The synthesis of high surface area cerium oxide and cerium oxide/silica nanocomposites by the silica aquagel-confined co-precipitation technique Irene López, Teresa Valdés-Solís 1 , Gregorio Marbán * Instituto Nacional del Carbón (INCAR), CSIC – c/Francisco Pintado Fe, 26, 33011 Oviedo, Spain article info Article history: Received 15 April 2009 Received in revised form 14 July 2009 Accepted 20 July 2009 Available online 23 July 2009 Keywords: Cerium oxide Nanosized Confined co-precipitation Aquagel abstract This paper describes the synthesis of high surface area cerium oxide and cerium oxide/sílica nanocom- posites, by means of silica aquagel-confined co-precipitation (SACOP). The obtained cerium oxide struc- tures have surface areas of up to 200 m 2 g 1 and are made up of aggregates of nanosized particles (2– 5 nm). By modifying certain specific synthetic parameters [the type of acid (HNO 3 or HCl), the silica/acid ratio (Si/H + ) and the cerium concentration (Ce/Si) in the synthesis suspension, the aquagel ageing time and the silica removal procedure] we were able to establish the optimum parameters for fabricating high surface area CeO 2 and to shed light on the different mechanisms involved in the novel SACOP technique. Ó 2009 Published by Elsevier Inc. 1. Introduction The preparation of high surface area metal oxide is currently the subject of extensive research due to their multiple applications in different areas (electronic devices, sensing, biomedicine, catalysis, etc.). In the field of catalysis, the well-known relationship between particle size and catalytic activity [1] is evidence of the importance of developing techniques to fabricate nanoparticles with tailored sizes. Furthermore, when conveniently immobilised in the appro- priate supports [2,3], catalyst nanoparticles present clear advanta- ges over conventional catalysts in terms of enhanced active phase distribution [4] and catalytic activity [5]. High surface area metal oxide can be obtained by various meth- ods, including precipitation [6–9], hydrothermal synthesis [10–12], solvothermal synthesis [13], sol–gel [14–16], microemulsion [17–22], template procedures [23–26], pyrolysis [27], etc. These methods are especially suitable for the preparation of nanometric metal oxide particles (below 100 nm). Recently we developed a novel template procedure (silica aquagel-confined co-precipitation (SACOP)) to synthesise (mixed) metal oxides with surface area val- ues that are considerably higher than those obtained by most of the other techniques referred to above [28]. SACOP is a modified silica-template route that is based on the forced precipitation of metal hydroxides (MOH) in a silica aquagel medium. Subsequent thermal treatment of the dried composite (MOH–SiO 2 ) causes metal oxide nanoparticles to form in the silica mesopores. This method results in a better metal distribution on the silica matrix than that achieved by the conventional hard-template route and allows higher metal oxide to silica mass ratios to be achieved in one single step. The final removal of the silica matrix produces either metal oxide nanoparticles or nanostructures, depending on the type of oxide used [28]. As SACOP is a novel procedure, it is necessary to perform a systematic investigation of the synthesis parameters that influence the final surface area (or particle size) of the oxides. This is the aim of this work. This information may also serve to clarify the mechanisms that cause SACOP to produce nanostructures with a lower effective particle size than those ob- tained by the conventional silica-template route [29]. Out of the numerous metal oxides that could have been em- ployed to perform this investigation, we selected cerium oxide due its unique redox and oxygen storage properties [30], which make it highly suitable for a variety of catalytic applications. For example, ceria is an additive in automobile exhaust catalysts (three way catalysts) and is also commonly used as a support in a large number of different catalysts because of its ability to facilitate the dispersion of base metals [31]. In this work we have analysed the effect of different synthesis parameters on the surface area (or effective particle size) of cerium oxide nanostructures prepared by SACOP, and we have tried to clarify the synthesis mechanisms of the nanostructures obtained. To this end we focused our attention on the following parameters: (i) the type of acid, (ii) the silica/acid molar ratio (Si/H + ), (iii) the cerium concentration in the synthesis suspension (Ce/Si), (iv) the aquagel ageing time and (v) the silica removal procedure. 1387-1811/$ - see front matter Ó 2009 Published by Elsevier Inc. doi:10.1016/j.micromeso.2009.07.014 * Corresponding author. Tel.: +34 985119090; fax: +34 985297662. E-mail address: greca@incar.csic.es (G. Marbán). 1 Present address: Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK. Microporous and Mesoporous Materials 127 (2010) 198–204 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso