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MÖSSBAUER STUDY OF SILICA SUPPORTED CATALYSTS
I. Dézsi, D. Nagy, M. Eszterle, L. Guczi
To cite this version:
I. Dézsi, D. Nagy, M. Eszterle, L. Guczi. MÖSSBAUER STUDY OF SILICA SUPPORTED CATA- LYSTS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-76-C2-77. �10.1051/jphyscol:1979226�.
�jpa-00218624�
JOURNAL DE PHYSIQUE
Colloque C2, supplkment au n " 3, Tome 40, mars 1979, page C2-76
MOSSBAUER STUDY OF S I L I C A SUPPORTED CATALYSTS
I . DCzsi, D.L. Nagy, M. ~ s z t e r l e ~ a n d L. ~ u c z i "
Central Research I n s t i t u t e for Physics,
H-1525Budapest
* ~ n s t i t u t e of Isotopes
H-1525Budapest, Hungary
Rbsum6.- On a Btudi& Fe, Fe-Ru e t Fe-Pt f o r t e m e n t d i s p e r s e s s u r d e s s u p p o r t s e n s i l i c e pour a n a l y s e r l ' 6 t a t L l e c t r o n i q u e e t s t r u c t u r e 1 du f e r pendant l a p r e p a r a t i o n d e s Q c h a n t i l l o n s . Une f o r t e d i s p e r - s i o n du f e r e s t obtenue s i on s e s e r t d'une s o l u t i o n P p H = 1 pour l 1 i m p r L g n a t i o n . La f o r t e d i s p e r - s i o n e s t r e t e n u e pendant l e s proced6s de c a l c i n a t i o n e t de r e d u c t i o n .
A b s t r a c t . - Highly d i s p e r s e d Fe, Fe-Ru and Fe-Pt s u p p o r t e d on s i l i c a have been s t u d i e d t o a n a l y s e t h e e l e c t r o n i c and s t r u c t u r a l s t a t e of i r o n d u r i n g sample p r e p a r a t i o n p r o c e d u r e . High d i s p e r s i t y of i r o n was a c h i e v e d by p e r f o r m i n g t h e i m p r e g n a t i o n w i t h t h e use of pH = 1 s o l u t i o n s . The h i g h d i s p e r s i t y was r e t a i n e d throughout t h e c a l c i n a t i o n and r e d u c t i o n p r o c e s s e s .
I n t r o d u c t i o n . - One o f t h e main t a s k s a t t h e prepa- r a t i o n of s u p p o r t e d mono- and b i m e t a l l i c c a t a l y s t s i s t o determine what a r e t h e f a c t o r s c o n t r o l l i n g t h e d i s p e r s i o n of m e t a l p a r t i c l e s i n t h e impregna- t i o n
-
c a l c i n a t i o n-
r e d u c t i o n p r o c e s s e s . ~ z s s b a u e r s p e c t r o s c o p y seems t o be a u s e f u l t o o l t o monitor a l l changes which o c c u r d u r i n g t h e p r e p a r a t i o n l i n e P r e v i o u s l y i t h a s been e s t a b l i s h e d t h a t t h e c a t a l y - t i c a c t i v i t y i n PtFe s y s t e m s u p p o r t e d on y -alumina and c h a r c o a l i s s t r o n g l y i n f l u e n c e d by t h e i n t e r a c - t i o n between t h e m e t a l and t h e s u p p o r t / 1 , 2 / . Opti- mum s t r e n g t h of i n t e r a c t i o n between metal and sup- p o r t i n c a l c i n e d s t a g e i s needed t o o b t a i n t h e h i g h e s t d i s p e r s i o n / 3 / . However, when t h e compound used f o r impregnation i s n o t s t r o n g l y adsorbed on t h e s u p p o r t , m i g r a t i o n of t h e a c t i v e component-
maybe i n i o n i c form
-
