Investigations of the state of Fe in H-ZSM-5

The state of Fe dispersion in Fe-H-ZSM-5 was investigated by means of infrared spectroscopy of adsorbed NO, temperature-programmed desorption (TPD) of NO, TPD of NH3, and H-2 temperature-programmed reduction (TPR). For samples with a low loading of Fe [e.g., Fe/Al ratio less than or equal to 0.56], Fe cations exchange with Bronsted acid protons on a one-to-one basis, while at Fe/Al ratios above this level small particles of FeOx are formed, Results of H-2 TPR and NO uptake experiments on Fe-ZSM-5 samples following a He pretreatment suggest that the primary form of Fe present on samples with an Fe/Al ratio less than or equal to 0.19 is Fe3+ [i.e., Fe3+(OH-)(2)]. An increase in the Fe loading above this level results in an increase in the concentration of Fe2+ cations formed via the autoreduction of Fe3+(OH-)(2) which is present at highly reducible alpha sites in ZSM-5. Three types of Fe2+ sites are observed on Fe-ZSM-5 during the room temperature adsorption of NO. They are defined as type I, II, and III sites. Type I sites adsorb NO as mononitrosyls (1767 cm(-1)) and dinitrosyls (1917 and 1806 cm(-1)). Types II and III sites adsorb NO solely as mononitrosyl species (1876 and 1856 cm(-1), respectively). While types II and III sites are found to be relatively resistant to oxidation and reduction, type I sites are significantly affected by these pretreatments. The most notable change occurs for the high-weight-loaded Fe-ZSM-5 sample in which CO reduction results in the redispersion of FeOx to type I sites, (C) 1999 Academic Press.