Oxides Mossbauer effect

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 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... 

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. 

De Endredy, A.S. (1963) Estimation of free iron oxides in soils and clays by a photolytic method. Clay Min. Bull. 5 209-217 de Faria, D.L.A. Venancio Silva, S. de Oliveira, M.T. (1997) Raman Microspectroscopy of some iron oxides and oxyhydroxides. J. Raman Spectrosc. 28 873-878 De Grave, E. Vandenberghe, R.E. (1986) 57Fe Mossbauer effect study of well-crystallized goethite (a-FeOOH) Hyp. Interact. 28 643-646... 

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). 

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 kinetics of bulk reactions. Mossbauer spectra of super-paramagnetic iron particles in applied magnetic fields can be used to determine particle sizes. In favorable cases, the technique also provides information on the structure of catalysts. The great advantage of Mossbauer spectroscopy is that its high-energy photons can visualize the insides of reactors in order to reveal information on catalysts under in-situ conditions. 

Although relatively little used in catalysis, Mossbauer spectroscopy has given important information on the state of iron and cobalt in Fischer-Tropsch and hydrodesulphurization catalysts. Mossbauer spectroscopy provides the oxidation state, the magnetic field and the lattice symmetry of a number of elements such as iron, tin, iridium, and cobalt, and can be applied in situ. We will first describe the theory behind the Mossbauer effect and explain how a nuclear technique gives information on the state of atoms. 

The combined results of EXAFS and Mossbauer effect spectroscopies have been used to deduce the oxidation state and local structure of iron in iron containing Na, K- and H-chabazites. In the mineral Na,K(Fe) chabazite iron is found to be present as oxo- or hydroxo-bridged Fe(III) oligimers, while in the H-chabazite it is found to be present as Fe(III) ions bridged by oxo- or hydroxo-groups to the zeolite frameworks... 

The Mossbauer effect is sensitive to the oxidation and spin state of iron and the environment around the iron nucleus therefore different chemical species yield different Mossbauer spectra. Furthermore all spin states and oxidation states of iron are accessible to the technique. There are three main components of a Mossbauer spectrum. The isomer shift arises from the electron density at the 57Fe nucleus the quadrupole splitting results from the electric field gradient produced by electrons and ions around the 57Fe nucleus, and the nuclear Zeeman splitting is sensitive to the spin state and magnetic coupling of the iron. 

The most common use of the Mossbauer effect in mineralogy and geology has been the determination of the oxidation states of iron in various minerals (2). The study of the Mossbauer spectral area also gives valuable information on the concentration of the different minerals in rocks (2). Recently the Mossbauer effect was applied to the study of iron-bearing minerals in coal to determine the amount of pyritic sulfur (3, 4, 5). 

In the present study Fe(Pc)(Py)2 encaged into Y-zeolite is studied under catalytic conditions of oxidation of hydroquinone to benzoquinone in acetic acid media. Most of the measurements were performed in frozen slurries. The changes of characteristic Mossbauer data are correlated with proposed steps of the removal of pyridine ligands. From the comparison of data obtained at different temperatures the probability of the Mossbauer effect is also considered for identification of various iron species. Finally, the results are correlated with the catalytic properties, as well. 

Murad, E. and Taylor, R. M. (1986) The oxidation of the hydroxycarbonate green rusts. In Long, G. J. and Stevens J. G. (eds.) Industrial applications of tlie Mossbauer effect. Plenum. Publ. Corp. 585-593. 

One aspect of Mossbauer spectroscopy that has not been widely exploited in phase transformation studies is time resolution. Studies with conventional techniques are possible over a wide range of time scales, starting from the intrinsic time scale of the Te Mossbauer effect (t <= 10 s) to investigate processes such as electron transfer, to time scales of t 10 s to measure diffusion, to longer timescales of t > 10 s to study phase transitions, oxidation and other chemical reactions. 

Boso, B., G. Lang, T. McMurry, and J.T. Groves (1983). Mossbauer-effect study of tight spin coupling in oxidized chloro-5,l0,15,20-tetra(mesityl) porphyrinatoiron(lll). J Chem. Phys. 79,1122-1126. 

Johnston, J.H. and Lewis, D.G. (1986). A study of the initially formed hydrolysis species and intermediate polymers and their role in determining the product iron oxides formed in the weathering of iron. In Industrial Applications of the Mossbauer Effect, Long, G. J. and Stevens, J.G (eds). Plenum Press, New York, pp. 565-593. 

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