The role of lanthanum oxide on Pd-only three-way catalysts prepared by co-impregnation and sequential impregnation methods

The effect of lanthanum oxides on the catalytic performance and the physicochemical properties of Pd-only three-way catalysts prepared by co-impregnation and sequential impregnation methods was studied by using hydrogen chemisorption, BET surface area, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that the roles of La closely depended on the order of La introduction in the preparation of the Pd catalysts. Pd-La/Al₂O₃ prepared by co-impregnation of La and Pd, kept its superior activity in spite of the significant loss of surface area of the alumina support after thermal aging at 1273 K, indicating that the primary role of La was a Pd stabilizer through the intimate interaction between La and Pd. However, on Pa/La/Al₂O₃, in which Pd was consecutively impregnated after the impregnation of La, La preferentially interacted with the alumina support as a form of La_xAl_yO₂, resulting in the stabilization of the alumina support during thermal aging. XPS results indicated that lanthanum oxide suppressed the formation of PdO interacting with alumina during thermal aging. In the case of Pd/La-Ce/Al₂O₃, the formation of the solid solution of (Ce_xLa_l-_x)O₂ was not strong enough to maintain the high activity and the good textural property after thermal aging.