Effect of zeolite framework on the NO operating cycle in Pd-based passive NOx adsorbers

Abstract

Density Functional Theory (DFT) simulations are performed to understand the impact of zeolite framework on the operating cycle of NO adsorption and desorption in Pd-based passive NO_x adsorbers. The binding energy on all sites exhibits the trend: BEA > ZSM-5 > CHA, which is consistent with reported trends in the NO_x desorption temperatures. Independent of the zeolite framework, NO binds strongly on Pd¹⁺ site, whereas its binding on Pd(II) sites (Pd²⁺, [PdOH]⁺, dimeric Pd) is considerably weaker. However, the binding strength on Pd¹⁺ and Pd²⁺ sites is similar in ZSM-5 and BEA. The free energy of activation for the reduction of Pd(II) species by NO is dependent on the zeolite framework. NO oxidation on [Pd-O-Pd]²⁺ has a low free energy barrier in ZSM-5 (29 kJ/mol) but exhibits the highest reaction barrier in CHA (77 kJ/mol). Further, the reoxidation of Pd¹⁺ to [Pd-O-Pd]²⁺ is facile in Ferrierite (FER) but has a high free energy barrier in Chabazite (CHA). A kinetic model is developed for Pd/CHA and Pd/BEA which predicts a single desorption peak for CHA and two desorption peaks for BEA. The site-specific NO binding energy trends and the energetics of interconversion between sites result in differences in the NO_x adsorption-desorption characteristics on different zeolite frameworks. The presence of H₂O decreases the NO binding strength on all Pd sites except Pd²⁺ in FER and [PdOH]⁺ in CHA. CO preferentially reduces the Pd(II) species when both CO and NO are co-adsorbed on [PdOH]⁺[PdOH]⁺ with BEA exhibiting the lowest barrier.