Surface Properties of Mixed-Metal Catalysts - ACS Publications

Chapter 18

Surface Properties of Mixed-Metal Catalysts D. W. Goodman

Downloaded by GEORGETOWN UNIV on September 7, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch018

Department of Chemistry, Texas A & M

University, College Station, T X 77843

The chemical behavior of monolayer coverages of one metal on the surface of another, i . e . , Cu/Ru, Ni/Ru, Ni/W, Fe/W, Pd/W, has recently been shown to be dramatically d i f f e r e n t from that seen for e i t h e r of the metallic components separately. These chemical alterations, which modify the chemisorption and c a t a l y t i c properties of the overlayers, have been correlated with changes i n the s t r u c t u r a l and e l e c t r o n i c properties of the b i m e t a l l i c system. The films are found to grow i n a manner which causes them to be strained with respect to t h e i r bulk lattice configuration. In addition, unique electronic interface states have been i d e n t i f i e d with these overlayers. These studies, which include the adsorption of CO and H on these overlayers as well as the measurement of the elevated pressure k i n e t i c s of the methanation, ethane hydrogenolysis, cyclohexane dehydrogenation reactions, are reviewed. 2

In a d d i t i o n to modification o f s u r f a c e s b y non-metals, t h e c a t a l y t i c p r o p e r t i e s o f m e t a l s can a l s o be a l t e r e d g r e a t l y b y the a d d i t i o n o f a second t r a n s i t i o n m e t a l ( 1 ) . I n t e r e s t i n b i m e t a l l i c c a t a l y s t s h a s r i s e n s t e a d i l y o v e r t h e y e a r s because o f the c o m m e r c i a l s u c c e s s o f t h e s e systems. T h i s s u c c e s s r e s u l t s from an enhanced a b i l i t y t o c o n t r o l the c a t a l y t i c a c t i v i t y and s e l e c t i v i t y by t a i l o r i n g t h e c a t a l y s t c o m p o s i t i o n ( 2 - 3 ) . A long-standing q u e s t i o n r e g a r d i n g such b i m e t a l l i c systems i s t h e n a t u r e o f t h e p r o p e r t i e s o f t h e mixed m e t a l system w h i c h g i v e r i s e t o i t s enhanced c a t a l y t i c performance r e l a t i v e t o e i t h e r o f i t s i n d i v i d u a l m e t a l components. These enhanced p r o p e r t i e s ( i m p r o v e d s t a b i l i t y , s e l e c t i v i t y and/or a c t i v i t y ) can be a c c o u n t e d f o r b y one o r more o f several p o s s i b i l i t i e s . F i r s t , t h e a d d i t i o n o f one m e t a l t o a second may l e a d t o an e l e c t r o n i c m o d i f i c a t i o n o f e i t h e r o r b o t h o f the m e t a l c o n s t i t u e n t s . T h i s e l e c t r o n i c p e r t u r b a t i o n can r e s u l t from d i r e c t b o n d i n g (charge t r a n s f e r ) o r from a structural 0097-6156/89/0411-0191$06.00/0 o 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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m o d i f i c a t i o n i n d u c e d by one m e t a l upon t h e o t h e r . Secondly, a m e t a l a d d i t i v e c a n promote a p a r t i c u l a r s t e p i n t h e r e a c t i o n sequence and, t h u s , a c t i n p a r a l l e l w i t h the h o s t m e t a l . Thirdly, the a d d i t i v e m e t a l c a n s e r v e t o b l o c k t h e a v a i l a b i l i t y o f c e r t a i n a c t i v e s i t e s , o r ensembles, p r e r e q u i s i t e f o r a p a r t i c u l a r r e a c t i o n step. I f t h i s " p o i s o n e d " r e a c t i o n s t e p i n v o l v e s an u n d e s i r a b l e r e a c t i o n product, t h e n t h e n e t e f f e c t i s a n enhanced o v e r a l l selectivity. F u r t h e r , t h e a t t e n u a t i o n b y t h i s mechanism o f a r e a c t i o n step l e a d i n g t o undesirable surface contamination will promote c a t a l