Synthesis of Supported Palladium Catalysts

The synthesis of supported palladium (Pd) catalysts is reviewed with emphasis on the chemistry of catalyst synthesis, oxidic and carbon support properties and case studies in the period 1990-2000. For the application of Pd onto oxidic supports aqueous precursor solutions are frequently used. The charge of the Pd precursor and the iso-electric point of the support are important properties since they affect both the dispersion and the distribution over large support bodies during synthesis. With carbon supports direct reduction of Pd precursors may occur, giving rise to large metallic Pd particles. Using the latter support lyophilicity can to a large extent affect Pd emplacement. For the application of Pd on both oxidic and carbon supports sol-gel, deposition-precipitation, deposition-reduction, ion exchange and impregnation methods have been used. In general, it turns out that the thermal treatment often dominates the primary application of the Pd-precursor in establishing the ultimate metal dispersion. Thermal treatment in an inert atmosphere at temperatures not exceeding 773 K prior to reduction is beneficial for the Pd dispersion. Furthermore, it is concluded that gasphase reduction leads to smaller Pd particles than liquid-phase reduction. In our opinion large progress in the development of highly loaded thermostable Pd-catalysts can be made by utilizing deposition-precipitation techniques in combination with anchoring sites on the support.