Effects of Rapid High Temperature Cyclic Aging on a Fully-Formulated Lean NO _x Trap Catalyst 2009-01-0634

In this study, high-temperature deactivation of a fully-formulated lean NO_x trap (LNT) is investigated with an accelerated aging protocol where accelerated aging is accomplished by rapid temperature cycling and by higher temperatures. Thermal aging is carried out in a bench-flow reactor at nominal temperatures of 700, 800, 900, and 1000°C using an aging cycle consisting of a 130s lean-phase and a 50s rich-phase. After a prescribed number of lean/rich aging cycles, the NO_x conversion of the aged LNT is evaluated at 200, 300, and 400°C. The NO_x performance is obtained at a GHSV of 30,000 h⁻¹ using an evaluation cycle consisting of a 60s lean-phase and 5s rich-phase. The effects of aging on the LNT washcoat are determined with EPMA, XRD, STEM/EDS, and BET. Aging at 700 and 800°C has a minimal effect on LNT performance and material properties. However, at aging temperatures of 900 and 1000°C reduction in surface area and sintering of PGM particles are observed and result in a drastic reduction in NO_x conversion. Additionally, after aging at 900°C and 1000°C the NO_x storage medium, BaCO₃, is no longer visible in the XRD patterns, even though a Ba-phase identified by EPMA still exists in all aged samples. BaAl₂O₄ is not identified at any aging temperatures; possibly due to stabilization effects provided by washcoat additives present in this particular LNT.