Effects of High Temperature Cyclic Aging on a Fully-Formulated Lean NO_x Trap Catalyst

The thermal deactivation mechanisms of a lean NO_x trap (LNT) are analyzed in this study. Accelerated lean/rich aging experiments are performed with a bench-flow reactor, and high temperature exotherms are achieved with pulses of reductants. Reduction in steady state NO_x conversion as a result of aging is evaluated at a range of temperatures using the same bench-flow reactor. The results of these evaluations are used to show that NO_x conversion in this study is a function of evaluating temperature, aging temperature, and number of aging cycles.

Surface characterization studies are used to determine changes in washcoat morphology. Electron probe microanalysis line scans and elemental maps show that migration or agglomeration of washcoat components (excluding PGM) does not occur. Substantial PGM growth as a result of aging is seen using scanning transmission electron microscopy/scanning electron microscopy/energy dispersive spectroscopy (STEM/SEM/EDS) and powder X-ray diffraction (P-XRD), but this growth, at least initially, has little effect on NO_x conversion. P-XRD is also used to study phase transitions of Ba and Al₂O₃. All BaCO₃ peaks quickly disappear after aging, and there is no subsequent growth of any BaAl₂O₄ peaks. The disappearance of crystalline BaCO₃ does not appear to have a large effect on NO_x storage or conversion. The Al₂O₃ peaks are stable, but large surface area reductions are measured with Brunauer, Emmett, and Teller (BET) surface area measurements. Both reductions in surface area and PGM particle size growth are shown to correlate well with reductions in NOx conversion.