Coaxial 3D Printing Enabling Mn/CHA@Pd/CHA Zeolite-Based Core–Shell Monoliths for Efficient Passive NO_x Adsorbers

Pd-based zeolites are extensively used as passive NO_x adsorbers (PNA) for cold-start NO_x emissions to meet stringent emission regulations. However, optimizing adsorber design to reduce Pd usage with substitution by non-noble metals that are prone to suffer from H₂O remains a significant challenge. Herein, the core–shell Mn/CHA@Pd/CHA zeolite monoliths based on non-noble metal/zeolite core are constructed using coaxial 3D printing technology and identified as efficient passive NO_x adsorbers for the first time. In the Mn/CHA@Pd/CHA monolith, the Pd/CHA shell effectively protected the Mn active sites in the core from H₂O, while the integration of the Mn/CHA core not only introduced efficient storage sites but also facilitated NO_x desorption, thereby achieving comparable adsorption properties and increased the NO_x desorption efficiency by 35% at 350 °C compared with that of Pd/CHA monolith. Furthermore, some non-noble metal-based zeolites (e.g., Co/CHA, Mn/MFI, Mn/BEA) and Pd-based zeolites (e.g., Pd/AEI) are also employed as cores and shells respectively to fabricate a series of core–shell zeolite monoliths via coaxial 3D printing, highlighting the benefits of incorporating non-noble metals into Pd-based zeolites for improving adsorption and desorption behaviors. This work provides a promising strategy for designing cost-effective PNA materials and contributes to improving the exhaust after-treatment technology.