Tailoring of Fe-ferrierite for N 2O 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 β 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 Mössbauer spectroscopy of 57Fe isotope enriched (96%) samples. The higher extent of Fe(III) ions reduced to Fe(II) and predominantly located in β 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 α 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 β sites were indicated as the active sites, but not all β-type Fe(II) ions exhibited equal activity. The highest unique activity in N2O decomposition is attributed to two cooperating Fe(II)-β⋯Fe(II)-β 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°C increased reduction of Fe(III) ions and occupation of the β 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 β-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 (7.5Å) of the most active Fe⋯Fe ion in β sites of Fe-ferrierite is in agreement with the lower activity of Fe-β and Fe-ZSM-5 zeolites.