Roles of the basic metals La, Ba, and Sr as additives in Al₂O₃-supported Pd-based three-way catalysts

Three-way catalytic converters are widely used to regulate emissions from gasoline-powered vehicles. Although significant effort over the past 40 years has resulted in the discovery of several metal additives that improve the thermal stability of three-way catalysts (TWCs), their effects on the actual catalytic process have not been studied systematically. The present work examines the roles of the typical basic metal additives La, Ba, and Sr in Pd-based TWC systems, using various spectroscopic and kinetic studies. Metallic Pd⁰ species on Sr/Al₂O₃ and Ba/Al₂O₃ supports were found to be more electron-rich than those on pristine Al₂O₃, whereas those on La/Al₂O₃ were more electron-deficient. Consequently, Pd/La/Al₂O₃ showed a lessened CO poisoning effect during NO reduction reactions. Evaluations were performed using powdered catalysts as well as monolithic honeycomb catalysts under conditions simulating actual use. Pd/La/Al₂O₃ was observed to promote the catalytic reduction of NO most efficiently, while Pd/Ba/Al₂O₃ exhibited the highest activity for the oxidations of CO and C₃H₆. The present data suggest that the optimal metal additive for a Pd-based TWC will be determined by the specific application. The selection of such metals should take into account not only the stability but also the promotional effect during the exhaust purification process.