N2O decomposition over [Fe]-MFI catalysts: Influence of the FexOy nuclearity and the presence of framework aluminum on the catalytic activity

• A set of MFI samples with framework Fe3+ and Al3+ ions were prepared and characterized.
• Magnetic susceptibility is very effective to characterize FexOy nanoparticles.
• Small-sized FexOy nanoparticles are the most active species for N2O decomposition.
• In high Fe loading preparations, the number of small-sized FexOy nanoparticles increases.
• Framework Al3+ ions enhance the catalytic activity of small-sized FexOy nanoparticles.

In this work, some [Fe]-MFI catalysts at different Fe loadings and Si/Al ratios have been prepared by loading only Fe3+ ions, in the case of [Fe]-S-1 specimens, or Fe3+ and Al3+ ions, in the case of [Fe]-ZSM-5 preparations, into the synthesis solutions prior to crystallization in order to have in both sets of materials the Fe3+ ions occupying the MFI zeolite framework sites. These catalysts have been characterized in either the as synthesized state or after thermal treatment in air at high temperature with different methods as porosimetry, UV–Vis diffuse reflectance spectroscopy (DRS), Gouy balance for magnetic susceptibility measurements. Moreover, an assay of 57Fe Mössbauer spectroscopy was also made on a few representative samples. The combined investigation with the above spectroscopic and magnetic techniques was useful to clarify the iron chemical state, and the nature of the FexOy nanoparticles formed upon dehydration at high temperature in the calcined samples. Our findings suggest that most of the FexOy nanoparticles are formed inside the MFI structure by gradual migration of Fe3+ ions from framework sites in consequence of the calcination in air at 823 K. These FexOy nanoparticles can have different nuclearity regardless the iron loading but, interestingly, likely due to the gradual migration, it seems that FexOy nanoparticles with lower nuclearity are formed even in the material with the highest iron loading and that they are located inside the interconnected channel systems of the MFI framework. The catalytic behavior for the N2O decomposition of the [Fe]-S-1 and [Fe]-ZSM-5 catalysts was also investigated. It was found that the catalytic activity is depending on the nuclearity of the FexOy nanoparticles formed inside the MFI structure and it is suggested the FexOy nanoparticles with lower nuclearity located inside the channel systems of the MFI framework are responsible for the catalytic activity. Finally, on the basis of a comparison between the catalysts with and without aluminum, that is, the [Fe]-ZSM-5 and [Fe]-S-1 materials, respectively, it was further confirmed that the framework aluminum has a beneficial effect on the N2O decomposition catalytic activity.

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