Cu-doped resorcinol–formaldehyde (RF) polymer and carbon aerogels Orsolya Czakkel a,b , Erik Geissler b , Imre M. Szilágyi c , Edit Székely d , Krisztina László a, * a Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, H-1521 Budapest, Hungary b Laboratoire de Spectrométrie Physique CNRS UMR 5588, Université J. Fourier de Grenoble, BP 87, 38402 St Martin d’Hères cedex, France c Materials Structure and Modelling Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Department of Inorganic and Analytical Chemistry, H-1111 Budapest, Szt. Gellért tér 4., Hungary d Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, H-1521 Budapest, Hungary article info Article history: Received 30 January 2009 Accepted 22 May 2009 Available online 29 May 2009 Keywords: RF aerogel Carbon aerogel SAXS/WAXS Gas adsorption Copper Doping abstract Introduction of transition metal salt(s) onto the surface of porous carbons may increase the selectivity and/or efficiency of these adsorbents in catalysis or separation. Carbon aerogels with low pressure drop are particularly suited for these applications. Moreover the sol–gel process used in the synthesis of the resorcinol–formaldehyde polymer gel (RF) precursors offers an extra opportunity for introducing metal ions. Salts of different metals modify both the macroscopic texture and the porosity, depending on the synthesis protocol. In this paper we show, by means of low temperature nitrogen adsorption measure- ments and SEM, as well as small- and wide-angle X-ray scattering (SAXS and WAXS), how the addition of copper acetate at three different stages influences not only the specific surface area but also the result- ing overall structure over a wide range of length scales. Posttreatment in either the polymer or the carbon aerogel stage provides a means of adjusting the copper content. While the Cu-containing carbon aerogels differ mainly in their micropore volume but not in the width of the distribution, their pore size window in the mesopore range can be tuned between 50 and 400 Å by the protocol of Cu addition. The synthesis protocol also determines the chemical form of the copper. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction The response characteristics of an adsorbent are a function not only of its total adsorption capacity but also of its response rate. The design of exchange structures in biology, such as that of plants or of animal respiratory systems, serves to demonstrate that, in or- der to achieve a high response rate in a limited volume, the archi- tecture must be optimized over a wide range of length scales. In many adsorption applications, therefore, a tailored combination of narrow porosity and open structures is called for. In this respect, the simultaneous macroporosity and micropo- rosity of carbon aerogels offer advantages over other forms of adsorbents for applications that require rapid access to the microp- ores. Meso- and macroporous carbons can be prepared from resor- cinol and formaldehyde under controlled conditions. In aqueous solution, resorcinol and formaldehyde undergo a polycondensation reaction, most often catalyzed by sodium carbonate, to yield a three-dimensional polymer matrix, the resorcinol–formaldehyde (RF) hydrogel. The morphology of the RF gels can be adjusted by modifying the concentration or the mixing ratio of the monomers and the catalyst in the precursor solution, as well as by changing the pH [1–8]. Most of these parameters act by controlling the mechanism and kinetics of the polycondensation process. In the subsequent drying step, the structure of the hydrogel can effec- tively be retained by careful removal of the solvent [9]. Water, however, owing to its physico-chemical properties, adversely af- fects the drying process, and consequently a solvent exchange step is often introduced before drying. Three kinds of drying techniques are generally employed: (1) drying in an inert atmosphere [1,10,11], (2) freeze drying [4–7], or (3) extraction with supercrit- ical CO 2 [12–14]. The carbon aerogels are prepared by carbonizing the solvent-free RF gel, a process that basically maintains the pri- mary structure of the dry gel. This treatment, however, results in a significant increase in the micropore volume. This RF aerogel synthesis route has the advantage that metal ions can easily be introduced, either in the sol/gel process or by impregnation of the network, before or after the carbonization step. When sodium carbonate is replaced by other metal salts of basic character (e.g., K þ ; Mg 2þ ; and Zr 4þ Þ, carbon with highly dis- persed metal oxide particles can be produced. In this way, the por- ous texture of the carbonaceous component can be combined with the basic properties of the oxides. Zr-containing carbon aerogels obtained by copolymerization of organic and inorganic (zirco- nium(IV)-propoxide) precursors display a more uniform distribu- tion of the metal in the carbon matrix than supported catalysts 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.05.059 * Corresponding author. Fax: +36 1 463 3767. E-mail address: klaszlo@mail.bme.hu (K. László). Journal of Colloid and Interface Science 337 (2009) 513–522 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis