NO_x reduction on fully formulated lean NO_x trap catalysts subjected to simulated road aging: Insights from steady-state experiments

Fully formulated lean NO_x trap (LNT) catalysts of the type Pt/Rh/BaO/Al₂O₃ were prepared with and without incorporation of CeO₂-ZrO₂ in the washcoat, and their NO_x reduction behavior was evaluated in steady-state, continuous flow experiments. In the fresh state, the CeO₂-ZrO₂ addition was found to exert little effect on NO_x reduction activity using H₂, CO, and NH₃ as the reductants. However, after simulated road aging, NO_x 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 the catalyst aging, indicative of a change in the relative surface coverages of N and H ad-atoms on the precious metal sites.