Chapter 29
Structure of Cobalt Sulfide Phase in Carbon-Supported Co and Co—Mo Sulfide Catalysts
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Studies by Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) S. M. A. M. Bouwens, D . C . Koningsberger, V . H . J. de Beer, and R. Prins
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Laboratory for Inorganic Chemistry and Catalysis, Eindhoven, University of Technology, P.O. Box 513, 5600 M B Eindhoven, Netherlands
An X-Ray absorption spectroscopy study has been carried out on a carbon-supported Co and Co-Mo sulfide cata lyst, the latter consisting of a fully sulfided Co-Mo-S (type II) phase. Detailed information on the structure of the cobalt sulfide phase is obtained by comparing the EXAFS and XANES spectra of the catalysts with those of Co S and CoS reference compounds. It is shown that the cobalt atoms in the Co-Mo-S phase have an octahedral-like sulfur coordination while the cobalt atoms in the sulfided Co/C catalyst have a larger frac tion of octahedral cobalt than Co S . On the basis of these results, the high HDS activity of a sulfided Co/C catalyst can be understood since i t appears that the structure of the cobalt sulfide phase in Co/C is in agreement with that in Co-Mo/C. In the Co-Mo-S phase, approximately one cobalt atom is in contact with one mo lybdenum atom at a distance of 2.85 Å. 9
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The EXAFS (Extended X-Ray A b s o r p t i o n F i n e S t r u c t u r e ) t e c h n i q u e i s nowadays a v a l u a b l e t o o l i n t h e c h a r a c t e r i z a t i o n o f c a t a l y s t s . The s t r e n g t h o f EXAFS i s t h a t i t c a n p r o v i d e s t r u c t u r a l i n f o r m a t i o n on h i g h l y d i s p e r s e d systems t h a t a r e d i f f i c u l t t o s t u d y w i t h o t h e r conv e n t i o n a l t e c h n i q u e s . With r e s p e c t t o h y d r o t r e a t i n g c a t a l y s t s ( e . g . , sulfided Co-Mo/Al 0 or Ni-Mo/Al 0 ) EXAFS has been able t o p r o v i d e d i r e c t i n f o r m a t i o n r e g a r d i n g t h e l o c a l s t r u c t u r e o f t h e phases p r e s e n t i n a w o r k i n g c a t a l y s t ( 1 - 3 ) . A l t h o u g h t h e s t u d y o f h y d r o t r e a t i n g c a t a l y s t s i n g e n e r a l h a s r e c e i v e d much a t t e n t i o n d u r i n g t h e l a s t decade, s e v e r a l i m p o r t a n t q u e s t i o n s r e g a r d i n g t h e s t r u c t u r a l c h a r a c t e r i s t i c s a r e s t i l l unanswered. One o f t h e s e c o n c e r n s t h e r o l e o f t h e p r o m o t e r Co and N i i o n s i n s u l f i d e d Co(Ni)-Mo c a t a l y s t s . 2
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Current address: Technisch-Chemisches Laboratorium, ETH-Zentrum, 8092 Zurich, Switzerland 0097-6156/89/0411-0319$06.00/0 e 1989 American Chemical Society
In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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Topsjrfe and coworkers showed t h a t t h e c o b a l t atoms a r e s i t u a t e d a t MoS c r y s t a l l i t e edges i n a s o - c a l l e d "Co-Mo-S" s t r u c t u r e , which s t r u c t u r e governs a l m o s t c o m p l e t e l y t h e h y d r o d e s u l f u r i z a t i o n (HDS) a c t i v i t y ( 4 , 5 ) . N e v e r t h e l e s s , t h e e x a c t s t r u c t u r e o f t h i s phase o r more p r e c i s e l y t h e l o c a t i o n o f t h e c o b a l t p r o m o t e r i s s t i l l unknown. F u r t h e r m o r e , t h e h i g h s p e c i f i c a c t i v i t y o f t h e Co-Mo-S s t r u c t u r e i s n o t u n d e r s t o o d . R e c e n t l y , Ledoux e t a l . (6) showed w i t h t h e use o f ^ C o NMR t h a t t h e p r o m o t i o n e f f e c t o f c o b a l t was c o r r e l a t e d w i t h t h e conc e n t r a t i o n o f c o b a l t s i t e s h a v i n g a d i s t o r t e d t e t r a h e d r a l symmetry. These c o b a l t s i t e s were s t a b i l i z e d by s o - c a l l e d " r a p i d o c t a h e d r a l " c o b a l t atoms a c t i n g as a g l u e between t h e t e t r a h e d r a l c o b a l t s i t e s and t h e MoS p h a s e . A l t h o u g h t h e " r a p i d o c t a h e d r a l " c o b a l t atoms c o u l d not be r e l a t e d t o t h e HDS a c t i v i t y , Ledoux e t a l . l a t e l y p r o p o s e d t h a t t h e y c o u l d be t h e o r i g i n o f v e r y a c t i v e s i t e s ( 7 ) . T h e i r e x p l a n a t i o n , b a s e d upon t h e t h e o r y o f H a r r i s and C h i a n e l l i ( 8 ) , was t h a t an e l e c t r o n t r a n s f e r t a k e s p l a c e between t h e " r a p i d o c t a h e d r a l " c o b a l t t o t h e molybdenum i o n r e s u l t i n g i n a s t r o n g a c t i v a t i o n o f t h e t h r e e s u l f u r i o n s sandwiched between them. 2
