Improved NOx Storage Performance from Ethylene with Zeolite-Based Low Temperature NOx Adsorbers: Evidence of a Chemical Interaction
Joseph Theis, Ford Motor Company
The NOx storage performance of zeolite-based Low Temperature NOx Adsorbers (LTNA) is significantly improved in the presence of exhaust reductants such as CO or C2H4. Other researchers have suggested that NO and the reductant co-adsorb on the Pd storage sites. Today we present evidence that NO and C2H4 interact chemically on the catalyst. The monolithic samples used in this study were prepared in-house and had washcoats consisting of 95% (by weight) of chabazite (CHA), beta (BEA), MFI (ZSM5), or ferrierite (FER) along with 5% Al2O3 to improve the washcoat adhesion. The samples were catalyzed with 96 gpcf Pd and 24 gpcf Pt. Rapid temperature ramp tests from 90oC to 470oC were performed under lean conditions with and without C2H4, with and without H2O, and with and without NO. Relative to the NOx storage performance on tests with NO and H2O but without C2H4, the NOx storage efficiency and the maximum NOx storage capacity of CHA, BEA, and ZSM5 were increased on tests with C2H4, and significant acetaldehyde (CH3CHO) was formed during the NOx storage period. Much less CH3CHO was formed on tests with C2H4 and H2O but without NO, and no CH3CHO was formed on tests without H2O. Thus, the generation of CH3CHO provides an indication that the NO and C2H4 do not merely co-adsorb but interact chemically on the catalyst in the presence of H2O, and this interaction results in the improved NOx storage performance. Good agreement was obtained between the additional NOx stored in the presence of C2H4 and the amount of CH3CHO formed. For FER, there was little benefit from the presence of C2H4, and no CH3CHO was produced.