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EXAFS STUDY OF NICKEL TETRACARBONYL AND Ni-CLUSTERS IN ZEOLITE Y
T. Bein, S. Mclain, G. Stucky, G. Woolery, D.E . Sayers, K. Moller
To cite this version:
T. Bein, S. Mclain, G. Stucky, G. Woolery, D.E . Sayers, et al.. EXAFS STUDY OF NICKEL TETRACARBONYL AND Ni-CLUSTERS IN ZEOLITE Y. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-277-C8-280. �10.1051/jphyscol:1986852�. �jpa-00226174�
JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment au n o 12, Tome 47, d6cembre 1986
EXAFS STUDY OF NICKEL TETRACARBONYL AND Ni-CLUSTERS IN ZEOLITE Y(')
T. BE IN*^**, S.J. MCLAIN*, G:D, STUCKY*, G. WOOLERY**, D . E . SAYERS* * * and K. MOLLER'
* E . I . Dupont d e Nemours and Company, C e n t r a l R e s e a r c h and D e v e l o p m e n t D e p a r t m e n t , E x p e r i m e n t a l S t a t i o n , W i l m i n g t o n , DE 1 9 8 9 8 , U.S.A.
t. *
Mobil R e s e a r c h and Development C o r p o r a t i o n , P a u l s b o r o R e s e a r c h L a b o r a t o r y , P a u l s b o r o , NJ 0 8 0 6 6 , U.S.A.
* * t
D e p a r t m e n t o f P h y s i c s , North C a r o l i n a S t a t e U n i v e r s i t y , R a l e i g h , NC 2 7 6 9 5 , U.S.A.
' D e p a r t m e n t o f C h e m i c a l E n g i n e e r i n g , U n i v e r s i t y o f D e l a w a r e , Newark, DE 1 9 7 1 6 , U.S.A.
ABSTRACT
A d s o r p t i o n and t h e r m a l d e c o m p o s i t i o n o f Ni(C0)4 i n t h e c a g e s y s t e m of z e o l i t e Y h a v e been s t u d i e d w i t h EXAFS, e l e c t r o n m i c r o s c o p y and I R s p e c t r o s c o p y . Ni(C0)4 is a d s o r b e d as a n i n t a c t m o l e c u l e i n b o t h c a t i o n - f r e e z e o l i t e Y a n g Nay. Symmetry c h a n g e s of t h e m o l e c u l e i n Nay a r e a s s i g n e d t o t h e f o r m a t i o n of Na --OC-Ni b r i d g e s . Thermal t r e a t m e n t of t h e Ni(C0)4/NaY a d d u c t l e a d s t o l o s s o f CO c o n c o m i t a n t w i t h t h e f o r m a t i o n o f a b i n o d a l N i phase. A m a j o r p a r t o f t h e N i f o r m s c l u s t e r s w i t h d i a m e t e r between 0.5 and a b o u t 1.5 nm, i n a d d i t i o n t o l a r g e r c r y s t a l l i t e s (5-30 nm), s t i c k i n g a t t h e o u t e r s u r f a c e o f t h e z e o l i t e m a t r i x ,
The N i - N i s c a t t e r i n g a m p l i t u d e i n d i c a t e s i n c r e a s i n g a v e r a g e p a r t i c l e s i z e w i t h i n c r e a s i n g t e m p e r a t u r e .
INTRODUCTION
T h e r m a l d e c o m p o s i t i o n o f s u p p o r t e d Ni(G0) a s a p r e c u r s o r f o r d i s p e r s e d N i c a t a l y s t s h a s been r e p o r t e d p r e v i o u s l y (1-3), i n a d a i t i o n t o a l i m i t e d number of a t t e m p t s t o a n c h o r mixed Ni(C0) L c o m p l e x e s d i r e c t l y on p a r t i a l l y h y d r o x y l a t e d o x i d e s u r f a c e s (4). L i t t l e is k n o 3 n m r e g a r d i n g t h e i n t e r a c t i o n of t h e complex w i t h t h e m a t r i x a n d f o r m a t i o n o f t h e m e t a l phase. I n a n a t t e m p t t o u n d e r s t a n d t h e r e a c t i v i t y o f n i c k e l c a r b o n y l compounds i n t h e c a g e s y s t e m of f a u j a s i t e s , we h a v e u s e d a c o m b i n a t i o n o f EXAFS, X-ray a b s o r p t i o n n e a r e d g e s p e c t r o s c o p y (XANES) a n d t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , The r e s u l t s a r e compared w i t h t h o s e o b t a i n e d w i t h I R s p e c t r o s c o p y . EXPERIMENTAL
M a t e r i a l s : Z e o l i t e Y f r o m S t r e m C h e m i c a l s ( L i n d e LZ-Y52), w i t h t h e u n 3 t c e l l com- p o s i t i o n Na55(A10 ) (SiO )137 xH20, was d e h y d r a t e d a t 670 H u n d e r 10 t o r r f o r 1 2 h o u r s ( h e a t i n g 2 r & 2 i / m l n ) and h a n d l e d u n d e r n i t r o g e n i n a drybox.
D e a l u m i n a t e d z e o l i t e Y 1 was p r e p a r e d a c c o r d i n g t o ( 5 ) a n d d e h y d r a t e d i n a s i m i l a r p r o c e d u r e . Ni(C0)4 from A l f a was u s e d a s r e c e i v e d .
Vapor p h a s e l o a d i n g : 0.5 g of d e h y d r a t e d z e o l i t e were weighed i n t o a q u a r t z s a m p l e h o l e r i n t h e d r y b o x , i n t r o d u c e d i n t o a v e r t i c a l q u a r t z r e a c t o r a n d e v a c u a t e d 10 -g t o r r f o r 3 0 min. Ni(CO)& was d e g a s s e d a t t h e same vacuum l i n e w i t h 3 f r e e z e - a t pump-thaw c y c l e s u n d e r low i n t e n s i t y l i g h t . A c a l c u l a t e d amount o f c a r b o n y l v a p o r was a d m i t t e d t o t h e z e o l i t e a n d a l l o w e d t o e q u i l i b r a t e f o r 30 min. a t 295 K , f o l l o w e d by e v a c u a t i o n f o r 30 min. G r a v i m e t r i c a n d m a n o m e t r i c c o n t r o l o f t h i s p r o c e s s r e v e a l e d t h a t t h e d o s i n g t e c h n i q u e a l l o w s a p r e d i c t a b l e l o a d i n g o f t h e z e o l i t e w i t h v o l a t i l e compounds.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986852
C8-278 JOURNAL DE PHYSIQUE
Thermal d e c o m p o s i t i o n : Samples o f t h e c a r b o n y l / z e o l i t e a d d u c t (300 mg) were h e a t e d u n d e r vacuum, w i t h a r a t e of 1 K/min, up t o 3 2 3 , 373 a n d 6 7 3 K ( s a m p l e s WAC323, VAC373 and VAC673) and h e l d a t t h e s e t e m p e r a t u r e s f o r 2 h o u r s and 1 2 h o u r s ( 6 7 3 K).
Methods: I R s p e c t r a were t a k e n w i t h a N i c o l e t 5DXB FT-IR s p e c t r o m e t e r i n a B a r n e s c o n t r o l l e d a t m o s p h e r e d i f f u s e r e f l e c t a n c e I R c e l l w i t h CaF windows.
EXAFS e x p e r i m e n t s were c a r r i e d o u t a t t h e XII-A b e a m l i n e a$ t h e N a t i o n a l S y n c h r o t o n L i g h t S o u r c e w i t h a s t o r e d e l e c t r o n e n e r g y o f 2.5 GeW a t c u r r e n t s between 40 a n d 100 mA. S p e c t r a of s a m p l e s , embe@d.in p a r a f f i n e o r s e a l e d i n p o l y e t h y l e n e c e l l s , were t a k e n i n a dewar w i t h K a p t o n wlndows a t ca. 100 K. The d a t a were a n a l y z e d a c c o r d i n g t o p u b l i s h e d p r o c e d u r e s (6). Background removal was a c c o m p l i s h e d u 3 i n g a c u b i c s p l i n e f u n c t i o n and t h e r e s u l t a n t EXAFS m o d u l a t i o n s were weighed by k and F o u r i e r t r a n s f o r m e d o v e r a r a n g e o f 2 t o 16 K.
T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s o f t h e z e o l i t e s were t a k e n on a H i t a c h i H600 i n s t r u m e n t from microtomed s e c t i o n s of 70 t o 90 nm t h i c k n e s s . The s a m p l e s were embedded i n Epon 8 1 2 u n d e r e x c l u s i o n of air.
