Highly Siliceous Shell-Protected Cu-Impregnated ZSM-5 Zeolite Cores for Robust Hydrocarbon Trapping

Although a Cu-impregnated ZSM-5 (zeolite socony mobil-five; MFI type) zeolite having a finite Si/Al ratio serves as a promising hydrocarbon (HC) trap in the fresh state, its hydrothermal stability should be significantly improved to ensure its long-term use in real vehicles. In this study, the Cu-impregnated ZSM-5 zeolite (i.e., core) and, accordingly, the active Cu species therein were protected by forming a highly siliceous MFI-type zeolite shell. Despite the inevitable modest decrease in the HC removal efficiency owing to the inactive shell part, the original key ability of the core part was well maintained after harsh hydrothermal treatments (HTs) at 800°C. Indeed, an optimal shell structure (obtained after 9-h epitaxial growth), which could cover the core part effectively, resulted in improved long-term HC removal efficiency during the cold-start tests (11.9% vs. nil with the core part alone). The adsorption capacities of two representative HCs (propene and toluene) were effectively correlated with the physicochemical properties (mainly related to the active Cu species and microporous zeolite structure, respectively). In addition, the shell structure effectively preserved HC oxidation ability after HT. Finally, we demonstrated the synergistic roles of the HC adsorption and oxidation abilities in determining the overall HC removal efficiency.