Pd/SAPO-34 passive NOx adsorbers: Stable Pd ion adsorption sites in six-member rings

Pd/zeolites with chabazite (CHA) topology are effective passive NOx adsorbers (PNAs), which are promising for the removal of NOx from diesel exhausts during cold start to meet increasingly stringent emission regulations. In this work, Pd/SAPO-34 zeolites with different preparation methods and Pd contents were developed and exhibited high NO adsorption performance. Three Pd species in Pd/SAPO-34, including well-dispersed PdOx clusters, Pd ions in the eight- and six-member rings (8MRs and 6MRs) of zeolites, are NO adsorption sites in the absence of H2O in the feed, demonstrated by a good linear relationship between the total Pd contents and NOx desorption amounts. However, in the presence of H2O, only Pd ions in 6MRs are stable NO adsorption sites and provide the continuous NO adsorption, due to the limited mobility of the Pd ions in 6 MRs with small pore size. This new finding points out a definite direction for the rational design of high-performance PNAs to eliminate NOx during the cold start of diesel vehicles.

Automated summary

Pd/SAPO-34 zeolites were developed as effective passive NOx adsorbers, with Pd clusters and ions identified as NO adsorption sites. The presence of water affected the stability of these sites, with only Pd ions in specific rings remaining as stable NO adsorption sites. This discovery provides a new direction for the rational design of high-performance PNAs for NOx removal during cold starts in diesel vehicles.