A Mössbauer and structural investigation of Fe-ZSM-5 catalysts: Influence of Fe oxide nanoparticles size on the catalytic behaviour for the NO-SCR by C3H8

In this work some results concerning a spectroscopic and structural characterization of two Fe-ZSM-5 (Si/Al = 25) catalysts (Fe = 1.31 wt% and 8.40 wt%), prepared by the ion-exchange method, are reported. The final catalysts, obtained by calcination in air at 773 K, were investigated by Mossbauer spectroscopy as well as by XRD. DRS and porosimetry together with the H-ZSM-5 parent material. In consequence of the treatment at high temperature in air, iron oxide species were formed in the Fe-ZSM-5 catalysts. Surprisingly, in spite of the rather high Fe loading, in both catalysts no large, or bulk, Fe oxide particles, which should be segregated at the external surface of the zeolite, have been found. On the contrary the iron oxide species were only in the form of nanoparticles, even in the largely over-exchanged preparation at high Fe content. The Fe oxide nanoparticles are varying in dimension and, depending on their size, they result to be differently located in the zeolite structure. The smaller Fe oxide-like nanoparticles, which prevails in the catalyst with minor Fe content (Fe = 1.31 wt%). are mainly confined inside the micropores whereas the Fe oxide-like nanoparticles characterized by a larger size, which appear to be more abundant in the catalyst at higher Fe loading (Fe = 8.40 wt%), are located on the zeolite external surface. The catalytic activity and selectivity for the SCR of NO by propane are remarkably influenced by the size of the Fe oxide nanoparticles. The catalyst with less Fe content, characterized by a larger amount of smaller nanosized FexOy like-species, is more active but less selective to CO2 than the excessively over-exchanged catalyst. This suggests that the FexOy nanoparticles larger in size, prevailing in the catalysts at higher Fe content, contribute very little, or not at all, to the catalytic activity but markedly improve the selectivity to CO2. (C) 2010 Elsevier BM. All rights reserved.