Catalytic performance of different types of iron zeolites in N2O decomposition

Abstract

A series of Fe/zeolites (ZSM-35, ZSM-5, beta, and mordenite) samples with Fe/Al molar ratios of 0.33 were prepared using a solid-state ion-exchange method. A combination of ultraviolet-visible diffuse reflectance, in-situ Fourier-transform infrared, and in-situ visible Raman spectroscopic techniques, with a transient response method, was used to investigate the influence of the zeolite framework on the catalytic properties of the Fe/zeolites in N2O decomposition. The results show that the catalytic activity of the Fe/zeolites-HT (HT denotes high-temperature treatment) samples is in the order Fe/ZSM-35-HT > Fe/beta-HT > Fe/ZSM-5-HT > Fe/mordenite-HT. There is a linear relationship between the rate of N2O decomposition and the concentration of binuclear iron sites. This indicates that binuclear iron sites are the active sites for N2O decomposition. A correlation between the formation of binuclear iron sites and Fe ion distribution among the cationic sites is proposed. Two Fe(II) cations located in two adjacent six-membered rings in a 10-membered ring channel (α sites) or in two neighboring six-membered rings in an eight-membered ring channel (β sites) of Fe/ZSM-35 are favorable for the formation of active binuclear iron sites. Similar structure can also be formed in two adjacent six-membered rings in polymorphs A and B of beta zeolite or in the six-membered rings at the intersection of the straight and sinusoidal channels of the ZSM-5 framework. For the Fe/mordenite-HT sample, most of the iron species are present as isolated iron cations, so it has the lowest activity in N2O decomposition.