The Schwank Group

-Characterization of lean NOx reduction catalysts-

Xiaoyin Chen and Johannes Schwank

Dispersed and Bulk-like NOx Storage Components

Quantification of dispersed and bulk-like NOx
storage components (BaO) on an alumina support.

Lean-Rich Cycling of NOx Storage

Exit gas concentration of lean-rich cyclic reaction for
NOx reduced by CO over Pt-BaO/Al2O3 model catalyst

Lean burn engines, such as diesel and lean burn gasoline engines, offer significant fuel economy benefits and reduced CO2 emission. A major challenge with these types of engines is the difficulty of dealing with NOx emissions. Under lean conditions, the currently used three-way emissions catalyst (TWC) is not effective in eliminating NOx. Lean NOx trap (LNT) catalysts provide one of the promising options to reduce lean NOx for lean burn gasoline engines, with the engine working under so-called lean-rich repeatable operation cycles. Typical cycles consist of a longer lean period where NO is oxidized to NO2 and then NO2 is captured by BaO forming barium nitrate, followed by a shorter rich period where the trapped NOx is released and then reduced by Pt.

Therefore, effective NOx storage is critical to practical application of LNTs because the storage capacity determines the fraction of time an engine can be operated in the more fuel-efficient lean mode. Collaborating with Ford, we found that thermal gravimetric analysis (TGA) coupled with FT-IR can be effectively used to quantify highly dispersed and bulk-like BaO supported on Al2O3. The results are useful for optimizing BaO loading strategies for LNT catalysts.

Steam generated during fuel combustion is one of the major components in automotive emissions. While steam effects have been widely investigated for TWC catalysts, less is known about how steam effects NOx reduction over LNT catalysts. Therefore, we are studying this steam effect under lean-rich cyclic operations in collaboration with General Motors. Of special interest is the generation of NH3 over Pt-BaO/Al2O3 under steam during the rich cycle. A method has been established using a dual-bed reactor system containing a zeolite catalyst down-stream of the LNT bed. The zeolite traps NH3 generated during the rich cycle. It also serves as selective catalytic reduction (SCR) catalyst to eliminate lean NOx by using desorbed NH3 as reductant when the rich cycle is switched to the lean cycle.

Go Back