Oxide studies Mossbauer

It has been shown that the addition of lead to a chromia-promoted magnetite WGS catalyst enhances the activity for WGS (4 ), A study of the solid state changes which occur upon this substitution was made to probe the active sites..of the catalyst. Through a combination of oxidation studies, Mossbauer spectroscopy, and X-ray diffraction line broadening, a model for the. catalyst was developed. It was concluded that Pb was present as Pb " at tetrahedral sites. The Pb substitution resulted in the expansion of the tetrahedral sites, contraction of the octahedral sites, and the oxidation of some Fe to Fe. The resulting octahedral cations became more covalent in nature, and since the octahedral cations have been reported to be the active sites for CO oxidation over ferrites... 

In this study we present the first in-situ formation of a ternary transition metal oxide within the pores of MCM-48 silica. XRD measurements showed the preservation of the host structure as well as the formation of 5-6 nm small particles. In addition, the analysis of nitrogen physisorption data revealed the existence of mesopores with smaller pore diameters and surface areas in comparison to the pristine phase, which can be attributed to the introduction of the Co/Fe/O phase into the pores. First qualitative XANES and EXAFS analyses support the formation of CoFe204 nanoparticles. TEM investigations on particle size and structure are in progress. To learn more about the properties of the oxide nanoparticles Mossbauer and magnetic measurements have to be carried out, which are planned for the future. 

The first and most studied Mossbauer nucleus, iron-57, displays specific catalytic behavior. Mossbauer investigations of supported microcrystallites of iron and its oxide have demonstrated the importance of the techniques in the investigation of surface structure and chemistry. The application to other nuclei that have important catalytic qualities indicates the potential importance of the study of supported microcrystallites by Mossbauer spectroscopy in future investigations of catalysts. Developments in experimental techniques enabling in situ investigations are enhancing the scope of the technique. 

The rigidity of the basic silicate structures exerts a greater controlled influence on cation site symmetries than do the individual cation charges, an important difference from the oxides where the anions have more freedom of movement. The ease with which Fe and Fe + cations can be distinguished and site occupancies determined in oxides by Mossbauer spectroscopy suggests a logical extension to similar studies in silicates, and considerable progress has been made in this direction. 

Attempts to study the nature of Fe in the MoFe protein have extended to oxidation studies on Avl and Kpl. From Mossbauer studies Orme-Johnson et al. (1977) defined four states of Fe in native dithionite-reduced Avl. These included (8 Fe/mol, Mg in Kpl) state D (9 Fe/mol equivalent to the epr silent 8-Fe group Mg in Kpl) Fe (3 Fe/mol equivalent to the 2-Fe group M4 in Kpl) and state S (1 Fe/mol) which may be an impurity (Munck et al., 1975). In a later study Zimmerman et al. (1978) modified these numbers they concluded that M, ccmtained 12 Fe in two apparently identical clusters. Species D and Fe " were replaced by four P clusters each containing three irons of the D type and one of the Fe " type. The remaining irons were in species S (2/mol) which, although inert under all conditions tested, were present in Cpl, Avl, and Kpl. Presumably, species S is not an impurity. The authors emphasized that active MoFe protein should contain 30 2 Fe/mol and that the P clusters, although spectroscopically unique, were probably closely related to conventional 4Fe4S clusters. 

Recent organometallic studies have been reported. Mossbauer spectra have shown that the Fej dimer reacts at 4K with CH4 to oxidatively cleave the C—H bond to form HFe2CH3. Zirconium atoms have also been shown to oxidatively cleave the C—H and C—C bonds of alkanes to form discrete organometallic species. ... 

Mossbauer spectroscopy has been used to characterize the iron clusters in fuscoredoxin isolated from D. desulfuricans (133). The authors explained why the iron nuclearity was incorrectly determined, and studied the protein in three different oxidation states fully oxidized, one-electron reduced, and two-electron reduced. The error made in determining the iron cluster nuclearity was caused by the assumption that in the as-purified fuscoredoxin, cluster 2 is in a pure S = state. This assumption was proven to be false and unnecessary. In fact, the observation of four resolved, equal intensity (8% of total Fe absorption) spectral components associated with the S = i species in the as-purified protein is consistent with cluster 2 being a tetranuclear Fe cluster. The 4x8 = 32% Fe absorption for the four components indicates that only 64% of clusters 2 are in the S = state (the total Fe absorption for cluster 2 is 50% of the total Fe absorption). The remaining clusters 2 are in a different oxidation state, the spectrum of which is unresolved from that of cluster 1. 

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

An important advance on these studies was the possibility of isolating AORs from Fe enriched media with obvious interest for an iron-sulfur center site labeling, with enhanced sensitivity of the Mossbauer studies. The work developed with bacterial systems is advantageous as compared with mammalian systems for isotopic labeling and opens the possibility of a direct measurement of substrate binding. Spectra of the enzyme in oxidized, partially reduced, benzaldehyde-reacted, and fully reduced states were recorded at different temperatures and with variable externally applied magnetic fields (222). In the oxidized enzyme, the clusters are diamag-... 

A Mossbauer study of the protein reacted with benzaldehyde (in parallel with EPR detection of Mo(V) signals) shows partial reduction of the iron—sulfur centers, indicating the involvement of the clusters in the process of substrate oxidation and rapid intramolecular electron transfer from the molybdenum to the iron—sulfur sites. 

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