FeRu/TiO2 and Fe/TiO2 catalysts after reduction and
Fischer-Tropsch synthesis studied by Mössbauer spectroscopy
Citation for published version (APA):
Kraan, van der, A. M., Nonnekens, R. C. H., & Niemantsverdriet, J. W. (1986). FeRu/TiO2 and Fe/TiO2 catalysts
after reduction and Fischer-Tropsch synthesis studied by Mössbauer spectroscopy. Hyperfine Interactions,
28(1-4), 899-902. https://doi.org/10.1007/BF02061589
DOI:
10.1007/BF02061589
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Published: 01/01/1986
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Hyperfine Interactions 28 (1986) 899-902
899
F e R u / T i O 2 A N D F e / T i O 2 C A T A L Y S T S A F T E R
R E D U C T I O N A N D F I S C H E R - T R O P S C H S Y N T H E S I S
S T U D I E D BY M { ) S S B A U E R S P E C T R O S C O P Y
A.M. VAN DER KRAAN and R.C.H. NONNEKENS
Interuniversitair Reactor Instituut, 2629 JB Delft, the Netherlands
J.W. NIEMANTSVERDRIET
Laboratory for Inorganw Chem~try and Catalysis, Eindhoven
University o f Technology, 5600 MB Eindhoven, the Netherlands
A series of TiO2-supported bimetallic FeRu catalysts with differ- ent Fe:Ru ratios (~; ]0:l; 3:1; I:I; 1:3) has been studied by means of in situ M~ssbauer spectroscopy. The influence of reduc- tion and Fischer-Tropsch synthesis on the state of iron in the FeRu/TiO 2 catalysts will be derived.
i. I N T R O D U C T I O N S u p p o r t e d b i m e t a l l i c c a t a l y s t s of i r o n a n d r u t h e n i u m h a v e s h o w n i n t e r e s t i n g s e l e c t i v i t i e s f o r f o r m a t i o n of l i g h t o l e f i n s , e t h y l e n e a n d p r o p y l e n e in t h e F i s c h e r - T r o p s c h s y n t h e s i s /I/. R e c e n t w o r k b y S t o o p et al / 2 / r e v e a l e d t h a t F e R u o n T i O 2 a n d S i O 2 w i t h a n a t o m i c r a t i o of F e : R u = 3:1 e x h i b i t e d s i g n i f i c a n t l y h i g h e r o l e f i n a n d l o w e r m e t h a n e s e l e c t i v i t i e s t h a n e i t h e r of t h e s i n g l e m e t a l s o n T i O 2 o r S i O 2 in F i s c h e r - T r o p s c h s y n t h e s i s a t 575 K a n d 1 atm. M o s s b a u e r i n - v e s t i g a t i o n s of r e d u c e d F e R u / S i O 2 c a t a l y s t s h a v e i n d i c a t e d t h a t i r o n is p r e s e n t in F e R u a l l o y a n d in i r o n (III) o x i d e / 3 - 7 / . H e r e w e r e - p o r t on the s t a t e of i r o n in F e R u / T i O 2 c a t a l y s t s d u r i n g r e d u c t i o n a n d a f t e r F i s c h e r - T r o p s c h s y n t h e s i s . 2. E X P E R I M E N T A L T h e c a t a l y s t s w e r e p r e p a r e d b y i m p r e g n a t i n g T i O 2 ( D e g u s s a , p 2 5 , 5 0 m 2 / g ) w i t h a q u e o u s s o l u t i o n (pH=l) of F e ( N O 3 ) 3 . 9 H 2 0 (Merck, p.a) a n d R u C I 3 . x H 2 0 ( F l u c a A . G . ) , u n t i l t h e i n c i p i e n t w e t n e s s p o i n t w a s r e a c h e d . T h e t o t a l m e t a l l o a d i n g w a s 5 w t % . B e t w e e n 5 a n d 15% o f t h e i r o n w a s 57Fe. S a m p l e s w e r e d r i e d in a i r a t 295 K f o r 72 h, in v a c u u m (i00 Pa) at 295 K f o r 24 h a n d in a i r a t 400 K f o r 24 h. A l l f u r t h e r t r e a t m e n t s w e r e d o n e in a M ~ s s b a u e r in s i t u r e a c t o r /8/. I s o m e r s h i f t s a r e r e p o r t e d r e l a t i v e to SNP. 3. R E S U L T S T h e M ~ s s b a u e r s p e c t r a of t h e d r i e d F e R u / T i O 2 c a t a l y s t s c o n s i s t o f a d o u b l e t w i t h IS = 0 . 