Electrochemical Surface Science - American Chemical Society

Chapter 5

Modeling the Aqueous—Metal Interface in Ultrahigh Vacuum via Cryogenic Coadsorption

Downloaded by TUFTS UNIV on November 23, 2015 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch005

Frederick T. Wagner and Thomas E. Moylan Physical Chemistry Department, General Motors Research Laboratories, Warren, MI 48090-9055

Three stages in the modeling of electrochemical interfaces in ultrahigh vacuum are described, and experimental examples of each stage, verified with a variety of surface spectroscopies, are given. The first stage is the adsorption of water alone, or water and a neutral probe molecule, on a well-defined metal surface. Hydrophilic coadsorption of CO and water on Rh(111) is contrasted with hydrophobic coadsorption on Pt(111), demonstrating that weakly-bound water can modify the adsorption of even strongly-bound, neutral species. The second stage of modeling is the introduction of solvated ionic species into the model double layer. Coadsorption of HF and water yields adsorbed H O ions; the solvation stoichiometries of ions in the f i r s t monolayer and in subsequent layers are determined. The third stage of modeling is establishment of potential control in UHV. Hydrogen coadsorption is used to deflect the effective potential of the water monolayer below the potential of zero charge. The unique ways in which UHV models can contribute to an improved molecular-scale understanding of electrochemical interfaces are discussed. +

3

U l t r a h i g h vacuum s u r f a c e s p e c t r o s c o p i e s can p r o v i d e f a r g r e a t e r b r e a d t h and depth o f i n f o r m a t i o n about s u r f a c e p r o p e r t i e s than c a n y e t be a c h i e v e d u s i n g i n s i t u s p e c t r o s c o p i e s a t t h e aqueous/metal i n t e r f a c e . A p p l i c a t i o n o f t h e vacuum t e c h n i q u e s t o e l e c t r o c h e m i c a l i n t e r f a c e s i s t h u s d e s i r a b l e , b u t has been plagued by q u e s t i o n s o f t h e r e l e v a n c e o f t h e emersed, evacuated s u r f a c e s examined t o t h e r e a l e l e c t r o c h e m i c a l i n t e r f a c e s . T h i s concern i s a c c e n t u a t e d by s u r f a c e s c i e n t i s t s ' o b s e r v a t i o n s t h a t i n UHV no m o l e c u l a r water remains on w e l l - d e f i n e d s u r f a c e s a t room t e m p e r a t u r e and above (1). Emersion and e v a c u a t i o n a t room t e m p e r a t u r e may o r may n o t produce s i g n i f i c a n t changes i n e l e c t r o c h e m i c a l i n t e r f a c e s ! depending on whether o r n o t water p l a y s a major r o l e i n t h e s u r f a c e c h e m i s t r y . 0097-6156/88/0378-0065$06.00/0 © 1988 American Chemical Society

