Gas-Phase Adsorption Model Studies of Electrode ... - ACS Publications

Chapter 4

Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 23, 2015 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch004

Gas-Phase Adsorption Model Studies of Electrode Surfaces 1

2

J. K. Sass and K. Bange 1

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Federal Republic of Germany Schott Glaswerke, Postfach 130367, D-6200 Wiesbaden 13, Federal Republic of Germany 2

Model adsorption experiments in ultrahigh v a c u u m (UHV.), a i m e d at s i m u l a t i n g t h e interfacial r e g i o n b e t w e e n a m e t a l a n d a n e l e c t r o l y t e , are d e s c r i b e d . It is s h o w n t h a t by m e a s u r i n g t h e w o r k f u n c t i o n c h a n g e , i n d u c e d by such a s y n t h e t i c a d s o r b a t e layer, a m e a n i n g f u l c o m p a r i s o n t o in situ e l e c t r o c h e m i c a l d a t a may be a c h i e v e d . T h e a g r e e m e n t w h i c h has b e e n o b t a i n e d , by such c o m p a r i s o n , f o r t h e specific a d s o r p t i o n o f t h e t w o h a l i d e s b r o m i d e a n d c h l o r i d e o n A g { 1 1 0 } is argued to provide definite evidence for the relevance of e l e c t r o c h e m i c a l surface science studies. O t h e r recent i n v e s t i g a t i o n s o f i n t e r e s t i n g coadsorption systems with electrochemical s i g n i f i c a n c e , in particular t h o s e w h e r e non -specific a d s o r p t i o n b e h a v i o u r w o u l d be e x p e c t e d , are briefly r e v i e w e d . Encouraging p r o g r e s s in observing and understanding molecular p h e n o m e n a at s o l i d - e l e c t r o l y t e interfaces has b e e n m a d e in r e c e n t years. S i g n i f i c a n t c o n t r i b u t i o n s t o this a d v a n c e m e n t h a v e c o m e f r o m the application of experimental procedures which were d e v e l o p e d for the s t u d y o f g a s - s o l i d i n t e r a c t i o n s in u l t r a h i g h v a c u u m / 1 / . T h e s e surface science t e c h n i q u e s are attractive b e c a u s e t h e y p r o v i d e a w i d e r a n g e o f m i c r o s c o p i c i n f o r m a t i o n a b o u t solid surfaces a n d a d s o r b a t e layers. T h e d r a w b a c k is, o f c o u r s e , t h a t t h e y c a n n o t be used directly at e l e c t r o c h e m i c a l interfaces a n d t h a t a certain m e a s u r e o f a m b i g u i t y is t h e r e f o r e i n h e r e n t in such s t u d i e s . Considerable effort had to be invested, for example, before c o n t a m i n a t i o n levels d u r i n g t r a n s f e r o f a s a m p l e f r o m a n e l e c t r o c h e m i c a l cell t o a v a c u u m c h a m b e r c o u l d b e a d e q u a t e l y assessed a n d c o n t r o l l e d 121. W i t h this p r o v i s i o n , e l e c t r o d e m o d i f i c a t i o n s by e l e c t r o c h e m i c a l processes c a n b e s t u d i e d in m u c h g r e a t e r d e t a i l t h a n is possible in situ a n d m a n y i n t e r e s t i n g results h a v e b e e n o b t a i n e d by such e x p e r i m e n t s . Intrinsic l i m i t a t i o n s o f this transfer m e t h o d arise, h o w e v e r , w i t h loosely b o u n d 0097-6156/88/0378-0054$06.00/0 ° 1988 American Chemical Society

In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


4.

