Effect of rare earth (La, Nd, Pr, Sm and Y) on the performance of Pd/Ce0.67Zr0.33MO2−δ three-way catalysts

Abstract

Ce–Zr–M (M=La, Pr, Nd, Sm and Y) ternary mixed oxides were first prepared by coprecipitation method combined with supercritical drying technique. The Pd-only three-way catalysts (TWC) were obtained by the incipient impregnation method, in which Ce–Zr–M ternary mixed oxides were used as supports. The samples were characterized by nitrogen adsorption–desorption, X-ray powder diffraction, X-ray photoelectron spectroscopy, oxygen storage capacity complete, H2-temperature programmed reduction techniques and transmission electron microscopy. The effects of rare earth addition on Pd-only three-way catalytic activities have also been investigated. The results show that rare earth doping improves remarkably structural and textural stability of mixed oxide. After aging treatment, the crystallite size of CeO2–ZrO2 is 21.4nm, but it decreases to 9.6, 9.3, 9.6, 10.4 and 9.4nm with doping of La, Pr, Nd, Sm and Y into CeO2–ZrO2, respectively. The incorporation of La, Nd, Pr, Sm and Y into ceria–zirconia lattice enhances the oxygen storage capacity and markedly improves the interaction between PdO species and support. After aging treatment, Pd-only three-way catalyst supported on Ce–Zr–M ternary mixed oxide exhibits superior activity for HC (C3H6+C3H8) and NOx conversion. Furthermore, compared to Y, Sm or La, more homogeneous ternary solid solution of Ce–Zr–M (M=Nd, Pr) is formed and the aged catalyst supported on CZ doped by Nd or Pr exhibits better influence on the thermal stability. Therefore, Ce–Zr–M (M=Nd, Pr) should be more suitable as support for TWC especially after aging treatment.