A Cu-impregnated ZSM-5 zeolite for active cold start hydrocarbon removal: Cation-type-dependent Cu species and their synergetic HC adsorption/oxidation functions

Abstract

In this study, we designed an active hydrocarbon (HC) trap with additional low-temperature HC oxidation ability. For this, we employed a wet impregnation method to add Cu+ ions (for HC adsorption) inside a zeolite socony mobil-5 (ZSM-5) zeolite and CuO particles (for HC oxidation) on the outer surface. To elucidate the effect of the zeolite support on the activity, two types of zeolite were used as supports: Na-form and H-form ZSM-5 s. Interestingly, in Na-form ZSM-5 s, the preferred formation of CuO particles occurred on the outer surface, whereas in H-form ZSM-5 s, Cu+ ions were placed inside the zeolite and tiny CuO particles formed on the outer surface. Accordingly, the Cu-impregnated H-form ZSM-5 showed much improved cold start test (CST) performance with respect to representative HCs (propene and toluene). In addition, hydrothermal treatments under various conditions were conducted to simulate different degrees of long-term driving. Furthermore, we found a clear correlation between the CST performances of the Cu-impregnated ZSM-5 zeolites and the physicochemical properties. To the best of our knowledge, we, for the first time, show that the chemical properties of Cu+ ions and Lewis acid sites, corrected by appropriate physical properties (in this case, the micropore volume), can account for the HC removal abilities of fresh and hydrothermally treated Cu-impregnated ZSM-5 zeolites. Furthermore, we demonstrated the effective oxidation ability of the tiny CuO particles (as small as ca. 3.2 nm) on the outer surface of Cu-impregnated H-form ZSM-5 s at low temperatures (ca. 210 °C), thus yielding an active HC trap. Finally, the preferred adsorption and low-temperature oxidation worked synergistically for HC removals.