Applied Catalysis A: General 274 (2004) 33–41 Synthesis of Rh nano-particles by the microemulsion technology Particle size effect on the CO + H 2 reaction Manuel Ojeda a , Sergio Rojas b , Magali Boutonnet b , Francisco J. Pérez-Alonso a , F. Javier Garc´ ıa-Garc´ ıa c , José Luis G. Fierro a,∗ a Institute of Catalysis and Petrochemistry (CSIC), C/Marie Curie, s/n. 28049 Madrid, Spain b Department of Chemical Engineering and Technology, Chemical Technology, Royal Institute of Technology (KTH), Teknikringen 42, SE 100 44 Stockholm, Sweden c Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE 10691 Stockholm, Sweden Received in revised form 7 May 2004; accepted 7 May 2004 Available online 2 July 2004 Abstract Several alumina-supported rhodium-based catalysts have been prepared using the microemulsion technology. The change of microemulsion properties led to catalysts with different Rh particle size and narrow size distribution while keeping the same metal loading. The metal particle size was determined by XRD and TEM. These catalysts were tested in the CO hydrogenation reaction in order to investigate the influence of the Rh particle size. The catalytic results indicated that turnover frequency (TOF) increased about four times when the Rh particle size increased approximately from <5 to 30 nm. Product selectivity also depended on particle size. As Rh particle size decreased, methane and oxygenated compounds formation was favored at expenses of the higher hydrocarbons (C 2+ ). XPS shows that the electronic metal–support interaction increases as the particle size decreases, leading to partially oxidized Rh atoms. These species are responsible of oxygenate formation, and as a result, its selectivity increases as rhodium particle size decreases. © 2004 Elsevier B.V. All rights reserved. Keywords: Rh; Microemulsion; Alumina; Particle size; CO hydrogenation 1. Introduction Rhodium-based catalysts are highly active in the synthe- sis of C 2 -oxygenated compounds from syngas [1,2]. Many investigations have been carried out in order to improve the knowledge about the influence of the catalyst nature on the catalytic behavior. Among others, the metal particle size is of great importance because CO hydrogenation is a structure-sensitive reaction [3,4]. Reactions involving either C–C bond breaking or formation are usually structure sensi- tive, and this implies that both catalytic activity and selectiv- ity can be dependent on the metal particle size [5]. There is a body of current agreement on the effect of the particle size on the performance of rhodium-based catalysts in the CO hydrogenation reaction [6,7]. Particle size dependence was usually observed when using catalysts with different metal ∗ Corresponding author. Tel.: +34-915854769; fax: +34-915854760. E-mail address: jlgfierro@icp.csic.es (J.L.G. Fierro). loadings. However, some authors [8] have clearly shown on industrial silica support that the difference in selectivity and activity with loading was not the result of differences in par- ticle size. In order to get a more reliable knowledge of the par- ticle size effect, it is mandatory to prepare a series of Rh-supported catalysts containing equal metal loading while displaying different particle size. To achieve this objective, the water-in-oil (w/o) microemulsion technology appears as a promising tool [9]. A microemulsion is a thermodynam- ically stable, optically clear dispersion of two immiscible liquids such as water and oil stabilized by the presence of a surfactant and, in some cases, a co-surfactant. This prepa- ration method allows to control the metal particle size, that is, to achieve a given particle size with a narrow particle size distribution regardless of metal content. Typically, a metal salt precursor is dissolved in water and dispersed in an oil phase, using a surfactant as stabilizing agent. Under particular conditions (constituent relative concentration and nature) a microemulsion is obtained. In the microemul- 0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.05.014