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A d i f f e r e n t model was p o s t u l a t e d by Duchet e t a l . (9) and by V i s s e r s e t a l . (10) who o b s e r v e d a h i g h a c t i v i t y f o r p u r e c o b a l t s u l f i d e s u p p o r t e d on a c t i v a t e d carbon and hence e x p l a i n e d t h e a c t i v i t y o f a s u l f i d e d Co-Mo/C c a t a l y s t c o m p l e t e l y by t h e v e r y high a c t i v i t y of the c o b a l t s i t e s . In t h e i r theory the authors i m p l i c i t l y assumed t h a t a c o b a l t s i t e i n s u l f i d e d Co/C has t h e same a c t i v i t y as a c o b a l t s i t e i n s u l f i d e d Co-Mo/C. However, t h i s does n o t have t o be t h e c a s e s i n c e t h e s t r u c t u r e o f c o b a l t s u l f i d e i n Co/C c a n be v e r y d i f f e r e n t from t h a t i n Co-Mo/C. I n o r d e r 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 c o b a l t s u l f i d e phase i n a s u l f i d e d Co/C and a Co-Mo/C c a t a l y s t we a p p l i e d i n t h i s s t u d y t h e EXAFS and XANES (X-Ray A b s o r p t i o n Near Edge S t r u c t u r e ) t e c h n i q u e s a t t h e Co K-edge.
Experimental The c a r b o n s u p p o r t u s e d was a N o r i t a c t i v a t e d c a r b o n (RX3 e x t r a ) having a surface a r e a o f 1190 m .g~ and a p o r e volume o f 1.0 c m . g ~ . The Co/C c a t a l y s t (4.1 wt% Co) was p r e p a r e d by p o r e volume i m p r e g n a t i o n w i t h an aqueous s o l u t i o n o f c o b a l t n i t r a t e (Merck p.a.) f o l l o w e d by d r y i n g i n a i r a t 383 K (16 h ) . The promoted c a t a l y s t (1.5 wt% Co, 7.7 wt% Mo) was p r e p a r e d i n a s p e c i a l way t o e n s u r e a maximum amount o f t h e Co-Mo-S phase ( 1 1 ) . Mossbauer s p e c t r o s c o p y o f t h i s promoted c a t a l y s t c l e a r l y showed t h a t o n l y t h e Co-Mo-S phase was p r e s e n t a f t e r s u l f i d i n g (11) and f u r t h e r m o r e t h a t t h i s Co-Mo-S i s p r o b a b l y a Co-Mo-S t y p e I I p h a s e , meaning a minor i n f l u e n c e o f a c t i v e p h a s e - s u p p o r t i n t e r a c t i o n ( 1 1 , 1 2 ) . The c a t a l y t i c a c t i v i t y o f t h e s u l f i d e d c a t a l y s t s was d e t e r m i n e d by a t h i o p h e n e HDS measurement a t 673 K and a t m o s p h e r i c p r e s s u r e , a s d e s c r i b e d e l s e w h e r e ( 1 0 ) . The t h i o phene HDS r e a c t i o n r a t e c o n s t a n t k p e r mol Co p r e s e n t (approximated as a f i r s t o r d e r r e a c t i o n ) was f o u n d t o be 17*10" s " f o r Co/C and 6 1 * 1 0 s " f o r Co-Mo/C. The X-Ray a b s o r p t i o n measurements were c a r r i e d o u t on f r e s h l y i n s i t u s u l f i d e d c a t a l y s t s a t l i q u i d n i t r o g e n t e m p e r a t u r e a t t h e SRS i n D a r e s b u r y (EXAFS s t a t i o n 9 . 2 ) . The s u l f i d a t i o n was c a r r i e d o u t i n a 2
