Absorbance 71000 71500 72000 72500 0.2 E (eV) Re/ γ -Fe 2 O 3 cata lyst s poss ess a char acte risti c a c t i v i t y o n m e t h a n o l p a r t i a l o x i d a t i o n r e a c t i o n t o produce methylal ( CH 2 ( CH 3 O ) 2 ) selectively [1]. The re ac ti on pr oc ee ds un de r a mi ld co nd it io n an d th e s e l e c t i v i t y i s m o r e t h a n 9 0 % . I n a d d i t i o n , t h e li fe ti me of th e Re / γ -F e 2 O 3 is qu it e lo ng . In ma ny c a s e s , r h e n i u m f o r m s s u b l i m a b l e o x x i d e s s u c h a s R e 2 O 7 u n d e r t h e o x i d a t i v e c o n d i t i o n s a n d c a n n o t b e u s e d a s o x i d a t i o n c a t a l y s t s . T h u s , R e O / γ - Fe 2 O 3 , an ac ti ve an d lo ng li vi ng rh en iu m ca ta ly st unde r oxid ativ e cond itio ns, is quit e unus ual. This m a y b e d u e t o a n u n u s u a l l o c a l s t r u c t u r e o f t h e rhenium species. T o e l u c i d a t e t h e s t r u c t u r e o f t h e r h e n i u m s p e c i e s , w e m e a s u r e d t h e R e K - e d g e E X A F S o f Re / γ -F e 2 O 3 an d Re / Al 2 O 3 at SP ri ng -8 . As Fe 2 O 3 support absorbs X-rays at the energy range of Re L I I I - e d g e ( 1 0 - 1 1 k e V ) , R e L I I I - e d g e E X A F S i s d i f f i c u l t t o m e a s u r e b e c a u s e o f i t s l a r g e background. On the other hand, as Fe 2 O 3 support does not absorb X-rays in the energy range of Re K -ed ge (71 - 72 keV ), Re K- edg e EXA FS can be measured with small background. Thus Re K- edge EXAFS is a useful technique to elucidate the local s t r u c t u r e a r o u n d R e s p e c i e s i n R e / γ - F e 2 O 3 catalysts. C a t a l y s t p r e p a r a t i o n h a s b e e n d e s c r i b e d e l s e w h e r e [ 1 ] . R e K - e d g e E X A F S s p e c t r a o f R e O x x / γ - F e 2 O 3 a n d R e O / A l 2 O 3 c a t a l y s t s w i t h v a r i o u s R e l o a d i n g s w e r e m e a s u r e d a t b e a m l i n e BL 01 B1 wi th th e tr an sm is si on mo de at ca . 10 K. The spectra were analyzed by use of the UWXAFS p a c k a g e [ 2 ] . A f t e r b a c k g r o u n d s u b t r a c t i o n , k 3 w e i g h t e d E X A F S f u n c t i o n s w e r e F o u r i e r tra nsf orm ed ( FT ) to the R-s pac e and fit ted in thi s space. The FT k range and fitting R-ranges were 4 0 – 1 4 0 n m - 1 a n d 0 . 1 – 0 . 4 n m , r e s p e c t i v e l y . Investigation of Local Structure of Re Active Site in Re/ γ γ -Fe 2 O 3 Catalysts by Re- K Edge EXAFS B a c k s c a t t e r i n g a m p l i t u d e s a n d p h a s e s h i f t s w e r e c a l c u l a t e d b y t h e F E F F 8 c o d e [ 3 ] . T h e l i n e broadening due to the resolution of monochromator an d co re -h ol e li fe ti me wa s es ti ma te d to be 15 eV and was taken into account to calculate the phase and amplitude. Figure 1 shows an X-ray absorption spectrum of R e ( 9 . 