Ceria-zirconia

Ceria-zirconia is a solid solution of cerium(IV) oxide (CeO2, also known as ceria) and zirconium oxide (ZrO2, also known as zirconia).[1]

Fluorite-like crystal structure of ceria and cubic zirconia

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 (pure zirconia normally only adopts a cubic structure at high temperatures). 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]

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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. Bibcode:2008JMatR..23.1105L. 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. arXiv:1012.3367. Bibcode:2011RSPSA.467.1925G. doi:10.1098/rspa.2010.0512. ISSN 1364-5021.
  6. 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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