Catalytic properties of layered gold-palladium colloids

Layered bimetallic gold-palladium colloids in the size range of 20-56 nm have been synthesized by the seed-growth method: gold seeds were covered by palladium layers of various thickness and vice versa. The outer metal was coordinated by trisulfonated triphenylphosphine and sodium sulfanilate ligands to stabilize the bimetallic particles to such an extent that they could be isolated in the solid state. Owing to the hydrophilic ligand shell, redispersion in water was possible in any concentration. High-resolution transmission electron microscopy and energy-dispersive X-ray analysis was used to characterize the colloids. Stabilized and nonstabilized gold-palladium and palladium-gold systems on a TiO2 support were used as heterogeneous catalysts for the hydrogenation of hex-2-yne to cis-hex-2-ene. Both the palladium-plated gold seeds and the gold-plated palladium particles showed considerably increased activities compared with the pure metals. The ligand shell seems not to influence the catalytic behavior because protected and unprotected colloids behave very similarly.