EPR investigation of Fe-exchanged beta-zeolites

Iron-exchanged beta-zeolites with an exchange level (300 Fe/Al, mol/mol) of 10-97 were characterized by electron paramagnetic resonance (EPR) after calcination and a redox pretreatment (H-2/N2O). According to the EPR spectra, three different types of iron species are present in the samples: isolated ions, polynuclear oxo-iron species (clusters), and superparamagnetic iron particles. Fe-exchanged beta-zeolites (Fe-BEA) with an iron exchange level lower than 24 are mainly constituted by isolated Fe(III) ions. These latter species can be located in the structure or in charge compensation of the BEA zeolite structure. The amount of interacting species above an iron exchange level of 24 increases strongly, and these species become predominant at an iron exchange level of 100. Oxidation of Fe2+-BEA by N2O leads to the generation of new oxo-iron species with superparamagnetic character. These species are considered as the active species in the selective catalytic reduction of N2O by NH3 over fully exchanged Fe-BEA.