6 3 + 0 . 0 3 m m / s a n d Q S = 0 . 8 2 + 0 . 0 5 m m / s , w h i c h is c h a r a c t e r i s t i c of h i g h - s p i n F e 3+ i o n s in h i g h l y d i s p e r s e d i r o n (III) o x i d e or o x y h y d r o x i d e . F i g u r e 1 s h o w s t h e M ~ s s b a u e r s p e c t r a of t h e c a t a l y s t s a f t e r i n t e r m e d i a t e r e d u c t i o n a t 400 K, f i n a l r e d u c t i o n a t 675 K a n d F i s c h e r - T r o p s c h s y n t h e s i s a t 5 7 5 K (CO/H 2 = 89 A s t h e s p e c t r a of 3 : 1 F e R u / T i O 2 a n d i:i F e R u / T i O 2 a r e s i m i l a r , o n l y t h o s e o f t h e f o r m e r a r e s h o w n . It a p p e a r s t h a t e x p o s u r e o f t h e c a t a l y s t s to H 2 a t 400 K d o e s n o t a f f e c t t h e c h e m i c a l s t a t e of i r o n in m o n o m e t a l l i c F e / T i O 2 , b u t it d o e s l e a d to p a r t i a l r e d u c t i o n of t h e F e 3+ in t h e b i m e t a l l i c F e R u / T i O 2 c a t a l y s t s . In i0:I, 3:1 a n d i:i F e R u / T i O 2 a l m o s t a l l F e 3+ p r e s e n t in t h e f r e s h c a t a l y s t s is c o n v e r t e d to h i g h - s p i n F e 2+ (IS = 1.36 + 0 . 0 3 m m / s , Q S = 2 . 1 0 + 0 . 1 5 m m / s ) b y
900 A . M . van der Kraan, et al., F e R u / T i O 2 and F e / T i O 2 catalysts after reduction 1 1 6 - 113- 4 0 0 ~
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b C d - i 0 - 8 - 6 -4 -2 0 2 4 6 8 I0 - I 0 - 8 - 6 -4 -2 0 2 4 6 8 I0 - DOPPLER VELOCITY (mms "1)Fig. l. Mossbauer spectra of FeRu/TiO 2 catalysts at T = 300 K after the treatments indicated, a) Fe/Ti02; b) I0:] FeRu/Ti02; c) 3:1FeRu/Ti02; d) ]:3 FeRu/TiO 2.
H 2 at 400 K, w h e r e a s r e d u c t i o n to F e 2+ a n d F e ~ o c c u r s in 1:3 F e R u / T i O 2. T h e p a r a m e t e r s of F e ~ IS = 0 . 2 6 ~ 0 . 0 3 m m / s a n d Q S = 0 . 2 0 + 0 . 0 4 m m / s c o r r e s p o n d to t h o s e o f F e ~ a t o m s in h c p - F e R u a l l o y s , as ~ e p o r t e d b y R u s h et al. /9/. A f t e r t h e f i n a l r e d u c t i o n o f t h e c a t a l y s t s at 6 7 5 K f o r 18 h, t h e M ~ s s b a u e r s p e c t r a of F e / T i O 2 a n d i0:I F e R u / T i O 2 a r e d o m i n a t e d b y a s i x - l i n e p a t t e r n d u e to ~ - F e , a n d to a c o m b i n a t i o n of a - F e a n d b c c - F e R u a l l o y , r e s p e c t i v e l y . T h e M ~ s s b a u e r s p e c t r a o f t h e r e d u c e d 3:1, i:i a n d 1:3 F e R u / ~ i O 2 c a t a l y s t s i n d i c a t e t h a t i r o n is p r e s e n t in t h r e e d i f f e r e n t s t a t e s : z e r o - v a l e n t i r o n in h c p - F e R u a l l o y , F e 2+ a n d F e 3+. T h e s e a s s i g n m e n t s a r e c o n f i r m e d b y in s i t u s p e c t r a a t 77 K a n d 4 K as in / i 0 / . T h e c o n t r i b u t i o n of t h e z e r o - v a l e n t i r o n to t h e s p e c t r a , a b o u t 6 - 7 % , is s u r p r i s i n g l y s m a l l in v i e w of t h e r e l a t i v e l y h i g h d e g r e e o f r e d u c t i o n in F e / T i O 2 a n d I0:i F e R u / T i O 2. A r e m a r k a b l e r e s u l t c o n c e r n i n g t h e 3:1, i:i a n d I:3 F e R u / T i O 2 c a t a l y s t s is t h a t a l m o s t a l l F e 3+ in t h e f r e s h c a t a l y s t s c a n b e r e d u c e d to F e 2+ b y H 2 at 400 K, w h e r e a s a f t e r r e d u c t i o n a t 675 K a c o n s i d e r a b l e f r a c t i o n of t h e i r o n is in t h e f e r r i c - s t a t e . T h i s i m p l i e s t h a t o x i d a t i o n of F e 2+ to F e 3+ h a s o c c u r r e d , u n d e r H2, at t e m p e r a t u r e s b e t w e e n 400 K a n d 6 7 5 K. T h e o x i d a t i o n is m o r e p r o - n o u n c e d w i t h i n c r e a s i n g r u t h e n i u m c o n c e n t r a t i o n . T h i s u n e x p e c t e d p h e n o m e n o n is i l l u s t r a t e d in m o r e d e t a i l in F i g . 