Catalytic performances of Co3O4/CeO2 and Co3O4/CeO2-ZrO2Composite oxides for methane combustion: influence of catalyst pretreatment temperature and oxygen concentration in the reaction mixture

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, Co 3 O 4 /CeO 2 and Co 3 O 4 /CeO 2 –ZrO 2 containing 30 wt.% of Co 3 O 4 . The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, 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 (12 000 and 60 000 mL g −1 h −1 ). The catalytic performance of Co 3 O 4 /CeO 2 and Co 3 O 4 /CeO 2 –ZrO 2 were compared with those of two pure Co 3 O 4 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, Co 3 O 4 /CeO 2 and Co 3 O 4 /CeO 2 –ZrO 2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co 3 O 4 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 Co 3 O 4 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 Co 3 O 4 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 Co 3 O 4 and by promoting the reduction at low temperature.