y s t a c t i v i t y and d u r a b i l i t y . The s t u d i e s r e v i e w e d h e r e a r e p a r t o f a c o n t i n u i n g e f f o r t ( 4 10) t o i d e n t i f y t h o s e p r o p e r t i e s o f b i m e t a l l i c systems w h i c h can be related to t h e i r superior c a t a l y t i c properties. A p i v o t a l question t o be a d d r e s s e d o f b i m e t a l l i c systems (and o f s u r f a c e i m p u r i t i e s i n g e n e r a l ) i s t h e r e l a t i v e i m p o r t a n c e o f ensemble ( s t e r i c o r l o c a l ) versus electronic (nonlocal o r extended) effects i n the modification of catalytic properties. I n gathering information to a d d r e s s t h i s q u e s t i o n i t h a s been advantageous t o s i m p l i f y t h e p r o b l e m b y u t i l i z i n g models o f a b i m e t a l l i c c a t a l y s t such as t h e d e p o s i t i o n o f m e t a l s on s i n g l e c r y s t a l substrates i n the clean environment f a m i l i a r t o surface s c i e n c e . These s t u d i e s were c a r r i e d o u t u t i l i z i n g t h e s p e c i a l i z e d a p p a r a t u s d e s c r i b e d i n r e f e r e n c e s (11-12). This device c o n s i s t s o f two distinct regions, a surface analysis chamber and a microcatalytic reactor. The custom b u i l t r e a c t o r , c o n t i g u o u s t o the s u r f a c e a n a l y s i s chamber, employs a r e t r a c t i o n b e l l o w s t h a t s u p p o r t s t h e m e t a l s i n g l e c r y s t a l and a l l o w s t r a n s l a t i o n o f t h e c a t a l y s t i n vacuo from the r e a c t o r t o t h e s u r f a c e a n a l y s i s r e g i o n . B o t h r e g i o n s a r e o f u l t r a h i g h vacuum c o n s t r u c t i o n , b a k e a b l e , and capable o f u l t i m a t e pressures o f l e s s than 2 X 10" Torr. Auger s p e c t r o s c o p y (AES) i s used t o c h a r a c t e r i z e t h e sample b e f o r e and after reaction. A second chamber was e q u i p p e d w i t h Auger spectroscopy, l o w energy e l e c t r o n d i f f r a c t i o n (LEED) and a mass s p e c t r o m e t e r f o r t e m p e r a t u r e programmed d e s o r p t i o n (TPD). Many s u c h model systems have been s t u d i e d b u t a p a r t i c u l a r l y a p p e a l i n g c o m b i n a t i o n i s t h a t o f Cu on Ru. Cu i s i m m i s c i b l e i n Ru which facilitates coverage determinations b y TPD (4) and c i r c u m v e n t s t h e c o m p l i c a t i o n o f d e t e r m i n i n g t h e 3-d c o m p o s i t i o n . The a d s o r p t i o n and growth o f Cu f i l m s on the Ru(0001) s u r f a c e have been s t u d i e d (4-10.13-20) by work f u n c t i o n f u n c t i o n measurements, LEED, AES, and TPD. The r e s u l t s from r e c e n t s t u d i e s (4-10) i n d i c a t e t h a t f o r submonolayer d e p o s i t i o n s a t 100K the Cu grows i n a highly dispersed mode, subsequently forming 2-d i s l a n d s pseudomorphic t o t h e Ru(0001) s u b s t r a t e upon a n n e a l i n g t o 300K. Pseudomorphic growth o f the copper i n d i c a t e s t h a t the copper-copper bond d i s t a n c e s are s t r a i n e d a p p r o x i m a t e l y 6% beyond the e q u i l i b r i u m bond d i s t a n c e s found f o r b u l k copper. A c o m p a r i s o n o f CO d e s o r p t i o n from Ru (2) from m u l t i l a y e r Cu ( 10ML) on Ru and 1ML Cu on Ru i s shown i n F i g . 1. The TPD f e a t u r e s o f t h e 1ML Cu (peaks a t 160 and 210K) on Ru a r e a t t e m p e r a t u r e s i n t e r m e d i a t e between Ru and b u l k Cu. T h i s s u g g e s t s t h a t t h e monolayer Cu i s e l e c t r o n i c a l l y p e r t u r b e d and t h a t t h i s p e r t u r b a t i o n m a n i f e s t s i t s e l f i n the b o n d i n g o f CO. An i n c r e a s e i n the desorption temperature relative to bulk Cu i n d i c a t e s a 10



