Introduction Ceria (CeO 2 ) is a rare earth oxide that has been attract- ing great interest in the last years due to the redox chem- istry of cerium and the high affinity of the element for oxygen that allows wide applications [1]. Particularly, the presence of CeO 2 has been found effective in the promotion of various catalytic reactions including, CO 2 activation, CO oxidation and CO/NO removal. Nowa- days, there is much interest in the study of different mixed oxides containing CeO 2 , like ceria/zirconia mixed oxides. The use of this type of catalysts as active components of the three-way catalysts for the treatment of exhaust gas from automobiles [2] has recently met a rapid increase. Owing to the number of possible applica- tions of this type of materials, a detailed study of their different types of active sites is of interest. Experimental Ceria–zirconia mixed oxides with different molar com- position were synthesised by Rhône–Poulenc using a precipitation route from nitrate precursors. The pure ceria sample was also provided by Rhône–Poulenc whereas the pure zirconia sample was supplied from Norton. The pure and mixed oxides were stabilised by calcination in air at 550°C for 4 h, and then pelleted using a hydraulic press (2.2⋅10 4 N cm –2 ). Finally, pel- lets were crushed and sieved to obtain grains with 0.3–0.5 mm diameter. The textural properties were determined by N 2 adsorption–desorption at –196°C in a Micromeritics ASAP 2010 equipment. The samples were previously evacuated overnight at 300°C under high vacuum. A Philips X’PER MPD-System X-ray diffractometer with CuK α radiation (λ=1.5406 ) and Ni filter was used for the X-ray diffraction (XRD) studies. Unit cell parameters were estimated from the intensities of lines (111) of CeO 2 and Ce x Zr 1–x O 2 (x>0.5), (101) of Ce x Zr 1–x O 2 (x≤0.5) and (111) and (101) of ZrO 2 (monoclinic and tetragonal structure, respectively). A Micromeritics AutoChem 2910 instrument was used for temperature-programmed investigations. Prior to analysis the samples were first calcinated at 550°C in a 5% O 2 /He stream and then cooled down in a N 2 flow. The temperature-programmed desorption (TPD) analysis required a previous saturation of the sample with the respective probe molecule followed by re- moval of physisorbed species with a desorption treat- ment in a He stream at 10 K min –1 heating rate from 100 to 550°C. The 10 K min –1 temperature-programmed reduc- tion (TPR) was carried out under an Ar+5% H 2 flow from 50 to 950°C. Results and discussion Table 1 summarises the textural properties of the investigated ceria–zirconia mixed oxide catalysts. Table 1 data show a surface area of about 100 m 2 g –1 in the composition range 50–80% CeO 2 , whereas 86 and 51 m 2 g –1 are the values found for 15% CeO 2 and pure ZrO 2 , respectively. Composition and pore size seem not clearly correlated to each other, possibly because of the transition from cubic to tetragonal 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2005 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 80 (2005) 225–228 CHARACTERISATION OF THE CATALYTIC PROPERTIES OF CERIA–ZIRCONIA MIXED OXIDES BY TEMPERATURE- PROGRAMMED TECHNIQUES J. I. Gutiérrez-Ortiz, B. de Rivas, R. López-Fonseca and J. R. González-Velasco * Chemical Technologies for Environmental Sustainability Group, Department of Chemical Engineering, Faculty of Science and Technology, Universidad del País Vasco/EHU, P.O. Box 644, 48080 Bilbao, Spain The catalytic properties of ceria–zirconia mixed oxides have been characterised using temperature-programmed techniques such as temperature-programmed desorption of ammonia and water, and temperature-programmed reduction with hydrogen. The acidity and hydrophobicity of these materials increased with zirconia content while the reducibility was maximum for Ce 0.5 Zr 0.5 O 2 sample. Keywords: catalyst, ceria–zirconia mixed oxides, temperature-programmed techniques (TPD, TPR) * Author for correspondence: iqpgovej@lg.ehu.es