Fig. 1. Improvement of the oxygen storage/release capacity ( OSC ) of CeO 2 -ZrO 2 with the same compos ition ratio ( Ce / Zr =1). The OSC was estima ted at 773 K. These sample s w e r e p r e p a r e d b y t h e f o l l o w i n g m e t h o d s . C Z 5 5 - 1 w a s p r e p a r e d b y t h e p r e c i p i t a t i o n pro ces s usi ng CeO 2 pow der and zir con yl nit rat e sol uti on. CZ5 5-2 was pre par ed by the coprecipitation process using cerium nitrate and zirconyl nitrate solutions. CZ55-3 was synthesized by the heating CZ55-2 at 1473 K under reductive condition. Oxygen storage/release capacity ( OSC ) is one o f t h e i m p o r t a n t f u n c t i o n s r e q u i r e d o f a u t o m o b i l e t h r e e - w a y c a t a l y s t s ( T W C s ) i n o r d e r t o e f f i c i e n t l y remove harmful compounds such as hydrocarbons, CO and NOx in automotive exhaust gases [1] . In the TWCs, CeO 2 is widely used as a promoter due to its high OSC according to the reversible reaction (2 CeO 2 ↔ Ce 2 O 3 + 1/2 O 2 ). However, the OSC p e r f o r m a n c e a n d t h e d u r a b i l i t y o f p u r e C e O 2 a r e s t i l l i n a d e q u a t e f o r p r a c t i c a l u s e . R e c e n t l y , o u r la bo ra to ry di sc ov er ed th at th e ad di ti on of Zr O 2 to C e O 2 w o u l d e n h a n c e t h e O S C o v e r t h a t o f p u r e Ce O 2 as we ll as im pr ov e th e th er ma l st ab il it y [2 ] . Thereafter, a considerable number of studies on the p h y s i c a l p r o p e r t i e s a n d s t r u c t u r e s o f C e O 2 - Z r O 2 h a v e b e e n m a d e b y m a n y r e s e a r c h g r o u p s ( e . g . [3] ). We have improved the OSC of CeO 2 -ZrO 2 by m o d i f i c a t i o n o f t h e p r e p a r a t i o n m e t h o d s ( F i g . 1 ) . C Z 5 5 - 1 , C Z 5 5 - 2 a n d C Z 5 5 - 3 h a v e i d e n t i c a l co mp os it io n ra ti o ( Ce / Zr =1 ), ye t th ey we re pr ep ar ed XAFS ANALYSIS OF THE LOCAL STRUCTURE OF CeO 2 -ZrO 2 MIXED OXIDES CZ55-1 CZ55-2 CZ55-3 0.8 0.4 0 Oxygen Storage/Release Capacity (m mol - O 2 /g) using different methodologies. Although the CeO 2 - Z r O 2 h a s b e e n w i d e l y u t i l i z e d f o r c o m m e r c i a l c a t a l y s t s , t h e k e y f a c t o r i n i m p r o v i n g O S C i s n o t clearly understood. The purpose of this study is to cl ar i fy th e re l at i on sh i p be tw ee n th e OS C an d th e structure of CeO 2 -ZrO 2 . We investigated the local structure around both of Ce and Zr of these CeO 2 - ZrO 2 by XAFS and clarified the cation-cation (cation = Ce, Zr ) network at atomic level [4] . Although several groups have reported XAFS an al ys es on Ce O 2 -Z rO 2 , al l of th em ha ve ut il iz ed C e L 3 - e d g e ( 5 . 7 k e V ) X A F S [ 5 ] w h i l e t h e r e h a v e been no studies employing Ce K -edge (40.5 keV ) X A F S . T h e u s a b l e d a t a r e g i o n o f C e L 3 - e d g e EXAFS is limited to ca. 3 - 9 Å -1 in electron wave number ( k ) because of the presence of Ce L 2 -edge (6.2 keV ). Since Ce and Zr contributions to EXAFS signals are remarkable in the high- k part, the XAFS measurement of the Ce K -edge with a wide k -range i s ne c e s s a r y in or d e r to ob t a i n th e pr e c i s e in f o r m a t i o n Fig. 2. Fourier-transformed k 3 χ data from Ce K-edge EXAFS. 