(570d) Mechanistic Aspects of Passive NOx Adsorption on Pd-SSZ-13: Microkinetic Modeling to Explore the Role of Monomeric and Dimeric Sites

Lean-burn designs of higher fuel-efficient diesel engines have resulted in exhaust emissions at considerably lower temperatures. However, leading emission control technologies available to abate NO_x,CO and hydrocarbon (HC) emission levels function at temperatures typically above 200°C. Pd-exchanged SSZ-13 has shown great promise in trapping NO_x at low temperatures and releasing them above 200°C. With the help of density functional theory (DFT) simulations combined with microkinetic modeling of simulated temperature programmed desorption (TPD) experiments, we have investigated the nature of relevant active sites.

Under low SAR, high Pd loading and dry conditions, Z₂Pd^II are the majority sites whereas dimeric hydroxylated Z₂(Pd^IIOH)₂ are minorities. Though the monovalent ZPd^I sites bind NO the strongest, they are not as stable as the hydroxylated Pd^II sites or Z₂Pd^II sites, suggesting that ZPd^I may dynamically form in response to temperature or feed changes. Using DFT simulations, we calculated the energetics of several competing mechanisms for the reduction of Pd^II to monovalent Pd^I, involving monomeric Pd^II in Z₂Pd^II getting reduced to Pd^I and dimeric hydroxylated Pd^II to dimeric monovalent Pd^I involving the reductants NO and CO, hydroxylated NOOH and COOH species, and the bridging site Z₂(Pd^II-O-Pd^II). We simulated a TPD experiment with a comprehensive microkinetic model including possible reoxidation pathways of dimeric monovalent Pd^I to Pd^II under dry conditions. This enabled us to identify dominant species and mechanisms within different temperature regimes (Figure 1). Our results indicate that monovalent Pd^I sites exist primarily as the dimeric site due to the high NO binding energy on Pd^I despite the highly oxidative environment under PNA conditions, whereas the monomeric site is mostly Pd^II. Our detailed mechanistic insight into the elementary steps describing the dynamic changes of active sites during NO trapping and NO₂ production suggests that the relative stability of reduced, oxidized and hydrated/hydroxylated cationic sites is paramount.