EX Α FS and Near Edge Structure IV July 7 - 11 , 1986

A d s o r p t i o n and thermal decomposition of IMi(CG)^ i n t h e cage system o f z e o l i t e Y have been s t u d i e d u i t h EXAFS, e l e c t r o n microscopy and IR s p e c t r o s c o p y , Ni(CO)^ i s adsorbed as an i n t a c t molecule i n both c a t i o n f r e e z e o l i t e Y and NaY. Symmetry changes of the molecule i n NaY are assigned t o the f o r m a t i o n o f Na —OC-IMi b r i d g e s . Thermal treatment of the Ni(CQ)^/NaY adduct leads t o l o s s o f CO co n c o m i t a n t u i t h t h e f o r m a t i o n of a b i n o d a l f\)i phase. A major p a r t o f t h e forms c l u s t e r s u i t h d i a m e t e r between 0.5 and about 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 the ou t e r s u r f a c e o f t h e z e o l i t e matrix., The i \ ] i i \ l i s c a t t e r i n g amplitude i n d i c a t e s i n c r e a s i n g average p a r t i c l e s i z e u i t h i n c r e a s i n g temperature. INTRODUCTION Thermal decomposition of supported Ni(CO), as a p r e c u r s o r f o r d i s p e r s e d (Mi c a t a l y s t s has 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 d i t i o n t o a l i m i t e d number o f a t t e m p t s t o anchor mixed N i ( C 0 ) n i _ m complexes 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 i s knoun r e g a r d i n g the i n t e r a c t i o n o f t h e complex u i t h t h e m a t r i x and f o r m a t i o n of the metal phase. I n an at t e m p t t o un 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 the cage system of f a u j a s i t s s , ue have used 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 near edge spectroscopy (XANES) and t r a n s m i s s i o n e l e c t r o n m icroscopy. The r e s u l t s are compared u i t h those o b t a i n e d u i t h IR 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 from Strem Chemicals ( L i n d e LZ-Y52), u i t h t h e u n ^ t c e l l comp o s i t i o n N a ^ A l O ^ ^ S i O p ) ̂ 7 xH^O, uas dehydrated a t 670 Κ under 10 t o r r f o r 12 hours ( h e a t i n g räie 2 R/mm) and handled under n i t r o g e n i n a dr y b o x . Dealuminated z e o l i t e Y uas prepared a c c o r d i n g t o ( 5 ) and dehy 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(CO)^ from A l f a uas used as r e c e i v e d . Vapor phase l o a d i n g : 0.5 g of dehydrated z e o l i t e uere ueighed i n t o a q u a r t z sample h o l d e r i n the drybox, 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 and evacuated a t 10 t o r r f o r 30 min. N i ( C 0 ) ^ uas degassed a t t h e same vacuum l i n e u i t h 3 f r e e z e pump-thau c y c l e s under l e u 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 vapor uas a d m i t t e d t o the z e o l i t e and a l l o u 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 u e d by evacuation f o r 30 min. G r a v i m e t r i c and manometric c o n t r o l o f t h i s process r e v e a l e d t h a t the dosing t e c h n i q u e a l l o u 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 u i t h v o l a t i l e compounds. C o n t r i b u t i o n 4081 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 adduct (300 mg) were heated under vacuum, u i t h a r a t e o f 1 K/min, up t o 323, 373 and 673 Κ (samples VAC323, UAC373 and 1/AC673) and h e l d a t thes e t e m p e r a t u r e s f o r 2 hours and 12 hours (673 K ) . Methods: IR s p e c t r a uere t a k e n u i t h a N i c o l e t 5DXB FT-IR sp e c t r o m e t e r i n a Barnes c o n t r o l l e d atmosphere d i f f u s e r e f l e c t a n c e IR c e l l u i t h CaF p u i n d o u s . EXAFS experiments uere c a r r i e d o u t a t t h e X11-A beamline a t the N a t i o n a l Synchroton L i g h t Source u i t h a s t o r e d e l e c t r o n energy o f 2.5 Gel/ a t c u r r e n t s betueen UD and 1G0 mA. S p e c t r a o f samples, embec^ed i n p a r a f f i n e or sea l e d i n p o l y e t h y l e n e c e l l s , uere taken i n a deuar u i t h Kapton uindous a t ca. 100 K. The data uere analyzed a c c o r d i n g t o p u b l i s h e d procedures ( 6 ) . Background removal uas accomplished u^ing 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 modulations uere ueighed by k and F o u r i e r t r a n s f o r m e d over a range o f 2 t o 16 K. Tr 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 uere taken on a H i t a c h i HSOO i n s t r u m e n t from microtomed s e c t i o n s o f 7G t o 90 nm t h i c k n e s s . The samples uere embedded i n Epon 812 under e x c l u s i o n o f a i r . 