Hydrogen Spillover: Effects on Pt Catalysed Ethylene Hydrogenation

G.M. W o n k , S.J. Teichner and J.E. Germain (Editors),Spillouer of Adsorbed Species 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

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H Y D R O G E N SPILLOVER:

EFFECTS ON P t CATALYSED E T H Y L E N E HYDROGENATION

H . SALTSBURG and M . E . MULLINS Department of Chemical Engineering, University o f Rochester, Rochester, N Y 14627

ABSTRACT The study of the hydrogenation of ethylene by a n alumina supported c a t a l y s t showed b o t h the existence of hydrogen s p i l l o v e r a n d a n enhanced reaction r a t e when s p i l l o v e r had occurred. Using a s o l i d s t a t e electrochemical technique t o monitor surface hydrogen, i t was concluded t h a t t h e s p i l l o v e r hydrogen was located a t the three phase boundary of metal, support, and gas. I t i s suggested t h a t the r a t e enhancements a r e not the r e s u l t of u t i l i z i n g t h i s e x t r a hydrogen, b u t a r e a r e s u l t of a l t e r i n g the i n i t i a l condition under which the reaction begins . INTRODUCTION Under c e r t a i n circumstances an amount of hydrogen g r e a t e r than a theoretical monolayer can be adsorbed onto supported metal s u r f a c e s . This phenomenon, hydrogen s p i l l o v e r , was f i r s t reported by Khoobiar ( r e f . 1) and has long been

known to be associated with the c a t a l y s t s u p p o r t , f o r i n i t s absence the bare metal does n o t display an " e x t r a " hydrogen storage capacity ( r e f . 2 ) . This e f f e c t has been noted on many supported metals including R u , Ni, Re, and P d , b u t the e f f e c t i s p a r t i c u l a r l y pronounced f o r alumina supported platinum. The d i s t r i b u t i o n o f the additional adsorbed hydrogen and t h e e f f e c t of t h a t hydrogen o n the c a t a l y t i c process i s o f p a r t i c u l a r s i g n i f i c a n c e because s p i l l o v e r i s t h o u g h t t o enhance some reaction r a t e s a n d t o a l t e r s e l e c t i v i t i e s ( r e f . 3 ) . A v a r i e t y of possible s t o r a g e locations have been suggested including absorpt i o n i n t o the bulk metal, d i f f u s i o n i n t o and p a r t i a l reduction of the support m a t e r i a l , and t r a n s p o r t t o and storage on the surface of the support material via a "porthole" a t the t h r e e phase i n t e r f a c e as proposed by Taylor ( r e f . 4 ) . Another p o s s i b i l i t y i s t h a t hydrogen may r e s i d e i n a " r e s e r v o i r " a t the t h r e e phase i n t e r f a c e of the support, metal, and gas. Figure 1 shows the l i k e l y points f o r hydrogen s t o r a g e on or within the c a t a l y s t . D i f f i c u l t y i n resolving the questions concerning this phenomena stem i n p a r t from the i n a b i l i t y to make d i r e c t observation of surface hydrogen under adsorpt i o n and reaction conditions. During the course of a study of ethylene hydrogenation ( r e f . 5 ) on alumina supported P t n o t only was s p i l l o v e r observed, b u t a n e f f e c t usually a t t r i b u t e d t o hydrogen s p i l l o v e r , enhanced reaction r a t e s , was a l s o observed. A systematic study of t h i s s p i l l o v e r using t r a d i t i o n a l o l e f i n

296

s u r f a c e t i t r a t i o n was c a r r i e d o u t w h i l e u s i n g a new e l e c t r o c h e m i c a l t e c h n i q u e t o m o n i t o r t h e thermodynamic a c t i v i t y o f s u r f a c e hydrogen so as t o determine t h e a p p r o p r i a t e end p o i n t , as w e l l as, the a c t i v i t y i t s e l f . b r i e f d e s c r i p t i o n o f t h i s procedure w i l l be presented.

I n the f o l l o w i n g a

I t w i l l be shown t h a t

t h e s t o r a g e p o i n t f o r t h e hydrogen i s on t h e s u p p o r t s u r f a c e , p r o b a b l y a t t h e t h r e e phase boundary ( g a s - m e t a l - s u p p o r t ) and t h a t t h e observed r a t e enhancements f o r e t h y l e n e h y d r o g e n a t i o n a r e n o t d i r e c t l y a t t r i b u t a b l e t o a u t i l i z a t i o n o f s t o r e d hydrogen.

