ALUMINA-SUPPORTED PD, AG AND PD-AG CATALYSTS - PREPARATION THROUGH THE POLYOL PROCESS, CHARACTERIZATION AND REACTIVITY IN HEXA-1,5-DIENE HYDROGENATION

Palladium and palladium-silver catalysts, supported on gamma-alumina, were prepared by precipitation in a liquid polyol and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM ) and X-ray photoelectron spectroscopy (XPS). The catalysts were teste d in the hydrogenation of hexa-1,5-diene in liquid phase, at 313 K. Th e addition of silver improved the hex-1-ene fractional selectivity in all cases, but lowered the catalyst activity when the Pd/Ag ratio was about unity. On the other hand a palladium-rich (Pd/Ag=4) bimetallic c atalyst exhibited a very good activity and a good selectivity to hex-1 -ene as well. The effects of silver addition are ascribed to the dilut ion of palladium atoms by silver atoms, which prevents the isomerisati on of hexa-1,5-diene. Hence, the polyol method, which has been used up to now to prepare metal powders with controlled morphology, appears o f interest in the elaboration of supported metal catalysts. (C) 1998 E lsevier Science B.V. All rights reserved