Engineering of Core-Shell Pd/SSZ-13@Al₂O₃ Zeolite: Unlocking Superior NO_x Adsorption and Chemical Durability

Pd-zeolites are promising passive NO_x adsorber (PNA) materials for mitigating cold-start emissions from lean-burn engines. However, their practical deployment is constrained by insufficient densities and dispersion of isolated Pd²⁺ active sites as well as their susceptibility to hydrothermal degradation and phosphorus poisoning encountered in vehicle exhaust environments. Herein, we develop a rationally engineered core-shell Pd/SSZ-13@Al₂O₃ composite, featuring a Pd/SSZ-13 core encapsulated within a mesoporous Al₂O₃ shell. This hierarchical architecture facilitates the controlled migration and dispersion of Pd²⁺ species, significantly enriching and stabilizing isolated Pd active sites within the zeolite core. Comprehensive characterization and density functional theory calculations confirm that the Al₂O₃ shell serves as a robust barrier, forming stable aluminum phosphate species that prevent phosphorus infiltration and safeguard both the zeolite framework integrity and Pd²⁺ active sites from environmental degradation. Catalytic evaluations revealed that Pd/SSZ-13@Al₂O₃ exhibited superior NO_x adsorption capacity, favorable NO_x desorption behavior, and exceptional stability under hydrothermal and phosphorus poisoning conditions, outperforming conventional Pd-zeolite catalysts. This work establishes a generalizable core-shell design strategy for stabilizing atomically dispersed active sites in harsh environments, offering broad implications for the development of durable catalytic materials in air pollution control and environmental remediation.