0 1 2 3 4 5 6 0 12 10 14 2 4 8 6 R (Å) Magnitude of FT (Å -4 ) Zr - Zr, Ce Z r - O CZ55-1 CZ55-2 CZ55-3 on Ce-Ce and Ce-Zr bonding. The high-energy X- ray at SPring-8 makes it possible to record XAFS spectra with an excellent signal-to-noise ratio at the K -edges of heavy elements [6 ]. Ce K -edge (40.5 keV ) and Zr K -edge (18.0 keV ) X A F S s p e c t r a w e r e m e a s u r e d a t B L 0 1 B 1 a n d BL16B2 . Measurements were carried out using a S i ( 3 1 1 ) d o u b l e c r y s t a l m o n o c h r o m a t o r i n a t r a n s m i s s i o n m o d e a t r o o m t e m p e r a t u r e . T h e d e t a i l e d p r o c e d u r e o f d a t a r e d u c t i o n h a v e b e e n de sc ri be d el se wh er e [7 ] . Fo ur ie r tr an sf or ma ti on s ( F T s ) w e r e p e r f o r m e d o n C e a n d Z r K - e d g e s EXAFS spectra in about 3.0 - 17.0 Å -1 region. The FTs of the Ce K -edge EXAFS spectra are presented in Fig. 2 . The position and amplitude of th e Ce -O pe ak s fo r CZ 55 -1 , CZ 55 -2 an d CZ 55 -3 are slightly different from each other. The CZ55-2 an d CZ 55 -3 ex hi bi t lo we r Ce -c at io n (c at io n = Ce , Zr ) pe ak in te ns it ie s th an th at of CZ 55 -1 . Ad di ti on al ly the Ce-cation peak of CZ55-3 appears to have split in two. The FTs of the Zr K -edge EXAFS spectra ar e sh ow n in Fi g. 3 . He re , th e sh ap e of FT s fo r C Z 5 5 - 1 , C Z 5 5 - 2 a n d C Z 5 5 - 3 a r e o b v i o u s l y d i f f e r e n t . A c c o r d i n g t o t h e r e s u l t s m e n t i o n e d above, it is thus suggested that the OSC exhibits a s i g n i f i c a n t c o r r e l a t i o n w i t h t h e l o c a l s t r u c t u r e a r o u n d C e a n d Z r . A q u a n t i t a t i v e c u r v e - f i t t i n g an al ys is wa s pe rf or me d fo r ca ti on -c at io n sh el ls in FTs to clarify this network. First, the Ce-cation shell f o r C Z 5 5 - 1 w a s f i t t e d w i t h a s i n g l e C e - C e s h e l l , while the Zr-cation shell was also fitted with a single Zr -Z r sh el l. Th us , CZ 55 -1 co ns is ts of pu re Ce O 2 a n d Z r O 2 e x i s t i n g s i m u l t a n e o u s l y ( F i g . 4 ( a ) ) . Se co nd ly , in th e ca se of CZ 55 -2 , no t on ly Ce -C e ( Zr-Zr ) but also Ce-Zr ( Zr-Ce ) shells were required t o o b t a i n a n a p p r o p r i a t e f i t f o r t h e c a t i o n - c a t i o n s h e l l a t t h e C e ( Z r ) K - e d g e . T h e C e - c a t i o n s h e l l was fitted with Ce-Ce (coordination number; CN = 8.0 ) and Ce- Zr ( CN = 3.6 ) she lls . The CN of the Ce -C e sh el l is la rg er th an th at of th e Ce -Z r sh el l. L i k e w i s e t h e C N o f t h e Z r - Z r ( C N = 3 . 