Chapter 9 Mechanism for H Dissociation on Transition 2
The Challenge of d and f Electrons Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 09/10/18. For personal use only.
Metal Clusters and Surfaces Itai Panas, Per Siegbahn, and Ulf Wahlgren Institute of Theoretical Physics, University of Stockholm, Vanadisvägen 9, S-11346 Stockholm, Sweden
Basic rules are set up for the requirements on a cluster for dissociatively chemisorbing H . These rules are built on previous experience from model calculations on transition metal complexes and surfaces. If these rules are combined with the knowledge of the atomic spectra of the transition metals, an understanding of both the low reactivity of the Co -Co clusters compared to the corresponding nickel clusters and the similarly large reactivities of Co and Ni can be obtained. It is found that all the Co -Co clusters have stable closed valence shell configurations. The present model for dissociating H does not involve the electron donation from the cluster to H that has been suggested previously. The correlation between ionization energies and reactivities of the clusters is shown to be a secondary consequence of the electronic structure of the clusters. 2
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A few years ago Smalley and coworkers were able to obtain detailed experimental information about the reactivity of specific transition metal clusters with hydrogen molecules (1). The results for copper and nickel clusters were essentially as expected from the known results for surface and metal complex activities. For copper no clusters were able to dissociate H whereas for nickel all clusters were active with a slow, steady increase of activity with cluster size. For the other transition metals studied, cobalt, iron and niobium, a completely different picture emerged. For these metals a dramatic sensitivity of the reactivity to cluster size was detected. No convincing explanation for these surprising results has hitherto been suggested. It should be added that there are no dramatic differences in the activity towards H for the metal surfaces (or the metal complexes) of nickel on the one hand and iron, cobalt and niobium on the other. In another set of interesting experiments Whetten et al 2
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0097-6156/89A)394-0125$06.00/0 ο 1989 American Chemical Society
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measured t h e i o n i z a t i o n e n e r g i e s as a f u n c t i o n o f c l u s t e r s i z e f o r i r o n (2) and niobium ( 3 ) . A s t r o n g c o r r e l a t i o n was o b s e r v e d between t h e s e i o n i z a t i o n e n e r g i e s and t h e r e a c t i v i t i e s o f t h e c l u s t e r s . The s i m p l e e x p l a n a t i o n g i v e n f o r t h i s c o r r e l a t i o n was t h a t hydrogen c h e m i s o r p t i o n r e q u i r e s charge t r a n s f e r from t h e c l u s t e r . T h i s p i c t u r e i s i n l i n e w i t h t h e o r e t i c a l Extended Hiickel r e s u l t s o b t a i n e d by S a i l l a r d and Hoffmann ( 4 ) , who found t h a t t h e c r i t i c a l f a c t o r i n s u r f a c e a c t i v a t i o n i s e l e c t r o n t r a n s f e r from t h e m e t a l . The c l u s t e r s w i t h a h i g h e r Fermi l e v e l s h o u l d thus hav^e an enhanced a b i l i t y t o g i v e e l e c t r o n d e n s i t y t o t h e a n t i b o n d i n g cr o r b i t a l o f . T h i s p i c t u r e i s , however, q u i t e d i f f e r e n t from