Complementary physicochemical characterization by SAXS and¹²⁹Xe NMR spectroscopy of Fe-ZSM-5: Influence of morphology in the selective catalytic reduction of NO

Fe-ZSM-5 catalysts were prepared by FeCl₃ sublimation (CVD method) and impregnation in aqueous media. To describe the morphology of the iron species, the materials were characterized by XRD, SAXS, and ²⁷Al and ¹²⁹Xe NMR spectroscopy. In the catalysts prepared by impregnation, XRD detected large hematite particles at the external surface of the zeolite. In contrast, the Fe₂O₃ phase was not detectable by XRD in the materials prepared by CVD, indicating much smaller species. The ¹²⁹Xe NMR results show that, in the CVD-prepared materials, the amount of free space inside the zeolite cavities was decreased appreciably by the presence of the Fe oxo species. Furthermore, ¹²⁹Xe NMR spectroscopy reveals a sharp peak for the CVD-prepared material with Fe (3.8 wt %), indicating that this catalyst exhibits strong adsorption sites that are not detectable in the sample of similar Fe content that was prepared by impregnation. Morphology appears to be strongly affected by the method of preparation of the catalyst. The fractal dimension of the metal phase was associated with the catalytic activity in the selective catalytic reduction (SCR) of NO using ammonia and n-decane as reducing agents.