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Figure 1. T P D results for C O adsorbed to saturation levels on clean Ru(0001), on multilayer Cu, and on a 1ML Cu covered Ru(0001). (Reproduced with permission from ref. 7. Copyright 1986 Academic.)



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s t a b i l i z a t i o n o f t h e CO on t h e monolayer Cu s u g g e s t i n g a c o u p l i n g o f t h e CO t h r o u g h t h e Cu t o t h e Ru. The magnitude o f t h e CO s t a b i l i z a t i o n i m p l i e s t h a t t h e e l e c t r o n i c m o d i f i c a t i o n o f t h e Cu b y t h e Ru i s s i g n i f i c a n t a n d s h o u l d be o b s e r v a b l e w i t h a band s t r u c t u r e p r o b e . Recent a n g u l a r r e s o l v e d p h o t o e m i s s i o n s t u d i e s (7) i n d e e d show a u n i q u e i n t e r f a c e s t a t e w h i c h i s l i k e l y r e l a t e d t o t h e a l t e r e d CO b o n d i n g on Cu f i l m s i n t i m a t e t o Ru. F i g u r e 2 shows t h e r e s u l t s ( 7 ) o f CO c h e m i s o r p t i o n on t h e Cu/Ru(0001) system as a f u n c t i o n o f t h e Cu c o v e r a g e . I n each case t h e e x p o s u r e c o r r e s p o n d s t o a s a t u r a t i o n c o v e r a g e o f CO. Most a p p a r e n t i n F i g u r e 2 i s a monotonic d e c r e a s e upon a d d i t i o n o f Cu o f t h e CO s t r u c t u r e i d e n t i f i e d w i t h Ru (peaks a t 400 and 480K) and an i n c r e a s e o f t h e CO s t r u c t u r e c o r r e s p o n d i n g t o Cu (peaks a t 160 and 210K). The b u i l d u p o f a t h i r d f e a t u r e a t -300K ( i n d i c a t e d b y t h e dashed l i n e ) i s a s s i g n e d t o c o r r e s p o n d t o CO d e s o r b i n g from t h e edges o f Cu i s l a n d s . I n t e g r a t i o n o f t h e 200, 275, and 300K peaks provides information regarding island sizes, that i s , perimeter-toi s l a n d a r e a r a t i o s , a t v a r i o u s Cu c o v e r a g e s . F o r example, a t # 0.66 t h e average i s l a n d s i z e i s e s t i m a t e d t o be a p p r o x i m a t e l y 50A i n diameter. T h i s i s l a n d s i z e i s c o n s i s t e n t w i t h an estimate o f t h e 2-d i s l a n d s i z e c o r r e s p o n d i n g t o t h i s coverage o f 40-60A d e r i v e d from t h e w i d t h o f t h e LEED beam p r o f i l e s (2). Model s t u d i e s o f t h e Cu/Ru(0001) c a t a l y s t have been c a r r i e d o u t (10) f o r m e t h a n a t i o n and h y d r o g e n o l y s i s r e a c t i o n s . These d a t a s u g g e s t t h a t copper m e r e l y s e r v e s as an i n a c t i v e d i l u e n t , b l o c k i n g s i t e s on a one-to-one b a s i s . S i m i l a r r e s u l t s have been found i n analogous studies (21) i n t r o d u c i n g silver onto a Rh(lll) methanation c a t a l y s t . S i n f e l t (22,) has shown t h a t copper i n a Cu/Ru c a t a l y s t i s c o n f i n e d t o the s u r f a c e o f ruthenium. R e s u l t s from t h e model c a t a l y s t s d i s c u s s e d h e r e t h e n s h o u l d be r e l e v a n t t o those on t h e corresponding supported, b i m e t a l l i c catalysts. S e v e r a l such s t u d i e s have been c a r r i e d o u t i n v e s t i g a t i n g t h e a d d i t i o n o f copper o r o t h e r Group I B m e t a l s on t h e r a t e s o f CO h y d r o g e n a t i o n (23-25) and ethane h y d r o g e n o l y s i s (25.) c a t a l y z e d b y r u t h e n i u m . I n general, t h e s e s t u d i e s show a marked r e d u c t i o n i n a c t i v i t y w i t h a d d i t i o n o f the Group IB m e t a l s u g g e s t i n g a more p r o f o u n d e f f e c t o f t h e Group IB m e t a l on r u t h e n i u m t h a n i m p l i e d from t h e model s t u d i e s . A c r i t i c a l parameter i n t h e s u p p o r t e d s t u d i e s i s t h e measurement o f the active ruthenium surface using hydrogen chemisorption techniques. H a l l e r and coworkers (26-27) have r e c e n t l y s u g g e s t e d t h a t hydrogen spillover d u r i n g c h e m i s o r p t i o n may o c c u r from r u t h e n i u m t o copper c o m p l i c a t i n g t h e assessment o f s u r f a c e Ru atoms. Recent s t u d i e s i n o u r l a b o r a t o r y (5-6) have shown d i r e c t l y t h a t s p i l l o v e r from Ru t o Cu c a n t a k e p l a c e a n d must be c o n s i d e r e d i n t h e hydrogen c h e m i s o r p t i o n measurements. H s p i l l o v e r would l e a d t o a s i g n i f i c a n t o v e r e s t i m a t i o n o f t h e number o f a c t i v e r u t h e n i u m m e t a l s i t e s and t h u s t o s i g n i f i c a n t e r r o r i n c a l c u l a t i n g ruthenium s p e c i f i c a c t i v i t y . I f t h i s i s indeed the case, the r e s u l t s o b t a i n e d on t h e s u p p o r t e d c a t a l y s t s , c o r r e c t e d f o r t h e o v e r e s t i m a t i o n o f s u r f a c e r u t h e n i u m , c o u l d become more comparable w i t h t h e model d a t a r e p o r t e d h e r e . Finally, the a c t i v a t i o n e n e r g i e s o b s e r v e d on s u p p o r t e d c a t a l y s t s i n v a r i o u s l a b o r a t o r i e s Cu

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Figure 2. T P D results corresponding to C O adsorbed to saturation levels on the clean Ru(0001) surface, and from this same surface containing various coverages of Cu. (Reproduced with permission from ref. 7. Copyright 1986 Academic.)



