Geometric and electronic structure of alpha-oxygen sites in Mn-ZSM-5 zeolites

Abstract

It is shown that alpha-oxygen sites are generated by the decomposition of N2O in ion-exchanged Mn-ZSM-5 zeolites. The catalytic activity decreases to zero in 1 h, and the amount of alpha-oxygen that is formed is reduced from similar to 30% alpha-oxygen sites per Mn atom in the initial N2O treatment to a value of similar to 4% in the subsequent cycles. The catalytic activity correlates with the intensity of a UV-vis band located at 18 500 cm(-1), suggesting that the alpha-oxygen sites are linked to Mn sites with a distorted tetragonal symmetry with extended Mn-O bonds with respect to bulk oxides. Scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) results indicate that the Mn sites are distributed unevenly over the ZSM-5 crystals, where most Mn atoms are found in the top similar to 2 nm from the crystal surface. The X-ray absorption spectroscopy (XAS) spectra indicate that, under reaction conditions, the Mn atoms have a 5-fold coordination and a valence of similar to 2.5, which is little affected by the N2O and NO treatments as only similar to 4% of these sites is active.