Synergy between varied Pd sites on CeO for enhanced low-temperature passive NO adsorption.

Passive NO adsorber (PNA) captures NO during cold-start phases and then releases it at operating conditions of the SCR catalyst, allowing NO elimination of diesel vehicle exhaust efficiently. The Pd/CeO 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 adsorption and desorption is crucial but still unclear. This study systematically studied the PNA performance of Pd/CeO catalysts having distinct Pd loadings. The 3.0%Pd/CeO, on which the aggregated Pd species (Pd) and highly dispersed single atoms (Pd) coexist on the CeO surface, exhibited the highest low-temperature NO adsorption capacity. The structure-performance relationship governing NO storage and release behaviors on Pd sites with distinct dispersion states was thoroughly studied. It revealed that the synergy of Pd and Pd sites contributed to optimal PNA performance of 3.0%Pd/CeO. At low temperature, Pd species supplied adequate sites for NO adsorption, and Pd site in PdCeO solid solutions mainly promoted interfacial active oxygen to activate NO to monodentate nitrite via the MvK mechanism. At high temperature, part of the unstable nitrites readily decomposes into NO on Pd sites, and the remaining part is further oxidized to nitrate species by Pd promoted active oxygen and then decomposed. Both the NO storage and release process were facilitated through the synergy between Pd and Pd sites of Pd/CeO. These findings provide a theoretical guideline for the controlled design of novel Pd/CeO based PNA materials via interfacial structural regulation.