Investigate the Impacts of Aging Temperature and Aging Gas On LNTs

LNTs have been widely investigated and developed for NO_x emission control, for both lean-burn gasoline and diesel vehicles. Thermal deactivation has been a major challenge in LNT development owing to the need to maintain close proximity of the precious metals to the trapping materials.  Aging and deactivation effects are compounded by sulfur poisoning, which aside from direct poisoning, results in accelerated thermal deactivation due to the high temperature conditions required to desulfate the trap.   

Temperature and exhaust gas composition are the major factors influencing precious metal sintering and overall NO_x trap performance.  Metal-support interactions are also important, as demonstrated by work reported by Toyota and others.  It has been found that high temperature exposure reduces precious metal dispersion; also the LNTs deactivate differently in lean, stoichiometric, and rich gas conditions even at the same aging temperatures. However, details regarding the combined effects of aging temperature and gas conditions on LNT deactivation are not clear. Significant technical challenges remain for LNT research on developing durable catalysts and understanding their deactivation mechanisms.

This paper systematically investigates the impact of aging temperatures and aging gas conditions on LNTs. Rather than using hydrothermal aging as employed widely in most previous LNT thermal durability studies, the aging conditions representing LNT deSO_x as well as DPF regeneration are applied in this paper. The testing temperatures, gas composition, and aging conditions were chosen based on the best estimation of real-world conditions. The aged LNTs were also characterized by TGA, TPR/TPO, H₂/CO chemisorption, XRD, and XPS to develop further understandings.

The results showed that the LNTs were aged in very different ways in lean and lean/rich cycling gases. Compared to aging temperatures, the aging gas conditions had more significant impacts on the LNTs during the aging. And, lean/rich cycling aging (even at temperatures characteristic of normal NO_x storage and regeneration conditions) caused greater deactivation of NO_x, HC, and CO activities than aging in lean or rich environments alone. The profiles of NH₃ and N₂O formation were very different between the LNTs aged in lean and in lean/rich cycling gases. The characterization results also showed differences between the LNTs aged in lean and in lean/rich cycling gases, including the state of the NO_x storage components, OSC, and accessible PGM surface area. The results indicate that there are several deactivation mechanisms for LNTs that respond differently to aging conditions. The study provides further insight into LNT deactivation mechanisms, thereby isolating design aspects that must be improved to develop more durable LNTs.