Mossbauer experiments active

These mixed metal systems have also been tested with the transient method for catalytic activity in the Fischer-Tropsch reaction. We would like to remark here that the nature of the cation, anion, and zeolite are all important factors in the Fischer-Tropsch reactions that we have studied. Further details of these catalytic studies can be found elsewhere (23). We do observe here, however, that some catalysts that are completely reduced to the metallic state are not necessarily the most active catalysts. Also, even though the Mossbauer experiments suggest that 400°C is sufficient for complete reduction, higher activation temperatures can increase the activity and selectivity of these reactions. We have also observed that the cation definitely changes the product distribution and the activity. 

We have reported here the preparations and treatment conditions that are needed to reduce Iron Ions to metallic Iron In zeolites. Although we have not Isolated highly-dis-spersed superparamagnetic Iron particles In zeolites, we have shown that these iron-containing zeolites are active catalysts in Fischer-Tropsch and in olefin isomerization reactions. The added insight that stems from the use of in-situ Mossbauer experiments has led to the preparation of new active catalysts that can be selectively activated. We presently are studying photochemical reactions of other metal carbonyl complexes in zeolites and believe that increased selectivity is a major benefit in these types of reaction. 

The most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

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. 

When sulfation is carried out in a mixture of SO2 arc O2 at higher temperatures, most probably the thermodynamically predicted iron(HI) sulfate phase is formed This phase is less active, because no Fe /Fe3 redox cycle is possible, unless decomposition of the sulfate takes place. However when subsequently the H2S / O2 mixture is passed through the catalyst bed, the ironfUI) sulfate reacts again completely or partially to the move active iron(ll) sulfate. Consequently the activity and selectivity return to their original level. To verify this reaction mechanism in-situ Mbssbauer measurements are in progress 9]. These experiments will also elucidate the observed discrepancy between the ex-situ Mossbauer measurements and XRD and wet chemical analysis. 

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