PdO/Al 2 O 3 –(Ce 1-X Zr X )O 2 catalysts: effect of the sol-gel support composition Gabriela Pe´rez Osorio, a, * Sergio Fuentes Moyado, b Vitalii Petranovskii, b and Andrey Simakov b a Posgrado en Fı´sica de Materiales, Centro de Investigacio ´n Cientı´fica y de Educatio ´n Superior de Ensenada, B.C., (CICESE), Apdo. Postal 2732, Ensenada, B.C., Me ´xico b Departamento de Cata ´lisis, Centro de Ciencias de la Materia Condensada, Universidad National Auto ´noma de Me ´xico (UNAM), Apdo. Postal 2681, Ensenada, B. C., Me ´xico Received 24 October 2005; accepted 23 May 2006 The contribution of (Ce 1-X Zr X )O 2 additives to alumina supports prepared by sol-gel and the catalytic properties of PdO/Al 2 O 3 – (Ce 1-X Zr X )O 2 catalysts (0.3 wt% Pd, 5 wt% (Ce 1-X Zr X )O 2 ) in CO oxidation was herein investigated. The addition of (Ce 1- X Zr X )O 2 to the support enhanced the surface area and decreased the size of Al 2 O 3 particles. The UV–Vis bands of PdO particles and Pd 2+ ions indicate that zirconia in (Ce 1-X Zr X )O 2 promotes palladium-support interactions by forming highly dispersed PdO particles. Temperature-programmed reduction (TPR) in hydrogen revealed that ceria enhanced the redox capacity of the supports while zirconia lowered the reduction temperature of palladium oxide species. The comprehensive study revealed that the Ce/Zr ratio was a key factor influencing the catalytic activity of samples in CO oxidation, because palladium oxide-support interactions had a significant effect in changing of the reducibility of samples. So, the PdO/Al 2 O 3 –(Ce 0.5 Zr 0.5 )O 2 exhibited the highest catalytic activity. KEY WORDS: Pd oxide supported catalysts; sol-gel alumina; ceria–zirconia mixed oxides; redox behavior CO oxidation. 1. Introduction Three-way catalysts (TWC) actually used for removal of pollution gases from automobile exhaust contain noble metals supported on mixed oxides, Pd-only TWC constitutes a new catalyst generation with improved performance compared to conventional rhodium-con- taining technologies due to palladium good resistance to thermal sintering [1,2]. Since the support plays a key role in the TWC performance, mixed oxides like alumina– ceria have been widely studied. Ceria acts as alumina surface area stabilizer [3] and as oxygen storage capac- itor (OSC) that operates under atmospheres oscillating between rich and lean conditions, due to its ability to undergo easy the Ce 3+ /Ce 4+ redox process [4]. The OSC is useful to adjust the air/fuel ratio achieving a high conversion efficiency of pollutants such as CO, hydro- carbons (HC) and NO X in the TWC [5]. Several studies have been devoted to ceria-containing materials where the presence of noble metal particles on the oxide sur- face promotes the OSC [6,7]. However, ceria suffer the loss of the OSC with decreasing surface area after exposing to moderate temperature treatments (above 350°C) [8]. The addition of zirconia (ZrO 2 ) to ceria (CeO 2 ) leads to a solid solution formation, which enhances ceria thermal sta- bility by structural and redox properties modifications [8,9]. The ceria–zirconia mixed oxides prepared by dif- ferent techniques like coimpregnation or cogellation offer a mutual stabilization with the alumina support [10] and promote the dispersion and catalytic activity of palladium [11,12]. The preparation method of the sup- port and the load of zirconia play an integral role in the catalytic performance of these complex systems. The aim of this work was to characterize the PdO/Al 2 O 3 – (Ce 1-X Zr X )O 2 catalysts by means of BET, XRD, TEM, DRS and H 2 -TPR and examine the influence of the addition of CeO 2 and ZrO 2 in the activity for CO oxi- dation. The catalytic activity was compared in terms of temperature when 50% of CO is converted to CO 2 (T 50% ). 2. Experimental 2.1. Catalyst preparation The Al 2 O 3 –(Ce 1-X Zr X )O 2 supports (X = 0, 0.25, 0.33, 0.5, 0.67, 0.75, 1) were prepared by the sol-gel method. Ethanol solutions of cerium (III) acetylaceto- nate and zirconium (IV) acetylacetonate mixed in desired proportion were added to the solution of alu- minum sec-butoxide in hexylenglycol under Ar atmo- sphere. The resulting sol was stirred and refluxed for 3 h at 94°C. The hydrolysis was performed by adding dropwise an appropriate amount of water. In order to *To whom correspondence should be addressed. E-mail: fuentes@ccmc.unam.mx Catalysis Letters Vol. 110, Nos. 1–2, August 2006 (Ó 2006) 53 DOI: 10.1007/s10562-006-0097-8 1011-372X/05/0800–0053/0 Ó 2006 Springer Science+Business Media, Inc.