, 1971 Gruverman, 1965-74 May, 1971 Wertheim, 1964). A schematic diagram of a typical Mossbauer spectrometer is shown 

As mentioned above, 57Fe is the most important isotope that exhibits the Mossbauer effect and Mossbauer spectra provide valuable information about the chemical environments of iron nuclei. At the trivial level it is able to provide quantitative discrimination between Fe11 and Fem non-invasively, a valuable technique particularly for unstable samples such as, for example, air-sensitive sediments. Also, because the technique is specific for individual isotopes, it is able to detect and identify small amounts of iron-rich phases in the presence of large quantities of other compounds. A good example here is the case of soil and mineral specimens, where the various oxide and oxyhydroxide species can all be distinguished from one another on the basis of their Mossbauer spectra at different temperatures (see e.g. Goodman, 1994). 

From the biological area, iron-sulfur clusters in biomolecules such as rubredoxin mononuclear Fe-S clusters (Rao et at., 1972), plant-type ferredoxin 2Fe-2S clusters (Johnson, 1975) and bacterial-type ferredoxin 4Fe-2S clusters (Thompson et at., 1974) are readily distinguished from one another by their Mossbauer spectra. The temperature dependence of relaxation effects can provide information about the types of internuclear interaction and can even lead to estimates of the distance between paramagnetic sites, for example, the two 4Fe-4S clusters in ferredoxin in Peptococcus aerogenes (Adman etal., 1973). 

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. 

There are only a few Mossbauer nuclei which are interesting in zeolite chemistry and, thus, candidates for application of Mossbauer spectroscopy in soUd-state ion exchange. However, among them is one of the most important elements, viz., iron, which has also attracted much attention in zeoUte chemistry as a key component of possible catalyst formulations. Mossbauer spectroscopy proved to be exceptionally successful in discriminating Fe + and Fe + cations residing on extra-framework sites after introduction of iron via soHd-state ion exchange. Moreover, Mossbauer spectroscopy provides information about the various coordinations of Fe + and Fe in zeoUte lattices (cf. Sect 5.3.4). 

When the complexity of organotin structures was first becoming appreciated, Mossbauer spectroscopy played a major part in elucidating the structures in the solid state. However, the spectra usually consist of singlets or doublets which are broad (typically 

The source of the y-rays is the 119mSn isotope which is prepared by the (n,y) reaction of 118Sn. It decays with a half life of 245 days to give the nuclear excited 119Sn. This has a spin / of /2, and a half life of 1.84 x 10 x s, and emits ay-ray of 23.875 keV in its transition to the ground state with spin I of 1 /2. It is usually incorporated into barium or calcium stannate, which give a line-width of about 0.33 mm s 4. Measurements are usually carried out at 77 K, to increase the recoil-free fraction of the emission and absorption for BaSnC 3, this is 0.8 at 77 K, and 0.55 at 300 K. 

The isomer shift gives a measure of the. y-electron density at the tin nucleus. As the nucleus emits or absorbs the y-ray, its radius changes, and the interaction with the. v-clcctrons which are close to the nucleus affects the separation between the ground state and the excited state. A decrease in the. v-electron density at the nucleus corresponds to a more positive isomer shift. 

Isomer shift values also depend on the electronegativity of the ligands, on the coordination number, and on the stereochemistry. Thus the series of alkylpentahalogeno-stannates, BuSnX Y5 2 shown in Table 2-4, may all be assumed to have similar octahedral structures, and the value of IS falls with increasing electronegativity of X and Y, 

as the ligand attracts electrons away from the tin.22 A similar trend can be distinguished as the alkyl groups are varied in, for example, the tetrahedral compounds f Sn, indicating that the electron releasing power increases in the sequence Me Et Pr Bu. 

From these parameters, a high charge density on the iron nucleus can be inferred. It is interesting to note that this situation is not reflected by the spectroscopic 

The assignment of oxidation states has more a formal character in the sense of electron counting rules [145]. In this context it should, however, be justified to use at least the term low valent silicon. 

Products of decomposition may be of such small particle size that superparamagnetism is exhibited [329] (e.g. by Fe203 [324,326] where the characteristic six-line spectrum of antiferromagnetic Fe203 is replaced by a doublet with an isomeric shift corresponding to Fe3+). 

Mossbauer spectra may also be used to study radiolytic decompositions [330], 

The information contained in ESCA (Electron Spectroscopy for Chemical Analysis) spectra [331] makes the method particularly suitable for determinations of surface compositions, chemical bonding of surface atoms and changes which occur at solid surfaces during reaction [312], Applications of this technique to the study of reactions of and between solids are awaited with interest. 

A sample is continuously heated at a constant rate (e.g. 10 C min ) while two changes are recorded (1) the temperature difference between an inert compound and the sample with a thermocouple (differential thermo analysis DTA) and (2) the weight loss measured with a balance (thermogravimetry TGA) (Mackenzie, 1957 Smykatz-Kloss, 1974). With DTA, information is obtained about endothermic and exothermic phase transformations (see Fig. 1-2), whereas with TGA adsorbed water and structural OH can be measured. 

All fine grained Fc oxides lose adsorbed water at characteristic temperatures of between 100 and 200 °C. Structural OH in gocthite and lepido-crocite is lost at 250-400°C by the dehydroxylation reaction 2OH O + H2O. Even fine grained oxides such as hematite contain some OH in the structure (Stanjek Schwertmann, 1992) and this is driven off over a wide temperature range. For Fe oxides endothermic peaks result from the release of adsorbed or structural water, whereas exothermic peaks come from phase transformations (e.g. maghemite to hematite) or from recrystallization of smaller crystals into larger ones. An example of this is observed during the transformation of ferrihydrite to hematite. 

