Lean NOx Capture and Reduction by NH3 via NO+ Intermediates over H-CHA at Room Temperature

The oxidation of NO to NO2 and the subsequent reduction by NH3 via a NO+ intermediate over a proton-type chabazite zeolite (H-CHA) were investigated by the combination of in situ infrared (IR) spectroscopy and density functional theory (DFT) calculations. The in situ IR spectral results indicate that the NO' species formed under a flow of NO + O-2 at 27-250 degrees C are more stable at lower temperatures over both H-CHA and copper-cation-exchanged CHA zeolite (Cu-CHA). The Arrhenius plot (T = 27-120 degrees C) shows a negative apparent activation barrier energy (-11.5 kJ mol(-1)) for the formation of NO+ species under the NO + O(2 )flow over H-CHA. The time course of the IR spectra at 27 degrees C shows that NO is oxidized by O-2 to NO2 and then further converted via N2O4 to NO+ and NO3. The subsequent exposure to NH3 at 27 degrees C reduces the NO species to N-2. DFT calculations revealed that Bronsted acid sites in zeolite pores promote the dissociation of N2O4 intermediates into NO and NO3- species with a low activation barrier (15 kJ mol(-1)). Moreover, the computed activation barrier for the reduction of NO+ species by NH3 was considerably low (6 kJ mol(-1)). The experimental and theoretical results of this study demonstrate the high potential of Cu-free H-CHA zeolites for promoting lean NOx capture to form NO+ species and the subsequent reduction by NH3 at room temperature.

Automated summary

The study found that NO' species formed on H-CHA and Cu-CHA are more stable at lower temperatures, with a negative apparent activation energy for the formation of NO+ species on H-CHA. DFT calculations showed that Bronsted acid sites in zeolite pores promote the dissociation of N2O4 intermediates and the reduction of NO+ species by NH3 has a considerably low activation barrier.