SASS AND BANGE

Gas-Phase Adsorption Model Studies

55

species, f o r e x a m p l e s o l v e n t m o l e c u l e s , w h i c h may d e s o r b a f t e r r e m o v a l o f t h e s a m p l e f r o m s o l u t i o n since t h e transfer is typically carried o u t at o r n e a r room temperature. T h e e x p e r i m e n t a l a p p r o a c h discussed in this article is, in c o n t r a s t , particularly a m e n a b l e t o i n v e s t i g a t i n g s o l v e n t c o n t r i b u t i o n s t o the i n t e r f a c i a l p r o p e r t i e s 131. Species, w h i c h electrolyte s o l u t i o n s are c o m p o s e d o f , are d o s e d in c o n t r o l l e d a m o u n t s f r o m t h e gas p h a s e , in u l t r a h i g h v a c u u m , o n t o c l e a n m e t a l substrates. Sticking is e n s u r e d , w h e r e necessary, by c o o l i n g t h e s a m p l e t o sufficiently l o w t e m p e r a t u r e . A g a i n s u r f a c e sensitive t e c h n i q u e s can be u s e d , to c h a r a c t e r i z e m i c r o s c o p i c a l l y t h e i n t e r a c t i o n o f s o l v e n t m o l e c u l e s a n d ionic species w i t h t h e solid s u r f a c e . E v e n w i t h o u t f u r t h e r c o n s i d e r a t i o n such i n f o r m a t i o n is certainly m o s t v a l u a b l e . T h e u l t i m a t e g o a l in these studies, h o w e v e r , is t o actually m i m i c structural e l e m e n t s o f t h e interfacial r e g i o n a n d t o be a b l e t o assess t h e e x t e n t t o w h i c h this may b e a c h i e v e d . A key e l e m e n t in c o n s i d e r i n g t h e p r o p e r t i e s o f t h e interfacial electric d o u b l e layer is t h e d i s t i n c t i o n b e t w e e n specific a n d n o n - s p e c i f i c a d s o r p t i o n . W i t h r e g a r d t o t h e feasibility o f U H V s i m u l a t i o n o f interfacial p r o p e r t i e s this d i s t i n c t i o n is also very i m p o r t a n t /A/. Ions w h i c h directly c o n t a c t t h e e l e c t r o d e s u r f a c e , such as t h e h a l i d e s for e x a m p l e , may b e d o s e d s e p e r a t e l y a n d , b e c a u s e o f e l e c t r o n i c e q u i l i b r a t i o n , n o t necessarily in i o n i c f o r m . U p o n s u b s e q u e n t d o s i n g o f s o l v e n t m o l e c u l e s o n l y lateral m o t i o n s o f t h e t w o a d s o r b a t e s are r e q u i r e d f o r a m i n i m i z a t i o n o f t h e e n e r g y . In c o n t r a s t , t h e p r e p a r a t i o n o f a fully s o l v a t e d ion o n a m e t a l surface in U H V is clearly a m o r e d e m a n d i n g task. A l t h o u g h s o l v a t e d ions may be g e n e r a t e d in t h e g a s p h a s e t h e r e w o u l d p r o b a b l y be insufficient intensity in a c o l l i m a t e d b e a m s u i t a b l e f o r U H V studies. In s e q u e n t i a l d o s i n g of b o t h species, o n t h e o t h e r h a n d , p r o h i b i t i v e l y large a c t i v a t i o n barriers f o r s o l v e n t e n c l o s u r e o f t h e i o n m a y p r e v e n t t h e s i m u l a t i o n . O n l y in o n e o f several c o a d s o r p t i o n studies o f alkalis a n d w a t e r , for e x a m p l e , w a s t h e r e any e v i d e n c e f o r a c o m p l e t e h y d r a t i o n shell a r o u n d an alkali ion IS/. T h e m a j o r d i f f e r e n c e in t h e e x p e r i m e n t a l c o n d i t i o n s of this study w a s t h a t instead o f s u b m o n o l a y e r alkali d e p o s i t s multilayers o f p o t a s s i u m a n d w a t e r r e a c t e d t o f o r m b u l k p o t a s s i u m h y d r o x i d e w h i c h t o s o m e e x t e n t w a s d i s s o c i a t e d a n d h y d r a t e d by a d d i t i o n a l w a t e r . In t h e o n l y o t h e r r e p o r t , r e l e v a n t t o n o n - s p e c i f i c a d s o r p t i o n , vibrational spectroscopy indicated the f o r m a t i o n of hydrated p r o t o n s f r o m t h e i n t e r a c t i o n o f h y d r o g e n f l u o r i d e a n d w a t e r /6/. In b o t h o f t h e s e studies e q u a l a m o u n t s o f cations a n d a n i o n s w e r e a p p a r e n t l y p r e s e n t , h o w e v e r , a n d it is n o t yet clear h o w a n excess o f h y d r a t e d ions might be g e n e r a t e d . For this r e a s o n , t h e e m p h a s i s in this article is d i r e c t e d m o r e t o w a r d s t h e s i m u l a t i o n o f specific a d s o r p t i o n a n d , in particular, t h e r e c e n t e n c o u r a g i n g comparison of electrochemical and UHV data for the interaction of b r o m i n e a n d c h l o r i n e w i t h A g { 1 1 0 } 11, 87. A brief o u t l i n e o f t h e c o n c l u s i o n s e m e r g i n g f r o m a l k a l i - w a t e r c o a d s o r p t i o n e x p e r i m e n t s is g i v e n t o illustrate basic m o d e s o f i o n - s o l v e n t i n t e r a c t i o n o n m e t a l surfaces a n d t o discuss f u t u r e d i r e c t i o n s o f this research.

