In situ DRIFTS and temperature-programmed technology study on NH3-SCR of NO_x over Cu-SSZ-13 and Cu-SAPO-34 catalysts

摘要

In situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTS), temperature-programmed desorption (TPD), and temperature-programmed surface reactions (TPSR) were employed to investigate the adsorption and reactive properties of Cu-SSZ-13 and Cu-SAPO-34 zeolite catalysts; these fully formulated washcoat cordierite monoliths were hydrothermally treated at 750 °C in the simulated feed gases. The intrinsic mechanism and reasons for the differences in NH3-SCR activity were proposed based on the characterization results. The in situ DRIFTS and TPD results showed that ammonia could adsorb on both the Lewis and Bronsted acidic sites on these two catalysts; the ammonia on the Bron-sted acidic sites might be active in the NH3-SCR reaction. For the different NO_x adsorption processes, the total NO_x desorption levels followed the following sequence: NO < NO+O2 < NO2 =NO2 + O2. The results confirm that the reaction pathways are totally different in the low and high temperature ranges on the Cu-SSZ-13 and Cu-SAPO-34 catalysts. In the low temperature range, the ammonium nitrates from the reaction between surface ammonia and nitrates are the key intermediates and are further reduced to form N2 and H2O by the NO gas. In the high temperature range, the gas-phase NO2 gradually become more important in the NH3-SCR reaction. The activity tests indicated that the Cu-SAPO-34 catalyst had a relatively higher DeNO_x performance than the Cu-SSZ-13 catalyst across the entire reaction temperature range, showing a double peak shapes with a dip point at approximately 390 °C Cu-SAPO-34 might retain many surface nitrate species and did not produce much more NO2 gas than Cu-SSZ-13; this species hindered the SCR reaction at 390°C.