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


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B u l k water can be k e p t on s u r f a c e s i n UHV by m a i n t a i n i n g a s u r f a c e t e m p e r a t u r e below 160 K. A wide range o f s p e c t r o s c o p i e s i n d i c a t e t h a t water l a y e r s formed a t t h e s e t e m p e r a t u r e s , even f o r submonolayer c o v e r a g e s , a r e e x t e n s i v e l y hydrogen-bonded and t h u s r e p r e s e n t an a s s o c i a t e d , condensed phase r a t h e r t h a n a c o l l e c t i o n o f i s o l a t e d adsorbed water m o l e c u l e s ( 1 ) . We can t h e r e f o r e hope t o s t u d y f u l l y h y d r a t e d s p e c i e s i n UHV on l i q u i d n i t r o g e n c o o l e d s u r f a c e s . The r e l e v a n c e o f such low-temperature i n t e r f a c e s t o normal aqueous s o l u t i o n s i s a t t e s t e d t o by f r o z e n e l e c t r o l y t e e x p e r i m e n t s ( 2 ) , i n which e l e c t r o c h e m i s t r y q u i t e s i m i l a r t o normal, room-temperat u r e aqueous b e h a v i o r has been observed down t o around 150 K. To t h e a u t h o r s ' knowledge no one has y e t succeeded, i n a U H V - e l e c t r o c h e m i c a l t r a n s f e r e x p e r i m e n t , a t c o o l i n g a sample d u r i n g emersion t o m a i n t a i n a l a y e r o f b u l k water on t h e s u r f a c e a f t e r e v a c u a t i o n , though s e v e r a l groups a r e now equipped f o r t h e a t t e m p t . However, an a l t e r n a t e approach a l l o w i n g UHV s t u d i e s o f aqueous i n t e r f a c e s i s a v a i l a b l e and forms t h e s u b j e c t o f t h i s paper. T h i s approach i s t o grow, from t h e vapor phase and w i t h o u t removing t h e sample from t h e UHV a n a l y t i c a l environment, models o f e l e c t r o c h e m i c a l d o u b l e l a y e r s c o n t a i n i n g a l l r e l e v a n t s p e c i e s . There a r e t h r e e major s t e p s t o such m o d e l i n g : (1) a d s o r p t i o n o f water a l o n e , o r water p l u s a n e u t r a l probe m o l e c u l e , (2) a d d i t i o n o f s o l v a t e d i o n i c s p e c i e s t o t h e UHV i n t e r f a c e , and (3) e s t a b l i s h m e n t o f c o n t r o l over t h e e f f e c t i v e e l e c t r o c h e m i c a l p o t e n t i a l of t h e UHV i n t e r f a c e . T h i s c h a p t e r w i l l d e s c r i b e , and p r o v i d e an e x p e r i m e n t a l i l l u s t r a t i o n o f , each o f t h e s e s t e p s . Experimental A l l e x p e r i m e n t s were performed i n a s t a i n l e s s s t e e l u l t r a h i g h vacuum chamber equipped 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), h i g h r e s o l u t i o n e l e c t r o n energy l o s s s p e c t r o s c o p y (HREELS), Auger e l e c t r o n s p e c t r o s c o p y (AES), and low energy e l e c t r o n d i f f r a c t i o n (LEED) ( 3 - 5 ) . Gases were impinged o n t o t h e Rh and P t s i n g l e c r y s t a l samples, c o o l e d t o 100 K w i t h l i q u i d n i t r o g e n , t h r o u g h c a p i I l a r y argay d o s e r s which a l l o w e d background p r e s s u r e s i n t h e 10" T o r r (10" Pa) range t o be m a i n t a i n e d d u r i n g d o s i n g . Water was a d m i t t e d i n t o t h e chamber from the vapor above a freeze-pump-thaw c y c l e d sample o f t h e pure l i q u i d h e l d i n a g l a s s ampoule. Anhydrous HF, condensed from a commercial anhydrous c y l i n d e r ( p r e s s u r i z e d w i t h He) i n t o a s t a i n l e s s s t e e l t u b e , was a l s o freeze-pump-thaw c y c l e d t o remove v o l a t i l e i m p u r i t i e s . S t e p 1 - Water p l u s a n e u t r a l probe m o l e c u l e , CO The f i r s t s t e p i n t h e UHV modeling o f aqueous/metal i n t e r f a c e s i s t h e a d s o r p t i o n o f water a l o n e , o r water w i t h a n e u t r a l probe m o l e c u l e , on the metal o f i n t e r e s t . T h i s s t e p s e t s up t h e hydrogen-bonded, h i g h d i e l e c t r i c c o n s t a n t medium which c o u l d do much t o d i s t i n g u i s h a q u e o u s - s o l i d from g a s - s o l i d i n t e r f a c e s by p r o v i d i n g an environment amenable t o i o n i c s p e c i e s . E x t e n s i v e UHV s t u d i e s o f water on a v a r i ety o f s u r f a c e s ( a b l y reviewed i n Ref. 1) have shown water t o be weakly a d s o r b e d , w i t h H 0-metal and HoQ-rLO i n t e r a c t i o n s summing t o 10-12 k c a l / m o l e . S i n c e water i s weakly bound compared t o many neut r a l m o l e c u l e s such a s CO, one might e x p e c t t h e p r e s e n c e o r absence of water t o have l i t t l e e f f e c t on t h e a d s o r p t i o n c h e m i s t r y o f such s t r o n g l y bound n e u t r a l s . W h i l e f o r CO on P t ( l l l ) t h e e x p e c t a t i o n o f 2


