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

Passive NO_x adsorber (PNA) captures NO_x during cold-start phases and then releases it at operating conditions of the SCR catalyst, allowing NO_x elimination of diesel vehicle exhaust efficiently. The Pd/CeO₂ catalyst exhibits excellent PNA performance in the cold-start stage of diesel vehicles. An insightful understanding of the mechanism by which different Pd species mediate NO_x adsorption and desorption is crucial but still unclear. This study systematically studied the PNA performance of Pd/CeO₂ catalysts having distinct Pd loadings. The 3.0%Pd/CeO₂, on which the aggregated Pd species (Pd_n) and highly dispersed single atoms (Pd₁) coexist on the CeO₂ surface, exhibited the highest low-temperature NO_x adsorption capacity. The structure-performance relationship governing NO_x storage and release behaviors on Pd sites with distinct dispersion states was thoroughly studied. It revealed that the synergy of Pd_n and Pd₁ sites contributed to optimal PNA performance of 3.0%Pd/CeO₂. At low temperature, Pd_n species supplied adequate sites for NO_x adsorption, and Pd₁ site in Pd_xCe_1-xO_2-σ solid solutions mainly promoted interfacial active oxygen to activate NO_x to monodentate nitrite via the MvK mechanism. At high temperature, part of the unstable nitrites readily decomposes into NO_x on Pd_n sites, and the remaining part is further oxidized to nitrate species by Pd₁ promoted active oxygen and then decomposed. Both the NO_x storage and release process were facilitated through the synergy between Pd_n and Pd₁ sites of Pd/CeO₂. These findings provide a theoretical guideline for the controlled design of novel Pd/CeO₂ based PNA materials via interfacial structural regulation.