Synthesis and characterisation of bimetallic Pd-Rh/alumina combustion catalysts

This paper reports the possibility of associating Pd with another noble met al (rhodium) which is able to form a thermally stable oxide to extend towar d higher temperatures the stability range of PdO. Bimetallic Pd-Rh/alumina catalysts were prepared and compared to reference Pd/alumina and Rh/alumina solids. The synthesis of the bimetallic catalysts was carried out by eithe r co-impregnation or stepwise impregnation of alumina from solutions of RhC l3 and H2PdCl4. When submitted to TPO/TPD cycles, Pd-0.75 Rh-0.25/alumina c atalysts obtained by the co-impregnation method exhibited oxidation and dec omposition peaks at the same temperatures as those for reference Pd/alumina samples. The amounts of oxygen involved at each step were also comparable for the two solids and lower than the expected stoichiometric atomic ratio O/Pd = 1. Conversely, Pd-0.75 Rh-0.25/alumina catalysts obtained by the ste pwise impregnation method exhibited improved stability of the PdO oxide, th e maximal decomposition temperature being increased from 730 to 800 degrees C. In addition, the amounts of oxygen involved at each step were also incre ased and exceeded the value O/Pd = 1. As shown by TPR, Rh2O3 and PdO partic les on the surface of catalysts obtained by the co-impregnation technique b ehave like the respective monometallic catalysts. When the catalyst was ela borated by stepwise impregnation, some Rh2O3 and PdO oxide particles were i n strong interaction and were simultaneously reduced at room temperature in a H-2/He medium. (C) 2001 Academic Press.