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a r e g e n e r a l l y unchanged b y t h e a d d i t i o n o f Group I B m e t a l (26-28) i n agreement w i t h t h e model s t u d i e s . These arguments suggest that Ru s p e c i f i c rates f o r m e t h a n a t i o n a n d ethane h y d r o g e n o l y s i s o n s u p p o r t e d Cu/Ru c a t a l y s t s a p p r o x i m a t e t h o s e v a l u e s f o u n d f o r p u r e Ru. As a consequence, t h e r a t e s f o r c y c l o h e x a n e d e h y d r o g e n a t i o n r e a c t i o n o n s u p p o r t e d Cu/Ru, s i m i l a r l y c o r r e c t e d , must exceed t h o s e s p e c i f i c r a t e s found f o r pure Ru. The u n c o r r e c t e d s p e c i f i c rates f o r cyclohexane d e h y d r o g e n a t i o n on t h e s u p p o r t e d Cu/Ru system r e m a i n e s s e n t i a l l y unchanged upon a d d i t i o n o f Cu t o Ru ( 1 0 ) . An a c t i v i t y enhancement f o r c y c l o h e x a n e d e h y d r o g e n a t i o n i n t h e mixed Cu/Ru system r e l a t i v e t o p u r e Ru i s most s u r p r i s i n g g i v e n t h a t Cu i s l e s s a c t i v e f o r t h i s r e a c t i o n t h a n Ru. F i g u r e 3 shows t h e e f f e c t o f t h e a d d i t i o n o f Cu t o Ru on t h e r a t e o f c y c l o h e x a n e d e h y d r o g e n a t i o n t o benzene. The o v e r a l l r a t e o f t h i s r e a c t i o n i s seen t o i n c r e a s e b y a p p r o x i m a t e l y an o r d e r o f magnitude a t a copper c o v e r a g e o f 3/4 o f a monolayer. This t r a n s l a t e s t o a Ru s p e c i f i c r a t e enhancement o f -40. Above t h i s coverage, the r a t e f a l l s t o an a c t i v i t y approximately equal t o t h a t o f C u - f r e e Ru. The o b s e r v a t i o n o f n o n - z e r o r a t e s a t t h e h i g h e r Cu c o v e r a g e s i s b e l i e v e d t o be c a u s e d b y t h r e e d i m e n s i o n a l c l u s t e r i n g o f t h e Cu o v e r l a y e r s (10) . S i m i l a r d a t a have been o b t a i n e d f o r t h i s r e a c t i o n on e p i t a x i a l and a l l o y e d A u / P t ( l l l ) s u r f a c e s ( 2 9 ) . The r a t e enhancement o b s e r v e d f o r submonolayer Cu d e p o s i t s may r e l a t e t o a n enhanced a c t i v i t y o f t h e s t r a i n e d Cu f i l m f o r t h i s r e a c t i o n due t o i t s a l t e r e d g e o m e t r i c ( 7 ) and e l e c t r o n i c ( 9 ) properties. A l t e r n a t i v e l y , a mechanism whereby t h e two m e t a l s c o o p e r a t i v e l y c a t a l y z e d i f f e r e n t s t e p s o f t h e r e a c t i o n may a c c o u n t for the a c t i v i t y promotion. F o r example, dissociative H a d s o r p t i o n on b u l k Cu i s u n f a v o r a b l e due t o a n a c t i v a t i o n b a r r i e r o f a p p r o x i m a t e l y 5 k c a l / m o l (.30) . I n t h e combined Cu/Ru system, Ru may f u n c t i o n as an atomic hydrogen source/sink v i a spillover t o / f r o m n e i g h b o r i n g Cu. A k i n e t i c a l l y c o n t r o l l e d s p i l l o v e r o f H from Ru t o Cu, d i s c u s s e d above, i s c o n s i s t e n t w i t h an o b s e r v e d optimum r e a c t i o n r a t e a t an i n t e r m e d i a t e Cu c o v e r a g e . F i n a l l y , we n o t e t h e d i f f e r e n c e s between a Ru(0001) c a t a l y s t w i t h o r w i t h o u t added Cu w i t h r e s p e c t t o a t t a i n i n g s t e a d y - s t a t e reaction rates. On t h e C u - f r e e s u r f a c e , an i n d u c t i o n time o f a p p r o x i m a t e l y 10 minutes i s r e q u i r e d t o achieve steady state activity. D u r i n g t h i s t i m e , p r o d u c t i o n o f benzene i s q u i t e l o w w h i l e t h e h y d r o g e n o l y s i s t o l o w e r a l k a n e s , p r i m a r i l y methane, i s s i g n i f i c a n t l y h i g h e r than a t s t e a d y - s t a t e . During t h i s i n d u c t i o n time t h e c a r b o n l e v e l (as d e t e r m i n e d by Auger s p e c t r o s c o p y ) r i s e s to a s a t u r a t i o n v a l u e c o i n c i d e n t a l w i t h the onset o f steady s t a t e reaction. T h i s b e h a v i o r s u g g e s t s t h a t a carbonaceous l a y e r on t h e m e t a l s u r f a c e e f f e c t i v e l y s u p p r e s s e s c a r b o n - c a r b o n bond s c i s s i o n , o r h y d r o g e n o l y s i s , on t h e Ru s u r f a c e . Cu a d d i t i o n l e a d s t o an enhanced r a t e o f benzene p r o d u c t i o n w i t h l i t t l e o r no i n d u c t i o n t i m e . That i s , t h e i n i t i a l r a t e o f cyclohexane h y d r o g e n o l y s i s , r e l a t i v e t o the Cu-free surface, i s suppressed. F u r t h e r , Cu reduces t h e r e l a t i v e c a r b o n b u i l d u p on t h e surface during reaction. Thus, Cu may p l a y a s i m i l a r r o l e as t h e carbonaceous l a y e r i n s u p p r e s s i n g c y c l o h e x a n e h y d r o g e n o l y s i s w h i l e c o n c u r r e n t l y s t a b i l i z i n g those i n t e r m e d i a t e s l e a d i n g t o t h e p r o d u c t 2