Concepts and Procedures T h e c o n s e c u t i v e stages o f a typical g a s - p h a s e - a d s o r p t i o n e x p e r i m e n t , in w h i c h t h e s i m u l a t i o n of an e l e c t r o c h e m i c a l i n t e r f a c e is t h e a i m , are s c h e m a t i c a l l y illustrated in F i g u r e 1. T h e d e p i c t e d s e q u e n c e illustrates in p a r t i c u l a r t h e case w h e n specific a d s o r p t i o n o f a n i o n is e x p e c t e d f r o m



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ELECTROCHEMICAL SURFACE SCIENCE

e l e c t r o c h e m i c a l d a t a . In t h e first step (Figure 1a) d o s i n g o f just t h e i o n i c species is p e r f o r m e d a n d its c o v e r a g e is d e t e r m i n e d . T h e next step i n v o l v e s p o p u l a t i n g nearest n e i g h b o u r sites of t h e a d s o r b e d ions w i t h s o l v e n t m o l e c u l e s (Figure 1b). F r o m t h e o r e t i c a l a n d e x p e r i m e n t a l w o r k r e l a t e d t o h e t e r o g e n e o u s catalysis o n e expects that s o l v e n t m o l e c u l e s e x p e r i e n c e s t r o n g p e r t u r b a t i o n s at t h o s e sites, a n d such a n effect has i n d e e d b e e n o b s e r v e d f o r surface interactions b e t w e e n p o l a r m o l e c u l e s a n d i o n i c species / 3 / W i t h a d d i t i o n a l s o l v e n t a d s o r p t i o n a stage may t h e n be r e a c h e d w h e r e a p p r o x i m a t e l y t h e first a d s o r b a t e layer is c o m p l e t e d (Figure 1c). T h i s s i t u a t i o n is o f particular interest f o r a c o m p a r i s o n w i t h e l e c t r o c h e m i c a l d a t a b e c a u s e it represents w h a t is c o m m o n l y c o n s i d e r e d t h e i n n e r layer in d o u b l e layer m o d e l s o f specific a d s o r p t i o n . T h e d e p o s i t i o n o f fully s o l v a t e d ions o n t o p o f this first layer, to s i m u l a t e t h e o u t e r o r d i f f u s e layer o f t h e i n t e r f a c e (Figure 1e), is n o t yet possible, as w a s p o i n t e d o u t earlier. B u t at least t h e p r e s e n c e o f a n a d j a c e n t c o n d e n s e d b u l k p h a s e can b e s i m u l a t e d by t h e a d d i t i o n o f a f e w multilayers of s o l v e n t in t h e final step o f t h e e x p e r i m e n t (Figure 1d). C o m p a r i n g F i g u r e 1d w i t h F i g u r e 1e, it is e v i d e n t t h a t t h e r e are t w o i m p o r t a n t f e a t u r e s o f t h e e l e c t r o c h e m i c a l interface w h i c h c a n n o t b e r e p r o d u c e d yet by t h e s i m u l a t i o n : t h e a b o v e m e n t i o n e d i o n i c excess c h a r g e in t h e d i f f u s e layer a n d t h e bulk electrolyte ions w i t h t h e i r s c r e e n i n g p r o p e r t i e s . F o r t u n a t e l y , t h e c o n d i t i o n of z e r o d i f f u s e layer c h a r g e c a n o f t e n be e x t r a c t e d f r o m e l e c t r o c h e m i c a l d a t a such t h a t t h e a b s e n c e o f t h e d i f f u s e layer d o e s n o t seriously d e p r e c i a t e t h e p u r p o s e o f t h e U H V e x p e r i m e n t . Similarly, it may be e x p e c t e d t h a t t h e structural p r o p e r t i e s o f t h e i n n e r layer, f o r a certain c o m p o s i t i o n , d o n o t d e p e n d o n t h e electrolyte c o n c e n t r a t i o n in t h e b u l k s o l u t i o n p h a s e . D e s p i t e t h e s e a r g u m e n t s a n d t h e c o n c e p t u a l attractiveness o f t h e p r o c e d u r e w h i c h is s k e t c h e d in F i g . 