Identification of the active sites for CO and C3H8 total oxidation over nanostructured CuO-CeO2 and Co3O4-CeO2 catalysts

Nanostructured Co3O4-CeO2 and CuO-CeO2 catalysts with the specific surface areas exceeding 100 m(2) g(-1) were synthesized by a surfactant-templated method. The catalytic performance of these catalysts was investigated using the total oxidation of CO and C3H8 as model reactions. The results show that the Co3O4-CeO2 catalysts are less active for CO oxidation but are more active for C3H8 oxidation as compared with the CuO-CeO2 catalysts. Moreover, the Co3O4-CeO4 catalysts exhibit a volcano-type performance for CO oxidation with the cobalt content increasing. The in situ diffuse reflectance infrared spectroscopy (DRIFTS) study shows that CO is adsorbed mainly as carbonyl (2106 cm(-1)) and bidentate carbonate (1568 and 1281 cm(-1)) on CuO-CeO2, and only as bidentate carbonate (1591 and 1268 cm(-1)) on Co3O4-CeO2. On the basis of the results of structural characterization, redox properties, and in situ DRIFTS study, the active sites for CO and C3H8 oxidation are identified, respectively. Carbon monoxide oxidation preferentially occurs at the interface between CeO2 and CuO or Co3O4, whereas propane oxidation takes place on the neighboring surface lattice oxygen sites in CuO or Co3O4 crystallites. The different requirements of the active sites are determined by the different reaction mechanisms and the rate-determining steps. It is also found that the introduction of a small amount of Pd to Co3O4-CeO2 can remarkably promote the CO oxidation activity, but it hardly enhanced the C3H8 oxidation activity of the catalyst. The different reaction mechanisms, on molecular level, are identified and discussed in detail.