Redispersion of PdOx to Pd ions in Pd/BEA and Pd/ZSM-5 during low-temperature NO adsorption in the presence of H2O and CO

Pd-based zeolites are promising materials for passive NOx adsorbers (PNA) to remove cold-start NOx emissions. H2O and CO are common components in cold-start exhaust. In this work, Pd/BEA and Pd/ZSM-5 with similar to 1 wt% Pd were prepared to investigate the effect of H2O and CO on the NO uptake at low temperature (120 degrees C). It was found that Pd sites in Pd/ZSM-5 and Pd/BEA can be almost fully utilized as NO adsorption sites by formation of Pd-nitrosyl complexes in the presence of CO and H2O. By combining the results of NO adsorption reactions and in-situ DRIFTS, it was found that the redispersion of PdOx to Pd2+ ions can occur during NOx adsorption in the presence of CO and H2O and NO uptake via formation of Pd-nitrosyl complexes. A mechanism for the improvement of low-temperature NO uptake by Pd/BEA and Pd/ZSM-5 in the presence of CO and H2O was proposed.