Robust, high-performance Cu-impregnated ZSM-5 zeolites for hydrocarbon removal during the cold-start period: Quantitative elucidation of effects of H⁺ and Na⁺ ions on performance and stability

Cu-containing zeolites are suitable for active hydrocarbon (HC) traps. Crucially, physicochemical properties of the zeolite could be finely tuned by varying the ion contents, though such approach has not been used intensively for HC trapping. Therefore, in this study, we adjusted the ratio of Na⁺ to H⁺ ions in ZSM-5 zeolites from 1 to 0 and, subsequently, wet-impregnated these zeolites with Cu. A low Na/Al ratio (≤0.4) resulted in marked cold-start test (CST) performance for HC removal in the fresh state (HC removal efficiency of 60.2 %-69.5 %). However, a hydrothermally treated counterpart having an optimal Na⁺ content (Na/Al ratio of 0.4) could well preserve the original active Cu⁺ ions along with zeolite structure and, accordingly, show the best CST performance after hydrothermal treatment (HT) at 800 °C (HC removal efficiency of 24.4 % vs. 10.8 % for the conventional H⁺-form-based one). Furthermore, we investigated adsorption behavior of water and HCs at a deep level and, further, revealed a clear correlation between the HC adsorption ability and CST performance and the representative physicochemical properties (mainly related to Cu⁺ ions) at all Na⁺ contents. Additionally, tiny CuO particles on the outer surface of the Cu-impregnated ZSM-5 zeolites having Na/Al ratios of 0–0.4 showed good low-temperature HC oxidation ability (ca. 175–182 °C). Although this HC oxidation ability was reduced after HT, an optimal Na/Al of 0.4 led to preservation of the original ability (ca. 283 °C vs. 320 °C for pure large CuO particles). Finally, we demonstrated a synergistic relationship between Na⁺ and H⁺ ion contents (Na/Al ratio of 0.4) that favored the formation of the desired Cu species, thus achieving marked HC adsorption and oxidation after HT.