Tailoring of the structure of Fe-cationic species in Fe-ZSM-5 by distribution of Al atoms in the framework for N2O decomposition and NH3-SCR-NOx

Abstract

The heterogeneity and redox behavior of Fe-cationic species in Fe-ZSM-5 catalysts for N2O decomposition and NH3-SCR-NOx were analyzed by Mössbauer and UV–Vis spectroscopy, TPR-H2, and reaction kinetic measurements. Iron was introduced into ZSM-5 zeolites of similar Si/Al but with different populations of close (Al–Si–Si–Al sequences in one ring – Al pairs) and far distant (single Al in different rings) Al atoms in the zeolite framework. It has been found that Al pairs in 6MRs of the framework greatly stabilize divalence of bare Fe(II) ions and [Fe(II)–O–Fe(II)]2+ complexes even in oxidizing atmosphere. These species were completely oxidized only with N2O and recognized as the most active sites in N2O decomposition. On the contrary, the prevailing concentration of single Al atoms resulted at comparable conditions in high population of Fe(III)-oxo species. Fe(III)-oxo species of low nuclearity with partially compensated positive charge by the zeolite framework facilitate the formation of highly active oxygen species in an O2-containing atmosphere and are the most active sites in NH3-SCR-NOx. These findings show that manipulation of the structure of Fe-cationic species can be achieved by the synthesis of zeolites with different distributions of Al atoms between Al pairs and single Al atoms in the framework. This represents a potential for tailoring of catalytic properties of Fe-zeolite based catalysts for abatement of nitrogen oxides.