The Operating Cycle of NO Adsorption and Desorption in Pd-Chabazite for Passive NO Adsorbers

Pd-doped chabazite (Pd/CHA) offers unique opportunities to adsorb and desorb NO in the target temperature range for application as a passive NO adsorber (PNA). The ability of Pd/CHA to trap NO emissions at low temperatures (<200 °C) is facilitated by the binding of NO species at various Pd sites available in the CHA framework. Density functional theory (DFT) simulations are performed to understand Pd speciation in CHA and the interaction of NO with Pd/CHA to explain the mechanisms of NO adsorption, oxidation, and desorption processes. The calculations are used to elucidate the important role of Pd¹⁺ cationic species, anchored at 6MR-3NN, in providing a strong (E_b = -272 kJ/mol) NO adsorption site in Pd/CHA. For NO release, the redox transformation of Pd species comes into play and Pd¹⁺ species are suggested to transform into cationic Pd²⁺, [PdOH]⁺, or [Pd-O-Pd]²⁺ species, all of which show significantly reduced NO binding (-116, -153, and -117 kJ/mol, respectively) as compared to Pd¹⁺. This enables NO desorption at the operating temperature of a downstream catalyst for subsequent catalytic reduction.