Mossbauer emissions studies

In Mossbauer emission spectroscopy one prepares the catalyst with the radioactive source element (e.g. Co) and uses a suitable moving single-line absorber of Fe to record the spectra. In this way one can also study Co-containing catalysts, although strictly speaking the information concerns the iron in the catalyst that forms by the Co —> Fe decay process. 

Table 7.2 Overview of Zn Mossbauer emission spectroscopy studies...

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. 

Mossbauer Measurements. Co-Mo catalysts cannot be studied directly in absorption experiments since neither cobalt nor molybdenum has suitable Mossbauer isotopes. However, by doping with 57Co the catalysts can be studied by carrying out Mossbauer emission spectroscopy (MES) experiments. In this case information about the cobalt atoms is obtained by studying the 57Fe atoms produced by the decay of 57Co. The possibilities and limitations on the use of the MES technique for the study of Co-Mo catalysts have recently been discussed (8., 25.). 

Mossbauer Spectroscopy. Figure 1 shows room temperature Mossbauer emission spectra of two of the unsupported Co-Mo catalysts which we have studied in the present investigation. It is observed that the MES spectra of the two catalysts are quite different. For the catalyst with the low Co/Mo ratio (0.0625), a quadrupole doublet with an isomer shift of 6=0.33 mm/s and a quadrupole splitting of AE =1.12 mm/s are observed (spectrum a). These parameters are very similar to those observed previously for the Co-Mo-S phase in other catalysts (6-9). Furthermore, the spectrum of an unsupported catalyst with Co/Mo = 0.15 is found to be essentially identical to spectrum (a). The MES spectrum (b) of the catalyst with Co/Mo =... 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

Topspe proposed that corner sites are responsible for direct sulfur extraction (A Do) (53-60), but the exact nature of corner sites is not known. What is known is that the active sites for sulfur removal constitute only about 10% of all of the Co(Ni)-Mo-S sites as identified by Mossbauer emission spectroscopy (MES) (57). Thus, there is something special about some of the Co-Mo-S sites. Further study in this area is greatly needed to clarify this issue, and it is recommended that, in the future, authors use terminology in a uniform manner. Some suggestions for standardization are made in later discussions. 

A Mossbauer spectrometer consists of a radioactive Co source on a transducer that continuously scans the desired velocity range, an absorber consisting of the catalyst and a detector to measure the intensity of the gamma radiation transmitted by the absorber as a function of the source velocity. This is the common mode of operation, called Mossbauer absorption spectroscopy, sometimes abbreviated as MAS. It is also possible to fix the Co containing source and move a single-line Fe absorber, in order to investigate Co-containing catalysts. This technique, called Mossbauer emission spectroscopy (MES), has successfully been applied to study Co-Mo hydrodesulphurization catalysts [42]. 

Recently, we have found that two techniques, Mossbauer emission spectroscopy (MES) (see e.g.. Refs (6, 8-13)) and extended X-ray absorption fine structure (EXAFS) (14-16), can provide some of the needed structural information. This has not only resulted in a better description of the structural state of the catalysts but it has also allowed a better understanding of the catalytic properties. In this connection, it should be stressed that both of the above techniques conveniently allow studies to be carried out under in situ conditions. 

Unfortunately, the requirements of recoil-free emission and resonant absorption and transmission through the absorber limit the useable energy range of the Mossbauer effect 7-ray to approximately 10-100 keV. Further, in order to obtain rather sharp absorption lines and a reasonable spectral resolution, the mean lifetime of the Mossbauer 7-ray precursor state should be between 1 ns and 100 ns. Further, the Mossbauer nuclide must have a sufficiently high isotopic abundance in the element to yield a usable signal-to-noise ratio over a reasonable acquisition time. Finally, the radioactive source containing the Mossbauer 7-ray precursor state must be easily prepared and have a mean lifetime of several weeks to be practical. These various requirements limit the number of nuclides available for typical Mossbauer spectral studies. 

An interesting application of Mossbauer emission spectroscopy is illustrated by the study of Co(pyridine)2Cl2 which was used as a source against a single-line absorber. The resulting emission spectra clearly reveal the presence of the phase transition and indicate that this polymeric material is able to withstand the damage associated with the electron capture decay of the Co. 

In 1956 and 1957, young physicist R. Mossbauer has performed the experiments concerning the scattering of the 129 keV y-ray of Ir by Ir and discovered an increase in scattering at low temperatures. Results obtained and his interpretations were published in 1958 [1-3], which is the beginning of the Mossbauer effects study and its development as the Mossbauer spectroscopy. The Nobel Prize for physics 1961 was awarded to him [4]. Mossbauer spectroscopy is the recoilless emission and the recoilless resonant absorption of the y-ray by the nucleus. After... 

In conventional Mossbauer spectroscopy one uses a single-line source, e.g. Co embedded in a rhodium matrix in the case of Fe spectroscopy, and the iron containing material under study as absorber. This technique is termed Mossbauer Absorption Spectroscopy (MAS) in order to distinguish it from the so-called source experiment, also known as Mossbauer Emission Spectroscopy (MES). In a MES... 

Almost every modem spectroscopic approach can be used to study matter at high pressures. Early experiments include NMR [ ], ESR [ ] vibrational infrared [33] and Raman [ ] electronic absorption, reflection and emission [23, 24 and 25, 70] x-ray absorption [Tf] and scattering [72], Mossbauer [73] and gems analysis of products recovered from high-pressure photochemical reactions [74]. The literature contains too many studies to do justice to these fields by describing particular examples in detail, and only some general mles, appropriate to many situations, are given. 

In spite of the development of physicochemical techniques for surface analysis, spectroscopic methods applicable to the study of bonding between adsorbed metal ion species and substrate are limited, especially those applicable to in situ measurement at interfaces between solid and aqueous phases (1,2). In previous papers, we showed that emission Mossbauer measurement is useful in clarifying the chemical bonding environment of dilute metal ions adsorbed on magnetic metal oxide surfaces (3,1 ) ... 

In absorption Mossbauer spectroscopy, a source nuclide in a standard form (usually in a metallic matrix) is coupled with a sample to be investigated. This method requires at least 100 pg of Fe or Sn in the usual experimental setup even if a Mossbauer sensitive enriched stable isotope Fe-57 or Sn-119 is employed. In emission Mossbauer spectroscopy, however, 1 mCi of Co-57 or Sb-119, which corresponds nominally to 120 ng of Co-57 or 1.4 ng of Sb-119, is sufficient to permit measurement. This technique enables study of very dilute systems, especially those with ions directly bound to the substrate. 

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