2, w h e r e s p e c t r a of I:I F e R u / T i O 2 a n d F e / T i O 2 a r e s h o w n as a f u n c t i o n of r e d u c t i o n t e m p e r a t u r e . A s Fig. 2 s h o w s , t h e d e g r e e of r e d u c t i o n in F e / T i O 2 i n c r e a s e s m o n o t o n i c a l l y 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 . In i:I F e R u / T i O 2 , h o w e v e r , r e d u c t i o n s t a r t s at l o w e r t e m p e r a t u r e s t h a n in F e / T i O 2 , b u t w h e n t h e t e m p e r a t u r e e x c e e d s 5 0 0 K, t h e d o u b l e t d u e to F e 3+ s h o w s a m a r k e d i n c r e a s e in i n t e n s i t y . T h e r e s u l t s s h o w t h a t t h e F e 3+ p r e s e n t in r e d u c e d F e R u / T i O 2 c a t a l y s t s is b y no m e a n s a r e s i d u e of F e 3+ in t h e f r e s h c a t a l y s t s , a n d s u g g e s t t h a t t h e f e r r i c
A . M . van der Kraan, et al., F e R u / T i O 2 a n d F e / T i O 2 catalysts after reduction 901 238- ' . . . . Fe~/T'i02 ' ' ' J H2'lh - l ~ I I 375 K 2.32~
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Fig.2. Mossbauer spectra of Fe/TiO$ and I:] FeRu/TiO 2 catalysts at T = 300 K after reduction in H 2 for ; h at the indlcated temperature.
i r o n r e f l e c t s t h e i n t e r a c t i o n o f t h e F e R u p a r t i c l e s w i t h t h e s u p p o r t . T h e F i s c h e r - T r o p s c h c o n d i t i o n s a l t e r t h e c h e m i c a l s t a t e o f t h e F e a n d F e R u / T i O 2 c a t a l y s t s s i g n i f i c a n t l y . A s s h o w n in F i g . 1 t h e e - F e p r e s e n t i n r e d u c e d F e / T i O 2 a n d i0:I F e R u / T i O 2 is c a r b u r i z e d to x - F e 5 C ~ , a n d t h e s m a l l a m o u n t o f F e 3+ in t h e s e s y s t e m s is c o n v e r t e d to F e 2+, ~s a l s o o b s e r v e d o n e x p o s i n g 3:1, i:i a n d 1:3 F e R u / T i O 2 c a t a l y s t s to F i s c h e r - T r o p s c h s y n t h e s i s . T h e z e r o - v a l e n t i r o n in h c p - F e R u a l l o y , h o w e v e r , is n o t a f f e c t e d b y s y n g a s . S i m i l a r b e h a v i o u r o f i r o n h a s b e e n r e p o r t e d f o r F e R h / S i O 2 c a t a l y s t s / i 0 , i i / . 4. D I S C U S S I O N T h e f a c t t h a t p a r t i a l r e d u c t i o n o f i r o n in H 2 a t 400 K o c c u r s in a l l F e R u / T i O 2 c a t a l y s t s a n d n o t i n F e / T i O 2 d e m o n s t r a t e s t h a t Ru e n h a n c e s t h e r e d u c i b i l i t y o f i r o n , s i m i l a r l y as in S i O 2- a n d A I 2 0 3 - s u p p o r t e d F e R u c a t a l y s t s /5/. S e v e r a l i n v e s t i g a t i o n s b e f o r e / 4 , 1 2 , 1 3 / h a v e s h o w n t h a t in s u p p o r t e d b i m e t a l l i c c a t a l y s t s c o n s i s t i n g o f i r o n a n d a n o b l e g r o u p V I I I m e t a l p r o m o t i o n of t h e r e d u c t i o n o f i r o n b y t h e n o b l e m e t a l t a k e s p l a c e . A s e x p l a i n e d b e f