The activities of Ir-in-CeO2 (Ir embedded in CeO2) and Ir-on-CeO2 (Ir dispersed on the surface of CeO2) in preferential CO oxidation were comparatively studied in terms of their specific structures with different amount of surface Ir. It was found that the surface exposed Ir sites on Ir-on-CeO2 were favorable for CO oxidation at low temperature (<100 degrees C). Encapsulating Ir in CeO2 inhibited the adsorption of H-2, and also facilitated the activation of surface CeO2 on Ir-in-CeO2, which promised a high CO conversion at high temperature (>120 degrees C). Then a dual bed system with the top Ir-in-CeO2 and the bottom Ir-on-CeO2 was developed to widen the temperature window. This dual bed system is unique in affording a high CO conversion in a wide temperature window, from 80 to 200 degrees C, which is superior to single bed catalyst. (C) 2011 Elsevier B.V. All rights reserved.