Pd/SAPO-34 passive NO x adsorbers: Stable Pd ion adsorption sites in six-member rings

Abstract

Pd/zeolites with chabazite (CHA) topology are effective passive NO _x adsorbers (PNAs), which are promising for the removal of NO _x 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 PdO _x clusters, Pd ions in the eight- and six-member rings (8MRs and 6MRs) of zeolites, are NO adsorption sites in the absence of H _2 O in the feed, demonstrated by a good linear relationship between the total Pd contents and NO _x desorption amounts. However, in the presence of H _2 O, 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 NO _x during the cold start of diesel vehicles.

Keywords

• Pd/SAPO-34
• passive NO x adsorbers
• adsorption sites
• Pd ions
• H2O poison