NOx reduction performance of fresh and aged Fe-zeolites prepared by CVD: Effects of zeolite structure and Si/Al2 ratio

Abstract

A series of Fe/zeolites with different pore structures (MFI, BEA, FER, LTL, MOR) and Si/Al2 ratios (6–240) were prepared by chemical vapor deposition (CVD). The activities for selective catalytic reduction by ammonia (NH3-SCR) of fresh and hydrothermally aged (700°C) samples were measured in order to determine the effect of zeolite species on reactivity and hydrothermal stability. SCR activity decreased with increasing Si/Al2 ratios for MFI (Si/Al2=40, 73 and 90) and MOR (Si/Al2=20, 30 and 240) samples. The SCR activity of fresh samples was dependent on the type of zeolite used as follows: BEA>MFI>FER>LTL>MOR. The order could not be explained by the amount of Fe loading or by specific surface area (SSA), although activity was well correlated with the higher temperature (HT) peak in temperature-programmed desorption (TPD) spectra of NO2 and with NO oxidation conversion under NH3-free conditions. The HT peak indicates the number of active Fe sites. Therefore, the major factor affecting SCR activity is the amount of active Fe available to catalyze the oxidation of NO. After hydrothermal aging, the activity of all samples decreased and the order of activity by zeolite type was: MFI>BEA>FER>LTL>MOR. Activity retention ratios estimated from conversions for fresh and aged samples were independent of the number of acid sites and SSA. In contrast, the retention ratios were correlated with the crystal size of the parent H-zeolites.