CAN Fe OXIDE NANOPARTICLES BE FORMED IN Fe-ZSM-5 ZEOLITES AT HIGH Fe LOADING ?

ln the last decade the Fe-MFl zeolites have been widely investigated because they are active catalysts in some important reactions, as those for the abatement of N2O and NO. ln spite of all these studies, the complex chemistry leading to many Fe species during the preparation steps and, in particular, upon calcination at high temperature, discloses unexpected findings which are still matter of investigation. We found that, by ion exchanging a H-ZSM-S (Si/Al=2S) zeolite with a solution containing Fe3* ions and acetate ions, remarkable Fe loadings (Fe=1.31 wto/o and Fe=8.40 wt%) can be easily reached. upon calcination of these two over-exchanged Fe-ZSM-S samples at 773 K, in spite of the high Fe content, no large/bulk Fe oxide particles, which should be segregated at the external surface of the zeolite, are present. On the contrary, the results of spectroscopic (DRS, Móssbauer) and structural (XRD, porosimetry) investigations show that in both the Fe-ZSM-S catalysts only nanosized FexOy, species, varying in dimension, have been formed, even in the preparation at very high Fe content. Depending on their size, these species are differently located in the zeolite. The smaller Fe oxide-like nanoparticles are mainly confined inside the MFI structure whereas the larger ones are formed on the zeolite external surface, as in particular it occurs in the catalyst at very high Fe loading. The fraction of smaller nanosized FexOy, species prevails in the catalyst with minor Fe content (Fe = 1.31 wt%), whereas those characterized by a larger size appear to be more abundant in the catalyst with higher Fe content (Fe = 8.40 wt%). The catalytic activity and selectivity for the SCR of NO by propane result to be remarkably influenced by the FexOy, nanoparticles size.