不同类型含铁分子筛上N2O催化分解反应

采用固态离子交换法制备了一系列Fe/Al摩尔比为0.33的Fe/分子筛(ZSM-35,ZSM-5,beta和mordenite (丝光沸石))样品,并利用紫外-可见漫反射光谱、原位红外光谱和可见拉曼光谱以及瞬态应答实验考察了分子筛种类对N₂O分解反应性能的影响.研究表明,经高温处理(HT)后分子筛的催化活性顺序为 Fe/ZSM-35 > Fe/beta > Fe/ZSM-5 > Fe/mordenite,与骨架外铝稳定的双核铁物种含量一致.这说明双核铁物种是高温处理后Fe/分子筛样品中N₂O分解的活性位,而它的形成与分子筛种类密切相关.Fe/ZSM-35分子筛十元环孔道中相邻两个α位和八元环孔道中相邻两个β位都能稳定两个Fe(II)离子而形成能高效分解N₂O的双核铁活性中心.Fe/beta和Fe/ZSM-5分子筛中只有落在相邻两个β位上的两个Fe(II)离子才能形成参与N₂O分解的高活性的双铁活性中心.Fe/mordenite-HT分子筛上的铁物种绝大部分以孤立Fe离子的形式存在,因此其催化N₂O分解反应活性很低.

Catalytic performance of different types of iron zeolites in N2O decomposition

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.