Mossbauer spectroscopy active site

XL30). Mossbauer spectroscopy (KFKIj was applied to follow the state of Fe species in the zeolites. Carbon monoxide and ammonia adsorption (monitored with FTIR) (EQUINOX 55) was used to determine the nature, concentration and acid strength of the active sites in the Fe-TON zeolites. 

The types of macrocycles most studied in which the active metal center is believed to be retained include Co, Fe, Ru porphyrins and related macrocycles. In these studies the optimal pyrolysis temperature is often reported to be between 400-800 °C. Above these temperatures, the active site begins to be destroyed, and activity decreases.49 An array of characterization techniques have been used to support these claims. XPS analysis has demonstrated that at the highest activity of samples, the surface composition of metal and nitrogen is also at its highest.78,96 Above the optimal treatment temperature, nitrogen and metal begin to disappear from the surface. Furthermore, Mossbauer spectroscopy and XAS have been used to... 

As will be explained in Chapter 7, spectroscopic methods are a powerful way to probe the active sites of the hydrogenases. Often spectroscopic methods are greatly enhanced by judicious enrichment of the active sites with a stable isotope. For example, Mossbauer spectroscopy detects only the isotope Fe, which is present at only 2.2 per cent abundance in natural iron. Hydrogen atoms, which cannot be seen by X-ray diffraction for example, can be studied by EPR and ENDOR spectroscopy, which exploit the hyperfine interactions between the unpaired electron spin and nuclear spins. More detailed information has been derived from hyperfine interactions with nuclei such as Ni and Se, in the active sites. In FTTR spec-... 

The rapid progress in the understanding of the active site of aconitase in the 1980 s has primarily originated from the work of H. Beinert and his collaborators. Three essential factors contributed to the success of this work 1) a ready and consistent source of enzyme (gram quantities), 2) a solid chemical and biochemical understanding of aconitase, and 3) close interactions with outstanding collaborators (most notably E. Munck s group for Mossbauer spectroscopy and B. M. 

If mixing in each site is not ideal, would differ from the real equilibrium constant by the quotient of activity coefficients and hence may depend on composition. The measurement of the site occupancy (the fraction of Fe and Mg in each of Ml and M2 sites) is not trivial. There are two methods to determine the intracrystalline site distribution. One is by Mossbauer spectroscopy (MS), in which there are a pair of outer and smaller peaks, which are due to Fe in Ml site, and a pair of inner and larger peaks, which are due to Fe in M2 site (Figure 2-3). The ratio of Fe in Ml site to Fe in M2 site is assumed to be the area ratio of the pair of Ml peaks to the pair of M2 peaks. Using total Fe content from electron microprobe analysis, and the ratio from Mossbauer spectroscopy, Fe(Ml) and Fe(M2) concentrations can be obtained. 

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. 

These tetrahedral distorted cobalt atoms can be observed by NMR as a pure phase on carbon supports in the absence of molybdenum and are thus stable these probably correspond to the Co sites observed by Topspe s group using Mossbauer spectroscopy because Craje et al. (93) found a similar Mossbauer doublet for both cobalt in CoMo catalysts and pure cobalt sulfide on carbon support. They are also active for HDS and confirm the findings of Prins and co-workers (94) and Ledoux (96). These different structures are in full agreement with the XANES experiments performed by Prins and co-workers (95) and Ledoux (96). These structures also led Ledoux et al. to an incorrect interpretation of the synergy effect (64). On poorly dispersed catalysts supported on silica or in bulk form, their presence and activity are large enough to explain the increase in activity when cobalt is added to molybdenum, but on well-dispersed catalysts i.e., on alumina or carbon support this interpretation is shown to be incorrect if the activity is carefully measured. 

Studies of hematin associates [94] indicate their inactivity in the catalase reaction. The interaction by the Fe-O-Fe mechanism, the existence of which was confirmed by the Mossbauer spectroscopy method, also disproves the idea of two hematin group interaction in the active site of catalase [92],... 

Freeze-quenching technique in combination with ESR and Mossbauer spectroscopy was used for monitoring intermediates in the reaction of substrate free 57Fe-P450C8Itl with peroxy acetic acid (Schunemann et al., 2000). In such a condition, the oxidant oxidized the enzyme active site iron (III) to iron (VI) and Tyr 96 into tyrosine radical, 90% and 10% from the starting material, respectively. Thus the tyrosine residue may be involved in the catalytic process. 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

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