RESULTS AND DISCUSSION
1. A d s o r p t i o n o f N i c k e l T e t r a c a r h o n y l i n F a u j a s i t e .
We p r e s e n t h e r e t h e first EXAFS s p e c t r a of Ni(C0I4 a d s o r b e d i n d i f f e r e n t f a u j a s i t e s . F i g . 1 compares b o t h n e a r e d g e s p e c t r a a n d t h e F o u r l e r t r a n s f o r m e d EXAFS m o d u l a t i o n s ( F t s ) o f Ni(C0)4 i n d e a l u m i n a t e d Y ' and NaY. The c o m p l e t e match of b o t h edge s p e c - t r a a n d t h e F t s between t h e two s a m p l e s i n d i c a t e s t h a t a d s o r p t i o n o f Ni(C0)4 i n t o t h e d i f f e r e n t f o r m s o f z e o l i t e Y l e a d s t o e s s e n t i a l l y i d e n t i c a l m e t a l c a r b o n y l s p e c i e s . No o x i d a t i o n of t h e N i i s o b s e r v e d upon a d s o r p t i o n i n t o t h e z e o l i t e . Comparison o f t h e s e e d g e s w i t h t h a t of n e a t Ni(C0l4 as w e l l a s c o m p a r i s o n of t h e m a g n i t u d e of t h e s t r o n g o u t e r s h e l l s c a t t e r i n g due t o l i n e a r CO i n d i c a t e s l i t t l e d i s t o r t i o n of t h e CO l i g a n d s .
The I R C O - s t r e t c h i n g p a t t e r n o f Ni(C0)4 i n Y f r e s e m b l e s t h a t of t h e n e a t c a bon 1, Ti
w h e r e a s i n Nay f o u r d i f f e r e n t b a n d s a p p e a r a t 2120, 2065, 2030 a n d 2010 cm- , I f combined w i t h t h e EXAFS r e s u l t s , t h i s o b s e r v a t i o n c a n be u n d e r s t o o d i n t e r m s o f symmetry c h a n g e s d u e t o ~ a + - - 0 ~ - ~ i i n t e r a c t i o n s i n Nay z e o l i t e which a r e a b s e n t i n Y 1 . Comparable i n t e r a c t i o n s a r e d i s c u s s e d f o r t h e a d s o r p t i o n o f m e t a l c a r b o n y l s on a l u m i n a and f o r c a r b o n y l - i o n p a i r i n g ( 7 , 8 ) .
2. Thermal D e c o m p o s i t i o n o f N i c k e l T e t r a c a r b o n y l i n F a u j a s i t e .
Nay l o a d e d w i t h 2.0 m o l e c u l e s o f t h e c a r b o n y l p e r s u p e r c a g e and h e a t e d up t o 3 2 3 K i n v a c u o f o r 2 h o u r s (WAC323) l o s e s w e i g h t c o r r e s p o n d i n g t o a c o o r d i n a t i v e l y u n s a t u r a t e d s y s t e m w i t h CO/Ni = 1, accompanied by a c o l o r c h a n g e f r o m w h i t e t o d a r k g r e y , The c o r r e s p o n d i n g XANES o f WAC323 i s compared w i t h t h e e d g e of NaY/Ni(C0)b/2 i n F i g . 2a. A d r a m a t i c r e d u c t i o n i n t h e f i r s t absorption.maximum i n d i c a t e s increased o c c u p a t i o n of bonding o r b i t a l s as e x p e c t e d upon r e l e a s e o f p i - a c c e p t o r l i g a n d s . T h e s e e d g e c h a n g e s a r e i n d i c a t i v e o f t h e f o r m a t i o n of m e t a l l i c N i , t h o u g h some r e s i d u a l bound GO is s t i l l observed.
If t h e s a m p l e is e v a c u a t e d a t 3 7 3 K f o r 2 h o u r s (WAC373), c o m p l e t e l o s s o f CO i s now o b s e r v e d a s e v i d e n c e d by c o m p a r i s o n of t h e e d g e s t o t h a t of N i f o i l . C o n t i n u e d h e a t i n g t o 6 7 3 K p r o d u c e s n o f u r t h e r c h a n g e s i n t h e e d g e r e g i o n . G r a v i m e t r i c s t u d i e s a l s o show t h a t a l l c a r b o n y l l i g a n d s a r e d e s o r b e d a t 373 6.
The F o u r i e r t r a n s f o r m of s a m p l e WAC323 i s compared t o t h a t o f WAC373 i n F i g . 2b.
S l i g h t d i f f e r e n c e s i n o u t e r s h e l l a m p l i t u d e s a r e v i s i b l e i n a d d i t i o n t o a s m a l l p e a k i n VAC323 c o r r e s p o n d i n g t o t h e Ni-6 d i s t a n c e f o u n d i n Ni(G0)4 ( F i g . 1).
