Improved NOx storage-reduction catalysts using Al2O3 and ZrO2–TiO2 nanocomposite support for thermal stability and sulfur durability

A nanocomposite of Al2O3 and ZrO2–TiO2 solid solution (AZT) was synthesized for NOx storage-reduction (NSR) catalysts Pt/Rh/Ba/K/AZT, and the effect of calcination temperature on thermal stability was investigated. The catalyst containing AZT calcined at 1073 K (AZT catalyst) had a high NOx storage capacity after thermal aging. This enhanced storage capacity was attributed to the fact that ZrO2–TiO2 solid solution (ZT) was crystallized and stabilized. The solid-state reaction of potassium, which was added as a NOx storage material, was inhibited in the AZT catalyst, relative to those containing AZT calcined at a low temperature. The AZT catalyst also showed excellent NOx storage performance after sulfur aging at 973 K or higher, compared with the catalyst containing physically mixed Al2O3 and ZT (physically mixed catalyst). Furthermore, AZT catalysts inhibited the solid phase reaction of potassium with support materials and kept a high ratio of active potassium, which can store NOx. Further, because the barium in the AZT catalyst prevented sulfur poisoning, the ratio of active barium in the AZT catalyst was also larger than that in the physically mixed catalyst, probably because of the low basicity and high Pt dispersion of AZT.