Co-containing NOx storage and reduction (NSR) catalysts were studied for their NOx storage and NOx conversion behavior. It was found that addition of 5% Co increased the NOx storage capacity of a 1% Rh/15% Ba catalyst by 50% and that of a 1% Pt/15% Ba catalyst by 100%. This promotional effect was attributed to the high oxidizing ability of Co, present in the form Of Co3O4, providing extra oxidation sites for NO to NO2 conversion and more contact area for NO2 Spillover to Ba storage sites. Empirical models were developed to predict the NOx conversion capacity of the catalyst as a function of total cycle time and the fraction spent in the lean phase. Using these models, it was found that 5% Co also improved the performance of 1% Pt/15% Ba and 1% Rh/15% Ba catalysts at higher lean fractions, allowing substantial improvement in overall fuel efficiency. The reduction capacity of Co was much less than that of noble metals, such that noble metals were still necessary (although at a reduced loading) for regenerating the catalyst in the rich phase. Along these lines, it was found that a 0.25% Pt/5% Co/15% Ba-containing catalyst showed better performance than a 1% Pt/15% Ba-containing catalyst. This implies that the cost of NSR catalysts can be substantially reduced by replacing some of the Pt with Co as the active oxidizing metal in NSR catalyst formulations. (c) 2006 Elsevier Inc. All rights reserved.