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benzene. I n a d d i t i o n , copper may s e r v e t o weaken t h e c h e m i s o r p t i o n bond o f benzene and thus l i m i t s e l f - p o i s o n i n g by a d s o r b e d p r o d u c t . T h i s l a t t e r p o s s i b i l i t y has been p r o p o s e d by S a c h t l e r and S o m o r j a i (29) t o e x p l a i n the r o l e o f Au i n A u / P t ( l l l ) c a t a l y s t s f o r t h i s reaction. A weakening o f benzene c h e m i s o r p t i o n s a t i s f a c t o r i l y a c c o u n t s f o r our o b s e r v a t i o n t h a t the r e a c t i o n changes from z e r o o r d e r i n c y c l o h e x a n e on Ru(0001) t o a p p r o x i m a t e l y f i r s t o r d e r upon the a d d i t i o n o f Cu. A second b i m e t a l l i c system w h i c h has been t h o r o u g h l y s t u d i e d i s n i c k e l a d s o r b e d onto t u n g s t e n (31-33) . On b o t h W(110) and W(100), N i i s a d s o r b e d l a y e r - b y - l a y e r . Annealing Ni layers with c o v e r a g e s l e s s t h a n 1.3 ML t o 1200K p r o d u c e s l i t t l e change i n t h e Ni(848eV)/W(179eV) AES r a t i o . However, f o r N i c o v e r a g e s above 1.3ML, a 1200K a n n e a l r e s u l t s i n a v e r y s l o w i n c r e a s e i n t h i s AES r a t i o w i t h coverage, i n d i c a t i n g e i t h e r a l l o y or 3-dimensional i s l a n d formation. CO a d s o r p t i o n (.32) as a f u n c t i o n o f N i c o v e r a g e on W(110) has been i n v e s t i g a t e d . As the N i c o v e r a g e i s i n c r e a s e d from 0.3 t o 1.0ML, a d s o r p t i o n on the W(110) s u b s t r a t e d e c r e a s e s , as e v i d e n c e d by a r e d u c t i o n i n the CO f e a t u r e s between 225 and 350K, w h i l e a f e a t u r e a t 380K becomes more p r o m i n e n t . The 380K CO TPD peak maximum f o r 1ML N i compares w i t h 430K f o r t h e CO TPD peak maximum f o r N i ( l l l ) (3J5) . I n c r e a s i n g the N i c o v e r a g e above 1.0ML results i n a b r o a d e n i n g o f the 380K CO TPD peak and i n the development o f a s h o u l d e r f e a t u r e , s u g g e s t i v e o f b u l k N i CO d e s o r p t i o n , a t -430K. CO c h e m i s o r p t i o n on the Ni/W(100) s u r f a c e as a f u n c t i o n o f N i c o v e r a g e i s s i m i l a r t o CO a d s o r p t i o n on Ni/W(110) . As the N i c o v e r a g e i s i n c r e a s e d from 0.3 t o 1.0ML, d e c r e a s i n g i n t e n s i t y i n the TPD f e a t u r e s a s s o c i a t e d w i t h W(100) i s e v i d e n t n e a r 300K. A t a Ni coverage o f 1ML, the CO TPD peak maximum i s r e d u c e d by a p p r o x i m a t e l y 50K from the c o r r e s p o n d i n g peak maximum on N i ( 1 0 0 ) (35). For c o v e r a g e s g r e a t e r t h a n 1ML, a c l e a r s h o u l d e r a t 420-450K i s o b s e r v e d , i n d i c a t i n g t h a t second and s u c c e s s i v e N i l a y e r s have chemisorptive p r o p e r t i e s very s i m i l a r to b u l k N i . Thus the W s u b s t r a t e s c l e a r l y a l t e r the c h e m i s o r p t i v e p r o p e r t i e s o f the f i r s t N i l a y e r , b u t have o n l y s l i g h t e f f e c t s on the second and subsequent layers. That CO c h e m i s o r p t i o n i s p e r t u r b e d on s t r a i n e d - l a y e r N i i s n o t s u r p r i s i n g i n v i e w o f CO c h e m i s o r p t i o n b e h a v i o r on o t h e r m e t a l o v e r l a y e r systems. F o r example, on Cu/Ru i t has been proposed t h a t c h a r g e t r a n s f e r from Cu t o Ru r e s u l t s i n d e c r e a s e d occupancy o f the Cu 4s l e v e l . T h i s e l e c t r o n i c m o d i f i c a t i o n makes Cu more " n i c k e l l i k e , " and r e s u l t s i n an i n c r e a s e i n the b i n d i n g energy f o r CO. S i m i l a r l y Cu/W (36.) a l s o e x h i b i t s charge t r a n s f e r t o the s u b s t r a t e and a i n c r e a s e i n CO b i n d i n g s t r e n g t h t o Cu. I n a n o t h e r case where the CO b i n d i n g energy i n c r e a s e s , N i / R u (.37) , an i n c r e a s e i n the d e n s i t y o f s t a t e s i s o b s e r v e d c l o s e t o the F e r m i l e v e l . The increased electron d e n s i t y may result i n increased metal-CO b a c k b o n d i n g , w h i c h i n t u r n would i n c r e a s e the b i n d i n g energy o f CO. In c o n t r a s t t o the above examples, CO on Ni/W i s less s t r o n g l y bound t o the N i monolayer than to b u l k N i . One e x p l a n a t i o n f o r t h i s e f f e c t i s t h a t the charge t r a n s f e r o b s e r v e d from N i t o W r e s u l t s i n a s h i f t o f the N i d l e v e l s , r e l e v a n t t o CO b o n d i n g , t o h i g h e r b i n d i n g e n e r g i e s ( i . e . f a r t h e r from the Fermi



