NOx uptake and release on Pd/SSZ-13: Impact Of Feed composition and temperature

The Passive NOx Adsorber (PNA) is an emerging technology to abate cold-start NOx emissions from lean combustion vehicles. Binding sites on Pd-exchanged zeolites adsorb NO at low temperature ( 120 °C) regimes, indicating competition of NO uptake with H2O. The additions of CO and C2H4 increase the lower temperature uptake and higher temperature release of NO, both features desirable for application. In contrast, H2, dodecane, and toluene have a negligible effect on the NO uptake. During NO uptake the co-generation of NO2, CO2 and CH3CHO, respectively with reductants NO, CO, and C2H4, provides evidence for the reduction of Pd2+ in the form of [Pd(OH)]+ bound to zeolitic anionic sites [Al-O-Si]− and/or PdO2 clusters. Supporting DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) measurements identify several potential surface species that are linked to the multiple NO + NO2 desorption peaks. Two prospective mechanistic schemes are proposed that are consistent with the uptake/release and DRIFTS data.