CO and Zeolite Caging Effects of Model Passive NOx Adsorbers
Kevin Gu, University of Virginia
Passive NOx adsorbers (PNAs) are proposed as one of the solutions to help address cold start NOx emissions. PNAs can trap NOx at low temperatures and release NOx at a reasonably higher temperature, thereby removing NOx from the exhaust during cold start. Subsequently, NOx can be released to a downstream NH3-SCR catalyst when the temperature is high enough such that SCR will again be used to reduce NOx, in a temperature range where urea can be injected and the reactions rates are appreciable. Pd ion-exchanged zeolites have been identified as efficient catalysts material for NOx adsorption and have been extensively evaluated under a variety of simulated exhaust conditions. However, it appears that the presence of CO induces changes in both adsorption and desorption characteristics and these observed changes differ between Pd ion-exchanged zeolites, which can be explained by the difference of zeolite cage opening sizes. The mechanistic understanding of the CO effect as well as the zeolite caging effects are assessed in this work both experimentally and computationally. Investigation of the nature of Pd2+ cations as one of the proposed NOx storage species is performed using diffuse reflectance Fourier transform spectroscopy (DRIFTS). Experimentally measured DRIFTS spectra and density functional theory (DFT) computed spectra are compared to confirm the energetics of the proposed surface intermediates. Ultimately, a systematic NO adsorption and desorption mechanism with the presence of CO is provided.