Co-containing NO_x storage and reduction catalysts were investigated to identify the mechanism of Co promotion. X-ray diffraction and temperature programmed reduction demonstrated that Co exists in a highly oxidized state (Co₃O₄) and that the surface oxygen could be removed from the catalyst around 300ºC. Electron microscopy showed that Co is more uniformly distributed over the catalyst surface, compared to Pt, with a particle size of 20-80 nm.

         In- situ IR studies performed in a transmission IR cell at atmospheric pressures and for realistic gas compositions illustrated that NO_x storage in the lean phase occurs on Co-containing NSR catalyst via the formation of nitrites and nitrates as surface intermediates. It was found that, similar to Ba catalysts containing Pt, the addition of Co to Ba catalysts (no noble metal present) enhances the nitrite to nitrate transition rate and also increases the overall formation of nitrates. Therefore, the promotional effect shown by Co is the result of the combination of increased NO to NO₂ oxidation and improved surface area for NO₂ spillover to the BaNO_x storage sites. In both the case of noble metal free Co/Ba catalysts and Pt/Co/Ba catalysts, the total amount of nitrates observed in the IR spectra correlated very well with the experimentally measured NO_x storage capacities. In situ results for the rich phase will also be presented.