t h e one we have o b t a i n e d f o r a c t i v a t i o n o f h" on s u r f a c e s . In our c a l c u l a t i o n s on c l u s t e r models we have n o t found any c o r r e l a t i o n between t h e a b i l i t y to break t h e bond i n h" and t h e c l u s t e r i o n i z a t i o n p o t e n t i a l . I n s t e a d t h e c r i t i c a l f a c t o r i s t h e o v e r l a p between cr and an o c c u p i e d c l u s t e r o r b i t a l o f t h e same symmetry. In p a r t i c u l a r f o r on-top d i s s o c i a t i o n , a r e q u i r e m e n t o f t h e on-top atom i s an e a s i l y a c c e s s i b l e atomic s t a t e w i t h s i n g l y o c c u p i e d 3d and 4s o r b i t a l s ( 5 - 7 ) . When t h i s c o n d i t i o n i s f u l f i l l e d , t h e on-top d i s s o c i a t i o n becomes t h e most f a v o r a b l e pathway. T h i s i s t h e r e a s o n why n i c k e l s u c c e e d s and copper f a i l s t o break t h e H bond ( 7 , 8 ) . The c o r r e l a t i o n between t h e i o n i z a t i o n e n e r g i e s and t h e r e a c t i v i t i e s o f the c l u s t e r s i s t h e r e f o r e , i n our o p i n i o n , t h e most s u r p r i s i n g o f a l l t h e above mentioned e x p e r i m e n t a l r e s u l t s . The s i m p l e model f o r H a c t i v a t i o n proposed by Whetten e t a l (2) was l a t e r q u e s t i o n e d by S m a l l e y and coworkers: ".. one wonders how a s i m p l e e l e c t r o s t a t i c model can p o s s i b l y e x p l a i n t h e known s e n s i t i v i t y o f H and N c h e m i s o r p t i o n t o p a r t i c u l a r s u r f a c e g e o m e t r i e s , and t o t h e p r e s e n c e o f open d o r b i t a l s a t the r e a c t i v e s i t e " and t h e y c o n t i n u e by r a i s i n g t h e q u e s t i o n " I s t h e c o r r e s p o n d e n c e between I.P. and r e a c t i v i t y c a u s a l , o r c a s u a l ? " ( 9 ) . To answer t h i s q u e s t i o n S m a l l e y e t a l s t u d i e d t h e r e a c t i v i t y p a t t e r n o f i o n i z e d c l u s t e r s . I t t u r n e d out t h a t the p o s i t i v e c l u s t e r s had a v e r y s i m i l a r r e a c t i v i t y p a t t e r n as t h e n e u t r a l s and sometimes r e a c t e d w i t h even g r e a t e r c r o s s s e c t i o n s even though t h e s e c l u s t e r s have a f a r l e s s tendency t o donate e l e t r o n i c charge t o the n e u t r a l adducts. T h i s f i n d i n g i s consequently i n c o n f l i c t with the simple e l e c t r o s t a t i c model o f Whetten e t a l . In t h i s paper we w i l l f i r s t r e v i e w our f i n d i n g s c o n c e r n i n g d i s s o c i a t i v e a c t i v a t i o n o f H by t r a n s i t i o n metal complexes and c l u s t e r s . Of major importance i n the p r e s e n t c o n t e x t i s our r e c e n t u n d e r s t a n d i n g o f t h e o s c i l l a t i o n s o f t h e atomic hydrogen c h e m i s o r p t i o n e n e r g i e s w i t h c l u s t e r s i z e (10). A l s o o f importance i s our u n d e r s t a n d i n g o f t h e dominant e l e c t r o n i c mechanism f o r b r e a k i n g the H bond ( 5 - 7 ) . I f t h e s e two f a c t o r s a r e combined w i t h t h e knowledge o f t h e atomic spectrum o f t h e metal atoms (11) a l a r g e s t e p towards a complete u n d e r s t a n d i n g o f t h e o s c i l l a t i o n s o f r e a c t i v i t y w i t h c l u s t e r s i z e can be o b t a i n e d . I t i s a l s o c l e a r why n i c k e l does not show t h e same type o f o s c i l l a t i o n s as i r o n and c o b a l t . I t i s f i n a l l y a l s o p o s s i b l e t o u n d e r s t a n d why t h e r e a c t i v i t i e s c o r r e l a t e w i t h t h e i o n i z a t i o n e n e r g i e s o f the c l u s t e r s w i t h o u t r a t i o n a l i z i n g t h i s c o r r e l a t i o n by t h e s i m p l e e l e c t r o s t a t i c model s u g g e s t e d by Whetten. 2
