Mössbauer Spectroscopy and Catalytic Studies of Iron Exchanged, Silicon Substituted Y Zeolite

L. APARICIO; J. DUMESIC; S. FANG; M. LONG; M. A. ULLA; W. MILLMAN; K. HALL

JOURNAL OF CATALYSIS

ACADEMIC PRESS INC ELSEVIER SCIENCE

Año: 1987 vol. 104 p. 381 - 395

0021-9517

Iron-exchanged zeolites with Si Al ratios ranging from 2.49 to 8.88 were characterized by Mössbauer spectroscopy and studied for the decomposition of N2O. Mössbauer spectra were collected at room temperature after pretreatments in H2 and O2 at 700 K, and after exposure of the samples to CO or H2O. Nitrous oxide decomposition activities were determined at 800 K under a N2O partial pressure of 3.5 kPa. Mössbauer spectra collected after pretreatment in H2 showed that silicon-substituted Fe-Y had a Larger fraction of iron cations in sites I′, II′, and/or II than Conventional Fe-Y, in which most iron cations were in site I. The silicon-substituted Fe-Y catalysts had higher turnover frequencies for the decomposition of N2O than conventional Fe-Y. Mössbauer spectra collected after pretreatment in O2 showed that a fraction of the iron cations in silicon-substituted Fe-Y was more difficult to oxidize to Fe3+ than the iron in conventional Fe-Y. This result suggests that pairs of iron cations in close proximity are needed for the oxidation process to occur readily. Finally, exposure of the zeolites to CO or H2O produced Mössbauer spectroscopy doublets due to iron cations associated with adsorbate molecules. These doublets indicate that ferrous cations at sites I′, II′, and/or II serve as adsorption centers for H2O, the adsorption process increasing the coordination of the ferrous cations, and that ferrous cations at site II can serve as adsorption Centers for CO as wel