Identification of adsorbed species on Cu-ZSM-5 under NH3 SCR conditions

The selective catalytic reduction of nitrogen oxides with ammonia as the reducing agent was studied using Fourier transform infrared (FTIR) spectroscopy. The adsorbed species found on a Cu-ZSM-5 powder during exposure to NO, NO2 or NH3 was compared to the adsorbed species identified during SCR conditions. A blocking effect caused by ammonia at 175 °C was investigated by a stepwise increase of the ammonia concentration, and the spectra indicated that the formation of nitrites or nitrates decreased as surface coverage of ammonia increased. No such effect was observed at 350 °C, since the oxidation of ammonia results in very low ammonia coverage. The effect of changes in the NO to NO2 ratio was also studied at 350 °C, and the species identified during SCR reaction indicated that the enhanced activity at equimolecular amounts of NO and NO2 possibly involves gas phase components as well as adsorbed species.