Study on the role of Fe species and acid sites in NH3-SCR over the Fe-based zeolites

Abstract

Fe doped ZSM-5 zeolite catalysts, prepared with solution ion-exchange method, calcinated at various temperatures and aged with/without H2O, were tested for activity in the selective catalytic reduction of NOx with NH3 (NH3-SCR) and characterized by XRD, NMR, UV‐Vis, XPS, H2-TPR, NH3-TPD and in-situ DRIFTs for structure and species change to investigate the role of Fe sites and acidity in SCR reaction. The results showed that the SCR activity for the Fe-ZSM-5 was mainly dependent on the active Fe species rather than acidity. Fe-oxo related Lewis sites were important for the low temperature SCR reaction, while high Brønsted acidity was not necessary for a high SCR activity. It should be noticed that the inactive free Fe2+ ions could be converted to isolated Fe3+ species by high temperature during calcination and aging, which could be inhibited by H2O due to dealumination during hydrothermal aging. This resulted in the decreased SCR activity after aging in the presence of H2O and the improved NO conversion after high-temperature calcination and thermal aging due to that the isolated Fe3+ acted as the main active site at temperature <500 ℃. The in-situ DRIFTs study showed that NH3 adsorption on acid sites was not the necessary step for the low-temperature SCR reaction but essential to NH3 oxidation, and the Eley-Rideal (E-R) mechanism was the main pathway for standard NH3-SCR.