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PANAS ET AL. Computational
H- Dissociation on Transition Metal Clusters and Surfaces details
The c a l c u l a t i o n s i n the p r e s e n t paper a r e v e r y s i m p l e and q u a l i t a t i v e . The purpose i s not t o produce f i n a l d e f i n i t e numbers but r a t h e r t o i l l u s t r a t e the s i m p l e p r i n c i p l e s s u g g e s t e d h e r e . C a l c u l a t i o n s are performed f o r both n i c k e l and c o b a l t c l u s t e r s . When the metal atoms are i n the atomic d s s t a t e (see below) the atoms are t r e a t e d c o m p u t a t i o n a l l y as o n e - e l e c t r o n systems by the use of E f f e c t i v e Core P o t e n t i a l s ( E C P ' s ) . T h i s i s a crude but q u a l i t a t i v e l y c o r r e c t d e s c r i p t i o n and t r e a t s the c o r e i n c l u d i n g the 3d s h e l l by p o t e n t i a l s and p r o j e c t i o n o p e r a t o r s . S i m i l a r l y f o r atoms i n the d s s t a t e a t w o - e l e c t r o n ECP i s used. The ECP's are c o n s t r u c t e d based on the f r o z e n o r b i t a l ECP t e c h n i q u e (12). In t h i s t e c h n i q u e some o f the c o r e o r b i t a l s are expanded i n the v a l e n c e b a s i s s e t and f r o z e n i n atomic shapes. T h i s r e d u c e s the demand on the a c c u r a c y of the ECP p o t e n t i a l s and the p r o j e c t i o n o p e r a t o r s . O n e - e l e c t r o n ECP's c o n s t r u c t e d by t h i s t e c h n i q u e f o r n i c k e l and copper have been shown t o g i v e r e s u l t s of q u a n t i t a t i v e a c c u r a c y f o r s u r f a c e problems, p a r t i c u l a r l y f o r hydrogen c h e m i s o r p t i o n which i s t r e a t e d here (13,14). In the p r e v i o u s s t u d i e s the o n e - e l e c t r o n ECP's i n c l u d e d a f r o z e n 3s o r b i t a l . In the p r e s e n t case, s t a t e s w i t h a l a r g e occupancy of 4p appeared f o r the s type c o n f i g u r a t i o n s i n a c l u s t e r s u r r o u n d i n g . In o r d e r t o improve the d e s c r i p t i o n of the 4p o r b i t a l s a f r o z e n 3p o r b i t a l was i n c l u d e d i n both ECP's. n
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The parameters of the ECP's were f i r s t d e t e r m i n e d by f i t t i n g o r b i t a l e n e r g i e s and shapes f o r the c o r r e s p o n d i n g atoms. The f i n a l c a l i b r a t i o n was made i n a comparison w i t h e c a l c u l a t i o n s on CoNir, w i t h the c e n t r a l c o b a l t atom both i n a d s s t a t e and i n a d s s t a t e , u s i n g an a l l - e l e c t r o n d e s c r i p t i o n f o r Co and a o n e - e l e c t r o n ECP d e s c r i p t i o n f o r N i . The c l u s t e r b i n d i n g e n e r g i e s at the a l l - e l e c t r o n l e v e l and a t the ECP l e v e l agreed t o w i t h i n a few kcal/mol. O r b i t a l s were g e n e r a t e d i n the s i n g l e - c o n f i g u r a t i o n SCF a p p r o x i m a t i o n and dynamical c o r r e l a t i o n was t r e a t e d i n o n e - r e f e r e n c e s t a t e C o n t r a c t e d CI (CCI) c a l c u l a t i o n s (15). The D a v i d s o n c o r r e c t i o n (16) was f i n a l l y added t o the CCI e n e r g i e s t o a c c o u n t f o r u n l i n k e d cluster contributions. n +