2 w t % ) / γ - F e 2 O 3 . E X A F S o s c i l l a t i o n s w e r e observed in a wide energy range even though the ne ar ed ge st ru ct ur e wa s sm ea re d by br oa de ni ng . F i g u r e s 2 s h o w F o u r i e r t r a n s f o r m e d E X A F S f u n c t i o n s of Re/ γ -Fe 2 O 3 and Re/ Al 2 O 3 catalysts together with the result of curve fitting analysis. Phase shift was n o t c o r r e c t e d i n t h e s e f i g u r e s . T a b l e I s h o w s s t r u c t u r a l p a r a m e t e r s o b t a i n e d b y c u r v e f i t t i n g analysis for Re/ γ -Fe 2 O 3 and Re/ Al 2 O 3 catalysts. In the EXAFS functions of Re/ γ -Fe 2 O 3 , a strong peak o f R e - O w a s o b s e r v e d a t 0 . 1 7 5 n m i n a l l R e l o a d i n g s . I n a d d i t i o n , a p e a k f o r R e - F e w a s observed at 0.382 nm in EXAFS of Re(9.2 wt %)/ γ - Fe 2 O 3 . On th e co nt ra ry , Re -O co nt ri bu ti on in th e EXAFS of Re/ Al 2 O 3 was a convolution of two Re-O b o n d i n g s , a t 0 . 1 7 5 n m a n d 0 . 2 n m . I n a d d i t i o n , F i g . 1 . R e K - e d g e X - r a y a b s o r p t i o n spectrum of Re (9.2 wt %)/ γ -Fe 2 O 3 . 53 (c) (d) (a) (b) 0 2 4 0 2 4 6 -12 -8 -4 0 4 8 12 -12 -8 -4 0 4 8 12 FT [ k 3 χ χ (k)] R (10 -1 nm ) Fig. 2. Fourier transformed Re K-edge E X A F S f u n c t i o n s ( k 3 χ ( k ) ) o f Re(1.7wt%)/ γ -Fe 2 O 3 (a) , Re(9.2wt%)/ γ - Fe 2 O 3 (b) , Re(2.0wt%)/Al 2 O 3 (c) , and Re(9.2wt%)/Al 2 O 3 (d) . Red and blue lines represent observed and calculated EXAFS functions, respectively. T. Shido, F. Nakagawa and Y. Iwasawa The University of Tokyo E-mail: shido @ chem.s.u-tokyo.ac.jp s e v e r a l p e a k s c o r r e s p o n d i n g t o R e O 2 w e r e observed in EXAFS of Re(9.2 wt %)/ Al 2 O 3 . A Re K - e d g e E X A F S a n a l y s i s r e v e a l e d t h a t t e t r a h e d r a l R e O 4 s t r u c t u r e w a s s t a b i l i z e d b y f o r m i n g R e - F e b i n a r y o x i d e o n γ - F e 2 O 3 s u p p o r t . T h e R e O 4 s t r u c t u r e i s u n s t a b l e o n t h e A l 2 O 3 s u r f a c e a n d c o n v e r t s t o R e O 2 . F i g u r e 3 s h o w s t h e p r o p o s e d structure of an Re-Fe binary oxide formed on Re/ γ - Fe 2 O 3 ca ta ly st . Th e Re O 4 st ru ct ur e bo un d to Fe i o n a n d t h e f o r m a t i o n o f s u b l i m a b l e R e 2 O 7 w a s p r o h i b i t e d . A R e K - e d g e E X A F S s t u d y r e v e a l e d that active ReO 4 species were stabilized by forming Re-Fe binary oxide on the γ -Fe 2 O 3 surface. Re – O : 0.175 nm Re – Fe : 0.382 nm Re Fe O Fig. 3. Proposed structure of Re active site on Re/ γ -Fe 2 O 3 catalysts. Table I . Structural parameters of supported Re catalysts delivered by curve fitting analysis of Re K -edge EXAFS. (a) coordination number (b) interatomic distance (c) Debye-Waller factor Re-O Re-O Re-Fe Re-O Re-O Re-O Re-O Re ( 1.7wt %)/ γ -Fe 2 O 3 (0, 1.0) 4 4 1 4 1 1 3 1.75 1.75 3.82 1.75 1.95 1.76 2.00 3 3 20 1 1 4 Sample ( ∆ ∆ E 0 / eV, R f (%)) shell CN (a) R (b) / 0 . 1 n m σ 2 (c) / 1 0 - 5 n m 2 Re ( 9.2wt %)/ γ -Fe 2 O 3 (-5, 0.9) Re ( 2.0wt %) / Al 2 O 3 (0, 0.8) 3 Re ( 9.2wt %) / Al 2 O 3 (7, 2.7) References [ 1 ] Y . - Z . Y u a n e t a l . , J . C h e m . S o c . C h e m . Commun. (2000) 1421. [2] E.A. Stern et al. , Physica B 208 (1995) 117. [3] A.L. Ankudinov et al. , Phys. Rev. B 58 (1998) 7565. [ 4 ] T . S h i d o , F . N a k a g a w a a n d Y . I w a s a w a , i n preparation. 54
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