A Density Functional Theory Modeling on the Framework Stability of Al-Rich Cu-SSZ-13 Zeolite Modified by Metal Ions

Recent progress in performance optimization of Cu-SSZ-13 catalyst for selective catalytic reduction of NO x

Nitrogen oxides (NO x ), which are the major gaseous pollutants emitted by mobile sources, especially diesel engines, contribute to many environmental issues and harm human health. Selective catalytic reduction of NO x with NH3 (NH3-SCR) is proved to be one of the most efficient techniques for reducing NO x emission. Recently, Cu-SSZ-13 catalyst has been recognized as a promising candidate for NH3-SCR catalyst for reducing diesel engine NO x emissions due to its wide active temperature window and excellent hydrothermal stability. Despite being commercialized as an advanced selective catalytic reduction catalyst, Cu-SSZ-13 catalyst still confronts the challenges of low-temperature activity and hydrothermal aging to meet the increasing demands on catalytic performance and lifetime. Therefore, numerous studies have been dedicated to the improvement of NH3-SCR performance for Cu-SSZ-13 catalyst. In this review, the recent progress in NH3-SCR performance optimization of Cu-SSZ-13 catalysts is summarized following three aspects: 1) modifying the Cu active sites; 2) introducing the heteroatoms or metal oxides; 3) regulating the morphology. Meanwhile, future perspectives and opportunities of Cu-SSZ-13 catalysts in reducing diesel engine NO x emissions are discussed.

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

Pd-doped chabazite (Pd/CHA) offers unique opportunities to adsorb and desorb NO_x in the target temperature range for application as a passive NO_x adsorber (PNA). The ability of Pd/CHA to trap NO_x emissions at low temperatures (<200 °C) is facilitated by the binding of NO_x 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.