Investigation of the redox properties of Fe-CHA catalysts by transient response methods

Abstract

Fe-exchanged zeolite catalysts have recently raised interest due to their higher de-N₂O activity compared to Cu-CHA. However, no methodology is currently available to assess their active species and redox properties under real conditions. Herein we investigate Fe-CHA catalysts coupling Transient Response Methods, previously used to investigate Cu-CHA, and Operando FT-IR, with the aim to gain insights in the Fe active species and their behavior under different reactive conditions. Since Fe is well reported in literature to undergo a redox cycle between oxidized and reduced forms, the analysis was divided in two sections focused on reducing and oxidizing methods. From H₂-TPR and NO+NH₃ reductive protocols, different responses compared to Cu-CHA were observed, evidencing that oxidation of Fe-CHA is less effective. Concerning oxidative protocols, NO₂ adsorption/desorption was identified as a key protocol for the titration of active Fe species, as NO₂ was stored as nitrate every two iron sites regardless of the speciation. This result was supported by Operando FT-IR tests. The NO₂ ads/des protocol was replicated on pre-reduced catalysts, revealing the strong ability of NO₂ to oxidize Fe sites, redisperse them and use them as storage sites. Finally, transient N₂O decomposition runs, followed by NO+NH₃ reduction or TPD, were used to probe Fe-CHA sites. These experiments confirmed the strong oxidizing power of N₂O, close to NO₂ and superior to O₂, showing also how the stability of the stored oxygen, strictly linked with the auto-reduction of the catalysts, as well as the Fe oxidation state, were dependent on the N₂O decomposition temperature.