Commercial LNT Formulation: A View from Inside

Owen  Bailey, Umicore Autocat USA Inc.

LNT formulations which have been commercialized for emission control on lean burn gasoline applications exhibit a diversity in washcoat composition, complexity, and architecture — a diversity which parallels that of advanced Pt/Rh TWC formulations. A combination of macroscopic and microscopic analytical techniques were used to characterize two specific LNT formulations and their corresponding catalyst designs.

In order to maximize NOx storage capacity, both LNTs featured extremely high washcoat loadings. In one case, a hexagonal cell structure was adopted to minimize washcoat thickness relative to a corresponding square cell geometry, and presumably maximize effective utilization of all NOx storage sites. This strategy has been reported to enhance sulfur release kinetics.

Both formulations also incorporated TWC-like materials based on cerium and zirconium which introduce oxygen storage capacity to the washcoat. While these thermally stabilized solid solutions enhance NOx reduction, they also increase the fuel economy penalty associated with regeneration. Both ceria-rich and zirconia-rich compositions can be detected by elemental mapping. The “acidity” of zirconia-rich oxides is reported to minimize sulfur storage and facilitate LNT desulfation.

One formulation consisted of a sandwich-type layered architecture while the other was visually and chemically homogeneous. In each case, individual oxide particles were similar in size. Ba was observed throughout each structure, however, in the layered structure an alkali component was localized as the meat in the sandwich. It was postulated that this design might have been adopted to minimize alkali migration either into the cordierite substrate, or outpouring from the catalyst.

Although highly dispersed Pt was observed throughout both formulations, and within each layer, rhodium was not detected except by bulk analysis. Based on other literature reports, it was conjectured that a zirconia-rich overcoat might serve as an excellent support for Rh and facilitate NOx reduction during rich regeneration. Rh in this environment has also been reported to facilitate steam reforming of hydrocarbons.

Ultimately, participants involved in the modeling of LNTs were challenged to validate performance enhancements by these constructs, as well as assess transformations of the materials following aging processes.