Role of α-Sites in the selective catalytic reduction of NO x with ammonia over Fe-ZSM-5 catalysts

The catalytic properties of Fe-ZSM-5 zeolites with different iron contents have been investigated in the selective catalytic reduction (SCR) of NO x with ammonia. The observed catalytic properties the zeolites are correlated with the concentration of the iron-containing sites that are stabilized in the zeolite and effect N2O decomposition (α-sites). The catalysts activated at a high temperature to increase the α-site concentration (by a factor of 5–10) are more active in NO x SCR with ammonia than the unactivated samples. However, the difference between the activities of the activated and unactivated catalysts is well below the difference between the α-site concentrations in these catalysts. The nonlinear relationship between these parameters is evidence that the α-sites in Fe-ZSM-5 is not the only factor determining the activity of Fe-ZSM-5 in NO x SCR with ammonia. The activated catalysts show a low activity in nonselective ammonia oxidation and, accordingly, a high selectivity in the target process at high temperatures.