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

Rahman, B. M. - Presenter, University of Houston
Grabow, L., University of Houston
Ambast, M., University of Houston
Harold, M., University of Houston
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 NOx,CO and hydrocarbon (HC) emission levels function at temperatures typically above 200°C. Pd-exchanged SSZ-13 has shown great promise in trapping NOx 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, Z2PdII are the majority sites whereas dimeric hydroxylated Z2(PdIIOH)2 are minorities. Though the monovalent ZPdI sites bind NO the strongest, they are not as stable as the hydroxylated PdII sites or Z2PdII sites, suggesting that ZPdI 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 PdII to monovalent PdI, involving monomeric PdII in Z2PdII getting reduced to PdI and dimeric hydroxylated PdII to dimeric monovalent PdI involving the reductants NO and CO, hydroxylated NOOH and COOH species, and the bridging site Z2(PdII-O-PdII). We simulated a TPD experiment with a comprehensive microkinetic model including possible reoxidation pathways of dimeric monovalent PdI to PdII under dry conditions. This enabled us to identify dominant species and mechanisms within different temperature regimes (Figure 1). Our results indicate that monovalent PdI sites exist primarily as the dimeric site due to the high NO binding energy on PdI despite the highly oxidative environment under PNA conditions, whereas the monomeric site is mostly PdII. 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.