The influence of the preparation procedures on the catalytic activity of Fe-BEA zeolites in SCR of NO with ammonia and N2O decomposition

Abstract

Three series of Fe-containing BEA zeolites are prepared by two-step postsynthesis (PS), ion exchange (IE) and conventional wet impregnation (Imp) procedures with various Fe content (1.0–4.0 Fe wt.%). In these series of zeolites iron is mainly present as pseudo-tetrahedral Fe(III) as evidenced by combined used of DR UV–vis, 57Fe Mössbauer and XPS investigations. Two kinds of pseudo-tetrahedral Fe(III) have been distinguished by 57Fe Mössbauer spectroscopy in all Fe-containing BEA zeolites. As evidenced by TPD of NH3, the (Imp)-FexHAlBEA and (IE)-FeHAlBEA have a significantly higher acidity than (PS)-FexSiBEA. This is related to the presence in the former both pseudo-tetrahedral Al(III)–O(H)–Si and Fe(III)–O(H)–Si framework sites and in the latter only pseudo-tetrahedral Fe(III)–O(H)–Si. (PS)-FexSiBEA, (Imp)-FexHAlBEA and (IE)-FeHAlBEA are found to be active and selective catalysts of N2O decomposition, with 100% conversion in the temperature range of 750–800K. Moreover, the samples are active in selective catalytic reduction of NO with ammonia, with selectivity toward N2 exceeding 90% for NO conversions of 80–93% in the temperature range of 550–800K. Isolated pseudo-tetrahedral Fe(III), present in three series of Fe-containing BEA zeolites, seems to be involved in both N2O decomposition and SCR of NO processes. The higher activity of (Imp)-FexHAlBEA and (IE)-FeHAlBEA than (PS)-FexSiBEA in the SCR of NO process suggests that the acidic Al(III)–O(H)–Si framework sites present in the former catalysts play also a significant role in this process.