r e s u l t s i n t h e a g g l o m e r a t i o n of m e t a l l i c p a r t i c l e s . A t p r e s e n t i t i s n o t comple- t e l y e l u c i d a t e d whether t h i s d e c r e a s e i n t h e d i s - p e r s i o n t a k e s p l a c e d u r i n g t h e c a l c i n a t i o n o r i n t h e r e d u c t i o n p r o c e s s . Here we wish t o t r a c e t h e s e e f f e c t s i n t h e PtFe and RuFe b i m e t a l l i c c a t a l y s t s s u p p o r t e d on s i l i c a g e l by u s i n g t h e ~ G s s b a u e r t e c h n i q u e .~ x ~ e r i m e n t a 1 . - I n t h e measurements s i l i c a s u p p o r t s ( S c i e n t i f i c SAS) of 50-100 mesh w i t h 564 m2/g were u s e d . The impregnations were c a r r i e d o u t by s o a k i n g t h e s u p p o r t w i t h t h e aqueous s o l u t i o n of t h e r e a - g e n t s . For ~ e " i m p r e g n a t i o n s Fe/N03
/
3 s o l u t i o n s w i t h a c a r e f u l l y c o n t r o l l e d pH = 1 were used. R U ~ + and P ~ ( N H ~ ) $ ] 2f i o n s were adsorbed on t o t h e s i - l i c a from RuC13, and Pt(NH3)s((XI)2 s o l u t i o n s , r e s - p e c t i v e l y . The soaked samples were d r i e d a t 25OC i n vacuum. The c a l c i n a t i o n o f t h e samples p l a c e di n t o t h e G s s b a u e r sample h o l d e r h o r i z o n t a l l y , were made a t 300°C i n f l o w i n g oxygen atmosphere. The r e d u c t i o n was made a t v a r i o u s t e m p e r a t u r e s i n p u r i - f i e d hydrogen. During t h e measurements t h e samples were k e p t i n oxygen o r i n hydrogen atmospheres de- pending on whether c a l c i n a t e d o r reduced samples were t o b e s t u d i e d .
The average p a r t i c l e s i z e s of t h e c a t a l y s t s formed on t h e s u p p o r t s were determined by c h e m i s o r p t i o n and small a n g l e X-ray d i f f r a c t i o n methods t o be i n t h e range of 30 t o 40
A.
A c o n v e n t i o n a l Mijssbauer s p e c t r o m e t e r w i t h 5 7 ~ o ( ~ r ) s i n g l e l i n e s o u r c e was used.D i s c u s s i o n of e x p e r i m e n t a l r e s u l t s . - F i g u r e 1 shows some t y p i c a l s p e c t r a of t h e 0.5 wt. % Fe/SiOz a ) ; 0 . 5 w t . % Fe
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0.6 wt. % Ru/SiOz b) and 0.2 wt. % Fe - 1 . 1 wt. % Pt/SiOn c ) samples a f t e r impregna- t i o n measured a t 77 K ( I ) , a f t e r c a l c i n a t i o n measu- r e d a t 298 K ( 2 ) and a f t e r r e d u c t i o n measured a t 298 K ( 3 ) .I n t h e impregnated samples o n l y an e x t r e m l y weak resonance <0.1 % c o u l d b e d e t e c t e d a t room tempera- t u r e i f t h e s o l u t i o n s had a pH = 1 v a l u e . A t h i g h e r pH v a l u e s a d o u b l e t resonance was observed a l r e a d y a t room t e m p e r a t u r e , a s a consequence of t h e hydro- l y s i s t o low d i s p e r s i t y f e r r i c h y d r o x i d e . The spec- t r a a
-
c 1 on f i g u r e 1 a r e t y p i c a l f o r i s o l a t e d paramagnetic F e 3 + i o n s ( p r o b a b l y i n t h e form of some f e r r i c aquo-complexes) w i t h l o n g s p i n - r e l a x a - t i o n t i m e s . The h y p e r f i n e f i e l d c o r r e s p o n d i n g t o t h e outermost l i n e s i s 572 2 2 kG. The s t r o n g t e m p e r a t u r e dependence o f t h e r e c o i l l e s s f r a c t i o n i s a consequence of a weak c a t i o n - s u p p o r t i n t e r a c - t i o n .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979226
Fig. 1 : The ~ g s s b a u e r s p e c t r a o f 0 . 5 wt. % F e / S i 0 2 a ) ; 0 . 5 w t . % Fe- 0.6 w t . % Ru/SiOn b) and 0 . 2 wt.
