NO-Assisted N2O Decomposition over ex-Framework FeZSM-5: Mechanistic Aspects

Abstract

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, O₂ 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 O₂.