The Effects of Steaming and Sodium Exchange on the Selective Catalytic Reduction of NO and NO2 by NH3 over HZSM-5

We have studied the selective catalytic reduction of NO and NO2 by NH3 over a series of HZSM-5 catalysts that have been modified by mild steaming and by sodium exchange. We find that the activity of an HZSM-5 sample for the reduction of NO is increased following a moderate steaming. Kinetic data collected for this sample can be accurately fit using the rate expression developed for unsteamed catalysts. Mild steaming increases the rate constants for SCR and ammonia oxidation by approximately a factor of two but requires no change in the ammonia adsorption constant, suggesting that steaming increases the rate by increasing the number of active sites. We also observe that the rate of NO reduction is not strongly affected by the level of sodium exchange. Given that mild steaming is known to increase the concentration of extra-framework alumina while sodium exchange is known to primarily affect the acidity of the framework Brønsted alumina, these observations strongly support our earlier conclusion that the active site for the selective catalytic reduction of NO by ammonia over HZSM-5 is highly acidic extra-framework alumina. We also find that while the fully exchanged sample is active for the reduction of NO2 by NH3, the rate is strongly dependent on the number of unexchanged framework Brønsted sites, suggesting that both framework and extra-framework alumina are active for NO2 reduction.