A new approach of CeO₂ and La₂O₃ effects on the three-way catalysts containing low precious metals

A series of three-way catalysts (TWCs), containing a small amount of precious metals (PMs, including Pt, Pd and Rh) and a large amount of promoters CeO₂ and La₂O₃, were prepared with different precursor compounds and various doped manners. Crystal phases, dispersion of cerium and lanthanum, textural structure and thermal stability of the catalysts were investigated by XRD, XPS and pore parameters determination. The catalytic performance was studied by the measurements of CO, C₃H₆ and NO conversions on dependence of temperature at stoichimetric number point (S = 1.00), and from S = 0.75 to 1.30 at 280°C or 340°C for fresh or aged samples, respectively. The correlation between the catalytic performance and the characteristics of fresh and aged samples were discussed. The results show mat the sample, in which CeO₂ and La₂O₃ are doped with mixed oxide powders, possesses poor dispersion and less thermal stability, and the conversions of NO and C₃H₆ are apparently lower man those of the samples aged at 850°C The main reason is due to the lanthanum enrichment on the surface. The precious metals and cerium may be covered and enveloped, and the PMs located on the internal microporous surface where no cerium and lanthanum exist, are easier to sinter and oxidize. For the sample doped with La(NO₃)₃ and Ce(NO₃)₃ aqueous solutions, high dispersion and thermal stable CeO₂-La₂O₃ solid solution on all the surface of microporous γ-Al₂O₃ is identified. The solid solution CeO₂La₂O₃ also possessed high dispersion in Ok sample doped with La₂O₃ powder and Ce(NO₃)₃ aqueous solution. The last two aged samples keep higher NO conversion at S≥1 region.