The thermal ageing and reactivation of Ba/CeO 2 and Ba/Al 2 O 3 based NO x -storage/ reduction (NSR) catalysts was studied on model catalysts and catalyst systems at the engine. The mixed oxides BaAl 2 O 4 and BaCeO 3 , which lower the storage activity, are formed during ageing above 850 °C and 900 °C, respectively. Interestingly, the decomposition of BaCeO 3 in an atmosphere containing H 2 O/NO 2 leads again to NO x -storage active species, as evidenced by comparison of fresh, aged and reactivated Pt-Ba/CeO 2 based model catalysts. This can be technically exploited, particularly for the Ba/CeO 2 catalysts, as reactivation studies on thermally aged Ba/CeO 2 and Ba/Al 2 O 3 based NSR catalysts on an engine bench showed. An on-board reactivation procedure is presented, that improved the performance of a thermally aged catalyst significantly.

Differences in the NOx storage-reduction (NSR) behavior of Pt/Ba/CeO2 and Pt/Ba/Al2O3 have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO2 exhibits inferior NOx storage and, particularly, reduction (regeneration) activity compared to the Al2O3 supported catalyst. The incomplete reduction of the stored NOx-species in Pt/Ba/CeO2 seems to be caused by a faster and more profound reoxidation of Pt particles during the lean period as evidenced by in situ X-ray absorption spectroscopy. Interestingly, the reduction activity could be significantly improved by a pre-reduction step at mild conditions. Exposure of the Pt/Ba/CeO2 catalyst to reducing H2 atmosphere in the temperature range 300–500°C lead to a moderate increase of Pt particle size which beneficially influenced the regeneration activity. In contrast, pre-reduction at temperatures above 500°C was unfavorable and resulted in a severe decrease of the regeneration activity, probably due to migration of the partially reduced CeO2 onto the surface of Pt particles.

The formation and stability of BaAl2O4 and BaCeO3 in Pt-Ba/Al2O3 and Pt-Ba/CeO2 based NOx storage-reduction (NSR) catalysts has been investigated using kinetic measurements, X-ray diffraction, thermal analysis and X-ray absorption spectroscopy. In as-prepared state, the Ba-component in the NSR catalysts was made up of amorphous BaO and BaCO3. The formation of BaAl2O4 started above 850°C, whereas the formation of BaCeO3 was already observed at 800°C and was faster than that of BaAl2O4. The stability of BaAl2O4 and BaCeO3 in various liquid and gaseous atmospheres was different. BaAl2O4 was rapidly hydrated at room temperature in the presence of water and transformed to Ba(NO3)2 and γ-alumina in the presence of HNO3, whereas BaCeO3 was decomposed to much lower extent under these conditions. Interestingly, BaCeO3 was transformed to Ba(NO3)2/CeO2 in the presence of NO2/H2O at 300–500°C. Also, the presence of CO2 led to decomposition of barium cerate, which has important consequences for the catalyst ageing under NOx-storage conditions and can be exploited for regeneration of thermally aged NSR-catalysts.

Nitrogen oxide storage catalysts are used for the removal of nitrogen oxides present of so-called lean-burn engines in the lean exhaust gas. In this case, the cleaning effect due to the fact that the nitrogen oxides from the storage material of the storage catalyst in the form of nitrates stored in a lean operating phase of the motor and decompose the nitrates formed beforehand in a subsequent rich operating phase of the engine and again freed of nitrogen oxides with the reducing gas fractions at the storage catalyst nitrogen, carbon dioxide and water are reacted. Storage catalysts are thermally aged due to high temperatures. The aging is due to sintering of the catalytically active noble metal components of the catalyst and to the formation of compounds of the storage components with the carrier materials. According to the invention, the compound forming the storage materials can be largely reversed again by treating the memory material with a nitrogen and / or carbon dioxide and steam-containing gas mixture at temperatures between 300 and 500 ° C. The reactivation can be carried out directly on the vehicle by adjusting an appropriate exhaust gas conditions and effected during a routine maintenance by expansion of the catalyst and treatment with a gas mixture in an appropriate apparatus.