Mössbauer study of carbon-supported alpha-iron catalysts

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Mössbauer study of carbon-supported alpha-iron catalysts

Citation for published version (APA):

Christensen, P. H., Moerup, S., & Niemantsverdriet, J. W. (1986). Mössbauer study of carbon-supported

alpha-iron catalysts. Hyperfine Interactions, 28(1-4), 911-914. https://doi.org/10.1007/BF02061592

DOI:

10.1007/BF02061592

Document status and date:

Published: 01/01/1986

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Hyperfine Interactions 28 (1986) 911-914

911 MOSSBAUER STUDY OF CARBON-SUPPORTED a-Fe

CATALYSTS

P.H. CHRISTENSEN and S.

M@RUP

Laboratory o f Applied Physics II, Technical University o f Denmark, DK-2800 Lyngby,

Denmark

J.W. NIEMANTSVERDRIET

Laboratory of Inorganic Chem~try, Eindhoven Unive~ity of Technology, 5600 MB Eindhoven,

The Netherlands

M6ssbauer spectra of carefully reduced carbon-supported iron catalysts show superparamagnetic ~-Fe at 80 K. The results indicate that the particle size depends on the reduction temperature. Effects of evacuation and CO chemisorption are discussed.

i. I N T R O D U C T I O N R e c e n t l y , J u n g et

a l .

/ i / h a v e r e p o r t e d t h a t v e r y s m a l l p a r - t i c l e s of m e t a l l i c i r o n (~-Fe) w i t h d i a m e t e r s in t h e 2-3 n m r a n g e c a n b e p r e p a r e d o n a c a r b o n s u p p o r t c a l l e d C a r b o l a c - i (C-l). M ~ s s - b a u e r s p e c t r a of t h e s e s a m p l e s m e a s u r e d b y N i e m a n t s v e r d r i e t et al. c o n f i r m e d t h a t m o s t of t h e i r o n is in t h e m e t a l l i c s t a t e a n d r e v e a l e d t h a t a b o u t 60 % o f t h e a - F e is s u p e r p a r a m a g n e t i c a t 80 K /2/. In a s u b s e q u e n t s t u d y / 3 / w e h a v e p r e p a r e d a n F e / C - i c a t a l y s t in w h i c h a l l ~ - F e is s u p e r p a r a m a g n e t i c a t 80 K a n d w e h a v e e s t i m a t e d t h e a v e r a g e p a r t i c l e d i a m e t e r b y m e a n s of t h e m a g n e t i c f i e l d d e p e n d e n c e of t h e M ~ s s b a u e r s p e c t r a . In t h i s p a p e r w e s h o w t h a t t h e s u p e r p a r a m a g n e t i c b e h a v i o u r o f t h e e - F e p a r t i c l e s d e p e n d s c r i t i c a l l y o n t h e r e d u c t i o n c o n d i t i o n s . S p e c t r a of t h e c a t a l y s t s in Hz, v a c u u m a n d C O s h o w t h a t t h e M ~ s s b a u e r s p e c t r a a r e s e n s i t i v e to t h e e n v i r o n m e n t . 2. E X P E R I M E N T A L S E C T I O N T h e F e / C - I c a t a l y s t s w e r e p r e p a r e d b y t h e i n c i p i e n t w e t n e s s im- p r e g n a t i o n , a s d e s c r i b e d e a r l i e r / 3 / . T h e s a m p l e s w e r e r e d u c e d in f l o w i n g h y d r o g e n e i t h e r a t 7 2 5 K f o r 16 h or, in a t w o - s t e p p r o c e - d u r e , a t 615 K f o r 16 h a n d a t 675 K f o r 16 h. D u r i n g t h e m e a s u r e - m e n t s t h e s a m p l e s w e r e k e p t in a n in s i t u M S s s b a u e r c e l l , w h i c h a l l o w s s p e c t r a to b e o b t a i n e d at t e m p e r a t u r e s b e t w e e n 80 K a n d 7 2 5 K w i t h a p p l i e d m a g n e t i c f i e l d s u p to a b o u t 1.0 T / 4 / . M ~ s s b a u e r s p e c t r a w e r e m e a s u r e d w i t h a c o n s t a n t a c c e l e r a t i o n s p e c t r o m e t e r . V e l o c i t i e s a r e g i v e n w i t h r e s p e c t t o t h e i s o m e r s h i f t o f e - F e a t r o o m t e m p e r a t u r e . 3. R E S U L T S A N D D I S C U S S I O N F i g u r e 1 s h o w s t h e 80 K s p e c t r a of t h e c a t a l y s t a f t e r r e d u c t i o n a t 675 K. T h e s p e c t r a w e r e o b t a i n e d i n h y d r o g e n a n d in v a c u u m a f t e r d e s o r p t i o n o f h y d r o g e n a t 5 1 0 K. S p e c t r a o f t h i s s a m p l e m e a - s u r e d in a p p l i e d m a g n e t i c f i e l d s u p to 1 . 0 T c o n f i r m e d t h a t t h e s i n g l e p e a k is d u e t o s u p e r p a r a m a g n e t i c a - F e , a n d , f u r t h e r m o r e t h a t t h e a v e r a g e d i a m e t e r of t h e p a r t i c l e s is 2 . 5 • n m / 3 / . T h e l i n e s in t h e s p e c t r a s h o w n in F i g . i, a r e r a t h e r b r o a d , b u t t h e l i n e w i d t h d e c r e a s e s w h e n h y d r o g e n is r e m o v e d , i n d i c a t i n g a n