When the oxide particles are very small (tens of nm), fluctuations of the electron spin direction may be so fast that the nucleus can no longer fol- 

For reviews on the subject dealing with Fe oxides the reader is referred to Murad and Johnston, (1987) and Chapter 7.5 in Cornell Schwert-mann, (1996). 

Fundamentals. The energy of a gamma quantum emitted or absorbed by an atomic nucleus usually differs somewhat fi om the actual energy difference between the nuclear states because of recoil and Doppler-broadening induced by thermal movement. At room temperature, the value differs by about 10 to 10 eV from the true energy value vo of the nuclear transition. The recoil energy r and the corresponding momentum p are 

Emission and absorption lines consequently differ by 2 1 eV as a typical 

Shifts of the absorption line can be induced by changes in the electron density around the atomic nucleus. These changes can be effected by filled inner-core orbitals and partially filled valence orbitals they are called isomer shifts. The change influences both the ground state and the excited state in sum, a change of the absorption energy is observed as depicted in Fig. 5.88. 

Further details and tabulated values can be found elsewhere [536, 537]. The various parameters, observables and properties are collected in the table below. 

Mossbauer parameter Observed quantity Obtained information 

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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. 

Suwalski L., Kucharski Z., Lukasik M., Luty W. Utilised of Mossbauer spectroscopy for measuring residual austenite in bearing steel MOC IMP No 67, 1985,... 

MS Mossbauer Spectroscopy [233-236] Chemical shift of nuclear energy states, usually of iron Chemical state of atoms... 

Levinthal C 1969. In Debruimer P, J C M Tsibris and E Munck (Editors) Mossbauer Spectroscopy in Biological Systems, Proceedings of a Meeting held at Allerton House, Monticello, Illinois, University of Illinois Press, Urbarra, p. 22. 

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. 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

A number of ferrites have been subjected to shock modification and studied with x-ray diffraction as well as static magnetization and Mossbauer spectroscopy [87V01], Studies were carried out on cobalt, nickel, and copper ferrites as well as magnetite (iron ferrite). 

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. 

N. Greenwood and T. C. Gibb, Mossbauer Spectroscopy, Chapman Hall, London, 1971, 659 pp. 

N. N. Greenwood and T. C. Gibb, Mossbauer Spectroscopy, Chapman Hall, London, 1971, 659 pp. T. C. Gibb, Principles of Mossbauer Spectroscopy, Chapman Hall, London, 1976, 254 pp. 

Apart from XeF, which is the light-emitting species in certain Xe/F2 lasers, there is no evidence for the existence of any odd-valent fluorides. Reports of XeFg have not been confirmed. Of the other halides, XeCl2, XeBr2 and XeCl4 have been detected by Mossbauer spectroscopy as products of the -decay of their... 

Quadrupole coupling constants for molecules are usually determined from the hyperfine structure of pure rotational spectra or from electric-beam and magnetic-beam resonance spectroscopies. Nuclear magnetic resonance, electron spin resonance and Mossbauer spectroscopies are also routes to the property. There is a large amount of experimental data for and halogen-substituted molecules. Less data is available for deuterium because the nuclear quadrupole is small. 

Reinvestigation of iron porphyrins by Mossbauer spectroscopy using synchrotron radiation 98PAC917. 

D. G. Rancoutt, H. H. A. Smith, and R. C. Thiel, Metastable compositionally and magnetically modulated state of Fe-Ni Invar and the associated super-mon nt dynamics from Mossbauer spectroscopy, J. Magn. Magn. Mat, 66 121 (1987). 

Because of the possibility of applying Mossbauer spectroscopy the solid-state chemistry of the Fe- substituted material is best understood [69, 72, 77]. Mossbauer spectroscopy confirms that the Fe in the pyroaurite type material is Fe(III). Glemser and co-workers have found that electrochemical oxidation of the material converts about 30% of the Fe(III) to Fe(IV) [69, 72], The results were... 

Miglierini, M., et al., Mossbauer Spectroscopy in Materials Science, Kluwer,... 

Platinum ammine complexes have been a fertile area for studying transinfluence. Table 3.21 lists data for a range of ammines showing how /(195Pt-15N) depends upon the trans-atom [153]. (A further selection of data can be found in R.V. Parish, NMR, NQR, EPR and Mossbauer Spectroscopy in Inorganic Chemistry, Ellis-Horwood, Chichester, 1991, pp. 76, 87.) Possibly the most detailed study (of complexes of tribenzylphosphine) examined over a hundred neutral and cationic complexes [154] (Table 3.22). 

Electrobalances suitable for thermogravimetry are readily adapted for measurements of magnetic susceptibility [333—336] by the Faraday method, with or without variable temperature [337] and data processing facilities [338]. This approach has been particularly valuable in determinations of the changes in oxidation states which occur during the decompositions of iron, cobalt and chromium oxides and hydroxides [339] and during the formation of ferrites [340]. The method requires higher concentrations of ions than those needed in Mossbauer spectroscopy, but the apparatus, techniques and interpretation of observations are often simpler. 

Product yields may also be determined by magnetic measurements, as in the formation of ferrites [340], where kinetic data were obtained at reaction temperature. Quantitative applications of Mossbauer spectroscopy have also been described [326]. 

G.M. Bancroft, Mossbauer Spectroscopy, McGraw-Hill, New York, 1973. 

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