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

Abstract

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 + O2 at 27–250 °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 °C) shows a negative apparent activation barrier energy (−11.5 kJ mol–1) for the formation of NO+ species under the NO + O2 flow over H-CHA. The time course of the IR spectra at 27 °C shows that NO is oxidized by O2 to NO2 and then further converted via N2O4 to NO+ and NO3–. The subsequent exposure to NH3 at 27 °C reduces the NO+ species to N2. DFT calculations revealed that Brønsted 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.