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912

P.H. Christensen, et al., M.S. o f carbon-supported ct-Fe catalysts

i n c r e a s e in t h e s u p e r p a r a m a g n e t i c r e l a x a t i o n f r e q u e n c y . In a c c o r - d a n c e w i t h p r e v i o u s s t u d i e s of c h e m i s o r p t i o n o n ~ - F e p a r t i c l e s / 5 , 6 / w e a t t r i b u t e t h i s e f f e c t to a n i n c r e a s e in t h e s u r f a c e c o n - t r i b u t i o n to t h e m a g n e t i c a n i s o - t r o p y e n e r g y c o n s t a n t w h e n h y d r o - g e n is c h e m i s o r b e d o n t h e s u r - f a c e of t h e p a r t i c l e s . T h e s p e c t r a of Fig. 1 c o n - t a i n , in a d d i t i o n to t h e s i n g l e t d u e t o s u p e r p a r a m a g n e t i c ~ - F e , a s h o u l d e r at a b o u t 2 m m s -I, w h i c h is a t t r i b u t e d to u n r e d u c e d i r o n in t h e f e r r o u s s t a t e /2/. F i g u r e 2 s h o w s t h e 80 K s p e c t r a of t h e F e / C - I c a t a l y s t in v a c u u m a n d a f t e r C O c h e m i s o r p - t i o n a t 190 K, m e a s u r e d in a n a p p l i e d m a g n e t i c f i e l d of 1 . 0 3 T. A s a r e s u l t of C O a d s o r p t i o n t h e s i x t u p l e t is r e d u c e d in i n t e n - s i t y a n d the r e l a t i v e a r e a o f t h e l i n e at a b o u t - 0 . 2 m m s -I h a s i n c r e a s e d . A l t h o u g h a d e t a i l e d i n t e r p r e t a t i o n is n o t y e t p o s - s i b l e , w e b e l i e v e t h a t t h e c h a n - g e s in t h e s p e c t r u m a r e i n d i c a - t i v e of a s t r o n g i n t e r a c t i o n b e t - w e e n C O a n d t h e i r o n a t o m s a t t h e s u r f a c e . F i g u r e 3 s h o w s t h e r o o m t e m - p e r a t u r e s p e c t r u m of t h e F e / C - I c a t a l y s t a f t e r t h e f i r s t r e d u c - t i o n s t e p a t 615 K m e a s u r e d in a 1 T f i e l d , a n d t h e s p e c t r u m a f t e r r e d u c t i o n at 675 K in a 0 . 5 5 T f i e l d . T h e r e l a t i v e a r e a of t h e p a r a m a g n e t i c c o m p o n e n t is l a r g e r a f t e r r e d u c t i o n at 615 K t h a n a f t e r r e d u c t i o n at 675 K. B o t h s p e c t r a a r e p a r t l y m a g n e t i c a l l y s p l i t d u e t o t h e a p p l i e d m a g n e t i c f i e l d s , a n d t h e t o t a l w i d t h s o f t h e s u p e r p a r a m a g n e t i c c o m p o n e n t s a r e q u i t e s i m i l a r . S i n c e t h e a p p l i e d f i e l d s d i f f e r b y a f a c t o r of a b o u t t w o w e c o n c l u d e t h a t t h e v o l u m e s a l s o d i f f e r b y a f a c t o r o f a p p r o x i m a t e l y two. T h u s t h e m e a n d i a m e t e r o f t h e p a r t i c l e s , p r e p a r e d b y r e d u c t i o n a t 615 K, is a b o u t 2.0 nm. F i g u r e 4 s h o w s s p e c t r a o f t h e s a m p l e w h i c h w a s r e d u c e d a t 7 2 5 K. T h e s p e c t r u m o b t a i n e d at 300 K in a n a p p l i e d m a g n e t i c f i e l d o f 1 . 0 3 T s h o w s t h a t a b o u t 50 % o f t h e i r o n is e - F e . T h e r e m a i n i n g i r o n is I I I I I I I I I : / H2 9 i _{. :} Vac.

