Effects of the Topological Structure of Different Cu-Zeolites on the Relationship between Soot and NO_x Removal on NH₃–SCRF

Abstract

Experimental studies were carried out to explore the effect of zeolite topology on NO x purification and soot oxidation in ammonia selective catalytic reduction coated diesel particulate filter (SCRF) catalysts. The catalytic activity of Cu-exchanged zeolites Cu/SSZ-13 (CHA topology), Cu/ZSM-5 (MFI), and Cu/ BEA(BEA) was evaluated in the NH 3 −SCRF. Experiments with XRD and ICP-SFMS demonstrated that the three types of samples retained their typical structural peaks after Cu modification, and the similar ratios of Cu/Al and Si/Al ensure that the primary cause of different catalytic performances was caused by different framework structures. As determined by laboratory experiments with synthetic gas, Cu/SSZ-13 exhibited an extended activity window and a higher deNO x efficiency at NO 2 /NO x below 0.5. In addition, Cu/ZSM-5 consistently exhibited superior CO 2 selectivity in soot-loading experiments, regardless of the NO 2 concentration. When loaded with soot, Cu/BEA exhibited exceptional deNO x performance at NO 2 /NO x = 0.5. NH 3 was observed to have a negative effect on soot oxidation by NO 2 , while the consumption of NO 2 by soot oxidation contributed to the NO 2 −SCR reaction being aided. Influenced by different zeolite structural features, the degree of diffusion of copper in the zeolite as well as the replacement sites are different, thus forming different ratios of copper species. For instance, the presence of Z2Cu sites, which can adsorb more NH 3 molecules than ZCuOH sites as indicated by the in situ DRIFTS results, and the presence of Cu + with stronger redox properties compared to Cu 2+ , can be influenced by these structural characteristics. Additionally, soot particles have the ability to act as adsorption sites for the NH 3 −SCR reaction.

Advancements in In-Cylinder and After-Treatment Strategies for Mitigating Pollutants from Diesel Engines: A Critical Review

“…From these two studies, it was comprehended that since the molecular size of NO x compounds is proportionate with the size of micropores in zeolites, mass transportation is limited, and this obstacle reduces catalytic performances. In order to overcome this, mesoporous compounds could be used such as in this case, which not only facilitates movement but also provides an enhanced surface for better catalytic performance …”