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

• The promotional effect of basic metal additives in Pd/ M /Al2O3 ( M = La, Ba, or Sr) for NO x reduction was investigated. • Basic metal additives modify the electronic states of Pd0 species. • The electron-deficient Pd0 species in Pd/La/Al2O3 suppressed the CO poisoning effect. • Addition of La is the most effective for NO reduction, while Ba was effective for CO and C3H6 oxidation. 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 Pd 0 species on Sr/Al 2 O 3 and Ba/Al 2 O 3 supports were found to be more electron-rich than those on pristine Al 2 O 3 , whereas those on La/Al 2 O 3 were more electron-deficient. Consequently, Pd/La/Al 2 O 3 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/Al 2 O 3 was observed to promote the catalytic reduction of NO most efficiently, while Pd/Ba/Al 2 O 3 exhibited the highest activity for the oxidations of CO and C 3 H 6 . 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.