...

I I I I I I I I I - 8 - 6 - 4 - 2 0 2 4 6 8

VeLocity (mm/s)

Figol. M6ssbauer spectra obtained at 80 K of the Fe/C-1 catalyst after reduction in H2 at 675 K for 16 h, and in vacuum after desorption of

t h e H 2 at 510 K. I I I I I I I I I J

P$

v . o .: ." - .- ".

'..p.

^

p~.;

9 / . c o ;

~,~./'~ :

I I I I I I I I | - 8 - 6 - 4 - 2 0 2 4 6 8

VeLocity (mm/s)

Fig.2. M6ssbauer spectra of the Fe/C-I catalyst after reduction at 625 K. The spectra were obtained at 80 K in an applied magnetic field of 1.03 T in vacuum after evacuation at 510 K and after chemisorption of CO at 190 K.

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P.H. Christensen, et al., M.S. o f carbon-supported ct-Fe catalysts

913 p r e s e n t in i r o n c a r b i d e s , p o s s i - b l y F e x C a n d x - F e s C 2 /7/. T h e 650 K s p e c t r a c o n t a i n , in a d d i - t i o n t o t h e ~ - F e s e x t e t , a s i n g - let, in a g r e e m e n t w i t h x - F e s C 2 b e i n g p a r a m a g n e t i c a b o v e 530 K

/ 7 / .

T h e s p e c t r u m , o D t a l n e d a t 3 0 0 K in a n a p p l i e d m a g n e t i c f i e l d of 0 . 0 1 T h a s b r o a d e r l i n e s t h a n t h e 1 . 0 3 T s p e c t r u m and, m o r e o v e r , a b r o a d c e n t r a l l i n e is p r e s e n t n e a r z e r o v e l o - c i t y . T h e s e f e a t u r e s c a n b e e x - p l a i n e d b y t h e p r e s e n c e of s u p e r - p a r a m a g n e t i c r e l a x a t i o n e f f e c t s a n d c o l l e c t i v e m a g n e t i c e x c i t a - t i o n s / 6 , 8 / . A c o m p a r i s o n w i t h t h e s p e c t r a s h o w n i n F i g . 1 s h o w s t h a t r e d u c t i o n a t 7 2 5 K l e a d s to a c o n s i d e r a b l y l o n g e r r e l a x a t i o n t i m e t h a n r e d u c t i o n at 675 K. T h e r e s u l t s c a n b e e x - p l a i n e d b y a n i n c r e a s e in t h e a v e r a g e d i a m e t e r to a b o u t i0 n m u p o n r e d u c t i o n a t 7 2 5 K. It is, h o w e v e r , a l s o p o s s i b l e t h a t t h e b e t t e r r e s o l v e d m a g n e t i c h y p e r - f i n e s p l i t t i n g in t h e r o o m t e m - p e r a t u r e s p e c t r a in F i g u r e 4 c a n b e e x p l a i n e d b y a n e n h a n c e m e n t o f t h e m a g n e t i c d i p o l e - d i p o l e i n t e r - a c t i o n b e t w e e n t h e p a r t i c l e s / 2 , 9 / . : : : I I I I I I

