Co₃O₄ particles grown over nanocrystalline CeO₂: Influence of precipitation agents and calcination temperature on the catalytic activity for methane oxidation

Crystalline cobalt oxides were prepared by a precipitation method using three different precipitation agents, (NH₄)₂CO₃, Na₂CO₃ and CO(NH₂)₂. Cobalt oxide nanoparticles corresponding to a Co₃O₄ loading of 30 wt% were also deposited over high-surface area nanocrystalline ceria by the same precipitation agents. The effect of calcination temperature, 350 or 650 °C, on the morphological and structural properties was evaluated. Characterization by BET, XRD, SEM, TEM, Raman spectroscopy, H₂-TPR, XPS and NH₃-TPD was performed and the catalytic properties were explored in the methane oxidation reaction. The nature of the precipitation agent strongly influenced the textural properties of Co₃O₄ and the Co₃O₄-CeO₂ interface. The best control of the particle size was achieved by using CO(NH₂)₂ that produced small and regular crystallites of Co₃O₄ homogeneously deposited over the CeO₂ surface. Such a Co₃O₄-CeO₂ system precipitated by urea showed enhanced low-temperature reducibility and high surface Co³⁺ concentration, which were identified as the key factors for promoting methane oxidation at low temperature. Moreover, the synergic effect of cobalt oxide and nanocrystalline ceria produced stable full conversion of methane in the entire range of investigated temperature, up to 700-800°C, at which Co₃O₄ deactivation usually occurs.