Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

摘要

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N~(2)conversion, minimal ammonia slip and null N~(2)O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al~(2)O~(3)and Pt–Ce–Ba/Al~(2)O~(3)were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N~(2)adsorption–desorption isotherms at 77?K, X-ray diffraction, NH~(3)temperature-programmed desorption, and H~(2)chemisorption to relate textural and chemical characteristics with the DeNO~(x)performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH~(3)–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al~(2)O~(3)is beneficial for the NO-to-NOx oxidation and NO~(2)storage, improving NO conversion at low temperature and reducing the NH~(3)slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH~(3)slip with superior N~(2)selectivity for an extended temperature windows, including as low as 220?°C, and maintaining performance even after severe hydrothermal aging.