Steam-activated FeMFI zeolites. Evolution of iron species and activity in direct N 2O decomposition

Abstract

In this paper the effect of the composition and steaming conditions of FeMFI catalysts on activity in direct N 2O decomposition is investigated. MFI zeolites with different framework compositions (FeAlSi, FeGaSi, and FeSi) and without iron (<0.002 wt% Fe in the final catalyst) were activated at different temperatures (673–1273 K) and partial steam pressures (20–500 mbar). Extra-framework Fe is essential for high catalytic activity in direct N 2O decomposition, while Lewis and Brønsted acidic sites play a minor role. Optimized activity was obtained at lower activation temperatures for the iron zeolites containing Al and Ga (873–923 K) than in the purely siliceous zeolite catalyst (1123 K), indicating relatively high stability of Fe in the silicalite framework. High partial steam pressures (>100 mbar H 2O in N 2) favor the extraction of framework iron, enabling the application of lower activation temperatures. The optimum temperature during steam activation is that at which extraction of framework iron is complete without extensive clustering of extra-framework iron species into oxide particles, as was demonstrated by transmission electron microscopy and electrochemical characterization of the samples. Additional experiments showed an increase in activity as a function of decreasing crystal size of the zeolite, indicating the presence of intracrystalline transport limitations. Furthermore, higher N 2O decomposition activities can be obtained by the application of a novel alkaline post-treatment of the steamed catalyst to further improve the catalyst accessibility by creation of mesopores. Implications of the results for the nature of the active Fe-site in direct N 2O decomposition are discussed. Our observations suggest that small intrazeolitic iron species in extra-framework positions are crucial in direct N 2O decomposition.