Rather, t h e r a t e enhancement i s c h i e f l y t h e e f f e c t o f

a l t e r a t i o n o f t h e i n i t i a l o p e r a t i n g c o n d i t i o n s imposed by t h e s u r f a c e hydrogen on t h e complex r e a c t i o n n e t w o r k .

F i g . 1. P o s s i b l e Hydrogen Storage a ) W i t h i n t h e metal b) On t h e s u p p o r t s u r f a c e c) W i t h i n the support d) A t t h e t h r e e phase boundary EXPERIMENTAL The a d s o r p t i o n / r e a c t i o n s t u d i e s were c a r r i e d o u t i n a f l o w c e l l ( o p e r a t e d as a CSTR) i n w h i c h t h e p l a t i n u m c a t a l y s t was a t h i n , porous f i l m formed o n t h e i n s i d e s u r f a c e o f t h e c l o s e d end o f t h e ceramic r e a c t o r t u b e .

The gas t i g h t

t u b e was c o n s t r u c t e d of b e t a " - a l u m i n a , a p r o t o n c o n d u c t i n g s o l i d e l e c t r o l y t e , and was p u t i n t o t h e c o n f i g u r a t i o n t y p i c a l o f a s i m p l e e l e c t r o c h e m i c a l concent r a t i o n c e l l ( F i g u r e 2) b y p l a c i n g an i d e n t i c a l p l a t i n u m r e f e r e n c e e l e c t r o d e on the c e l l exterior.

U s i n g hydrogen a t one atmosphere as a r e f e r e n c e gas, t h e

thermodynamic a c t i v i t y o f s u r f a c e hydrogen i n t h e presence o f a c a t a l y t i c r e a c t i o n c o u l d be m o n i t o r e d b y measuring t h e open c i r c u i t EMF o f t h e c e l l and employing t h e a p p r o p r i a t e N e r n s t e q u a t i o n .

D e t a i l s o f t h e r e a c t i o n system

and e l e c t r o c h e m i c a l t e c h n i q u e used a r e d e s c r i b e d elsewhere ( r e f . 6 ) .

297

I-'"

J

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F i g . 2. B a s i c C e l l C i r c u i t E-@"-alumina, CED c a t a l y s t - e l e c t r o d e , RED r e f e r e n c e e l e c t r o d e , SS s t a i n l e s s s t e e l chamber, F I - f e e d i n l e t , FO R I - r e f e r e n c e i n l e t , RO - o u t l e t .

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RESULTS AND DISCUSSION:

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outlet,

HYDROGEN STORAGE

During the study o f t h e ethylene hydrogenation reaction, d i f f e r e n t rates o f t h e r e a c t i o n were observed under a p p a r e n t l y i d e n t i c a l c o n d i t i o n s .

The enhanced

r a t e o f ethane p r o d u c t i o n was observed i f t h e p l a t i n u m c a t a l y s t was t r e a t e d w i t h pure hydrogen (1 atm) a t temperatures i n excess o f 723 K f o r p e r i o d s g r e a t e r t h a n one h o u r .

The hydrogen a c t i v i t y ( a s measured b y t h e EMF), however,

d i d n o t change s i g n i f i c a n t l y once an i n i t i a l s t e a d y s t a t e was a c h i e v e d ( w i t h i n

10 t o 15 m i n u t e s ) and, on t h e b a s i s o f t h e a c t i v i t y measurements, no d i f f e r e n t i a t i o n between t h e s p i l l o v e r and normal s t a t e s c o u l d be made. The q u a n t i t y o f hydrogen adsorbed i n b o t h t h e normal ( s a t u r a t i o n ) and s p i l l o v e r s t a t e was determined b y t i t r a t i o n o f t h e hydrogen s a t u r a t e d c a t a l y s t s u r f a c e w i t h e t h y l e n e u s i n g t h e method d e s c r i b e d p r e v i o u s l y by Sermon ( r e f . 7 ) . Pulses o f e t h y l e n e were i n t r o d u c e d u n t i l t h e measured s u r f a c e a c t i v i t y approached zero, and t h e ethane produced by t h e c a t a l y t i c r e a c t i o n o f s u r f a c e hydrogen and e t h y l e n e was measured u s i n g gas chromatographic t e c h n i q u e s . M o n i t o r i n g o f t h e hydrogen a c t i v i t y d u r i n g o l e f i n t i t r a t i o n enables one t o a s s u r e t h e complete removal o f s u r f a c e hydrogen f r o m t h e c a t a l y s t w i t h o u t s i g n i f i c a n t l y o v e r s h o o t i n g t h e c o r r e c t amount o f o l e f i n r e q u i r e d t o complete t h e t it r a t i o n . The r a t i o o f ethane produced i n t h e two s t a t e s was found t o be 1 : l . g between t h e non s p i l l o v e r and s p i l l o v e r s t a t e s r e s p e c t i v e l y .

