NO-assisted N2O decomposition over ex-framework FeZSM-5:: mechanistic aspects

The decomposition of N2O over an ex-framework FeZSM-5 catalyst is strongly promoted by NO. Activity data show that the promoting effect of NO is catalytic, and that besides NO2, O-2 is formed much more extensively in the presence, than: in the absence of NO. Transient in situ FT-IR/MS measurements indicate that NO is strongly adsorbed on the catalyst surface up to at least 650 K, showing absorption frequencies at 1884 and 1876 cm(-1). A change in gas phase composition from NO to N2O results in the formation of adsorbed NO2, identified by a sharp IR band at 1635 cm(-1). Switching back to the original NO gas phase induces a rapid desorption Of NO2, restoring the original NO absorption frequencies. During the IR measurements, bands typical of nitro- or nitrate groups were not observed. Multi-Track (a TAP-like technique) experiments show that the presence of NO or NO2 on the catalyst surface significantly enhances the rate of oxygen desorption at the time of N2O exposure to the catalyst. The spectral changes and transient experiments are discussed and catalytic cycles are proposed, to explain the formation of NO2 and the (enhanced) formation of oxygen. The latter can be either explained by an indirect effect (electronic, steric) of NO adsorbed on sites neighboring the active sites, or by a direct effect involving reaction of adsorbed NO2 groups with neighboring oxidized sites yielding O-2.