The photocatalytic oxidation of CO into CO2 With oxidants such as NO, N2O and O-2 proceeded efficiently on a MO/SiO2 with high Mo dispersion under UV light irradiation. It was found that the reaction rate greatly depended on the kind and concentration of the oxidant. Photoluminescence investigations reveal the close relationship between the reaction rate and the relative concentration of the photo-excited Mo6+- oxide species, i.e. charge transfer-excited-triplet state (Mo5+-O-)*, under steady-state reaction conditions. Moreover, the photocatalytic oxidation of CO with O-2 in excess H-2 was carried out to test suitability for applications to supplying pure H-2. This reaction was seen to proceed efficiently on MO/SiO2 with a high CO conversion of similar to 100% and CO selectivity of 99% after 180 min under UV light irradiation, showing higher photocatalytic performance than TiO2 (P-25) photocatalyst. UV-vis, XAFS, photoluminescence and FIF-IR investigations revealed that the high reactivity of the charge transfer-excited-triplet state (Mo5+-O-)*, with CO as well as the high reactivity of the photoreduced Mo-oxide species (Mo4+-species) with O-2 to produce the original Mo-oxide species (Mo6+=O2-), played a crucial role in the reactions. (c) 2005 Elsevier B.V. All rights reserved.