T h i s f i g u r e must be

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regarded with caution as there a r e problems inherent t o the ethylene t i t r a t i o n technique due t o the competing ethylene disproportionation The surface d i s s o c i a t i o n of ethylene produces approximately surface hydrogen, and a stoichiometric equivalent amount of phase. Sermon ( r e f . 7 ) reported values of 1.23 to 1.34 f o r

reaction ( r e f . 5 ) . 20 t o 30% e x t r a ethane i n the gas

the adsorbed atomic hydrogen t o platinum s i t e r a t i o ( n H / n P t ) based upon alkene t i t r a t i o n of Si02

supported platinum; however, t h i s 30% excess hydrogen could be a r e s u l t of the concurrent disproportionation. Even considering the amount of ethane t h a t could be produced as a r e s u l t of the s i d e r e a c t i o n , the r a t i o observed in these studies i s s t i l l n o t u n i t y . This r e s u l t i s i n agreement with the l a r g e number of s t u d i e s reported in the l i t e r a t u r e which show t h a t some excess hydrogen i s present, p a r t i c u l a r l y i n a1 umina supported c a t a l y s t s . Other d i r e c t surface t i t r a t i o n methods applied t o the c a t a l y s t , p a r t i c u l a r l y acetylene adsorption, did i n d i c a t e t h a t a n over prediction of surface area by 20-30% was a r e s u l t of the a1 kene t i t r a t i o n . The reaction of ethylene with preadsorbed surface hydrogen occurs e s s e n t i a l l y instantaneously, r e a d i l y consuming a l l the electrochemically observable hydrogen. Such r a t e s suggest t h a t a l l the hydrogen, including the s p i l l o v e r amount, must be present e i t h e r on the alumina surface o r a t the t h r e e phase i n t e r f a c e where i t i s r e a d i l y a v a i l a b l e . There was no spontaneous subsequent recovery i n the hydrogen a c t i v i t y .

I f hydrogen i s s t o r e d within the i n t e r i o r of the elec-

t r o l y t e e i t h e r i n atomic o r proton form, t h e reported r a t e s o f diffusion f o r hydrogen through t h e e l e c t r o l y t e (lo-' cm/sec) a r e too slow t o account f o r t h e

r a p i d reaction r a t e . The f a i l u r e of t h e hydrogen a c t i v i t y t o recover following t h i s rapid r e a c t i o n i n d i c a t e s t h a t a s i g n i f i c a n t amount of hydrogen within t h e e l e c t r o l y t e i s not a v a i l a b l e t o migrate back t o the metal. Reports of reverse s p i l l o v e r of hydrogen have not appeared i n the l i t e r a t u r e . In o r d e r t o t e s t d i r e c t l y t h e r o l e of the support m a t e r i a l , a n additional amount o f alumina was introduced i n t o the system i n the form of 3 grams of 2 mm diameter p e l l e t s . In contact w i t h the platinum s u r f a c e , they increased the amount of ethane produced i n the t i t r a t i o n by an average of approximately 10%. Alumina p e l l e t s added so as not t o d i r e c t l y contact the metal surface do not enhance the apparent extent of the r e a c t i o n . Enhancement of hydrogen storage by increasing the amount of support i n contact with the c a t a l y s t shows t h a t the "extra" storage l i e s on the alumina o r a t the i n t e r f a c e , b u t not within the metal s t r u c t u r e . The thermodynamic hydrogen a c t i v i t y measured was the same i n the absence of additional alumina as i t was with the additional support material i n s p i t e o f the differences i n ethane production r a t e . I t has usually been assumed t h a t d i s s o c i a t i v e hydrogen adsorption occurs f i r s t on the metal followed by migration of the atoms t o the support. Recent findings ( r e f s . 5 , 8 ) i n d i c a t e

299

t h a t t h e hydrogen adsorbed on p l a t i n u m i s much l e s s s t r o n g l y bound t o t h e metal t h a n p r e v i o u s l y t h o u g h t , and t h a t t h e adsorbed hydrogen possesses s u f f i c i e n t m o b i l i t y t o r e a d i l y a l l o w such m i g r a t i o n t o t h e m e t a l - s u p p o r t boundary.

