Ceria-zirconia

Fluorite-like crystal structure of ceria and cubic zirconia

Definition

Ceria-zirconia is the name given to a solid solution of cerium (IV) oxide CeO2 (ceria) and zirconium oxide ZrO2 (zirconia).[1]

Crystal structure and stability

The crystal structure adopted by ceria-zirconia depends on the Zr/Ce ratio and temperature. At very low Zr concentrations, ceria-zirconia exhibits the cubic fluorite structure, which is common to both pure ceria and cubic zirconia. However, at higher Zr contents, other crystal structures are formed, including two different tetragonal phases at intermediate Zr concentrations, and a monoclinic phase at very high Zr concentrations.[2]

There is both experimental[3][4] and theoretical[5] evidence showing that the decomposition of ceria-zirconia into Ce-rich and Zr-rich oxides is thermodynamically favorable in a wide range of solid solution compositions, which means that ceria-zirconia is metastable with respect to phase separation.

Technological importance

Ceria-zirconia is widely used as a component in current three-way catalytic converters.[6] The ceria-based component of the converter has several functions, including promoting the dispersion of the noble metals in the catalyst, but also storing and releasing oxygen.[7] The incorporation of zirconium in modern converters, forming ceria-zirconia, improves the performance of the catalyst by enhancing the resistance of the material to sintering, and simultaneously increasing the ability of the oxide to accommodate oxygen vacancies in its structure.[6]

References

  1. ↑ MEMPRO Materials. "Ceria Zirconia Mixed Metal Oxide".
  2. ↑ Yashima, Masatomo; Arashi, Haruo; Kakihana, Masato; Yoshimura, Masahiro (1994). "Raman Scattering Study of Cubic-Tetragonal Phase Transition in Zr1-xCexO2 Solid Solution". Journal of the American Ceramic Society. 77 (4): 1067–1071. doi:10.1111/j.1151-2916.1994.tb07270.x. ISSN 0002-7820.
  3. ↑ Di Monte, Roberta; Fornasiero, Paolo; Desinan, Stefano; Kašpar, Jan; Gatica, José M.; Calvino, José J.; Fonda, Emiliano (2004). "Thermal Stabilization of CexZr1-xO2Oxygen Storage Promoters by Addition of Al2O3: Effect of Thermal Aging on Textural, Structural, and Morphological Properties". Chemistry of Materials. 16 (22): 4273–4285. doi:10.1021/cm048829q. ISSN 0897-4756.
  4. ↑ Lee, Theresa A.; Stanek, Christopher R.; McClellan, Kenneth J.; Mitchell, Jeremy N.; Navrotsky, Alexandra (2011). "Enthalpy of formation of the cubic fluorite phase in the ceria–zirconia system". Journal of Materials Research. 23 (04): 1105–1112. doi:10.1557/jmr.2008.0143. ISSN 0884-2914.
  5. ↑ Grau-Crespo, R.; de Leeuw, N. H.; Hamad, S.; Waghmare, U. V. (2011). "Phase separation and surface segregation in ceria-zirconia solid solutions". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 467 (2131): 1925–1938. doi:10.1098/rspa.2010.0512. ISSN 1364-5021.
  6. 1 2 Di Monte, Roberta; Kašpar, Jan (2005). "Heterogeneous environmental catalysis – a gentle art: CeO2–ZrO2 mixed oxides as a case history". Catalysis Today. 100 (1-2): 27–35. doi:10.1016/j.cattod.2004.11.005. ISSN 0920-5861.
  7. ↑ Trovarelli, Alessandro (1996). "Catalytic Properties of Ceria and CeO2-Containing Materials". Catalysis Reviews. 38 (4): 439–520. doi:10.1080/01614949608006464. ISSN 0161-4940.
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