NO reduction with isobutane on Fe/ZSM-5 catalysts prepared by different procedures

摘要

Fe/ZSM-5 catalysts were prepared from Na/ZSM-5 zeolite by ion exchange with Fe~(+2) in solution or in the solid state and characterized by XRD, ~(27)A1 MAS-NMR, EPR, XANES and Moessbauer spectroscopy (MOS-S). The catalysts were evaluated by the reduction of NO with isobutane. EPR, XANES and MOS-S data of all thermally treated Fe/ZSM-5 showed the Fe atoms as Fe~(3+). The EPR signals with g=4.29 or 4.23 were attributed to a mononuclear Fe~(3+) in tetrahedral coordination. The MOS-S data revealed the coexistence of Fe~(3+) cationic species in charge-compensation sites and extra-framework precipitated Fe species, such as α-goethite or hematite; the different IS and QS MOS-S values of these cationic species were attributed to Fe~(3+) in tetrahedral or octahedral coordination. It was proposed that the active sites for the NO reduction with isobutane were the Fe cationic species balancing the charge of the structure. In this sense, the catalytic activity was correlated with the Fe~(3+) cations with tetrahedral coordination. As these cationic species were identified at lower temperatures than were used for catalysis, they may have suffered modifications in their nature. The difficulty in identifying Fe species at higher temperatures impeded determination of the exact relation between the nature and activity of those species.