Abstract: A modified hydrothermal process method based on using urea instead of water as the solvent was used to prepare CeOHCO₃. Pure CeOHCO₃ with a single crystalline structure was produced by varying the experimental conditions. CeO₂ particles obtained from these CeOHCO₃ precursors were tested for CH₄ oxidation. The temperatures for 90% methane conversion were 604 and 647 °C for CeO₂ catalysts obtained from hexagonal and orthorhombic CeOHCO₃, respectively, indicating that the CeO₂ catalyst from hexagonal CeOHCO₃ (CeO₂-A) was more active than that from the orthorhombic form (CeO₂-D). The specific surface area and pore volume of CeO₂-A were 45 m²/g and 0.35 cm³/g, respectively, which were higher than those of CeO₂-D. H₂-TPR showed a much lower reduction temperature and enhanced reducibility with CeO₂-A. XPS and O₂-TPD results demonstrated there were more oxygen vacancies on the surface of CeO₂-A than on CeO₂-D, which implied increased oxygen mobility. The CeOHCO₃-structure dependent activity was investigated and found to originate from the morphologies of the CeOHCO₃ precursors. Hexagonal CeOHCO₃ had a rod-like shape while orthorhombic CeOHCO₃ had a sphere-like morphology. After calcination, the obtained CeO₂ had the morphology of the precursor. The difference in morphology gave CeO₂ catalysts with different texture, structure, reducibility, and thus catalytic activity.

Key words: hexagonal, cerium hydroxide carbonate, cerium oxide, rod-like, methane oxidation