Effect of Ce-Zr mixed oxides on the chemical state of Rh in alumina supported automotive exhaust catalysts studied by XPS and XRD

Several gamma -Al2O3 supported Rh catalysts containing Ce-Zr mixed oxides in various proportions were prepared by different manufacturing methods. Both fresh and aged catalysts were investigated, as well as Rh/Ce-Zr mixed oxide powders. Aging was performed in air at 1000 degreesC for 3 h. Disposition of the Rh-oxide formed in the aging towards reduction was investigated by exposing the catalysts to 400 mbar of H-2 at 300 degreesC for 30 min. Rhodium supported on pure alumina was found to sinter easily and react irreversibly with the alumina support at elevated temperatures to form an irreducible oxide phase. On the contrary, rhodium supported on Ce-Zr mixed oxide was observed to remain in a reducible form also in high temperature aging. It was also found that Ce-Zr mixed oxide is capable of stabilizing Rh particles against sintering at elevated temperatures. However, incorporation of Ce-Zr mixed oxide to gamma -Al2O3 supported catalysts was found to be ineffective in preventing the reaction between rhodium. and alumina. After 3 h aging in air at 1000 degreesC an irreducible rhodiurn oxide, which is likely RhO2 in intimate contact with alumina surface, was formed regardless of the type of Ce-Zr mixed oxide additive and manufacturing method of the catalyst. (C) 2001 Elsevier Science B.V. All rights reserved.