198

CHARACTERIZATION A N D CATALYST D E V E L O P M E N T

Copper coverage (monolayers) Figure 3. Relative rate of reaction versus surface Cu coverage on Ru(0001) for cyclohexane dehydrogenation to benzene. P = 101 Torr. H /cyclohexane = 100. T = 650K. (Reproduced with permission from ref. 10. Copyright 1987 Elsevier.) T

2

t — ' — i — • — i —

1

— i — ' — r

4» .9 M L N l / W ( 1 0 0 1 -3 1

0

I •

I

1.2

1.4

•

l

1.6

•

1.8

2.0

2.2

2.4

1000/T (K" ) 1

Figure 4. Arrhenius plot for C H synthesis over several different Ni coverages on W(110) and W(100) at a total reactant pressure of 120 Torr, H / C O = 4/1. (Reproduced from ref. 41. Copyright 1987 American Chemical Society.) 4

2



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Surface Properties ofMixed-Metal Catalysts

l e v e l ) . I n d e e d , s u c h an e f f e c t h a s b e e n o b s e r v e d i n t h e c a s e o f Ni/Nb(110) a n d Pd/Ta(110) ( 3 8 ) . S i m i l a r l y , r e s u l t s on o t h e r group V I I I m e t a l - W systems (39-40) have shown a d e c r e a s e i n t h e CO binding strength. The c a t a l y t i c a c t i v i t y o f s t r a i n e d l a y e r N i on W(110) f o r m e t h a n a t i o n a n d ethane h y d r o g e n o l y s i s h a s been s t u d i e d as a f u n c t i o n o f N i c o v e r a g e (41) . The a c t i v i t y p e r N i atom s i t e f o r methanation, a s t r u c t u r e i n s e n s i t i v e r e a c t i o n , i s independent o f the N i c o v e r a g e ( F i g . 4) and s i m i l a r t o t h e a c t i v i t y f o u n d f o r b u l k Ni. The a c t i v a t i o n energy f o r t h i s r e a c t i o n i s l o w e r on t h e s t r a i n e d m e t a l o v e r l a y e r , however, v e r y l i k e l y r e f l e c t i n g t h e l o w e r b i n d i n g s t r e n g t h o f CO on t h e b i m e t a l l i c system. I n c o n t r a s t , ethane h y d r o g e n o l y s i s , w h i c h i s a s t r u c t u r e s e n s i t i v e r e a c t i o n o v e r b u l k N i , d i s p l a y e d marked structural e f f e c t s o n t h e Ni/W system (41) . We have o b s e r v e d , as shown i n F i g u r e 5, t h a t t h e s p e c i f i c r a t e , o r r a t e p e r s u r f a c e m e t a l atom, but n o t t h e a c t i v a t i o n energy, i s a s t r o n g f u n c t i o n o f metal c o v e r a g e o n t h e Ni/W(110) s u r f a c e , s u g g e s t i n g t h a t t h e c r i t i c a l

1.5

1.7

1.9

2.1

1

1000/T (K" ) Figure 5. Arrhenius plots of the rates of ethane hydrogenolysis versus Ni coverage on W(110) at a total pressure of 100 Torr, H ^ C ^ = 100. (Reproduced from ref. 41. Copyright 1987 American Chemical Society.)



