Unveiling the elusive roles of Cu species in determining the hydrocarbon trap performance during cold start in Cu-impregnated MFI type zeolites

Abstract

High-performance zeolite-based hydrocarbon (HC) traps were developed by impregnating MFI zeolites with Cu species. The corresponding cold-start test (CST) results revealed that the optimal Cu loading (ca. 5-7 wt%) led to high CST performance. To the best of our knowledge, our unprecedented complementary investigation based on both experimental and theoretical approaches revealed that Cu+ ions rather than Cu2+ ions were key for securing high HC adsorption (especially propene) in the presence of copious amounts of water vapor. Moreover, tiny CuO nanoparticles (<= 4 nm) on the exterior zeolite surface resulted in effective HC oxidation. This preferential HC adsorption and oxidation synergistically contributed to effective hydrocarbon trapping, making the Cu-impregnated MFI zeolites active HC traps. However, after hydrothermal treatment, a lower Cu content (3 wt% instead of 5-7 wt%) was rather desirable for preserving the marked CST performance, indicating that the original physicochemical properties were more damaged at higher Cu content.