Mössbauer spectroscopy and catalytic studies of iron-exchanged, silicon-substituted Y-zeolite

Abstract

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 N 2O. Mössbauer spectra were collected at room temperature after pretreatments in H 2 and O 2 at 700 K, and after exposure of the samples to CO or H 2O. Nitrous oxide decomposition activities were determined at 800 K under a N 2O partial pressure of 3.5 kPa. Mössbauer spectra collected after pretreatment in H 2 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 N 2O than conventional Fe-Y. Mössbauer spectra collected after pretreatment in O 2 showed that a fraction of the iron cations in silicon-substituted Fe-Y was more difficult to oxidize to Fe 3+ 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 H 2O 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 H 2O, 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 well.