0 ) w a s n o t equal to that of the Zr-Ce ( CN = 4.0). This indicates that the CeO 2 -ZrO 2 solid solution in CZ55-2 forms, b u t C e r i c h d o m a i n a n d Z r r i c h o n e s t i l l r e m a i n Magnitude of FT (Å -4 ) 0 1 2 3 4 5 6 0 12 2 10 4 8 6 14 C e - Ce, Zr C e - O CZ55-1 CZ55-2 CZ55-3 R (Å) Fig. 3. Fourier-transformed k 3 χ data from Zr K-edge EXAFS. Fig. 4. Model illustration of the cation-cation network for the CeO 2 -ZrO 2 samples with the same chemical composition ( Ce / Zr = 1). CZ55-1 consists of pure CeO 2 and ZrO 2 . Ce rich domain and Zr rich one in CZ55-2 still remain. Ce 0.5 Zr 0.5 O 2 solid solution in CZ55-3 forms homogeneously at the atomic level. (a) CZ55-1 (b) CZ55-2 (c) CZ55-3 ( Fig. 4 (b) ). Finally, the Ce-cation shell for CZ55-3 was fitted with Ce-Ce ( CN = 6.0) and Ce-Zr ( CN = 6.0) shells. The Zr-cation shell also was fitted with Zr-Zr ( CN = 6.0) and Zr-Ce ( CN = 6.0) shells. The e v a l u a t e d v a l u e s c o r r e s p o n d w i t h t h e C e / Z r composition ratio = 1 of the sample. It is clear that t h e C e 0 . 5 Z r 0 . 5 O 2 s o l i d s o l u t i o n i n C Z 5 5 - 3 f o r m s h o m o g e n e o u s l y a t a t o m i c l e v e l ( F i g . 4 ( c ) ) . W e conclude from these results that the OSC increases with enhanced homogeneity of the CeO 2 -ZrO 2 solid solution at the atomic level. Thus the relationship between the OSC and the c a t i o n - c a t i o n n e t w o r k o f C e O 2 - Z r O 2 h a s b e e n e l u c i d a t e d , h o w e v e r , t h e m e c h a n i s m o f t h e O S C Y a s u t a k a N a g a i a , T a k a s h i Y a m a m o t o b a n d Tsunehiro Tanaka b (a) Toyota Central R&D Labs., Inc. (b) Kyoto University E-mail: e1062 @ mosk.tytlabs.co.jp improvement is not yet completely understood. We c o n s i d e r t h a t t h e c o n f i g u r a t i o n o f t h e o x y g e n a r o u n d Z r a n d C e i s m o d i f i e d b y e n h a n c i n g t h e homogeneity of CeO 2 -ZrO 2 solid solution at atomic level. This modification should generate additional active oxygen for the OSC improvement. In order to st ud y th is po ss ib il it y we pl an to in ve st ig at e th e oxy gen sto rag e/r ele ase beh avi or of hom oge neo us Ce O 2 -Z rO 2 so li d so lu ti on by XA FS an d cl ar if y th e configuration of oxygen. Through this research, we have determined an i m p o r t a n t r u l e t h a t c a t a l y s t d e s i g n a t t h e a t o m i c l e v e l i s n e c e s s a r y i n o r d e r t o d e v e l o p h i g h performance catalysts of practical use. [ 3 ] T . O m a t a e t a l . , J . S o l i d S t a t e C h e m . 1 4 7 (1999) 573. [4] Y. Nagai, T. Yamamoto, T. Tanaka, S. Yoshida, T. Nonaka, T. Okamoto, A. Suda and M. Sugiura, J. Synchrotron Rad. 8 (2001) 616. [5] G. Vlaic et al. , J. Catal. 168 (1997) 386. [6] T. Uru ga et al. , J. Syn chr otr on Rad . 6 (19 99) 143. [7] T. Tanaka et al. , J. Chem. Soc., Farad. Trans. 84 (1988) 2987. References [1] S. Matsumoto, Toyota Tec. Rev. 44 (1994) 10. [2] M. Ozawa et al. , J. Alloys Comp. 193 (1993) 73.
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