A Comparison of Two Types of ZSM-5 on the Activity of Cu/ZSM-5 Catalysts in Selective Catalytic Reduction of NOx with NH3: Simulated Exhaust and Engine Bench Testing Study

Two types of catalysts (CuZ-A and CuZ-B) containing 3 wt.% Cu on commercial ZSM-5 zeolites but different mesopore structures were prepared by the incipient-wetness impregnation method. The activity of these catalysts for SCR of NO_x by NH₃ (NH₃-SCR) has been investigated in the powder form and in the honeycomb monolith form by engine bench tests with simulated exhaust. Compared to the CuZ-A catalyst, the CuZ-B catalyst exhibited much better NO_x activity at low temperatures (＜450℃) in the simulated exhaust, especially in the presence of water vapor. The activity of the CuZ-B catalyst was obviously higher than that of the CuZ-A catalyst regardless of space velocity and NH₃/NO ratio. The engine bench testing results showed that the light-off temperature（T_50%）of NO_x on the CuZ-B catalyst at 25,000 h^-1 and 40,000 h^-1 was about 230 ℃, and the T_50% decreased to 215 °C at 15,000 h^-1. The BET and XRD results showed that the lower crystallinity (84.1%) of ZSM-5 and a special mesopore structure for the CuZ-B catalyst lead to a higher BET surface area (608 m²×g^-1), and thereby provide more activity sites. TPR, XPS, and TPD results revealed that the Cu²⁺ ions located at the ion-exchanged sites and [Cu-O-Cu]⁺ were the main sites for NO_x reduction activity rather than the surface CuO_x species. The higher activity of CuZ-B might be attributed not only to the improved redox property of the Cu species, but also to the enhanced strong Brönsted acidic sites. We speculate that the NH₃ storage stability may be the main reason to get the higher NO_x reduction activity in engine bench testing for the CuZ-B catalyst, thanks to its high BET area, mesopore structure, and strong acidity. This catalyst looks promising from an application perspective.