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For t r a n s i t i o n metal atoms t h e r e are t h r e e d i f f e r e n t atomic o c c u p a t i o n s which u s u a l l y l e a d to s t a t e s which are e n e r g e t i c a l l y low l y i n g . These c h e m i c a l l y d i f f e r e n t s t a t e s are i n g e n e r a l terms denoted d s , d s and d . B u l k t r a n s i t i o n m e t a l s (and s u r f a c e s ) of i r o n , c o b a l t , n i c k e l and copper a r e dominated by the d s atomic o c c u p a t i o n so t h a t t h e r e i s e s s e n t i a l l y one s e l e c t r o n c o n t r i b u t i n g t o the c o n d u c t i o n band. The d l e v e l s w i l l be e n e r g e t i c a l l y deeper and w i l l o f t e n be c h e m i c a l l y r a t h e r i n e r t . T h i s p i c t u r e i s p a r t i c u l a r l y t r u e f o r the m e t a l s t o the r i g h t i n the p e r i o d i c t a b l e such as n i c k e l and copper. For the i s o l a t e d atoms of n i c k e l and copper the d s s t a t e i s a l s o the lowest ( j - a v e r a g e d ) n
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s t a t e but t h i s i s o f t e n not the case f o r o t h e r atoms (11). For c g b a l t the d s s t a t e i s 0.43 eV (10.0 k c a l / m o l ) lower than the d s s t a t e and f o r i r o n the d s s t a t e i s even lower, 0.86 eV (19.8 k c a l / m o l ) below t h a n the d s s t a t e . I t i s immediately o b v i o u s t h a t t h i s i m p o r t a n t d i f f e r e n c e between the atomic s t a t e s of i r o n and c o b a l t on the one hand and n i c k e l and copper on the o t h e r i s one key t o the u n d e r s t a n d i n g of the e x p e r i m e n t a l f i n d i n g t h a t the r e a c t i v i t y o s c i l l a t e s f o r the two former m e t a l s but not f o r the l a t t e r two. The q u e s t i o n t h a t has p u z z l e d us f o r the p a s t y e a r s i s only how. In p a s s i n g i t c o u l d be i n t e r e s t i n g t o note t h a t the importance of a c c u r a t e l y d e s c r i b i n g the above mentioned lowest s t a t e s of the t r a n s i t i o n m e t a l s has been r e c o g n i z e d by quantum c h e m i s t s f o r a l o n g time. But t o o b t a i n a c o r r e c t s p l i t t i n g between t h e s e s t a t e s has t u r n e d out t o be a f o r m i d a b l e c o m p u t a t i o n a l t a s k . S i n c e the number o f d - e l e c t r o n s d i f f e r s between t h e s e s t a t e s the c o r r e l a t i o n energy o f the d - s h e l l w i l l e n t e r d i r e c t l y i n the energy s p l i t t i n g . Only r e c e n t l y has an a c c e p t a b l e e r r o r o f 0.1 eV been r e a c h e d c o m p u t a t i o n a l l y f o r n i c k e l (17). L a r g e m u l t i - r e f e r e n c e CI c a l c u l a t i o n s w i t h about 20 r e f e r e n c e s t a t e s and a b a s i s s e t i n c l u d i n g g - f u n c t i o n s were r e q u i r e d . 2
