Influence of extra-framework Al on the structure of the active iron sites in Fe/ZSM-35

The influence of extra-framework Al via AlCl3 deposition on the structure of the active Fe sites in Fe/ZSM-35 was investigated by 29Si MAS NMR, UV–visible diffuse reflectance spectroscopy, infrared spectroscopy, UV Raman spectroscopy, and 57Fe Mössbauer spectroscopy combined with catalytic decomposition of N2O. It is found that high-temperature treatment considerably modifies the iron speciation. UV–visible diffuse reflectance spectra and in situ infrared spectra of NO adsorption show that a significant amount of clustered and cationic Fe species is transformed into extra-framework Fe–O–Al mixed oxide species by high-temperature treatment. UV Raman spectra and 57Fe Mössbauer spectra suggest the generation of a binuclear Fe site stabilized by extra-framework Al species with a feature Raman band at 875 cm−1, which is associated with the high activity of N2O decomposition. A possible explanation is that the addition of extra-framework Al species is beneficial for the formation of binuclear active Fe sites bound to extra-framework Al.

By a combination of the transient-response method, steady-state kinetics, in situ IR, and in situ Raman spectroscopic techniques, the effect of extra-framework Al on the structure of active iron sites in Fe/ZSM-35 catalysts and the activity for steady-state N2O decomposition was studied. Both the number of binuclear active iron sites and the activity for N2O decomposition increase linearly with the quantity of introduced extra-framework Al.Figure optionsDownload high-quality image (138 K)Download as PowerPoint slideHighlights
► The N2O decomposition activity increases with increased AlCl3 deposition.
► High-temperature treatment strongly modifies the iron speciation.
► Extra-framework Al species are beneficial for the formation of binuclear active iron sites.
► The intensity of the Raman band at 875 cm−1 increases with increased AlCl3 deposition.