The Mossbauer Effect in Supported

N. R. Smyrl and Gleb Mamontov The Mossbauer Effect in Supported... 

An iron-exchanged mordenite was also studied by Meisel et al. (182), who incorporated Fe3+ into the zeolite structure. Upon calcination at temperatures greater than 500 K the appearance of Fe2+ was noted in the Mdss-bauer spectrum, and for calcination temperatures higher than 770 K the formation of a-Fe203 was observed to take place inside the mordenite. For the iron- mordenite system, it can now be seen that the Mossbauer effect provides information about the chemical state, symmetry, interaction strength with the support, and location on the support of the resonant iron ions. This information enhances the understanding of the catalytic activity of this and other zeolites (178). 

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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. 

Iron-supported-on-MgO catalysts behave in some ways differently from the above catalyst systems. That is, while the catalytic activity of these metallic-iron particles for the atmospheric-pressure ammonia synthesis depends markedly on particle size in the range 1.5-10 nm (206), the Mossbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine splitting) are independent of iron particle size in this range (97). This thus rules out an electronic effect in the interpretation of the effect of particle... 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

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