200


r e a c t i o n s t e p i n v o l v e s t h e need f o r a s i n g l e , s t e r i c a l l y u n h i n d e r e d N i atom. On t h e Ni/W(100) s u r f a c e t h e s p e c i f i c r e a c t i o n r a t e was i n d e p e n d e n t o f N i c o v e r a g e . I n a d d i t i o n , t h e r a t e on b u l k N i ( 1 0 0 ) , Ni/W(110) i n t h e l i m i t o f z e r o c o v e r a g e , and Ni/W(100) were a l l e q u a l , as were t h e a c t i v a t i o n e n e r g i e s . T h i s i m p l i e s t h a t on Ni/W(100) t h e N i atom geometry i s s u f f i c i e n t l y open t o a l l o w u n h i n d e r e d a c c e s s t o each N i atom. A p p a r e n t l y o n t h e Ni/W(110) s u r f a c e o n l y i s l a n d edges and i n d i v i d u a l atoms d i s p l a y a c t i v i t y s i m i l a r t o the Ni(100) surface; the i s l a n d i n t e r i o r s , i n c o n t r a s t , e x h i b i t b e h a v i o r s i m i l a r t o N i ( l l l ) w h i c h h a s a much lower s p e c i f i c r a t e and h i g h e r a c t i v a t i o n energy. As t h e N i c o v e r a g e i s r e d u c e d , the number o f a c t i v e , N i ( 1 0 0 ) - l i k e atoms i n c r e a s e s , l e a d i n g t o a n increase i n the s p e c i f i c rate. The a c t i v a t i o n energy, however, remains unchanged. We have s t u d i e d s e v e r a l o t h e r m e t a l o v e r l a y e r s on W(110), W(100), and Ru(0001) s u b s t r a t e s ( 4 2 ) . Table 1 l i s t s properties o f the m e t a l o v e r l a y e r s , and t h e e f f e c t o f t h e s u b s t r a t e on CO chemisorption. I n general only the f i r s t monolayer grows pseudomorphically, though more t h a n one monolayer may be s t a b l e b e f o r e t h r e e d i m e n s i o n a l i s l a n d s a r e formed (e. g. Cu/Ru grows two stable layers). The b i n d i n g s t r e n g t h o f CO i s always a l t e r e d from the b u l k m e t a l , though t h e magnitude o f t h e e f f e c t i s s e e m i n g l y more dependent on t h e m e t a l o v e r l a y e r , t h a n on t h e degree o f s t r a i n induced by t h e s u b s t r a t e (represented as t h e atom density m i s m a t c h ) . As w i t h Ni/W and Cu/Ru, t h e e f f e c t on CO b i n d i n g energy e x t e n d s p r i m a r i l y t o o n l y t h e f i r s t monolayer; subsequent l a y e r s e x h i b i t behavior close to the bulk metal. T a b l e 1.

Comparison o f S t r a i n e d - M e t a l O v e r l a y e r

Systems

P s e u d o m o r p h i c / Change A d s o r b a t e S u b s t r a t e Atom D e n s i t y E p i t a x i a l i n CO Mismatch/ML L a y e r s Desorption T

Cu Cu Ni Ni Ni Pd Pd Pd Fe Fe

Ru(0001) W(110) W(110) W(100) Ru(0001) W(110) W(100) Ta(110) W(110) W(100)

6% 20 21 42 15 10 35 18 9 35

1/2 1/1 1/1 1/1 1/1 1/1 2/2 1/1 1/2 2/2

50K 80 -50 -50 50 -200 -170 -230 -50 -60

ACKNOWLEDGMENT We acknowledge w i t h p l e a s u r e t h e p a r t i a l s u p p o r t o f t h i s work b y the Department o f Energy, O f f i c e o f B a s i c Energy S c i e n c e s , D i v i s i o n of Chemical Sciences.