The

a b i l i t y o f hydrogen t o m i g r a t e across t h e alumina s u r f a c e i s l e s s apparent; t h e r e f o r e , t h e most l i k e l y s t o r a g e l o c a t i o n f o r hydrogen appears t o be a t t h e t h r e e phase boundary, perhaps as a r e s u l t o f some t y p e o f m i c r o c a p i l l a r y condens a t i o n . RESULTS AND D I S C U S S I O N :

ENHANCED REACTIVITY

I n view o f t h e r e l a t i v e l y small i n c r e a s e i n t h e amount o f hydrogen s t o r e d , i t i s d i f f i c u l t t o see how s t e a d y s t a t e r e a c t i o n r a t e s c o u l d be enhanced s i g n i f -

icantly.

S c h l a t t e r ( r e f . 9 ) a t t e m p t e d t o show t h a t t h e appearance o f enhanced

r a t e s f o r hydrocarbon systems a r e due t o t h e f o r m a t i o n o f carbonaceous r e s i d u e s and a r e n o t an e f f e c t o f hydrogen s p i l l o v e r .

Such r e s i d u e s were found t o be

formed d u r i n g t h e course o f t h e h y d r o g e n a t i o n r e a c t i o n , b u t seem t o have a r e l a t i v e l y small e f f e c t on t h e s h o r t t e r m c a t a l y s t a c t i v i t y and were i n s t e a d a s s o c i a t e d w i t h s m a l l e r ( 5 t o 10%) changes i n t h e l o n g t e r m r e a c t i v i t y ( s e v e r a l hours) o f the c a t a l y s t .

F u r t h e r , t h e r e a c t i o n enhancement e f f e c t was n o t e d f o r

f r e s h l y cleaned c a t a l y s t s u r f a c e s w i t h l i t t l e o r no carbon b u i l d u p .

T h i s does

n o t r u l e o u t an e f f e c t o f t h e carbonaceous l a y e r , b u t a n o t h e r c a n d i d a t e f o r t h e o r i g i n o f t h e enhanced r a t e s seems more l i k e l y . During the study o f the ethylene hydrogenation reaction, m u l t i p l e steady r e a c t i o n s t a t e s were observed near t h e r e a c t i o n r a t e maxima (165-225 C) ( r e f . 5 ) . N e i t h e r n o n i s o t h e r m a l n o r t r a n s p o r t e f f e c t s a r e b e l i e v e d t o be necessary i n o r d e r t o e x p l a i n t h e e x i s t e n c e o f these s t a t e s .

Rather i t i s t h e i n h e r e n t

s t r u c t u r e o f t h e r e a c t i o n network d e s c r i b i n g t h e d e t a i l e d mechanism which p e r m i t s d i f f e r e n t s t e a d y s t a t e s t o e v o l v e depending on t h e i n i t i a l c o n d i t i o n s . Experiments i n d i c a t e d t h a t e i t h e r a1 t e r i n g t h e i n i t i a l f e e d c o m p o s i t i o n t o t h e r e a c t o r o r changing t h e r e a c t o r temperature h i s t o r y p r i o r t o r e a c t i o n c o u l d produce d i f f e r e n t , r e p r o d u c i b l e , s t a b l e r e a c t i o n s t a t e s .

Mathematical a n a l y s i s

o f t h e d i f f e r e n t i a l e q u a t i o n s r e p r e s e n t i n g t h e mechanism f o r e t h y l e n e hydrogenat i o n proposed b y M u l l i n s ( r e f . 5) i n d i c a t e s t h a t such m u l t i p l e s t e a d y s t a t e s a r e permitted. rates.

The r e s u l t i s a q u a n t i t a t i v e d i f f e r e n c e i n t h e observed r e a c t i o n

The d e t a i l s o f t h i s r e a c t i o n a n a l y s i s a r e d e s c r i b e d i n d e t a i l i n t h e

d i s s e r t a t i o n by Rumschitzki ( r e f . 1 0 ) . The e x i s t e n c e o f t h e s e m u l t i p l e s t e a d y s t a t e s o f f e r s an a l t e r n a t i v e explanat i o n f o r t h e observed r a t e enhancement:

i.e.

hydrogen p r o v i d e a d i f f e r e n t i n i t i a l c o n d i t i o n work.

