Ceria zirconia solid solution nanopowders have been widely used as automobile three-way catalysts to adjust the local oxygen environment in order to reduce pollution. The origin of this application is the ability of cerium-based oxides to reversibly form mixed +3 and +4 valence oxides leading to excellent oxygen storage capacity. Considerable attention has been paid to understand the fundamental mechanisms that may improve the low temperature reducibility and reduction fraction after high temperature reduction treatment [1]. The redox behavior of Ce is difficult to observe, as partially reduced cerium oxide is unstable at low temperatures and/or in high oxygen partial pressure. For this reason, we have undertaken a detailed in situ environmental TEM study of the dynamic nanostructural and nanochemical changes that take place in ceria zirconia during redox cycles [2]. We are investigating correlations between the structure, chemistry and reducibility of individual ceria zirconia nanoparticles and hope to identify the critical factors that may improve their performance [2-3].

High surface area samples of 50%CeO₂50%ZrO₂ samples were prepared by a spray freezing method [3]. Samples were calcined at 500^oC for 5h in air and then subjected to one redox cycle (reduced in H₂ at 1000^oC for 2.75hs and subsequently re-oxidized in air). In situ nanocharacterization was performed in an environmental scanning transmission electron microscopy (ESTEM) Tecnai F20, operated at 200KV and equipped with a Gatan imaging filter (GIF) and annular dark-field (ADF) detector [4]. Ceria-zirconia powder was dispersed over Pt grids and loaded into the microscope in a Gatan heating holder. The samples were heated progressively up to 600^oC in 1.5 Torr of dry H₂. Time and temperature resolved high resolution electron microscopy (HREM), ADF images and electron energy-loss spectra (EELS) were recorded to follow the structural and chemical changes during the reduction. We used the Zr M_4,5 edge at 180eV and CeM_4,5 at 883 and 901 eV signals to determine the Ce/Zr concentration ratio. The Ce M_4,5 integrated white line ratio was used to determine the Ce oxidation state changes in individual nanoparticles [2].

A combination of high resolution images, electron diffraction and EELS of individual nanoparticles in a nominal solid solution of ceria zirconia was recorded at different temperatures in 1.5 Torr of H₂. Nanoparticles with similar size (~20nm) were sometimes observed to have different redox property. For example, a comparison of reduction behavior of two particles as a function of temperature confirmed that activity in one particle is higher as its oxidation state changed from +4 to +3 while negligible change was observed for the other. Starting valence state of reducible particle was +4 while the other particle exhibited a lower valence state at room temperature. High resolution images show that structure of one particle has changed during ex-situ heat treatment in reducing environment, probably due to oxygen vacancy ordering. Statistical measurements obtained from 10 individual particles show that the activity of Ce may be directly related to the variation in local chemical composition (Ce/Zr ratio) and was observed to drop with increasing Ce concentration. Detailed results will be presented and discussed in terms of the correlation between redox activity, structure and chemistry of individual nanoparticles.

References

[1] Fornasiero P. et al, J. Catal. 164(1) (1996) 173; Yeste M.P. et al, Chem. Mater. 18 (2006) 2750.

[2] Sharma R. et al, Phil. Mag. 84 (2004) 2731.

[3] Wang R.G. et al, J. Phys. Chem. B. 110(37) (2006) 18278.

[4] Sharma R., J. Mat. Res. 20 (2005) 1695.

[5] The support from the National Science Foundation (NSF-CTS-0306688) and the use of TEM at the John M. Cowley Center for High Resolution Microscopy at Arizona State University are gratefully acknowledged.