An optimal combination of Na⁺ and H⁺ co-ions for high-performance Cu-impregnated *BEA zeolites: Unravelling the trade-off between the cold-start hydrocarbon trap activity and hydrothermal stability

A Cu-impregnated *BEA (denoted as BEA) type zeolite can serve as an effective hydrocarbon (HC) trap, but its long-term hydrothermal stability should be improved for real uses. However, designing robust high-performance HC traps, which is closely related to the confined environment that accommodates active Cu species and strengthens the zeolite structure, remains elusive. In this study, to the best of our knowledge, we, for the first time, systematically varied the Na content in the BEA zeolite prior to Cu impregnation. Intriguingly, we found a trade-off between the cold-start test (CST) activity (a simplified trap test for simulated HC emissions) and hydrothermal stability of the Cu-impregnated BEA zeolites. Specifically, increasing Na content decreased the CST performance monotonically, whereas it increased the hydrothermal stability. The decreased CST performance could be attributed to the less formation of the active Cu species (both Cu cations inside for preferred HC adsorption and CuO particles outside for facile HC oxidation). In addition, the improved hydrothermal stability benefited from dual features: Na⁺ ions could alleviate the zeolite structural collapse/damage during hydrothermal treatment (HT) at 800 °C by reducing the amount of most easily hydrolyzed Si-O(H)-Al bonds and avoid the formation of large CuO particles, which are known to destroy the zeolite structure during HT. As a net, we revealed that the Cu-impregnated BEA zeolite with an optimal Na/Al ratio of ca. 0.7 was key to achieving marked CST efficiencies in the HT state (22 %) in contrast to the H-form-based counterpart that exhibited extremely poor performance (i.e., nil).