NOx Reduction on Fully Formulated Lean NOx Trap Catalysts Subjected to Simulated Road Aging: Insights from Steady-State Experiments

Fully formulated lean NOx trap (LNT) catalysts of the type Pt/Rh/BaO/Al2O3 were prepared with and without incorporation of CeO2-ZrO2 in the washcoat, and their NOx reduction behavior was evaluated in steady-state, continuous flow experiments. In the fresh state, CeO2-ZrO2 addition was found to exert little effect on NOx reduction activity using H2, CO, and NH3 as the reductants. However, after simulated road aging, NOx reduction activity was significantly impaired for the CeO2-ZrO2-free catalyst, whereas the performance of the CeO2-ZrO2-containing analog was affected to only a minor degree. These differences are explained on the basis of high-resolution transmission electron microscopy measurements showing that Pt supported on CeO2-ZrO2 remained highly dispersed after aging, whereas Pt supported on BaO/Al2O3 underwent significant sintering. In addition, the Pt/CeO2-ZrO2 component did not accumulate sulfur during aging, unlike Pt/BaO/Al2O3 for which significant sulfation of the Ba phase occurred. For both catalysts, selectivity to NH3 in NO and NO2 reduction by H2 increased after catalyst aging, indicative of a change in the relative surface coverages of N and H ad-atoms on the precious metal sites.

The addition of CeO2-ZrO2 to a Pt/Rh/BaO/Al2O3 lean NOx trap catalyst affords significant benefits with respect to steady-state NOx reduction activity after catalyst aging. These benefits are explained on the basis of HRTEM analyses.Figure optionsDownload as PowerPoint slide