Effect of La Addition on the Performance of Three-Way Catalysts Containing Palladium and Rhodium

Three-way catalysts containing palladium and/or rhodium were prepared using gamma-Al2O3 doped with lanthanum oxide as a support. All the samples were obtained by an incipient wetness impregnation of the support with an aqueous solution of nitrates. In order to investigate the metal-support interaction, the support was additionally calcined at 800 degrees C before the impregnation procedure. Characterization of the support thermally treated within a range of 600-1000 degrees C by low-temperature nitrogen absorption, X-ray diffraction analysis, and electron paramagnetic resonance spectroscopy has revealed that the treatment conditions strongly affect the textural properties, the phase composition and the concentration of electron-donor sites on the surface of the support. Deposition of metals by the impregnation of initial support with a joint solution of Pd and Rh nitrates has led to formation of small Pd-Rh alloyed nanoparticles with strong metal-metal interaction, which was confirmed by a testing reaction of ethane hydrogenolysis. No alloy formation was observed in the case of mechanical mixing of the separately prepared Pd-only and Rh-only catalysts as well as in the case of preliminary calcined support impregnated with a joint solution of Pd and Rh nitrates. Bimetallic Pd-Rh catalyst of alloyed type was shown to be the most promising in terms of catalytic performance and thermal stability.