T h i s ~ e a k is t e n t a t i v e l v a s s i q n e d t o r e s i d u a l CO l i q a n d s which a r e o n l v removed upon
e v a c u a t i o n a t 373 K. . - -
The e l e c t r o n m i c r o ~ r a o h of WAC323 ( F ~ Q 3) shows o c t a h e d r a l N i c r v s t a l l i t e s o f 5 - 3 0 nm d i a m e t e r ; s t i c k i n g a t t h e alter s u r f a c e of t h e z e o l i t e - m a t r i x .
C o n t r i b u t i o n s of t h e 3 r d a n d 4 t h Ni-Ni s h e l l i n t h e EXAFS F t s ( F i g . 2b) must b e d u e t o t h e l a r g e p a r t i c l e s t h a t a r e d e t e c t e d i n t h e TEM. The f i r s t N i - N i s c a t t e r i n g a m p l i t u d e amounts t o o n l y 50% o f t h e b u l k v a l u e , and t h e u p p e r l i m i t o f b u l k - l i k e p a r t i c l e s i n t h i s s y s t e m is t h e r e f o r e 50%. T h i s a s s u m t i o n would i m p l y a t o m i c d i s - p e r s i o n - most l i k e l y w i t h oxygen c o o r d i n a t i o n - of t h e r e m a i n i n g N i f r a c t i o n . S i n c e n o oxygen c o o r d i n a t i o n is d e t e c t e d i n t h e F t s , a c o n s i s t e n t model i n v o l v e s a b i n o d a l N i p h a s e w i t h a m a j o r s i z e f r a c t i o n a t a b o u t z e o l i t e s u p e r c a g e d i m e n s i o n s a n d a f r a c t i o n o f l a r g e (5 - 30 nm) c r y s t a l l i t e s s t i c k i n g a t t h e o u t e r s u r f a c e of t h e z e o l i t e m a t r i x .
R + ~ ( r i l
Fiqure 1 and 2. Ni K edge spectra (A) and Fourier EXAFS moduLations (El).
1. Ni(CO)& in Nay (-1 and dealuminated V' (--).
2A. Edges a f NaV/Ni(CO) (2 per supercage) f --) and sample WAC323 (-).
28. Fts of samples ~ ~ ~ 3 5 3 (-) and WAC373 (--I.
Fiqure 3.
Transmission electron micrograph of sample WAC323.
JOURNAL DE PHYSIQUE
Comparison of t h e l a t e r s t a g e s of t h e r m a l t r e a t m e n t ( s a m p l e s WAC373 and WAC673) r e v e a l s a s i g n i f i c a n t i n c r e a s e of N i - N i n e i g h b o r p o p u l a t i o n s . T h i s is i n d i c a t i v e of an a g g l o m e r a t i o n p r o c e s s a s e x p e c t e d f o r e l e v a t e d t e m p e r a t u r e s .
EXAFS s t u d i e s c o n c e r n i n g t h e c h a r a c t e r i z a t i o n of s u p p o r t e d n i c k e l m e t a l p a r t i c l e s have been r e p o r t e d ( 9 , 10). Hydrogen r e d u c t i o n of Ni(II)/Si(OHI4 g e l s y i e l d e d N i p a r t i c l e s w i t h d i a m e t e r s between 2 and 10 nm ( 9 ) , whereas l a r g e r p a r t i c l e s were produced u s i n g a g a s e v a p o r a t i o n t e c h n i q u e (10). The magnitude of t h e EXAFS r a d i a l d i s t r i b u t i o n f u n c t i o n was found t o d e c r e a s e s u b s t a n t i a l l y w i t h d e c r e a s i n g a v e r a g e p a r t i c l e s i z e (10).
I t becomes c l e a r t h a t c a r e f u l decomposition of Ni(C0l4 a d s o r b e d i n z e o l i t e s p r o v i d e s a v a l u a b l e technique f o r t h e p r e p a r a t i o n of s m a l l n i c k e l p a r t i c l e s .
AMNOWLEDGEMENTS
The a u t h o r s would l i k e t o acknowledge t h e f o l l o w i n g people: Dr. S t e v e Heald, L a r r y F e r e r i a and J a c k S c r o f a n i of Brookhaven N a t i o n a l L a b o r a t o r y a s s i s t e d i n d a t a c o l l e c t i o n . Dr. David Corbin of DuPont p r e p a r e d t h e d e a l u m i n a t e d Y ' z e o l i t e , Michael Van K a v e l a a r p r e p a r e d e l e c t r o n m i c r o g r a p h s , and David R o t h f u s s of DuPont p r o v i d e d g e n e r a l t e c h n i c a l a s s i s t a n c e . K. M. is g r a t e f u l f o r a g r a n t from Deutsche Forschungsgemeinschaft. T h i s work was s u p p o r t e d i n p a r t by DOE g r a n t DE-AS05 fflER10742.
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