Journal of Molecular Catalysis A: Chemical 239 (2005) 249–256 Phase composition, reducibility and catalytic activity of Rh/zirconia and Rh/zirconia-ceria catalysts J.A. Wang a,∗ , T. L´ opez b , X. Bokhimi c , O. Novaro c,1 a Laboratorio de Cat´ alisis y Materiales, SEPI-ESIQIE, Instituto Polit´ ecnico Nacional, Col. Zacatenco, C. P. 07738 Mexico D.F., Mexico b Departamento de Qu´ ımica, Universidad Aut´ onoma Metropolitana-I, A. P. 55-534, 09340 M´ exico D.F., Mexico c Instituto de F´ ısica, Universidad Nacional Aut´ onoma de M´ exico, A. P. 20-364, 01000 M´ exico D.F., M´ exico Received 20 April 2005; received in revised form 9 June 2005; accepted 10 June 2005 Abstract Phase concentrations, crystalline structures and surface or bulk reduction properties of zirconia and ceria-doped zircoina nanophases prepared via a sol–gel method were studied by X-ray diffraction (XRD) and temperature-programmed reduction (TPR) techniques. Rietveld refinement showed that both the phase composition and crystallite size of the solids depended on ceria content which related to the degree of surface and bulk reducibility. The pure zirconia and the solid doped with 1 wt% ceria consisted of tetragonal and monoclinic phases, while, the solids doped with 25 and 50 wt% ceria contained only one phase with cubic structure. Cerium incorporation into zirconia led to a crystalline structure distortion and reducibility enhancement of the resultant solids. The Rh loaded zirconia and ceria-zirconia solids would dissociate hydrogen and spillover it onto the support, lowering the temperature for both surface and bulk reduction of the support. A structural dependence of the catalytic activity for CO oxidation upon the catalysts support was observed. Catalytic activity of the Rh loaded zirconia-ceria solid solution with cubic phase is quite higher than for the catalysts with the support containing tetragonal and monoclinic phases of zirconia. The latter exhibited an inducing period in the reaction temperatures below 180 ◦ C on the catalytic activity profile that might be a result of the relatively low reducibility of these support. © 2005 Elsevier B.V. All rights reserved. Keywords: Rhodium; Ceria-zirconia catalyst; Rietveld refinement; CO oxidation; Sol–gel synthesis 1. Introduction Ceria and zirconia are very important catalytic materials that have been widely applied as catalyst supports, active components or promoters in many catalytic reactions [1,2]. As an example, zirconium oxide doped with iron and man- ganese and promoted by sulfate, is a promising catalyst for skeletal isomerization of hydrocarbons at low reaction tem- perature [3,4]. Zirconia has been also used in solid oxide fuel cells and oxygen sensors due to its defective structure [5,6]. One of the most important applications of ceria is as pro- moter in the current three-way catalysts for elimination of ∗ Corresponding author. Tel.: +52 55 57206000x55124; fax: +52 55 55862728. E-mail addresses: wang j a@yahoo.com, jwang@ipn.mx (J.A. Wang). 1 Member of El Colegio Nacional de M´ exico. exhaust gases from automobiles [7,8]. The promoting effect is originally attributed to its large oxygen storage capac- ity and its ability of transferring lattice oxygen from bulk to surface [9,10]. It is also reported that cerium oxide may enhance metal dispersion and stabilize the support [11,12]. Although zirconium and cerium oxides play a variety of roles in many different catalytic reactions, the main drawback of these two materials is their thermal instability. For instance, the three-way catalysts usually are performed at temperatures above 500 ◦ C, when zirconia or ceria are used as support, the catalysts may be easily deactivated at high temperature by diminishment of active surface or growth of crystallite size resulted from serious thermal sintering and phase transfor- mation [13]. In the application of environmental catalysis, the findings to enhance thermal stability of the zirconia and ceria materials by suppressing the crystal growth are attrac- tive subjects. 1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2005.06.021