Preferential oxidation (PROX) of CO in hydrogen-rich steam reformates was investigated using CuO-CeO2, CuO-CeO2-ZrO2, and CuO-ZrO2 catalysts. CuO (1-10 wt%) samples supported over high-surface-area (S-BET = 117-172 m(2)/g), cubic CeO2, CeO2-ZrO2, and ZrO2 were synthesized by coprecipitation. The composition of the support markedly influenced the PROX activity. Both CuO-CeO2 and CuO-CeO2-ZrO2 exhibited higher activity and selectivity in CO oxidation than CuO-ZrO2. The adverse influence of H2O was accentuated in catalysts containing ZrO2. Below 423 K and over CuO-CeO2 with less than 5 wt% CuO, the presence of H2O in the feed suppressed CO oxidation. H2O had a negligible effect on H-2 oxidation. The catalysts showed stable activity in long-term experiments with the realistic feeds. The catalysts were characterized by XRD, surface area, TPR, diffuse reflectance UV-visible, EPR, and magnetic-susceptibility techniques. While a small amount of copper might be incorporated in the CeO2/ZrO2 fluorite lattice (forming a solid solution), most of it was at the surface of the support as isolated monomeric (types I and II) and dimeric (type IV) copper oxo species, nano-sized copper clusters containing magnetically interacting copper ions (type III) and a CuO-like, bulk phase. While the isolated copper oxo species exhibited reversible reduction-oxidation behavior, the interacting copper and CuO-Iike phases exhibited irreversible reduction behavior. The amount and reducibility of CuO on different supports correlated with their CO oxidation activities and increased in the order CuO-ZrO2 less than or equal to CuO-CeO2-ZrO2 < CuO-CeO2. (C) 2003 Elsevier Inc. All rights reserved.