Effect of zeolite structure on the selective catalytic reduction of NO with ammonia over Mn-Fe supported on ZSM-5, BEA, MOR and FER

Abstract

A series of catalysts based on Mn-Fe loaded zeolites was prepared by impregnation and their activity in the selective catalytic reduction of NO with ammonia (NH3-SCR) was investigated. The highest catalytic conversion was recorded for MnFe-ZSM-5 (MnFe-Z), followed by MnFe-BEA (MnFe-B) and MnFe-MOR (MnFe-M), while MnFe-FER (MnFe-F) showed a very poor activity over the entire temperature range. In order to evidence a correlation between the structure and acidity of the zeolites and NO conversion, the prepared samples were characterized by various techniques (ICP-AES, N2 physisorption at 77 K, XRD, 27NMR, Raman, FTIR spectroscopy of adsorbed ammonia, H2-TPR, DRS UV–Vis, EPR and XPS). The superior catalytic activity of MnFe-Z at low temperature is attributed to the abundance of Mn4+ concentration as revealed by XPS, the highest NH3-L/NH4+ ratio indicative of the contribution of metals in generating Lewis acidic centers as evidenced by IR-NH3, and the better reducibility of manganese and iron on ZSM-5 which increases the kinetics for red-ox cycles as confirmed in TPR analysis. Fe3Mn3O8 mixed oxide phase is also detected by XRD and XPS and can be associated with the high reducibility of MnFe-Z which generates a high oxidation ability favoring NO to NO2 oxidation. Raman spectroscopy was also used to confirm the existence of a strong synergy between metals and ZSM-5 support revealed by the shift in the signal position and the decrease in band intensities. The results showed that the zeolite framework and acidity generate catalysts with different textural and structural properties which influence the metal dispersion and speciation and hence influence the catalytic performances.