Passive NOx Adsorption - Pd/ZSM-5



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Vehicles are the source of a large portion of worldwide NOx (NO and NO2) emissions that have far-reaching ramifications on climate, ecological balance and human health. Although vehicles are presently equipped with catalytic converters that significantly reduce NOx at high temperatures (> 200°C) before emission, the majority of the NOx escapes during the cold-start period of an engine – the time taken by the converter to reach its required operating temperature (> 200°C). Passive NOx Adsorbers (PNA) have shown potential to prevent cold-start NOx emissions. Specifically, Pd/ZSM-5 - medium pore ion-exchanged zeolite - can act as a PNA by trapping NOx at lower temperatures (< 200°C) and releasing it at higher temperatures (> 200°C) for catalytic reduction. Density Functional Theory (DFT) simulations of passive NOx adsorption to Pd/ZSM-5 were performed to identify active sites in ZPd and calculate the binding free energies (BFE) of adsorbates – NO, NO2, O2 and CO - at different temperatures. BFE data revealed that adsorbate binding becomes weaker with increasing temperature. Comparison of BFE data for Pd/ZSM-5 with corresponding data for Pd/SSZ-13 (small pore zeolite PNA) showed that the former is a stronger adsorbent.