9 F -

\

:

a -~. % .: b 89 , 0 . 5 5 T I I I I I I I I I - 8 - 6 - 4 - 2 0 2 4 6 8 V e l o c [ t y ( m m / s )

Fig.3. Room temperature M6ssbauer spectra of the Fe/C-I catalyst after

a) reduction at 615 K measured in 1.03 T applied magnetic field and

b) after reduction at 675 K measured in 0.55 T applied magnetic field. I I I I I I I I I

:

~" ~"

% 6 5 0 K : 0 . 0 1 T %

9 ,--,.

:,-,...-,,,.

300 K " 9 " . " " O. Ol T 9 : " " " , , v , . : - . ..

~"

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I I I ) I I I I I '

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..

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3 0 0 K .. : ~. .. ! . 0 3 T I I I I I I I I I I I I I I I I I I

- 8 - 6 - 4 - 2

0

2

4

6

8 - 8 - 6 - 4 - 2

0

2

4

6

8

V e l o c l t y ( m m / s ) V e l o c l t y ( m m / s )

Fig.4. M6ssbauer spectra of the Fe/C-1 catalyst after reduction at 725 K. Left: without applied magnetic field; right: with 1 T applied magnetic field. Upper spectra obtained at 650 K and lower spectra obtained at 300 K.

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914

P.H. Christensen, et al., M.S.

of carbon-supported a-Fe catalysts

T h e c a r b u r i z a t i o n o f t h e i r o n , s e e n f r o m t h e p r e s e n c e o f c a r - b i d e s i n F i g . 4, i s p r e s u m a b l y d u e t o t h e f a c t t h a t C - I is s i g n i f i - c a n t l y g a s i f i e d i n H 2 a b o v e a b o u t 6 7 5 K / i 0 / . D u r i n g t h e g a s i f i c a t i o n C H 4 i s f o r m e d a n d t h e s e m o l e c u l e s a c t a s c a r b u r i z i n g a g e n t s f o r ~ - F e . 4. C O N C L U S I O N S T h e p r e s e n t s t u d y h a s s h o w n t h a t c a r e f u l d r y i n g a n d r e d u c t i o n o f c a r b o n - s u p p o r t e d i r o n c a t a l y s t s r e s u l t s i n f o r m a t i o n o f v e r y s m a l l e - F e p a r t i c l e s , w h i c h a r e w e l l s u i t e d f o r s t u d i e s o f s u r f a c e e f f e c t s . T h e p a r t i c l e s i z e s e e m s t o i n c r e a s e w i t h i n c r e a s i n g r e d u c t i o n t e m p e r a t u r e a n d c a r b i d e f o r m a t i o n b e c o m e s i m p o r t a n t a t r e d u c t i o n t e m p e r a t u r e s a b o v e 6 7 5 K. A C K N O W L E D G E M E N T S T h e a u t h o r s a c k n o w l e d g e s u p p o r t f r o m t h e N e t h e r l a n d s O r g a n i z a - t i o n f o r t h e A d v a n c e m e n t o f P u r e R e s e a r c h ( Z . W . O . ) a n d t h e D a n i s h C o u n c i l f o r S c i e n t i f i c a n d I n d u s t r i a l R e s e a r c h . R e f e r e n c e s

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Sci. Instrum. 12 (1979) 439.

/ 5/ S. M~rup, B.S. Clausen and H. Tops~e, J. Physique Colloq. 41 (1979) CI-331. / 6/ S. M~rup, H. Tops~e, B.S. Clausen, Phys. Scripta, 25 (1982) 713.

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/ 8/ S. M~rup, J.A. Dumesic and H. Tops~e, in Applications of M6ssbauer Spectro-

scopy, Vol. II, ed. R.L. Cohen (Academic Press, New York, 1980), p.l. / 9/ S. M~rup, J. Magn. Magn. Mat. 37 (1983) 39.