1 c o n v i n c i n g e v i d e n c e f o r t h e r e l e v a n c e o f a p a r t i c u l a r gas p h a s e a d s o r p t i o n e x p e r i m e n t can o n l y b e o b t a i n e d by direct comparison to electrochemical data T h e electrode potential a n d the w o r k f u n c t i o n c h a n g e are t w o m e a s u r a b l e q u a n t i t i e s w h i c h are particularly u s e f u l f o r such a c o m p a r i s o n . In b o t h m e a s u r e m e n t s t h e v a r i a t i o n o f t h e electrostatic p o t e n t i a l across t h e interface can be o b t a i n e d a n d c o m p a r e d by p r o p e r l y r e f e r e n c i n g these t w o values 11L T o g e t h e r w i t h t h e i o n i c excess c h a r g e in t h e d o u b l e layer, w h i c h in t h e U H V e x p e r i m e n t w o u l d b e e x p r e s s e d in t e r m s o f c o v e r a g e o f t h e i o n i c species, t h e m a c r o s c o p i c electrical p r o p e r t i e s o f t h e interfacial c a p a c i t o r can t h u s b e c h a r a c t e r i z e d in both environments. Such a c o m p a r i s o n has f o r m e d t h e basis, f o r e x a m p l e , f o r t h e assertion t h a t t h e d o u b l e layer can be e m e r s e d essentially intact f r o m s o l u t i o n /67. A c o m m o n a m b i g u i t y , a l t h o u g h f o r d i f f e r e n t reasons, in b o t h e m e r s i o n a n d U H V m o d e l e x p e r i m e n t s is t h e d i f f e r e n c e in t h e a m o u n t o f s o l v e n t p r e s e n t e i t h e r at t h e e m e r s e d or s y n t h e s i z e d i n t e r f a c e , c o m p a r e d t o t h e in-situ s i t u a t i o n . In t h e U H V t h e t o t a l a m o u n t o f s o l v e n t a d s o r b e d , a n d its d i s t r i b u t i o n i n t o t h e first a n d s u b s e q u e n t layers, can in m a n y instances directly b e d e t e r m i n e d , b u t this i n f o r m a t i o n is difficult to o b t a i n a n d n o t yet a v a i l a b l e f o r t h e e m e r s e d a n d t h e real i n t e r f a c e . T o g a t h e r such missing pieces in t h e interfacial p u z z l e is t h e m o t i v a t i o n f o r t h e w o r k d e s c r i b e d in this p a p e r . O n e i m p o r t a n t p r e r e q u i s i t e for any m o d e l o f t h e d o u b l e layer is, f o r e x a m p l e , t h e density o f s o l v e n t m o l e c u l e s in t h e i n n e r layer as a f u n c t i o n o f t h e c h a r g e o n t h e interfacial c a p a c i t o r .



4.

SASSAND BANGE


_

~ _ ~ ~ -Solvated _

F i g u r e 1. A s c h e m a t i c r e p r e s e n t a t i o n of the synthesis o f the e l e c t r o c h e m i c a l d o u b l e layer in U H V : a) a d s o r p t i o n o f specifically a d s o r b e d ions w i t h o u t s o l v e n t ; b) a d d i t i o n o f h y d r a t i o n w a t e r ; c) c o m p l e t i o n o f t h e i n n e r layer; d) a d d i t i o n o f s o l v e n t multilayers, e) m o d e l f o r t h e d o u b l e layer at a n e l e c t r o d e surface in s o l u t i o n .


