Synergy between varied Pd sites on CeO₂ for enhanced low-temperature passive NO_x adsorption

Area of Science:

• Catalysis
• Materials Science
• Environmental Engineering

Background:

• Diesel vehicles emit NOx, necessitating efficient aftertreatment systems.
• Passive NOx adsorbers (PNA) capture NOx during cold starts for later SCR catalyst processing.
• The precise mechanism of NOx adsorption/desorption on Pd/CeO2 catalysts, particularly the role of different Pd species, remains unclear.

Purpose of the Study:

• To systematically investigate the passive NOx adsorber (PNA) performance of Pd/CeO2 catalysts with varying Pd loadings.
• To elucidate the structure-performance relationship governing NOx storage and release on Pd sites with distinct dispersion states.
• To understand the synergistic mechanism between different palladium species (aggregated and single-atom) in Pd/CeO2 catalysts for NOx elimination.

Main Methods:

• Synthesis and characterization of Pd/CeO2 catalysts with distinct Pd loadings.
• Evaluation of PNA performance, focusing on low-temperature NOx adsorption capacity.
• Detailed analysis of the structure-performance relationship, correlating Pd species dispersion with NOx storage and release mechanisms.

Main Results:

• The 3.0%Pd/CeO2 catalyst, featuring coexisting aggregated Pd (Pd_n) and single-atom Pd (Pd₁) species, demonstrated the highest low-temperature NOx adsorption capacity.
• A synergistic effect between Pd_n and Pd₁ sites was identified as crucial for optimal PNA performance.
• Pd_n sites provided adsorption sites, while Pd₁ sites in Pd_xCe_1-xO_2-σ solid solutions activated NOx via the MvK mechanism and facilitated subsequent oxidation and decomposition of adsorbed species.

Conclusions:

• The synergy between aggregated and single-atom palladium species on Pd/CeO2 is key to efficient NOx storage and release.
• Understanding and controlling the interfacial structure of Pd/CeO2 catalysts can lead to improved PNA materials.
• These findings offer a theoretical basis for designing advanced Pd/CeO2-based PNA materials for diesel exhaust aftertreatment.