EVIDENCE OF NANOSTRUCTURED IRON SPECIES IN [Fe]-S-1 AND [Fe]-ZSM-5 CATALYSTS

Fe-ZSM-5 and Fe-S-1 catalysts are active for some reactions of great concern as the one-step selective oxidation of benzene to phenol by N2O [1], the selective catalytic reduction (SCR) of NO and N2O by hydrocarbons,and the catalytic N2O decomposition [2-4]. The catalytic activity has been found to depend on the nature of the iron species formed upon calcination in air at high temperature. ln order to clarify the formation process and the nature of the FexOy-like species, we prepared a Fe-ZSM-S (1.78 Fe wt%) and Fe-S-1 catalysts (O.54+1.76 Fe wt%) with iron originally introduced in the MFI framework by isomorphous substitution during the synthesis. The samples have been characterized by porosimetry, DRS, powder X-ray diffraction (XRPD) and 57 Fe Móssbauer spectroscopy. It is found that FexOy nanoclusters are formed inside the S-1 zeolite by gradual migration of iron initially located in the framework tetrahedral site of the zeolite structure. Microporosity and XRPD results suggest that, even if the iron content is increased in the S-1 matrix, no large FexOy particles are formed. On the other hand, for the Fe-ZSM-5 sample the oxo-iron clusters have a higher nuclearity and they are still present inside the zeolitic channels. The catalytic data for the N2O decomposition indicate that the activity of the Fe-S-1 samples is affected by the nuclearity of the FexOy nanoclusters, with a beneficial influence of aluminum for the Fe-ZSM-S sample. [1] G. I. Panov, Cattech. 4, 18 (2000). [2] G. Berlier, G. Ricchiardi, S. Bordiga and A. Zecchina, J. Catal. 229, 127 (2005). [3] F, Kapteijn, G. Marban, J. Rodriguez-Mirasol and J.A. Moulijn, J. Catal. 167,256 (1997) [4] G. Fierro, G. Ferraris, M. lnversi, M. Lo Jacono and G. Moretti, Studies in Surf. Sci. and Catal. 135, 4964 (2001).