57

58


Coadsorption of Bromine and Water on Aqf 110)


Br/Aq (110) In p r e p a r a t i o n f o r t h e c o a d s o r p t i o n e x p e r i m e n t s it is i m p o r t a n t t o study t h e n a t u r e o f t h e i n t e r a c t i o n o f t h e ionic species, by itself, w i t h t h e m e t a l s u r f a c e . A s e l e c t r o n e g a t i v e species, t h e h a l o g e n s are w e l l k n o w n t o attract c h a r g e f r o m t h e m e t a l substrate a n d to f o r m a d i p o l e layer w h i c h increases t h e w o r k f u n c t i o n . Since t h e h a l o g e n s are typically e x p o s e d t o t h e m e t a l s u r f a c e in m o l e c u l a r f o r m in a surface science e x p e r i m e n t a n d t h e s t i c k i n g c o e f f i c i e n t varies w i t h a d s o r b a t e c o v e r a g e t h e n e e d f o r c o v e r a g e c a l i b r a t i o n arises. F i g . 2 s u m m a r i z e s w o r k f u n c t i o n m e a s u r e m e n t s a n d L E E D results w h i c h w e h a v e used f o r this p u r p o s e /9A T h e p (2x1) a n d c (4 x 2) L E E D p a t t e r n s , w h i c h a p p e a r in t h e c o v e r a g e r a n g e s i n d i c a t e d in F i g . 2, c o r r e l a t e very w e l l w i t h t h e c o n c e p t o f a linear increase o f t h e w o r k f u n c t i o n 4>: A t a b o u t t w o thirds o f t h e m a x i m u m w o r k f u n c t i o n c h a n g e , t h e p(2x1) L E E D p a t t e r n , w h i c h is first seen at - 0.25 m o n o l a y e r s , d i s a p p e a r s a n d , at a slightly h i g h e r c o v e r a g e , is r e p l a c e d by a c (4x2) p a t t e r n . W e o b t a i n a s a t u r a t i o n c o v e r a g e o f 0.75 for Br o n A g { 1 1 0 } a n d c a n a s s u m e a l i n e a r r e l a t i o n s h i p b e t w e e n w o r k f u n c t i o n a n d Br c o v e r a g e .

H2O/Br/Aqf110) T h e key issue in s i m u l a t i o n e x p e r i m e n t s , a n d t h e most d i f f i c u l t t o a d d r e s s , is t h e t r a n s f e r a b i l i t y o f t h e c o n c l u s i o n s d r a w n f r o m t h e results o b t a i n e d in v a c u u m . In this section w e shall t h e r e f o r e e x a m i n e in s o m e d e t a i l a recently p r o p o s e d p r o c e d u r e / 9 / w h i c h permits o n e t o directly c o m p a r e p r o p e r t i e s o f t h e s y n t h e t i c interfacial layers p r e p a r e d in v a c u u m a n d t h o s e p r e s e n t at in situ e l e c t r o c h e m i c a l interfaces. For reasons o f space l i m i t a t i o n s it will n o t be possible to review h o w some of the information presented subsequently m a y b e o b t a i n e d by s t a n d a r d surface science m e t h o d s / 1 / . As p o i n t e d o u t a b o v e t h e c o m p a r i s o n b e t w e e n b o t h sets o f results relies u p o n t h e physical e q u i v a l e n c e o f t h e t w o m e a s u r e a b l e q u a n t i t i e s w o r k f u n c t i o n (surface science) a n d e l e c t r o d e p o t e n t i a l (electrochemistry) 111. T h i s e q u i v a l e n c e has b e e n r e a l i z e d in electrochemistry s o m e t i m e a g o a n d has b e e n e x p l o i t e d t o a n a l y z e m e a s u r e d values o f t h e p o t e n t i a l o f z e r o c h a r g e 111 a n d o f w o r k f u n c t i o n c h a n g e s u p o n e m e r s i o n o f e l e c t r o d e s at f i x e d p o t e n t i a l /8/. In t h e s i m u l a t i o n e x p e r i m e n t s t h e a p p r o a c h is q u i t e similar in t h a t o n e p r e p a r e s a w e l l - d e f i n e d c o m p o s i t i o n o f t h e s y n t h e t i c e l e c t r o c h e m i c a l a d s o r b a t e layer a n d t h e n o b t a i n s t h e electrostatic p o t e n t i a l d r o p across it by a w o r k f u n c t i o n m e a s u r e m e n t . In t h e p r e v i o u s section t h e p r o c e d u r e f o r d e t e r m i n i n g t h e b r o m i n e c o v e r a g e has b e e n d e s c r i b e d O b t a i n i n g t h e a b s o l u t e w a t e r c o v e r a g e a f t e r a p a r t i c u l a r d o s i n g e v e n t f r e q u e n t l y requires m o r e s u b t l e c o n s i d e r a t i o n s , in p a r t i c u l a r w h e n t h e r e is n o t e m p e r a t u r e d i s t i n c t i o n in t h e r m a l d e s o r p t i o n s p e c t r o s c o p y (TDS) b e t w e e n H2O m o l e c u l e s directly b o u n d t o t h e A g {110} s u r f a c e a n d t h o s e residing in s u b s e q u e n t multilayers /10/. By e x p l o i t i n g t h e k n o w n s t o i c h i o m e t r y of t h e hydroxyl f o r m a t i o n a n d recombination processes o n A g { 1 1 0 } f r o m t h e r e a c t i o n o f a d s o r b e d o x y g e n a n d H2O / 1 1 / it is, h o w e v e r , possible t o c o p e w i t h this p r o b l e m , such t h a t t h e relative w a t e r c o v e r a g e s , w h i c h m a y be o b t a i n e d by T D S , can be c o n v e r t e d into a b s o l u t e c o v e r a g e s o f H2O. As a n a d d i t i o n a l b o n u s , t h e a p p l i c a t i o n of T D S also p r o v i d e s i n f o r m a t i o n o n t h e a m o u n t s o f w a t e r p r e s e n t in d i f f e r e n t local e n v i r o n m e n t s o f t h e c o m p o s i t e d o u b l e layer, since w a t e r b o u n d t o b r o m i n e d e s o r b s at a h i g h e r



