Abstract
Pd-based zeolite materials have gained significant attention as passive NOx adsorbers (PNAs) for diesel-engine cold-start NOx mitigation due to their ability to store NOx at low temperatures. Pd/ZSM-5 is a promising PNA candidate, however, the NO adsorption mechanism over this material is not well understood. This study combines flow reactor experiments and surface spectroscopy to investigate NO adsorption under a variety of conditions. The state of hydration of the Pd cations played an important role in determining the impact of H2O and CO concentration on the PNA performance. Below 150 簞C, the inhibition effect of H2O on NO adsorption was mitigated by CO. Due to dehydration, neither H2O nor CO had an impact on NO uptake at 150 簞C, where maximum NO storage capacity of the PNA was also observed. The observed gas composition and temperature effects on NO adsorption and formation of surface intermediates ultimately inform a proposed mechanism.