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2 on a metal c l u s t e r two independent c r i t e r i a must be s a t i s f i e d . F i r s t , the r e a c t i o n must be e x o t h e r m i c . We w i l l d i s c u s s the c o n d i t i o n s f o r e x o t h e r m i c i t y a t the end of t h i s s e c t i o n and i n the next s e c t i o n . Second, a d i s s o c i a t i o n mechanism must e x i s t such t h a t the b a r r i e r s f o r d i s s o c i a t i o n w i l l not be too h i g h . D u r i n g the p a s t y e a r s we have s t u d i e d the H d i s s o c i a t i o n both f o r s i n g l e t r a n s i t i o n metal atoms and complexes and f o r c l u s t e r s as models f o r s u r f a c e s . S i n g l e i r o n , c o b a l t , n i c k e l and copper atoms have been s t u d i e d (5,6) as w e l l as models o f n i c k e l and copper s u r f a c e s (7,8). For s i n g l e metal atoms t h e r e are two r e q u i r e m e n t s f o r a low b a r r i e r : f i r s t the metal atom must be i n a d s state, including open s h e l l d - o r b i t a l s , and second one s i n g l y occuped d - o r b i t a l must be l o w - s p i n c o u p l e d towards the 4 s - e l e c t r o n . For n i c k e l the o n l y J o w - l y i n g s t a t e w i t h a low b a r r i e r f o r d i s s o c i a t i n g h" i s the D(d s) s t a t e . T h i s s t a t e i s o n l y 9.8 k c a l / m o l above the ground s t a t e . The lowest F ( d s ) and D ( d s ) s t a t e s have v e r y h i g h b a r r i e r s . From the r e q u i r e m e n t t h a t t h e r e must be open s h e l l d - o r b i t a l s , i t i s c l e a r t h a t the copper atom w i l l not d i s s o c i a t e H The f i r s t s t a t e of copper t h a t does d i s s o c i a t e H i s the d sp s t a t e which i s 312 k c a l / m o l above the ground s t a t e . I r o n and c o b a l t w i l l have d i s s o c i a t i v e s t a t e s v e r y s i m i l a r t o the D state o f n i c k e l but t h e s e s t a t e s w i l l be f a i r l y h i g h l y e x c i t e d . For c o b a l t the F s t a t e i s 21 k c a l / m o l above the ground s t a t e and f o r i r o n the d i s s o c i a t i v e F s t a t e i s e x c i t e d by as much as 34 k c a l / m o l . 2
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The r e a s o n t h a t the above mentioned s t a t e s o f n i c k e l , c o b a l t and i r o n are d i s s o c i a t i v e a r e t w o - f o l d . F i r s t , the l o w - s p i n c o u p l i n g between the d - s h e l l and the s - e l e c t r o n a l l o w s f o r an e f f i c i e n t s d - h y b r i d i z a t i o n which w i l l reduce the r e p u l s i o n between the d o u b l y o c c u p i e d c r - o r b i t a l o f H and the metal atom as h\> approaches.
9. PANAS ET A K
H Dissociation on Transition Metal Clusters and Surfaces 129 2
Second, t h e s i n g l y o c c u p i e d d - o r b i t a l and t h e s i n g l y o c c u p i e d s - o r b i t a l can s t a r t t o form t h e two bonds t o t h e hydrogen atoms as the H bond i s broken. The above f i n d i n g f o r s i n g l e metal atoms i s d i r e c t l y t r a n s f e r a b l e t o t h e case o f on-top d i s s o c i a t i o n o f Hg on metal c l u s t e r s and on s u r f a c e s ( 7 , 8 ) . The on-top d i s s o c i a t i o n o f H on a n i c k e l s u r f a c e c o n s e q u e n t l y p r o c e e d s w i t h o u t any b a r r i e r ( 8 ) , whereas t h e same d i s s o c i a t i o n pathway has a v e r y l a r g e b a r r i e r f o r a copper s u r f a c e . F o r copper o t h e r d i s s o c i a t i o n pathways w i l l have lower b a r r i e r s , such as d i s s o c i a t i o n over a h o l l o w s i t e . The b a r r i e r f o r t h i s d i s s o c i a t i o n can be e s t i m a t e d t o be o f t h e o r d e r o f 12-18 k c a l / m o l f o r a copper s u r f a c e . The same b a r r i e r w i l l be somewhat r