NOx Storage and Reduction on Pt/Alumina Catalysts: Effects of Alkaline-Earth and Rare-Earth Metal Additives

A series of Pt/Al2O3 catalysts, modified with alkaline-earth or rare-earth metal oxides, were prepared by the incipient-wetness impregnation method. The catalysts were characterized by N2 sorption, scanning electron microscopy, and transmission electron microscopy. The performance and durability of NOx storage of the catalysts were investigated under a lean-burn/rich-burn cycle process. The effects of reaction temperature on the capacity for NOx and duration of the catalysts were also studied. The transient experiments consisted of a storing phase using a lean gas mixture (NO/O2/N2/C3H6/CO) and a regeneration phase during which the O2 flow was switched off. The catalysts consisted of nonuniform aggregates of very small and distinct particles. The barium-promoted catalyst had better storage capability and duration. On the other hand, the cerium-promoted catalyst had a higher NO storage capability than those without cerium. Comparing the influences of cerium and lanthanum additives, Pt2.5Ce30.5Ba33.4Al100 had a storage capacity of 1020 × 10^-6 mol/g at 30 min, but Pt2.5La30.5Ba33.4Al100 had a storage capacity of 341 × 10^-6 mol/g. The catalysts containing both barium and cerium demonstrated high NO storage/conversion. The best compositions of the catalysts for high NO storage capacity were Pt2.5Ce30.5Ba33.4Al100 (1020 mol/g) and Pt2.5Ce22.5Ba41.7Al100 (911 (mu)mol/g). The operating temperature also had a pronounced effect on both NO storage and reduction. The best operating range was 350-400 °C.