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 . Lue p r e s e n t here t h e f i r s t EXAFS s p e c t r a o f Ni(CO)^ adsorbed 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 both near edge s p e c t r a and t h e F o u r i e r t r a n s f o r m e d EXAFS modulations ( F t s ) o f Ni(CO)^ i n de a l u m i n a t e d Y' and NaY. The complete match of both edge spec­ t r a and the F t s betueen t h e t u o samples i n d i c a t e s t h a t a d s o r p t i o n of Ni(CO)^ i n t o t h e d i f f e r e n t forms o f z e o l i t e Y lead s t o e s s e n t i a l l y i d e n t i c a l m etal 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 o f t h e N i i s observed 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 these edges u i t h t h a t o f neat Ni(CO)^ as u e l l as comparison of the magnitude o f 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'to 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 o f t h e CO l i g a n d s . The IR C O s t r e t c h i n g p a t t e r n o f Ni(CG), i n Y' resembles t h a t of t h e neat c a r b o n y l , uhereas i n NaY f o u r d i f f e r e n t bands appear a t 2120, 2065, 2030 and 2010 cm" . I f combined u 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 can be understood i n terms o f symmetry changes due t o Na —OC-Ni i n t e r a c t i o n s i n NaY z e o l i t e u h i c h are absent i n Y'. 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 the a d s o r p t i o n o f metal c a r b o n y l s on alumina 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 Decomposition 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 loaded u i t h 2,0 molecu l e s o f t h e c a r b o n y l per supercage and heated up t o 323 Κ i n vacuo f o r 2 hours (VAC323) l o s e s u 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 system u i t h CG/Ni = 1, accompanied by a c o l o r change from u h i t e t o dark grey. The c o r r e s p o n d i n g XANES of WAC323 i s compared u i t h t h e edge of NaY/Ni(C0)^/,-j 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 the f i r s t a b s o r p t i o n maximum i n d i c a t e s i n c r e a s e d o c c u p a t i o n o f banding o r b i t a l s as expected upon r e l e a s e of p i a c c e p t o r l i g a n d s . These edge changes are i n d i c a t i v e of the f o r m a t i o n of m e t a l l i c N i , though some r e s i d u a l bound CO i s s t i l l observed. I f t h e sample i s evacuated a t 373 h f o r 2 hours (VAC373), complete l o s s of CO i s nou observed as evidenced by comparison of the edges t o t h a t of Ni f o i l . Continued h e a t i n g t o 673 Κ produces no f u r t h e r changes i n t h e edge 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 shou t h a t a l l c a r b o n y l l i g a n d s are desorbed a t 373 K. The F o u r i e r t r a n s f o r m o f sample 1/AC323 i s compared t o t h a t of 1/AC373 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 are v i s i b l e i n a d d i t i o n t o a sma l l peak i n V/AC323 c o r r e s p o n d i n g t o t h e Ni-C d i s t a n c e found i n N i ( C 0 ) ^ ( F i g . 1 ) . T h i s peak i s t e n t a t i v e l y a s s i g n e d t o r e s i d u a l CO l i g a n d s uhich are o n l y removed upcn 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 g r a p h o f 1/AC323 ( F i g 3) shous o c t a h e d r a l Ni c r y s t a l l i t e s of 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 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 . C o n t r i b u t i o n s o f t h e 3 r d and ktU N i N i s h e l l i n the EXAFS Fts ( F i g . 2b) must be due 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 the 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 upper 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 system i s t h e r e f o r e 50%. T h i s assumtion uould i m p l y atomic d i s ­ p e r s i o n most l i k e l y u i t h oxygen c o o r d i n a t i o n of the remaining Ni f r a c t i o n . S i n c e no oxygen c o o r d i n a t i o n i s d e t e c t e d i n the 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 phase u i t h a major s i z e f r a c t i o n a t about z e o l i t e supercage dimensions and a f r a c t i o n o f l a r g e ( 5 3 0 nm) c r y s t a l l i t e s s t i c k i n g a t the o u t e r surface of t h e z e o l i t e m a t r i x .