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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.

The determination of carbonyl compounds in gaseous samples is usually accomplished by enrichment methods, in which 2,4-dinitrophenyl-hydrazine (DNPH) as a derivatization reagent has become established to a large extent. However, the conventional methods of DNPH-impingers and of DNPH-cartridges are applicable to emission measurements in a limited way only, depending on the NO(2)-concentration in the exhaust gas. It could be proved that DNPH-derivatives, as well as DNPH, are also decomposed by NO(2) at a different speed, in which the hydrazones of unsaturated carbonyl compounds are probably more sensitive than those of the saturated carbonyl compounds. In view of this fact, the collecting methods had to be modified to avoid losses with the enrichment. The analysis of the compounds is carried out by HPLC with an effective gradient-system which is able to separate and detect the carbonyl compounds in exhaust gas within 16 min. Furthermore, a simple working-up procedure is presented which facilitates a parallel analysis by GC.

Calcination of a Pt/Ba/CeO 2 catalyst at 700°C and subsequent reduction in hydrogen, carbon monoxide or propene at 350-550°C resulted in a considerable improvement of its NO x storage-reduction (NSR) properties compared to those of a freshly prepared Pt/Ba/CeO 2 catalyst. This behavior is traced back to the temporary formation of BaPtO 3 perovskite which leads after reduction to well-distributed Pt particles in intimate contact with the barium-containing phases. The oxidation and reduction of platinum is reversible which can be exploited for the design of ''self-regenerating'' NSR-catalysts under lean ([600°C) and rich ([400°C) reaction conditions. The formation of the BaPtO 3 -perovskite may not only be interesting for NSR-catalysis, but generally for Pt-based catalysts where a high dispersion of Pt is important.