e d u c e d f o r a n i c k e l s u r f a c e by 5-10 k c a l / m o l ( i t i s p o s s i b l e t h a t t h i s r e d u c t i o n c o u l d be somewhat l a r g e r i f t h e e f f e c t o f t h e y e t u n i n v e s t i g a t e d d - o r b i t a l p a r t i c i p a t i o n t u r n s out t o be l a r g e r t h a n e x p e c t e d ) . The p r e s e n c e o f t h e s e r e l a t i v e l y h i g h b a r r i e r s i s t h e r e a s o n why no copper c l u s t e r was found t o d i s s o c i a t e H The e x o t h e r m i c i t y f o r H d i s s o c i a t i o n would p r o b a b l y have been s u f f i c i e n t f o r d i s s o c i a t i o n on some c l u s t e r s , s i n c e t h e e x o t h e r m i c i t y i s s u f f i c i e n t f o r a copper s u r f a c e ( 1 4 ) . For metal s u r f a c e s t h e d s s t a t e w i l l dominate t h e atomic d e s c r i p t i o n even i f o t h e r s t a t e s a r e dominant f o r t h e i s o l a t e d atoms. T h i s means t h a t f o r both i r o n and c o b a l t s u r f a c e s , a b a r r i e r l e s s d i s s o c i a t i o n over on-top s i t e s i s e x p e c t e d . F o r metal c l u s t e r s w i t h more than a s i n g l e atom a t l e a s t one atom i s e x p e c t e d to be i n t h e d s s t a t e , which means t h a t a d i s s o c i a t i o n mechanism w i t h a low b a r r i e r s h o u l d e x i s t i n p r i n c i p l e f o r a l l t h e s e i r o n , c o b a l t and n i c k e l c l u s t e r s . F o r t h e s i n g l e metal atoms t h e low s p i n c o u p l i n g between t h e d- and t h e s - h o l e was found t o be i m p o r t a n t t o reduce t h e r e p u l s i o n t o H The same t y p e o f s p i n - c o u p l i n g i s a l s o i m p o r t a n t f o r c l u s t e r s and s u r f a c e s . F o r c l u s t e r s the s p i n - c o u p l i n g w i l l i n general require a small e x c i t a t i o n energy, which can cause t h e appearence o f s m a l l b a r r i e r s i n the reaction with H (8). We have emphasized t h e importance o f open d - o r b i t a l s and a p r o p e r atomic s t a t e i f H s h o u l d d i s s o c i a t e w i t h a low b a r r i e r on a t r a n s i t i o n metal s u r f a c e . F o r c l u s t e r s , however, t h e same type o f d i s s o c i a t i o n p u t s up a n o t h e r r e q u i r e m e n t , which w i l l t u r n out t o be even more s i g n i f i c a n t i n t h e p r e s e n t c o n t e x t : There must be a t l e a s t one open s h e l l v a l e n c e o r b i t a l ( o f s - c h a r a c t e r ) on t h e c l u s t e r , o t h e r w i s e t h e s d - h y b r i d i z a t i o n w i l l n o t t a k e p l a c e ( 8 ) . F o r an i n f i n i t e s u r f a c e , t h i s r e q u i r e m e n t can always be s a t i s f i e d s i n c e s t a t e s w i t h open v a l e n c e o r b i t a l s must a t l e a s t be r e a c h a b l e by a low energy e x c i t a t i o n . F o r c l u s t e r s , t h e same type o f e x c i t a t i o n may be much more e x p e n s i v e . S i n c e a l l n i c k e l c l u s t e r s d i s s o c i a t e H i t seems c l e a r t h a t a d i s s o c i a t i v e s t a t e i s r e a c h a b l e i n a l l c a s e s f o r n i c k e l . The q u e s t i o n t h a t has w o r r i e d us f o r t h e p a s t y e a r s i s why the same type o f s t a t e s do n o t always seem r e a c h a b l e f o r i r o n and c o b a l t c l u s t e r s . The answer t o t h i s q u e s t i o n i s d i s c u s s e d i n s e c t i o n VI. 2
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