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In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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10% in H flow, flow rate 60 ml.min under atmospheric p r e s s u r e , t h e t e m p e r a t u r e was i n c r e a s e d l i n e a r l y f r o m 293 t o 673 K (8.5 K.min" f o r Co/C, 2 K.min" f o r Co-Mo/C) and k e p t constant a t 673 K f o r 2 h (Co/C) o r 1 h (Co-Mo/C). The p r e p a r a t i o n o f t h e model compounds CogSg and C o S h a s been d e s c r i b e d i n (13) and ( 1 4 ) , r e s p e c t i v e l y . The p u r i t y o f t h e s e compounds was checked by XRay d i f f r a c t i o n . Phase s h i f t s and b a c k s c a t t e r i n g a m p l i t u d e s from r e f e r e n c e compounds were u s e d t o c a l c u l a t e t h e EXAFS c o n t r i b u t i o n s . F o r t h e Co-S EXAFS s i g n a l s C o S was u s e d , f o r t h e Co-Co c o n t r i b u t i o n s t h e N i - N i c o o r d i n a t i o n i n N i O (Merck p.a.) was chosen, a n d f o r t h e CoMo contribution in the Co-Mo/C catalyst we took ((C H ) P) Ni(MoS ) as a model compound. The u s e o f a N i a b s o r b e r and b a c k s c a t t e r e r i n s t e a d o f Co i s j u s t i f i e d s i n c e c a l c u l a t i o n s o f Teo and Lee (15) showed t h a t p h a s e s and b a c k s c a t t e r i n g a m p l i t u d e s o f n e a r e s t and n e x t - n e a r e s t neighbours i n t h e p e r i o d i c t a b l e hardly d i f f e r . 2
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Results I n F i g u r e 1 (a,b) t h e F o u r i e r - T r a n s f o r m e d (FT) EXAFS s p e c t r a o f t h e Co/C and t h e Co-Mo/C c a t a l y s t together with t h e Co^Sg r e f e r e n c e compound a r e p l o t t e d . The a b s o l u t e FT spectrum o f Co^Sg (Fig. 1 ( a ) ) e x h i b i t s two p e a k s . The f i r s t peak i s a t t r i b u t e d t o combined Co-S and Co-Co c o o r d i n a t i o n s , t h e second one o n l y t o a Co-Co c o o r d i n a t i o n (denoted C o - C o ( 2 ) , t o d i f f e r e n t i a t e i t from t h e Co-Co(1) c o o r dination i n the f i r s t peak). In bulk Co^Sg 8/9 o f t h e c o b a l t atoms a r e t e t r a h e d r a l l y c o o r d i n a t e d a n d 1/9 a r e o c t a h e d r a l l y c o o r d i n a t e d by s u l f u r s . The Co-S c o o r d i n a t i o n d i s t a n c e s a r e i n t h e range 2.13-2.39 A. The Co-Co c o o r d i n a t i o n d i s t a n c e o f Co-Co(1) i s 2.50 A, f o r Co-Co(2) t h e d i s t a n c e i s 3.51 A. From F i g u r e 1 (a) i t i s a p p a r e n t t h a t i n t h e c a t a l y s t s p e c t r a t h e f i r s t peak i s s h i f t e d t o lower r values compared t o t h a t i n Co^Sg. T h i s s h i f t i s l a r g e r f o r CoMo/C t h a n f o r Co/C. I t i s furthermore c l e a r t h a t a Co-Co(2) c o o r d i n a t i o n i s a l s o p r e s e n t i n t h e Co/C c a t a l y s t , b u t n o t i n t h e p r o moted c a t a l y s t . On t h e o t h e r hand, t h e l a t t e r catalyst shows an a d d i t i o n a l peak which i s n o t p r e s e n t i n COgSg and Co/C and, cons e q u e n t l y , might be a s c r i b e d t o Mo b a c k s c a t t e r e r s . The i m a g i n a r y FT s p e c t r a i n F i g u r e 1 (b) show t h e s e p a r a t e Co-S and Co-Co(1) c o o r d i n a t i o n s which cannot be d i s c e r n e d i n t h e a b s o l u t e FT s p e c t r a . I t appears t h a t t h e Co-S peak a m p l i t u d e i n c r e a s e s i n t h e order Co^Sg Co/C Co-Mo/C, whereas t h e Co-Co(1) peak a m p l i t u d e d e c r e a s e s i n t h e same o r d e r . Data a n a l y s i s was c a r r i e d o u t by i s o l a t i n g t h e f i r s t and second peak i n t h e a b s o l u t e FT s p e c t r a and f i t t i n g t h e r e s u l t i n g EXAFS f u n c t i o n s u s i n g Co-S and Co-Co EXAFS c o n t r i b u t i o n s . F o r a d e t a i l e d d e s c r i p t i o n o f t h e d a t a a n a l y s i s p r o c e d u r e we r e f e r t o ( 1 6 ) . The s t r u c t u r a l p a r a m e t e r s (N,R and A 3d t r a n s i t i o n (17,18). T h i s t r a n s i t i o n h a s been o b s e r v e d t o be more i n t e n s e i n t e -
In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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CHARACTERIZATION AND CATALYST DEVELOPMENT
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Imaginary k -weighted Fourier-Transforms ( Ak=3.0 -10.9 A" ) of t h e EXAFS data of CogSg, Co/C and Co-Mo/C: (a) a b s o l u t e part, (b) i m a g i n a r y p a r t . 3
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In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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BOUWENSETAL.
Structure of Cobalt Sulfide Phase
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