Hafnium Doped Ceria Nanocomposite Oxide as a Novel Redox

The emergence of a new Ce0.6Zr0.4O2 (PDF-ICDD 38-1439) cubic phase accounts for these shifts due to more incorporation of Zr4+ into the ceria lattice ...
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1878

2007, 111, 1878-1881 Published on Web 01/17/2007

Hafnium Doped Ceria Nanocomposite Oxide as a Novel Redox Additive for Three-Way Catalysts Benjaram M. Reddy,*,† Pankaj Bharali,† Pranjal Saikia,† Ataullah Khan,† Ste´ phane Loridant,‡ Martin Muhler,§ and Wolfgang Gru1 nert§ Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007, India, Institut de Recherches sur la Catalyse-CNRS, 2 AVenue Einstein, 69626 Villeurbanne Cedex, France, and Lehrstuhl fu¨r Technische Chemie, Ruhr-UniVersita¨t Bochum, D - 44780 Bochum, Germany ReceiVed: December 12, 2006; In Final Form: December 29, 2006

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.

1. Introduction In recent years, considerable interest has been devoted to the synthesis and structural characterization of cerium oxidecontaining composite oxides for numerous applications ranging from catalysis to biomaterials, fuel cell technologies, gas sensors, and solid-state electrolytes.1-3 The key factor to their catalytic applications is the ability of CeO2 (Ce4+) to be reversibly reduced to Ce2O3 (Ce3+) at moderate temperatures (