Redox Behavior Below 1000K of Pt-Impregnated CeO 2 -ZrO 2 Solid Solutions: An In-Situ Neutron Diffraction Study
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IArgonne National Laboratory, Argonne, IL 60439, U. S. A. Nagoya Institute of Technology, Tajimi, Gifu, 507, Japan.
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Figure 3. The unit-cell volumes of the tetragonal and monoclinic phases of Ceo. 1 Zro.902 (a) and Ce 01.Zr 0 .9 0 2 /Pt (b) versus temperature. The samples were first heated in 2%0 2 /Ar (+) and then switched to I%CO/Ar and continued heating to 7000 C (o). Each data point represents 1h data collection. CONCLUSION The lattice parameters of the tetragonal and monoclinic phases of Ce 01.Zr 0 .902 with and without Pt impregnation increase linearly with increasing temperature over the 25o-700o C range. From Fig. 1 the thermal expansion coefficients for the tetragonal phase at -300° C are ota = 9.5 X 10-6 /°C and c = 13.1 X 10-6 /°C. These values are comparable with those reported for Sc, In or Yb stabilized zirconia.[9] Switching from an oxidizing atmosphere for 25°-400'C to a reducing one for 400'-700' C in heating the Ce-ZrO2 sample does not induce an anomaly in the thermal expansion. Since the observed d-spacing reflects the long-range order structure in the crystalline grains, the lack of anomaly implies that the reduction of Ce ions, if any, occurs mainly on the surfaces the oxide particles. If Ce-ZrO 2 is impregnated with Pt metal, on the other hand, an increase of the axial lattice parameter (c) of the tetragonal phase when switching from oxidizing to reducing conditions was observed. Consequently, the tetragonality decreases. This suggests a gradual (over several hours) conversion of the 4+ to 3+ state of the Ce ions. This process probably occurs initially in the interfacial region between the oxide and Pt particles and then migrates to the bulk of the oxide lattice. This phenomenon can also be seen visually from the change of color (from light yellow before to gray after reduction) only for the Pt-impregnated sample. The original color can be recovered by heating the gray sample under an 0 2 -rich atmosphere. The origin for the color change in zirconia resulted from heat treatments was a subject of debate previously. [10-12] In the present case, it is undoubtedly caused by the redox process of Ce ions. ACKNOWLEDGMENTS Work performed at Argonne National Laboratory is supported by the U. S. DOE-BES under Contract No. W-31-109-ENG-38.
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