Author's personal copy Supercritical deposition of Pt on SnO 2 -coated Al 2 O 3 foams: Phase behaviour and catalytic performance G. Incera Garrido a , F.C. Patcas a, * , G. Upper b , M. Tu ¨rk b , S. Yilmaz c , B. Kraushaar-Czarnetzki a a Institute of Chemical Process Engineering CVT, University of Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany b Institute of Technical Thermodynamics and Refrigeration ITTK, University of Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany c Department of Chemical Engineering, Izmir Institute of Technology, Urla 35497, Izmir, Turkey Received 15 November 2007; received in revised form 14 December 2007; accepted 17 December 2007 Available online 27 December 2007 Abstract Deposition and reduction of an organometallic platinum complex from a supercritical Pt(COD)Me 2 /CO 2 solution was carried out to produce Pt/ SnO 2 catalysts supported on Al 2 O 3 foams for CO oxidation at moderate temperatures. The phase behaviour of the complex in supercritical carbon dioxide was investigated to find the optimum pressure and temperature conditions for the deposition. For the Pt(COD)Me 2 /CO 2 mixture, the melting point decreased with increasing pressure from 378 K at 0.1 MPa to 360 K at 25.6 MPa. Additional investigations showed that the solubility of Pt(COD)Me 2 in CO 2 increases from 5.9  10 4 mol/mol at 11.2 MPa and 313 K to 3.4  10 3 mol/mol at 29.6 MPa and 353 K. The supercritical deposition yielded catalysts with highly dispersed platinum nanoparticles of approx. 3 nm having a narrow size distribution and thus, a superior activity towards oxidation of carbon monoxide in comparison to a catalyst prepared by the conventional aqueous impregnation of Pt. # 2007 Elsevier B.V. All rights reserved. Keywords: Supercritical fluid; Platinum; Nanoparticle; CO oxidation; Ceramic foam 1. Introduction The catalytic oxidation of carbon monoxide is an important purification step downstream to, e.g. reforming and combustion processes. In the context of fuel cell technology, in particular, CO removal gained increasing importance since the catalysts of the low-temperature fuel cells are extremely sensitive to poisoning by CO. Here, the CO concentration needs to be reduced from about 1% to below 10 ppm in the presence of excess hydrogen. The preferential oxidation (PROX) of CO, i.e. the selective oxidation of CO to CO 2 with minimum H 2 consumption, could be an option to reach this goal. However, PROX has to be carried out at low temperatures in order to avoid the occurrence of the reverse water gas shift-reaction which produces CO [1] and, therefore, requires highly active catalysts. The system Pt/SnO 2 has been studied for many years because it has shown to catalyze the CO oxidation at moderate temperatures, where neither platinum nor tin dioxide alone are catalytically active. The high activity is ascribed to a synergistic bifunctional mechanism in which Pt provides the adsorption sites for CO, while oxygen adsorbs dissociatively on SnO 2 [2,3]. The reaction between the adsorbed species occurs at the Pt/SnO 2 boundary. If this synergistic mechanism holds, it should be of extreme importance for a high catalytic activity to realize a large boundary surface area, i.e. to achieve a high dispersion of the platinum particles on the tin dioxide phase. The preparation of fine dispersed platinum particles of 1– 2 nm size on g-alumina supports is a well-studied process. The precursor here is chloroplatinic acid and the fine dispersion of platinum is achieved by an intercourse of ion exchange and chloride interaction with the amphoteric carrier. In the case of SnO 2 , platinum is usually deposited by means of wet impregnation of the SnO 2 washcoat with aqueous solutions of platinum tetramine nitrate [4]. Owing to the relatively low surface area of SnO 2 and the capillarity effects during drying, the achievement of high Pt dispersions is difficult. Here, we report on the deposition and reduction of an organometallic www.elsevier.com/locate/apcata Available online at www.sciencedirect.com Applied Catalysis A: General 338 (2008) 58–65 * Corresponding author. Tel.: +49 721 608 4134; fax: +49 721 608 6118. E-mail address: Florina.Patcas@ciw.uni-karlsruhe.de (F.C. Patcas). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.12.019