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LITERATURE CITED 1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Grimley, T. B.; T o r r i n i , M., J . Phys. Chem. 1973, 87, 4378. S i n f e l t , J . H. B i m e t a l l i c Catalysts: Discoveries, Concepts, and Applications; John Wiley & Sons: NY, NY, 1983. Schwab, G. M., Disc. Faraday Soc. 1950, 8, 166. Yates, J . T., J r . ; Peden, C. H. F.; Goodman, D. W., J . Catal. 1985, 94, 576. Goodman, D. W.; Yates, J . T., J r . ; Peden, C. H. F., Surf. Sci. 1985, 164, 417. Goodman, D. W.; Peden, C. H. F., J . Catal. 1985, 95, 321. Houston, J . E.; Peden, C. H. F.; B l a i r , D. S.; Goodman, D. W., Surf. S c i . 1986, 167, 427. Houston, J . E.; Peden, C. H. F.; Feibelman, P. J . ; Goodman, D. W., I&EC Fundamentals 1986, 25, 58. Houston, J . E.; Peden, C. H. F.; Feibelman, P. J . ; Hamann, D. R., Phys. Rev. Lett. 1986, 56, 375. Peden, C. H. F.; Goodman, D. W., J . Catal. 1986, 100, 520; i b i d , 1987, 104, 347. Goodman, D. W.; Kelley, R. D.; Madey, T. E.; Yates, J . T., J r . , J . Catal. 1980, 64, 479. Goodman, D. W. Ann. Rev. Phys. Chem. 1986, 37, 425; Accts. Chem. Res. 1984, 17, 194; J . Vac. S c i . Tech. 1982, 20, 522. Shimizu, H.; Christmann, K.; E r t l , G., J . Catal. 1980, 61, 412. Vickerman, J . C.; Christmann, K.; E r t l , G., J . Catal. 1981, 71, 175. Shi, S. K.; Lee, H. I.; White, J . M., Surf. S c i . 1981, 102, 56. Richter, L.; Bader, S. D.; Brodsky, M. B., J . Vac. S c i . Techn. 1981, 18, 578. Vickerman, J . C.; Christmann, K., Surf. S c i . 1982, 120, 1. Vickerman, J . C.; Christmann, K.; E r t l , G.; Heiman, P.; Himpsel, F. J . ; Eastman, D. E., Surf. S c i . 1983, 134, 367. Bader, S. D.; Richter, L., J . Vac. S c i . Technol. 1983, A1, 1185. Park, C.; Bauer, E.; Poppa, H., Surf. Sci., submitted f o r publication. Goodman, D. W.; Heterogeneous Catalysis (Proceedings of IUCCP Conference), Texas A&M University, 1984. S i n f e l t , J . H.; V i a , G. H.; L y t l e , F. W., Catal. Rev.-Sci. Eng. 1984, 26, 81. Datye, A. K.; Schwank, J . , J . Catal. 1985, 93, 256. Bond, G. C.; Turnham, B. D., J . Catal. 1976, 45, 128. Luyten, L. J . M.; Eck, M. V.; Grondelle, J . V.; Hooff, J . H. C. V., Phys. Chem. 1978, 82, 2000. Rouco, A. J . ; Haller, G. L.; O l i v e r , J . A.; Kemball, C., Catal. 1983, 84, 297. Haller, G. L.; Resasco, D. E.; Wang, J . , J . Catal. 1983, 84, 477. S i n f e l t , J . H. J . Catal. 1973, 29, 308. Sachtler, J.W.A.; Somorjai, G.A., J . Catal. 1984, 89, 35. Balooch, M.; C a r d i l l o , M. J . ; M i l l e r , D. R.; Stickney, R. E., Surf. S c i . 1975, 50, 263.


202 31. 32. 33. 34. 35. 36. 37.


38. 39. 40. 41. 42.

CHARACTERIZATION AND CATALYST DEVELOPMENT Berlowitz, P. J . ; Goodman, D. W, Surf. S c i . 1987, 187, 463. Balooch, M.; C a r d i l l o , M. J . ; M i l l e r , D. R.; Stickney, R. E., Surf. S c i . 1975, 50, 263. Kolaczkiewicz, J . ; Bauer, E., Surf. S c i . 1984, 144, 495. Christmann, K.; Schober, O.; E r t l , G., Chem. Phys. 1974, 60. Goodman, D. W.; Yates, J . T.; Madey, T. E., Surf. S c i . 1980, 93, 135. Hamedeh, I.; Gomer, R., Surf. S c i . 1985, 154, 168. Houston, J . E.; White, J . M.; Berlowitz, P. J . , Goodman, D. W., Surf. Sci., 1988. 205, 7. Ruckman, M. W.; Strongin, M.; Pan, X., i b i d . Prigge, D.; Schlenk, W.; Bauer, E., Surf. S c i . 1982, 123. 698. Judd, R. W.; Reichelt, M. A.; Lambert, R. M., to be submitted for p u b l i c a t i o n . Greenlief, C. M.; Berkowitz, P. J . ; Goodman, D. W., J . Phys. Chem. 1987, 91, 6669. Berlowitz, P. J . ; Peden, C. H. F.; Goodman, D. W., Mat. Res. Soc. Symp. Proc. 1987, 83, 161.

R E C E I V E D July 17, 1989


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