J. of Supercritical Fluids 49 (2009) 369–376 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu Supercritical CO 2 as a reaction and impregnation medium in the synthesis of Pd–SiO 2 aerogel inverse opals María José Tenorio a , María José Torralvo b , Eduardo Enciso a , Concepción Pando a , Juan Antonio R. Renuncio a , Albertina Caba ˜ nas a,∗ a Departamento de Química-Física I, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain b Departamento de Química Inorgánica, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain article info Article history: Received 9 October 2008 Received in revised form 17 March 2009 Accepted 19 March 2009 Keywords: Palladium Inverse opals Supercritical fluids Aerogels Catalyst Nanomaterials abstract Pd–SiO 2 aerogel inverse opals were prepared for the first time using supercritical CO 2 (scCO 2 ). Colloidal crystals formed by 3D-arrays of monodisperse spherical polymer particles (opals) were used as tem- plates. In one approach, a pre-made large surface area SiO 2 aerogel inverse opal prepared in scCO 2 was impregnated with palladium hexafluoroacetylacetonate [Pd(hfac) 2 ] in scCO 2 and thermally treated to get Pd–SiO 2 . In another approach, tetraethylorthosilicate and Pd(hfac) 2 dissolved in scCO 2 reacted directly on the polymeric template in one step. Both, impregnation and reaction experiments were carried out at 40 ◦ C and 85bar. After the removal of template, large surface area porous materials replicating the structure of the original template were obtained (inverse opals). These materials presented hierarchical porosity with ordered macropores of mesoporous walls. Palladium (between 1 and 3mol% by EDX) was incorporated uniformly throughout the SiO 2 matrix forming small clusters (by TEM). The effect of Pd incorporation on the aerogel porosity was studied. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Opals are formed by monodisperse spherical colloidal parti- cles assembled in three-dimensional ordered arrays (3D-arrays). Because of the high contrast between the refractive index of the particles and the voids of the crystal, these materials exhibit opales- cence and resemble the natural opals. Synthetic opals formed by monodisperse SiO 2 or polymer particles (latex) can be used as tem- plates to infiltrate precursors, which after reaction and elimination of the template yield the inverse replica (inverse opal). Inverse opals exhibit a number of interesting properties that derive from their special structural features. If the skeletal structure is on the length scale of optical wavelengths the inverse opal exhibit pho- tonic properties, which are important for the design of photonic crystals, pigments and sensors, among others. Furthermore, inverse opals are highly structured porous systems which allow transport of species through the pores as well as in the solid phase and are used in fuel cells, microreactors, electrochemical cells, catalysis and separation [1,2]. Recently, supercritical fluids have received much attention as reaction and processing medium in material synthesis [3]. Super- critical CO 2 (scCO 2 ) is by far the most frequently used fluid because ∗ Corresponding author. Tel.: +34 91 3945225; fax: +34 91 3944135. E-mail address: a.cabanas@quim.ucm.es (A. Caba ˜ nas). it is cheap, non-toxic and non-flammable and has relatively low critical temperature and pressure (T c = 31 ◦ C, P c = 73.8 bar) [4]. Fur- thermore many ceramic precursors such as metal alkoxides dissolve at moderate pressure and temperature in scCO 2 [5]. The low viscosity, high diffusivity relative to liquids and very low sur- face tension of scCO 2 promote infiltration in complex geometries and mitigate mass transfer limitations common to liquid-phase processes. CO 2 is a gas at ambient pressure and is eliminated completely upon depressurization. All these advantages have been exploited in a new method to produce SiO 2 aerogel inverse opals in scCO 2 developed at our laboratory [6–8]. The method involves the reaction of silicon alkoxides dissolved in scCO 2 on 3D-latex array templates. In this paper we show that the method can be extended to produce PdO–SiO 2 and Pd–SiO 2 aerogel inverse opals. Supported Pd catalysts are used in reduction and oxidation reac- tions as well as in hydrogenation, dehydrogenation, debenzylation, hydrocracking, carbonylation and other carbon–carbon coupling reactions [9]. In order to improve the catalytic activity and selec- tivity of the catalyst, the metal must be evenly dispersed on a large surface area support such as a mesoporous SiO 2 aerogel. The pres- ence of macropores in the catalyst facilitates the transport of the reactants to the catalyst surface. On the other hand, structured cat- alysts of defined geometry and pore size distribution are necessary to model catalytic processes. Preparing Pd–SiO 2 inverse opals of large surface area may well serve to these purposes. 0896-8446/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2009.03.011