Catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides for methane combustion: Influence of catalyst pretreatment temperature and oxygen concentration in the reaction mixture

Abstract

The influence of catalyst pre-treatment temperature (650 and 750°C) and oxygen concentration (λ=8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co3O4/CeO2 and Co3O4/CeO2–ZrO2 containing 30wt.% of Co3O4. The catalytic materials prepared by the co-precipitation method were calcined at 650°C for 5h (fresh samples); a portion of them was further treated at 750°C for 7h, in a furnace in static air (aged samples).Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12000 and 60000mLg−1h−1). The catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 were compared with those of two pure Co3O4 oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co3O4/CeO2 and Co3O4/CeO2–ZrO2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co3O4 phase in the temperature range of 750–800°C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co3O4 heated up to 800°C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ=8, stoichiometric λ=1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co3O4 decomposition into CoO was observed.Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co3O4 and by promoting the reduction at low temperature.