(509bo) The Operating Cycle of NO Adsorption and Desorption in Pd-Chabazite for Passive NOx Adsorbers

Pd/CHA is touted to be the most suitable catalyst for passive NO_x adsorbers (PNAs) because of its high low-temperature NO_x adsorption and its release in the target temperature range. The ability of Pd/CHA in trapping NO_x emissions at low temperatures (< 373 K) is facilitated by the binding of NO_x species on a variety of Pd sites available in the CHA framework. To explain the mechanisms of NO adsorption, oxidation and desorption, density functional theory (DFT) simulations are undertaken to understand Pd speciation in CHA and the interaction of NO with different Pd sites (Pd⁺², Pd⁺¹, [PdOH]⁺ and [Pd-O-Pd]⁺²) in Pd/CHA. The calculations are used to elucidate the main role of Pd⁺¹ cationic species, anchored at 6MR-3NN, in providing a strong (E_ads = -272 kJ/mol) NO adsorption site. The redox transformation of [PdOH]⁺ and [Pd-O-Pd]⁺² sites to Pd⁺¹ site upon NO interaction is accompanied by NO₂ formation. For NO release, Pd⁺¹ species are suggested to undergo reoxidation into cationic Pd⁺², [Pd(OH)]⁺¹ or [Pd-O-Pd]⁺² species, all of which show significantly reduced NO binding (-116, -153 and -117 kJ/mol respectively) as compared to Pd⁺¹, thus enabling NO desorption at an operational temperature of downstream reduction catalyst for the subsequent reduction. For a functioning PNA material, it is highly desirable that NO should bind strongly on the adsorption sites to trap maximum NO at low temperatures and further desorb within a desired temperature range for reduction. As suggested by our theoretical calculations, Pd⁺¹ is the optimum binding site for NO adsorption in Pd/CHA whereas cationic Pd⁺², [Pd(OH)]⁺¹ or [Pd-O-Pd]⁺² species are suitable sites for NO desorption which is analogous to the requirement of a heterogeneous catalyst material, where Sabatier principle dictates that the catalyst surface should neither bind too strong nor too week, so as to complete the catalytic cycle giving maximum turnovers.