自動車触媒用酸素貯蔵材料CeO ZrO ... - jssrr.jp

1 480 1192 TEL: 0561 63 5283 FAX: 0561 63 6150 E-mail: e1062 ( ) CeO2 ZrO2 XAFS XAFS Analysis of CeO2 ZrO2 Mixed Oxides as OxygenStorage Promoters in Automotive CatalystsYasutaka NAGAITOYOTA Central R&D Labs., oxygen storage/release capacity(OSC)is one of the important functions required for automobile three-waycatalysts in order to e ciently remove harmful compounds such as hydrocarbons, CO and NOx in automotive exhaustgases. In this report, three types of CeO2 ZrO2(Ce:Zr 1:1 molar ratio)compounds with di erent OSC were character-ized by means of the Ce K-edge and Zr K-edge XAFS.

2 In order to investigate the relationship between the OSC and localstructure, the quantitative EXAFS curve- tting analysis was applied. By enhancing the homogeneity of the Ce and Zr a-toms in the CeO2 ZrO2solid solution, the OSC performance increased. Additionally, the local oxygen environmentaround Ce and Zr was remarkably modi ed by enhancing the homogeneity of the CeO2 ZrO2solid solution. It was postu-lated that the enhancement of the homogeneity of the CeO2 ZrO2solid solution and the modi cation of the oxygen en-vironment would be the source for the OSC improvement.. CO NOx HC 1977 1/10 1)

3 6 1 2 1 2 O2 CeO2 ZrO2.

4 CeO2 ZrO2 Fig. 1 / A/F (C) 2002 The Japanese Society for Synchrotron Radiation Research Figure 1. Removal e ciencies for three-way catalysts as a functionof air to fuel ratio of engine exhaust 2. Three-way catalyst 3. Improvement of an oxygen storage/release capacity(OSC)of CeO2 ZrO2with the same composition ratio(Ce/Zr 1).The OSC was estimated at 773 K. These samples were prepared byfollowing methods. CZ55 1 was prepared by the precipitationprocess using CeO2powder and zirconyl nitrate solution. CZ55 2was prepared by the coprecipitation process using cerium nitrate andzirconyl nitrate solutions.

5 CZ55 3 was synthesized by the heatingCZ55 2 at 1473 K under reductive condition. ( ) A/F CO, HC, NOx Fig. 2 A/F CeO2 CeO2 Ce4 Ce3 V CeO2 CeO2 x x/2O2 x 0 Oxygen Storage/ReleaseCapacity; OSC / CeO2 OSC CeO2 OSC CeO2 1980 CeO2 ZrO2 OSC 2 4) OSC Fig.

6 3 CeO2 ZrO2 .CeO2 ZrO2 XAFS CeO2 ZrO2 CeO2 ZrO2 5 11) CeO2 ZrO2 OSC Ce Zr Ce/Zr 1 OSC Fig. 3 3 CeO2 ZrO2 CZ55 1 CZ55 2 CZ55 3 OSC XAFS(X-ray AbsorptionFine Structure) Ce Zr 12 14) Ce K edge Zr K edge keV XAFS SPring-8 BL01B1 BL)17) BL16B2 13 BL)13) XAFS CeO2 ZrO2 6,15) Ce XAFS Ce L3 edge keV Ce K edge keV Figure 4.

7 Fourier-transformedk3xdata of(a)Ce K-edge and(b)Ce L3-edge EXAFS of CeO2 5. Fourier-transformedk3xdata of Zr K-edge EXAFS ofCeO2 6. Zr K-edge xanes spectra of CeO2 ZrO2samples. ( ) Ce L3 edge EXAFS k CeL3 edge Ce L2 edge keV 3 9 1 EXAFS Ce Zr 10 1 16) Ce Ce Ce Zr k Ce K edge SPring-8 X K edge 17) . Ce K edge Zr K edge EXAFS XAFS Ce K edge EXAFS Fig. 4(a) Ce O Ce-cation(cation Ce, Zr) Ce O CZ55 1, CZ55 2 CZ55 3 CZ55 2 CZ55 3 Ce-cation CZ55 1 CZ55 3 Ce-cation 2 Ce-cation Ce L3 edge Fig.

8 4 b CZ55 2 CZ55 3 Ce L3 edge EXAFS Ce-cation Ce-cation Ce K edge Zr K edge EXAFS Fig. 5 Zr O Zr-cation Ce K edge CZ55 1 CZ55 2 CZ55 3 Ce Zr OSC . Zr K edge xanes Zr K edge xanes Fig. 6 A B A CZ55 3 CZ55 1 CZ55 2 pre-edge 1s 4d 1s 4d ZrO2 Zr O tetragonal ZrO2 CZ55 1 xanes Figure 7.

9 Model illustration of cation-cation network for the CeO2 ZrO2samples with the same composition(Ce/Zr 1).CZ55 1consistsofpureCeO2and ZrO2solid solution forms in CZ55 2, but the Ce rich domain andZr rich one still remain. The 3 homogeneously forms at an atomic 1. Results of curve- tting analysis for Ce-cation shellsa SampleBondCNR Ds2b 2 cubic CeO2c Ce 1Ce 2 0 1 CZ55 2Ce 4 0 2 Ce 5 0 13 CZ55 3 0 2 Ce 3 0 3 a The standard deviation is given in Relative Debye-Waller factorc Standard compoundTable 2. Results of curve- tting analysis for Zr-cation shellsa SampleBondCNR Ds2b 2 cubic ZrO2c Zr Zr 1Zr 2 0 1 CZ55 2Zr 6 0 19 Zr 3 0 3 CZ55 3Zr 4 0 6 Zr 3 0 1 a The standard deviation is given in Relative Debye-Waller factorc 8mol Y-doped cubic ZrO2as standard compound ( )tetragonal ZrO2 18) A CZ55 3 Zr O CZ55 1 CZ55 2 B CZ55 1 B CZ55 2 2 CZ55 3 Li Y2O3 ZrO2 B 19)

10 Li Y2O3 ZrO2 Y2O3 3mol 20 mol B Y2O3 A Li ZrO2 CeO2 CZ55 1 CZ55 2 CZ55 3 CeO2 ZrO2 . CeO2 ZrO2 Ce/Zr CeO2 ZrO2 Ce/Zr Ce K edge Zr K edge cation-cation Table 1 Table 2 CeO2 ZrO2 cation-cation Fig. 7 CZ55 1 Ce-cation Ce Ce Zr-cation Zr Zr Ce Ce cubic CeO2 CZ55 1 CeO2 ZrO2 CeO2 ZrO2 Fig.