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

Abstract

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.