4. SASSANDBANGE


59

t e m p e r a t u r e t h a n t h a t residing at still u n m o d i f i e d areas o f t h e s u b s t r a t e s u r f a c e /12/. W i t h a p p r o p r i a t e analysis o f w o r k f u n c t i o n d a t a , o b t a i n e d at d i f f e r e n t b r o m i n e c o v e r a g e s as a f u n c t i o n o f w a t e r c o v e r a g e , it is t h e r e f o r e p o s s i b l e t o i d e n t i f y t h e particular c o n t r i b u t i o n o f surface h y d r a t i o n w a t e r a n d f r e e w a t e r in t h e i n n e r layer t o t h e electrostatic p o t e n t i a l d r o p at t h e i n t e r f a c e /9/. In F i g u r e 3 t h e results o f such d a t a analysis are s h o w n . A t b r o m i n e c o v e r a g e s b e l o w ~~ 0.25, t w o w a t e r m o l e c u l e s are b o u n d t o t h e specifically a d s o r b e d h a l i d e i o n a n d t h e o r i e n t a t i o n o f this surface h y d r a t i o n w a t e r (cf. F i g u r e 1) is s e e n t o be such t h a t t h e n o r m a l c o m p o n e n t s o f t h e p e r m a n e n t d i p o l e m o m e n t , w i t h smaller c o n t r i b u t i o n s f r o m c h a r g e t r a n s f e r a n d t h e e l e c t r o n i c p o l a r i z a b i l i t y /13/, essentially c o m p e n s a t e t h e d i p o l e layer o f o p p o s i t e polarity i n d u c e d by t h e ionic h a l o g e n a d s o r b a t e (see curve b in F i g u r e 3). O f c o u r s e , it must be r e a l i z e d t h a t this e f f e c t o f t h e t w o a d s o r b a t e s c o u n t e r b a l a n c i n g e a c h o t h e r ' s d i p o l e layer is a p u r e c o i n c i d e n c e and t h a t t h e i m p o r t a n t c o n c l u s i o n to be d r a w n f r o m this result is t h e invariability o f t h e effective d i p o l e m o m e n t o f t h e c o m p o s i t e b r o m i d e p l u s - h y d r a t i o n - w a t e r c o m p l e x at b r o m i d e c o v e r a g e s b e l o w —0.25. P r o v i d e d t h a t this result is o f a m o r e g e n e r a l n a t u r e , a n i m p o r t a n t b u i l d i n g b l o c k o f t h e i n n e r layer w o u l d h a v e b e e n i d e n t i f i e d by t h e U H V s i m u l a t i o n experiments. F o l l o w i n g t h e c o n c e p t u a l o u t l i n e of t h e d o u b l e layer s i m u l a t i o n in U H V , s k e t c h e d in F i g u r e 1, curve c in F i g u r e 3 s h o w s t h e w o r k f u n c t i o n c h a n g e d u e t o t h e a d s o r b e d b r o m i d e , t h e surface h y d r a t i o n shell a n d also t h e w a t e r o c c u p y i n g areas o f t h e A g { 1 1 0 } surface w h e r e t h e i n f l u e n c e o f t h e b r o m i d e , w h i c h is o n l y s h o r t - r a n g e , c a n n o t exert itself (cf. F i g u r e 1c). T h e s h a p e o f this curve is n o t difficult to i n t e r p r e t w h e n w e recall t h a t t h e h y d r a t e d surface b r o m i d e d o e s n o t c o n t r i b u t e t o t h e w o r k f u n c t i o n c h a n g e a n d t h a t t h e " f r e e " w a t e r is less tightly b o u n d t h a n t h e h y d r a t i o n w a t e r . Since