Hafnium Doped Ceria Nanocomposite Oxide as a Novel Redox Additive for Three-Way Catalysts

Abstract

Catalytic oxidation of CO over CexHf1 - xO2 (CH; 8:2 mole ratio) and CexZr1 - xO2 (CZ; 1:1 mole ratio) nanocomposite oxides reveals that the former combination exhibits more conversion at much lower temperature than the latter. Thermal and textural stability of both the combinations, obtained by a coprecipitation method and calcined at 773 and 1073 K, were examined by X-ray diffraction, Raman spectroscopy, and CO-temperature programmed reduction techniques. The XRD results disclose formation of a stable Ce0.8Hf0.2O2 cubic phase in the case of the CH sample, whereas CZ suffers phase segregation from Ce0.75Zr0.25O2 to Ce0.6Zr0.4O2 at 1073 K. Oxygen storage capacity (OSC) and CO-TPR measurements reveal a high OSC and low-temperature reducibility for the CH sample due to the defective fluorite structure generated upon incorporation of smaller Hf4+ cations (0.78 Å ionic radius), in comparison to Zr4+ (0.84 Å), into the ceria cubic lattice. The observed high activity of Ce0.8Hf0.2O2 was proven to be due to the generation of lattice defects, formation of more oxygen vacancies, and easy reducibility.