The behavior of Pt during aging of Pt/Ba/CeO2 and Pt/Ba/Al2O3 NOx storage-reduction catalysts (NSR) was studied using model catalysts with high and low Pt loadings. Pt composite formation, due to the possible reaction between BaO and platinum oxides, was observed in several cases and was elucidated by a series of analytical techniques, including X-ray diffraction (XRD), thermal analysis (TA), X-ray absorption spectroscopy (XANES, EXAFS), and electron microscopy. During calcination of mechanical mixtures of BaCO3 and PtO2, BaPtO3 was formed at ca. 600 degrees C, transforming to BaPtO2.38 above 800 degrees C. Investigation of Pt/Ba/CeO2 and Pt/Ba/Al2O3 model catalysts with high and low Pt loadings revealed that in the case of Pt/Ba/CeO2, the mixed oxide BaPtO3 was formed at relatively low temperature (600-700 degrees C) in oxidizing atmosphere. Above 800 degrees C, BaPtO3 reacted further with BaCeO3 to form a double perovskite, Ba2PtCeO6. In contrast, for Pt/Ba/Al2O3, only the sintering of Pt, with no mixed Pt-Ba oxides, was found. The recovery of the catalytically active metallic Pt species could be achieved in the aged Pt/Ba/CeO2 catalyst by reduction with hydrogen at relatively low temperature. Finally, investigation of the NOx storage and reduction activity of the fresh, aged, and reduced catalyst confirmed that this treatment is beneficial for catalyst reactivation. (c) 2007 Elsevier Inc. All rights reserved.