t h e r e is o n l y limited space a v a i l a b l e in t h e first layer, i.e. t h e i n n e r layer in p h e n o m e n o l o g i c a l m o d e l s f o r a n i o n specific a d s o r p t i o n /14/, t h e f r e e w a t e r is g r a d u a l l y d i s p l a y e d f r o m this i n n e r layer a n d t h e w o r k f u n c t i o n rises b e c a u s e o f t h e c o n c o m i t a n t loss o f t h e f r e e H2O m o l e c u l e s w h i c h w h e n p r e s e n t i n d u c e a w o r k f u n c t i o n d e c r e a s e /1 SI. A s o m e w h a t c u r i o u s effect arises w h e n a d d i t i o n a l w a t e r is d o s e d o n t o p o f this s y n t h e t i c i n n e r layer, in t h a t t h e w o r k f u n c t i o n is o b s e r v e d t o e x h i b i t s u b s t a n t i a l f u r t h e r decreases. This implies t h a t t h e w a t e r m o l e c u l e s in t h e m u l t i l a y e r s a b o v e t h e i n n e r layer assume s o m e m e a s u r e o f p r e f e r e n t i a l o r i e n t a t i o n , i n d u c e d by t h e p r e s e n c e o f t h e a d s o r b e d b r o m i d e in t h e first layer. This result is p r o b a b l y c o n n e c t e d to structure m a k i n g a n d structure b r e a k i n g , o r h y d r o p h o b i c a n d h y d r o p h i l i c , p r o p e r t i e s o f soild surfaces, b u t will n o t be discussed in d e t a i l h e r e . As p o i n t e d o u t a b o v e , t h e d a t a p r e s e n t e d in F i g . 3c c o r r e s p o n d t o t h e s i m u l a t i o n o f t h e i n n e r layer in h a l i d e specific a d s o r p t i o n , f o r t h e case o f z e r o d i f f u s e layer c h a r g e . F r o m d i f f e r e n t i a l capacity m e a s u r e m e n t s e q u i v a l e n t i n f o r m a t i o n , i.e. t h e electrostatic p o t e n t i a l d r o p across t h e i n n e r layer as a f u n c t i o n o f h a l i d e c o v e r a g e , can be o b t a i n e d a n d used t o o b t a i n v a l u e s f o r z e r o d i f f u s e layer c h a r g e . In F i g u r e 4 t h e full line is a r e p r e s e n t a t i o n o f such d a t a f o r b r o m i d e specific a d s o r p t i o n o n a A g {110} e l e c t r o d e /16/. T h e a g r e e m e n t b e t w e e n t h e s e m e a s u r e m e n t s a n d t h e c o r r e s p o n d i n g w o r k f u n c t i o n d a t a t a k e n in U H V is surprisingly g o o d , w h e n r e c a l l i n g t h a t t h e y w e r e o b t a i n e d at very d i f f e r e n t t e m p e r a t u r e s o f - 100 K a n d — 300 K. A l s o s h o w n in F i g u r e 4 is a similar c o m p a r i s o n , b a s e d o n r e c e n t d a t a /17/, f o r c h l o r i d e specific a d s o r p t i o n o n A g { l 10}. A g a i n a g r e e m e n t is o b t a i n e d , a l b e i t o v e r a smaller c o v e r a g e r a n g e , b e t w e e n t h e results f o r t h e s y n t h e t i c i n n e r layer a n d t h e o n e in s o l u t i o n .


60


Bromine Coverage 0

>

0.25

0.5

B r

0.75

-1250


Gas-Phase Adsorption Model Studies of Electrode ... - ACS Publications

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