The effect of mesopore structure to Cu-ZSM-5 catalyst：simulated exhaust and engine bench testing

Zhenguo  Li, Tsinghua university and China automotive technology&research center

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 NOx 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 NOx activity at low temperatures (<450C) 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 (T50%) of NOx on the CuZ-B catalyst at 25,000 h-1 and 40,000 h-1 was about 230 C, and the T50% decreased to 215C 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 NOx reduction activity rather than the surface CuOx 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 Bronsted acidic sites. We speculate that the NH3 storage stability may be the main reason to get the higher NOx 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.

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