o r e / 1 4 / , t h e n o b l e m e t a l p r o v i d e s t h e s i t e s w h e r e H 2 is d i s s o c i a t e d i n t o H a t o m s w h i c h d i f f u s e to c o n t i g u o u s i r o n o x i d e , w h e r e F e 3+ is r e d u c e d to F e 2+ o r F e o. T h u s , t h e r e s u l t s i n d i c a t e t h a t a l l i r o n c o n t a i n i n g p a r t i c l e s in t h e f r e s h c a t a l y s t s c o n t a i n o r a r e i n c o n t a c t w i t h r u t h e n i u m . In s p i t e o f t h e p r e s e n c e of r u t h e n i u m a n d i t s r e d u c t i o n - p r o m o t i n g a b i l i t i e s , r e d u c t i o n o f 3:1, i:I a n d 1:3 F e R u / T i O 2 c a t a - l y s t s at 675 K d o e s n o t l e a d to f o r m a t i o n o f s u b s t a n t i a l a m o u n t s o f z e r o - v a l e n t i r o n as in t h e F e / T i O 2 a n d i0:i F e R u / T i O 2 c a t a l y s t s . T h e e x t e n t to w h i c h i r o n i n F e R u / T i O 2 c a t a l y s t s is r e d u c e d a t 675 K, e v e n d e c r e a s e s w i t h i n c r e a s i n g R u c o n t e n t . M i n a i e t al / i i / o b s e r v e d s i m i l a r t r e n d s f o r a s e r i e s o f F e R h / S i O 2 c a t a l y s t s , w h i l e B e r r y e t al /5/, o n t h e o t h e r h a n d , f o u n d t h e o p p o s i t e f o r A l 2 0 3 - s u p p o r t e d
902 A . M . van der Kraan, et al., F e R u / T i O 2 a n d F e / T i O 2 catalysts after reduction F e R u c a t a l y s t s . W e s u g g e s t t h a t t h e f o r m a t i o n o f F e 3+ b y b a c k o x i d a - t i o n of F e 2 + d u r l n g r e d u c t i o n a t m o r e e l e v a t e d t e m p e r a t u r e s , s h i f t s to l o w e r t e m p e r a t u r e s b y i n c r e a s i n g r u t h e n i u m c o n t e n t . In a n a l o g y w i t h c h e m i s o r p t i o n - i n d u c e d s u r f a c e s e g r e g a t i o n in a l l o y s it s e e m s l i k e l y t h a t t h e f o r m a t i o n of F e 3+ d u r i n g r e d u c t i o n is a s s o c i a t e d w i t h s e g r e g a t i o n o f i r o n i n t h e F e R u p a r t i c l e s t o t h e s u p p o r t , w h e r e it is s t a b i l i z e d as F e 3+. A s y s t e m a t i c i n v e s t i g a t i o n of t h e r e d u c t i o n b e h a v i o u r o f F e R u o n d i f f e r e n t s u p p o r t s is p r e s e n t l y in p r o g r e s s . M ~ s s b a u e r s p e c t r a o f t h e c a t a l y s t s a f t e r F i s c h e r - T r o p s c h s y n - t h e s i s s h o w t h a t c a r b u r i z a t i o n o c c u r s o n l y in F e / T i O 2 a n d i r o n - r i c h i0:i F e R u / T i O 2. N e a r l y a l l z e r o - v a l e n t i r o n p r e s e n t in e - F e a n d b c c - F e R u a l l o y is c o n v e r t e d i n t o • w h i c h c a r b i d e is a l s o t h e m a i n c o m p o n e n t in u n s u p p o r t e d F e c a t a l y s t s a f t e r F i s c h e r - T r o p s c h s y n t h e - s i s / 1 5 / . T h e c a t a l y s t w i t h t h e h i g h e s t o l e f i n s e l e c t i v i t y is 3:1 F e R u / T i 0 9 /2/. It is i n t e r e s t i n g to n o t e t h a t t h i s c a t a l y s t c o n t a i n s i r o n p r e ~ o m i n e n t l y as F e 2+ a n d F e 3+, t h a t t h e c o n t r i b u t i o n o f F e ~ in h c p - F e R u to t h e M ~ s s b a u e r s p e c t r u m a t 295 K is o n l y a b o u t 6% a n d t h a t c a r b u r i z a t i o n o f t h i s F e o d o e s n o t o c c u r . R e f e r e n c e s
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