CuOx/CeO2 catalysts were prepared by adsorption-impregnation method. CO conversion was tested over the catalysts pretreated under different conditions for preferential CO oxidation in H2, and the catalysts were characterized with X-ray photoelectron spectroscopy and temperature programmed reduction. Experimental results show that there are two kinds of copper, which are Cu+ and Cu 2+ in calcined CuOx/CeO2. Among them, the Cu+ is the key active component for CO oxidation. The main reason is as follows: CO is activated by copper for CO oxidation over CuOx/CeO2, while CO can not be activated by Cu 2+ . Only when Cu 2+ is reduced to Cu+ or Cu0, the copper may be active for CO oxidation, moreover, the experimental results show that the reduction of Cu 2+ does not lead to an increase of catalytic activity. So the active species is Cu+ in CuOx/CeO2 catalysts.