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s p i l l o v e r hydrogen and normal f o r t h e complex r e a c t i o n n e t -

The s p i l l o v e r c o n d i t i o n induces a new, h i g h e r p r o d u c t i o n r a t e , s t e a d y

s t a t e much i n t h e manner t h a t changing t h e i n i t i a l feed s t r e a m c o m p o s i t i o n was

300

found t o produce d i f f e r e n t s t e a d y s t a t e s .

I t i s d i f f i c u l t t o e x t r i c a t e the

i n t e r d e p e n d e n t r o l e s o f s u r f a c e s t r u c t u r e a1 t e r a t i o n s d u r i n g c a t a l y s t p r e t r e a t m e n t and t h a t o f t h e m u l t i p l e r e a c t i o n paths on t h e observed r e a c t i o n r a t e s , b u t c l e a r l y t h e i n i t i a l r e a c t i o n c o n d i t i o n s may p l a y a m a j o r r o l e i n d e t e r m i n i n g t h e f i n a l steady s t a t e . CONCLUSIONS

The use o f an e l e c t r o c h e m i c a l t e c h n i q u e t o observe s u r f a c e hydrogen has made i t p o s s i b l e t o f o l l o w t h e t i t r a t i o n o f s u r f a c e hydrogen by e t h y l e n e .

Direct

measurement shows t h a t t h e manner o f p r e t r e a t m e n t a f f e c t s t h e hydrogen adsorpt i o n t h a t i s observed.

The most l i k e l y l o c a t i o n f o r s t o r a g e o f hydrogen i n

excess o f a monolayer i s a t t h e t h r e e phase boundary o f gas, m e t a l , and s u p p o r t . The r o l e o f t h e new d i f f e r e n t hydrogen a d s o r p t i o n s t a t e s i n r e a c t i o n r a t e enhancement may be t o p r o v i d e two d i f f e r e n t i n i t i a l c o n d i t i o n s f o r t h e r e a c t i o n sequence, which i n t u r n l e a d s t o two d i f f e r e n t s t e a d y s t a t e r a t e s .

Such m u l t i -

p l e s t e a d y s t a t e s have been observed f o r t h e e t h y l e n e h y d r o g e n a t i o n r e a c t i o n by i n d u c i n g s i m i l a r changes i n t h e i n i t i a l r e a c t a n t c o n c e n t r a t i o n s .

I f this i s

t h e case no s i g n i f i c a n t hydrogen s t o r a g e i s r e q u i r e d t o e x p l a i n t h e change i n reaction rates typically attributed to spillover.

REFERENCES

J . Khoobiar, J . Phys. Chem., 68 (1964) 411. Boudart, Advances i n C a t a l y s i s , 20 (1969) 153. H . S i n f e l t and P. J . Lucchesi, J . Amer. Chem. SOC., 85 (1963) 3365. F. T a y l o r , S. J . Thomas and G. Webb, J . C a t a l y s i s , 12 (1968) 150. E. Mu1 1 i n s , Hydrocarbon Reactions o v e r T r a n s i t i o n M e t a l s : O b s e r v a t i o n S u r f a c e Hydrogen, (Ph.D. T h e s i s ) U n i v e r s i t y o f Rochester (1983). 6 H. S a l t s b u r g and M. E . M u l l i n s , Proceedings o f t h e New York Academy o f Sciences : C a t a l y t i c T r a n s i t i o n Metal H y d r i d e s , I n Press. 7 P . A. Sermon and G . C. Bond, C a t a l y s i s Reviews, 8 (19731, 211. 8 P. R. N o r t o n and P. J . R i c h a r d s , S u r f a c e Science, 41, (1974) 293. 9 J . C. S c h l a t e r and M. J , B o u d a r t , C a t a l y s i s , 24 (1972) 482. 10 D. S . Rumschitzki, On t h e Theory o f M u l t i p l e Steady S t a t e s i n I s o t h e r m a l CSTR's, (Ph.D. T h e s i s ) U n i v e r s i t y o f C a l i f o r n i a , B e r k e l e y (1983). 1 2 3 4 5

S. M. J. G. M. of