Role of α-Sites in the selective catalytic reduction of NOx 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 NOx 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 NOx 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 NOx 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.