% Fe - 1.1 wt. % Pt/SiO3 c ) impregnated / I / , c a l - c i n a t e d ( 2 ) , and reduced ( 3 ) samples. S p e c t r a ( I ) were measured a t 77 K, s p e c t r a ( 2 ) , and ( 3 ) a t
298 K. The v e l o c i t y s c a l e i s g i v e n r e l a t i v e t o
a
-Fe a t 298 K.The c a l c i n a t e d samples show a q u a d r u p o l e d o u b l e t w i t h isomer s h i f t s ( r e l a t i v e t o a - i r o n ) 0.35 2 0.01 m / s a ) , 0.26
+
0.03 m / s b ) and 0.26+
0 . 0 3 mm/s c ) and q u a d r u p o l e s p l i t t i n g 1.01+
0.01 m/s a ) 0.78+
0.01 mm/s b) and 1.41+
0.02 m / s c ) . The isomer s h i f t and q u a d r u p o l e s p l i t t i n g v a l u e s a r ea l s o i n d i c a t e s t h a t p a r t i c l e s s i z e s o f t h e Fe203 a r e l e s s t h a n 8 0
1
141. T h i s h i g h d i s p e r s i t y c o u l d o n l y be e n s u r e d i f t h e i m p r e g n a t i o n was performed w i t h s o l u t i o n o f pH = 1 .The r e d u c t i o n o f t h e Fe/SiOz and Fe-Ru/SiOz samples was performed i n H2 atmosphere a t 700°C f o r 4 h o u r s . The Fe i n t h e Fe-Pt sample c o u l d b e c o m p l e t e l y r e - duced t o F ~ ( o ) a t 500°C. The isomer s h i f t of t h e s i n g l e l i n e ( c 3 ) i s 0.35
+
0.02 m / s c o r r e s p o n d i n g t o t h e isomer s h i f t o f t h e 32 a t % Fe-Pt a l l o y 151.The same h o l d s i f t h e r e d u c t i o n i s performed a t room t e m p e r a t u r e . These r e s u l t s s u g g e s t t h e e x i s - t e n c e of a s t r o n g Fe-Pt i n t e r a c t i o n .
The reduced Fe/SiOn sample ( a 3) c o n t a i n s ~ e ( ' ) a n d
~ e ' + s p e c i e s . About 20 % of t h e i r o n i s i n h i g h d i s p e r s i t y F ~ ( o ) S t a t e , t h e r e s t i n ~ e ( 1 . ~ . ~ +=
1.07
+
0.02 mm/s Q.S. = 1.17 5 0.01 m / s ) s t a t e . I t may b e s u r p r i s i n g t h a t t h e Fe-Ru/Si02 sample (b 3) c o u l d n o t b e reduced t o F ~ ( o ) . The hydrogen t r e a t m e n t r e s u l t e d i n % I 6 % ~ e / I . S . ~ += 1.07+
0.02 mm/s, Q.S. = 1.85
+
0.01 m / s and t h e r e s t i n F ~ ~ + ( I . s . = 0.422
0.01 m / s , Q.S. = 1.02+
0.01 mm/s) s t a t e .R e f e r e n c e s
/ I / Bartholomew, C.H. and Boudart, M., J. C a t a l y s i s
29 (1973) 278.
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/ 2 / G a r t e n , R.L. i n Mijssbauer E f f e c t Methodology Ed. by Gruverman, I.J. and S e i d e l , C.W. Vol.10.
Plenum P r e s s New-York 1977.
/ 3 / Dumesic, J.A. and Topsoe, H . , Adv. C a t a l .
6
(1976) 121.
141
Kiindig, W., ~ & m e l , H . , C o n s t a b a r i s , G. and L i n d q u i s t , R.H., Phys. Rev.142
(1966) 327./ 5 / P a l a i t h , D . , Kimball, C.W. P r e s t o n , R.S. and C r a n g l e , J . , Phys. Rev.
178
(3969). 795.t y p i c a l f o r h i g h l y d i s p e r s e d FenOs. These samples showed no magnetic s p l i t t i n g down t o 77 K which
