Tailoring of Fe-ferrierite for N2O decomposition: On the decisive role of framework Al distribution for catalytic activity of Fe species in Fe-ferrierite

Two parent ferrierites with similar Si/Al ratio (8.5 and 10.5), a commercial and a laboratory specimen with different distribution of Al atoms in the framework, were used for the preparation of model Fe-ferrierite catalysts for N2O decomposition. As proven by UV-Vis analysis of the fully exchanged Co(II)-ferrierites, the two ferrierites differ in the distribution of framework Al between Al-O-(Si-O)(2)-Al sequences in one ring (Al pairs) and single Al atoms in different rings (Al-O-(Si-O)(n > 2)-Al sequences). The part of the framework Al atoms arranged in Al pairs mostly in beta six-member rings was 66% and 30% of Al for the two ferrierites. Two series of Fe-ferrierites with low concentration between 0.4 and 2.1 wt.% Fe were prepared, containing Fe ions in cationic sites, and valence and siting analyzed by Mossbauer spectroscopy of Fe-57 isotope enriched (96%) samples. The higher extent of Fe(III) ions reduced to Fe(II) and predominantly located in beta sites was found in the ferrierite with the higher content of Al pairs, while that with prevailing single Al atoms possessed higher content of Fe(III) ions and Fe(II) in alpha sites compared to the former ferrierite. It has been shown that the activity in N2O decomposition is in the whole Fe concentration range higher for the Fe-ferrierite with higher proportion of Al pairs. The Fe(II) ions in beta sites were indicated as the active sites, but not all beta-type Fe(II) ions exhibited equal activity. The highest unique activity in N2O decomposition is attributed to two cooperating Fe(II)-beta ... -Fe(11)-beta ions each balanced by Al pair in six-member ring in an opposite wall of ferrierite cavity. For both series of Fe-ferrierites the vacuum heat-treatment at 700 degrees C increased reduction of Fe(III) ions and occupation of the beta sites by Fe(II) ions, dehydroxylation of OH groups with formation of Lewis sites and boosted the N2O decomposition, but not proportionally to the change of the beta-type Fe(II) ions. For the activity increase the effect of cooperation of the Fe active sites with Al-Lewis acid sites was suggested, operating through stabilization of the surface NOx species on Lewis sites. The optimum distance (7.5 angstrom) of the most active Fe ... Fe ion in p sites of Fe-ferrierite is in agreement with the lower activity of Fe-beta and Fe-ZSM-5 zeolites. (C) 2011 Elsevier Inc. All rights reserved.