Low temperature selective CO oxidation in excess of H 2 over Pt/CeAZrO 2 catalysts Hyun-Seog Roh, H.S. Potdar, Ki-Won Jun*, Sang Yun Han, and Jae-Woo Kim Chemical Technology Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Korea Received 19 July 2003; accepted 10 January 2004 Pt/CeAZrO 2 catalysts have been designed and applied to selective CO oxidation at low temperature. Both tetragonal and cubic phase CeAZrO 2 supports were prepared by co-precipitation method to get high surface area materials after calcination at 500 °C for 6 h in air. Selective CO oxidation was conducted using stoichiometric amounts of O 2 . Cubic CeAZrO 2 supported Pt catalyst exhibited 78% CO conversion and 96% CO 2 selectivity even at 60 °C, while Pt/Al 2 O 3 catalyst showed less than l% CO conversion at the same condition. The higher CO conversion and CO 2 selectivity (to CO 2 as opposed to H 2 O) of Pt/CeAZrO 2 catalyst is mainly due to the high oxygen storage capacity of CeAZrO 2 and nano-crystalline nature of cubic Ce 0.8 Zr 0.2 O 2 . KEY WORDS: CO oxidation; low temperature; co-precipitation; Pt; CeAZrO 2 . 1. Introduction Proton-exchange membrane (PEM) fuel cells have a series of advantages such as low operating temperature, sustained operation at high current density, low weight, compactness, potential for low cost and volume, long stack life, fast start-ups, suitability to discontinuous operation. Consequently, PEM fuel cells are leading candidates for mobile power applications or for small stationary power units [1–5]. The ideal fuel for PEM fuel cells is pure hydrogen, with less than 50 ppm CO. Low temperature PEM fuel cells utilize the hydrogen produced by an external reformer. The H 2 -rich reformate typically contains several mol% CO which will poison the anode of PEM fuel cells. Thus, CO should be selectively oxidized while minimizing oxidation of H 2 . Like other catalytic reactions, the challenges of CO oxidation are activity and selectivity. Since the selective CO oxidation is carried out after the low-temperature shift reactor (~200 °C), it should operate at low temperatures [6]. The low-temperature CO oxidation has been re- viewed extensively [7]. However, the number of publi- cations on the selective CO oxidation in H 2 -rich atmospheres has only increased as a result of the interest in PEM fuel cells [1,6]. Catalysts investigated include Pt [6,8–11], Ru [9], and Au [12]. It was reported that using a low O 2 concentration minimizes hydrogen oxidation [8]. Son and Lane [6] reported that CO conversions were improved at low temperatures (100– 200 °C) and stoichiometric ratio of O 2 /CO for Ce- promoted Pt/c-Al 2 O 3 . CeO 2 has been considered as the most important rare earth element in catalysis. It plays an important role in three-way catalysis (TWC) and fluid catalytic cracking (FCC) [13,14]. Therefore, much effort has been dedicated to studying the role of ceria. As a result, it is known that the high oxygen storage capacity (OSC) of CeO 2 improves catalytic performance by storing oxygen dur- ing oxidation and releasing it during reduction [13,14]. Ce-containing mixed oxides have attracted much attention as oxidation catalysts because of their unique redox properties and high OSC [13,14]. It has been reported that the addition of ZrO 2 to CeO 2 leads to improvements in OSC of CeO 2 , redox property, thermal resistance and promotion of metal dispersion [14–16], resulting in better performance in CO oxidation [17] and combustion of methane [18]. This was found to be due to the partial substitution of Ce 4+ with Zr 4+ in the lattice of CeO 2 which results in a solid solution formation [15,17]. Because of better thermal stability as well as their enhanced oxygen mobility, the Ce 1)x AZr x O 2 system has appeared as a promising support to be used in catalytic oxidation systems. It has been reported that the cubic phase of CeAZrO 2 has more OSC and is more easily reducible than the tetragonal phase [17,19,20]. Meeyoo and co-workers [17] reported that cubic Ce 1)x AZr x O 2 (x < 0.5) showed better CO oxidation performance than tetragonal Ce 1)x AZr x O 2 (x > 0.5) owing to the advantages of cubic CeAZrO 2 in the absence of H 2 . It is also known that CeAZrO 2 prepared by the co-precipitation method has higher surface area [21]. Based on the above results, we have designed cubic Ce 0.8 Zr 0.2 O 2 supported Pt catalyst and applied to the selective CO oxidation reaction. In this study, the * To whom correspondence should be addressed. Catalysis Letters Vol. 93, Nos. 3–4, March 2004 (Ó 2004) 203 1011-372X/04/0300–0203/0 Ó 2004 Plenum Publishing Corporation