(440d) 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.