Alumina-supported Pd, Ag and Pd±Ag catalysts: Preparation through the polyol process, characterization and reactivity in hexa-1,5-diene hydrogenation Emerson Andrade Sales a , Be Âatrice Benhamida b , Virginie Caizergues b , Jean-Pierre Lagier b , Fernand Fie Âvet b , Franc ,ois Bozon-Verduraz b,* a Instituto de Quõ Âmica, Universidade Federal da Bahia, Campus Universita Ârio de Ondina, 40170-290 Salvador-Bahia, Brazil b Laboratoire de Chimie des Mate Âriaux Divise Âs et Catalyse, Universite  Paris 7, case 7090, 2, place Jussieu, 75251 Paris Cedex 05, France Received 16 October 1997; received in revised form 14 April 1998; accepted 14 April 1998 Abstract Palladium and palladium±silver catalysts, supported on g-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 tested in the hydrogenation of hexa-1,5-diene in liquid phase, at 313 K. The 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 catalyst exhibited a very good activity and a good selectivity to hex-1-ene as well. The effects of silver addition are ascribed to the dilution of palladium atoms by silver atoms, which prevents the isomerisation of hexa-1,5-diene. Hence, the polyol method, which has been used up to now to prepare metal powders with controlled morphology, appears of interest in the elaboration of supported metal catalysts. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Hexa-1,5-diene; Liquid phase hydrogenation; Palladium±silver catalysts; Polyol process 1. Introduction Selective hydrogenation reactions have been used since the beginning of this century, namely (i) for a product value improvement; (ii) to eliminate dienes and acetylenic compounds, sources of contamination of the ®nal product or to avoid operational problems during the subsequent stages of the processes, like plugging due to polymer formation. For instance, acetylene removal from polyethylene grade ethylene by selective hydrogenation is a very important reac- tion, as it deals with the puri®cation of the most important petrochemical product, ethylene. Cobalt, nickel, molybdenum, chromium and palladium cata- lysts are reported in the literature [1±7], but only the palladium catalytic systems have been employed suc- cessfully in commercial operations. During the last years, many new petrochemical plants started opera- tion in China, Korea and Japan, and this important capacity increase caused the ethylene price break- down. This situation became more critical due to Applied Catalysis A: General 172 (1998) 273±283 *Corresponding author. Fax: 33-1-44276137; e-mail: bozonver@ccr.jussieu.fr 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00124-0