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

By D. Radu, P. Glatzel, A. Gloter, O. Stephan, B.M. Weckhuysen and F.M.F. de Groot


It is shown that R-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 R-oxygen that is formed is reduced from ∼30% R-oxygen sites per Mn atom in the initial N2O treatment to a value of ∼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 R-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 ∼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 ∼2.5, which is little affected by the N2O and NO treatments as only ∼4% of these sites is active

Year: 2008
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