Journal of Colloid and Interface Science 317 (2008) 166–174 www.elsevier.com/locate/jcis Study of Ni–Ag/SiO 2 catalysts prepared by reduction in aqueous hydrazine R. Wojcieszak, S. Monteverdi, J. Ghanbaja, M.M. Bettahar ∗ UMR 7565, Catalyse Hétérogène, Faculté des Sciences, Université Henri Poincaré, Nancy-I BP 239, 54506 Vandoeuvre-lès-Nancy Cedex, France Received 21 June 2007; accepted 13 September 2007 Available online 18 September 2007 Abstract We have studied bimetallic Ni–Ag (Ni + Ag = 1 wt%) catalysts supported on crystallized silica and prepared by aqueous chemical reduction with hydrazine at 353 K. Two protocols of reduction were used. Prepared catalysts were characterized by means of XRD, TEM, STEM, H 2 chemisorption and H 2 -TPD. Their catalytic activity was studied in the gas-phase hydrogenation of benzene. The most important feature of the results obtained is the synergistic effect between Ni and Ag which led to improvement of dispersion and reactivity of nickel in the presence silver for precipitated catalysts. Silver is inactive in the test-reaction. Precipitated bimetallic catalysts give rise to total conversion from 373 K, a temper- ature at which conversion hardly reaches 30% for the impregnated catalysts. Dispersion and activity pass through a maximum of monotonically decrease with precipitated and impregnated catalysts, respectively. Deactivation was observed for bimetallic catalysts, particularly with precipi- tated samples. These results could be explained by the mechanism of metal reduction in the hydrazine media. As a result, various Ni–Ag species formed where Ni and Ag phases were separated clusters or interacted as heteroatomic groupings on the carrier surface. These grouping would be responsible of the high performances of the precipitated catalysts.  2007 Elsevier Inc. All rights reserved. Keywords: Supported bimetallic catalysts; Nickel; Silver; Silica; Hydrazine; Benzene hydrogenation 1. Introduction Nanoparticles of transition or noble metals have attracted much attention because of their unusual properties compared with the conventional polycrystalline materials. Whereas in the macromolecular solid, surface atoms contribute only a rela- tively small fraction of the total number of metal atoms, the nanoparticles contain almost all surface atoms [1]. It follows that such atoms have lower coordination numbers than in the bulk and as a consequence are expected to exhibit greatly en- hanced activity to all manner of substrates [1]. The synthesis of metal nanoparticles has been focus of numerous studies in the last decade. Some very successful techniques have been de- veloped for producing gold [2–4], silver [5–7], nickel [8–12], platinum [13], copper [14] and other materials as nanoparticles. Recently, increasing interest has been shown in preparing metal alloys such as Au/Pd [15], Au/Pt [16], Fe/Ni [17], Ni/Cu [18,19] and Ni/Ag [20–23]. It has been demonstrated that even * Corresponding author. E-mail address: mohammed.bettahar@lcah.uhp-nancy.fr (M.M. Bettahar). with small particles bimetallic clusters are vastly superior to their monometallic counterparts [1,15]. Nickel supported catalysts are widely used in heterogeneous catalysis due to their high hydrogenating properties. Many parameters determine its catalytic activity in hydrogenation processes. The activity strongly depends on the nature of the support which may modify the properties of the active phase. The extent of metal–support interaction and support acidity seems to play a crucial role in complex chemistry of nickel supported catalysts [24–27]. Silver supported catalyst is con- sidered as an excellent catalyst in epoxidation and oxidation reaction [28,29]. However, it is not active in the hydrogenation processes. Nevertheless, incorporation of silver to palladium catalysts improves the hydrogenation properties. It was shown that incorporation of silver into palladium catalysts increases their selectivity in the acetylene hydrogenation [30]. Hydrazine is a powerful strong reductant widely used in var- ious chemical operations. A series of striking results has been obtained where hydrazine is used as a reducing agent for the production of finely divided metals [31–36]. Degan and Macek [35] used hydrazine as a reducing agent to prepare nickel pow- ders in the submicrometer size range from nonaqueous solu- 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.031