Total oxidation of propene at low temperature over Co3O4-CeO2 mixed oxides: role of surface oxygen vacancies and bulk oxygen mobility in the catalytic activity

Co3O4, CeO2 and Co3O4-CeO2 mixed oxides with Co/Ce nominal atomic ratio 0.1:5, prepared by coprecipitation method with sodium carbonate, were tested in the oxidation of propene under lean condition and the catalyst stability was checked by performing three consecutive heating-cooling cycles. Characterization of the textural properties were performed by surface area measurement BET, X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. Among the Co3O4-CeO2 mixed oxides, Co3O4 (30 wt%)-CeO2 (70 wt%) gives the best activity attaining full propene conversion at 250 8C. This sample is characterized by the presence of Co3O4 particles well dispersed and in good contact with ceria according to BET and XRD data and as evidenced by SEM micrographs. Oxygen temperature-programmed desorption (O2-TPD) and C3H6-temperature-programmed reduction (C3H6-TPR) experiments were carried out in order to study the surface and bulk oxygen mobility and to correlate it to the activity. At temperature around 200 8C, O2-TPD experiments showed the desorption of mobile surface oxygen species for the most active samples, Co3O4 and Co3O4 (30 wt%)-CeO2 (70 wt%). C3H6-TPR experiments for both of the oxides also evidenced a high reactivity at low temperature, especially, for Co3O4 (30 wt%)-CeO2 (70 wt%) giving at 345 8C an intense peak of CO2 formation. Conversely, the ceria sample showed by C3H6-TPR much less pronounced oxygen bulk mobility, starting to react with propene above 500 8C and forming only CO. In this case, the catalytic activity of ceria was explained in terms of formation of surface oxygen vacancies which are relevant to the propene oxidation in presence of gaseous oxygen.