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s m a l l water e f f e c t s i s borne o u t , on R h ( l l l ) a t t r a c t i v e CO-water i n t e r a c t i o n s s i g n i f i c a n t l y modify t h e a d s o r p t i o n c h e m i s t r y , s u g g e s t ing p o s s i b l e r a m i f i c a t i o n s f o r e l e c t r o c a t a l y s i s ( 5 ) . TPD o f CO p l u s water on R h ( l l l ) and P t ( l l l ) . F i g u r e 1 shows how i n c r e a s i n g predoses o f CO on R h ( l l l ) c o n t i n u o u s l y s h i f t t h e t h e r m a l d e s o r p t i o n o f 1/2 monolayer o f DgO t o h i g h e r t e m p e r a t u r e s , i n d i c a t i n g net a t t r a c t i v e i n t e r a c t i o n s between water and CO on t h i s s u r f a c e . F i g u r e 2 g i v e s t h e r e s u l t s o f t h e analogous e x p e r i m e n t s on P t ( l l l ) ; on t h i s s u r f a c e even t h e lowest CO p r e e x p o s u r e s s h i f t water des o r p t i o n t o lower t e m p e r a t u r e s , i n d i c a t i n g n e t r e p u l s i v e i n t e r a c t i o n s between water and CO. On both s u r f a c e s t h o o n l y d e s o r b i n g s p e c i e s were water and CO; t h u s no e v i d e n c e f o r an i r r e v e r s i b l e water gas s h i f t r e a c t i o n was seen. The lack o f and/or desorption, taken in t h e c o n t e x t o f t h e known thermal b e h a v i o r o f f u n c t i o n a l g r o u p s , a l s o a r g u e s a g a i n s t s u r f a c e r e a c t i o n t o form adsorbed s p e c i e s such as f o r m y l , f o r m a t e , o r c a r b o n a t e i n t h e t e m p e r a t u r e range below which water d e s o r b s (TS210 K ) . HREELS o f CO p l u s water on R h ( l l l ) . More s p e c i f i c i n f o r m a t i o n about the a c t u a l s p e c i e s p r e s e n t a f t e r c o a d s o r p t i o n i s p r o v i d e d by h i g h r e s o l u t i o n e l e c t r o n energy l o s s s p e c t r o s c o p y (HREELS), which y i e l d s v i b r a t i o n a l d a t a r o u g h l y analogous t o t h o s e o b t a i n e d by s u r f a c e i n f r a r e d s p e c t r o s c o p y . F i g u r e 3 shows R h ( l l l ) v i b r a t i o n a l s p e c t r a f o r o n e - h a l f monolayer o f D~0 w i t h i n c r e a s i n g CO p r e d o s e s ( 5 ) . The DgOa l o n e spectrum ( F i g u r e 3a) i s c h a r a c t e r i z e d by a l i b r a t i o n a l ( h i n d e r e d r o t a t i o n a l ) mode a t 580 cm" , the.D-O-D s c i s s o r i n g mode a t 1190 cm" , and 0-D s t r e t c h e s around 2480 cm" ( 4 ) . On R h ( l l l ) Jow c o v e r ages o f CO a l o n e g i v e a s i n g l e C-0 s t r e t c h a t 2020-2070 cm" ascribed t o CO bound a t o p a s i n g l e Rh atom. For h i g h e r c o v e r a g e s o f CO a l o n e a second^peak, a s c r i b e d t o CO i n a t w o - f o l d b r i d g e s i t e , grows i n a t 1830 cm , b u t i t s h e i g h t never exceeds 70% o f t h a t o f t h e a t o p CO pealj. I n t h e p r e s e n c e o f water s m a l l CO c o v e r a g e s y i e l d n o t t h e 2020 cm a t o p peak_jeen i n t h e absence of w a t e r , b u t r a t h e r a new peak around 1620 cm" . Even a t h i g h CO c o v e r a g e , t h e CO+D2O spectrum i s d i s t i n c t from t h e C O j a l o n e spectrum, as w i t h water p r e s e n t t h e i n t e n s i t y o f t h e 1830 cm" ( b r i d g e d ) peak exceeds t h a t o f t h e 2040 cm (atc-p) peak, and a s h o u l d e r on t h e low-frequency s i d e o f t h e 1830 cm peak i n d i c a t e s t h a t some o f t h e CO i s s t i l l i n t h e form which produced t h e 1620 cm" peak a t low.CO c o v e r a g e . We b e l i e v e t h a t t h e ~1620 cm" peak a r i s e s from CO bound i n a t h r e e - f o l d h o l l o w s i t e , by a n a l o g y t o rhodium c l u s t e r compounds y i e J d i n g a CO s t r e t c h a t 1600-1685 cm" (6-7) and t o t h e 1655-1700 cm" CO s t r e t c h o b s e r v e d f o r CO+benzene/Rh(lll) ( 8 ) , where LEED i n t e n s i t y a n a l y s i s has p l a c e d t h e CO i n t h e t h r e e - f o l d h o l l o w . A HREELS peak around 1620 cm" c o u l d a l s o a r i s e from r e a c t i o n o f CO and water t o produce f o r m a t e , f o r m y l , o r c a r b o n a t e , a l l o f which s h o u l d have v i b r a t i o n a l modes i n t h i s f r e q u e n c y range. However, none o f t h e o t h e r modes e x p e c t e d f o r such s p e c i e s were o b s e r v e d , and t h e l a c k o f Hg o r COg d e s o r p t i o n p r o d u c t s a l s o argue a g a i n s t t h e p r e s e n c e o f such s p e c i e s . XPS d a t a a r e a l s o c o n s i s t e n t w i t h a w a t e r - i n d u c e d s h i f t i n CO b i n d i n g s i t e . A d d i t i o n o f water t o low c o v e r a g e s o f CO on R h ( l l l ) s h i f t s t h e XPS 0 I s s i g n a l due t o CO from 531.7 t o 530.0 eV b i n d i n g energy, as e x p e c t e d f o r a s h i f t from an a t o p t o a m u l t i p l y - b o u n d s i t e (9). Thus, w a t e r , though weakly bound t o R h ( l l l ) , s h i f t s t h e much 1


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i

r

Rh(111) x L CO

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1 L

D 0 2


x

150 T

200 (K)

250

F i g u r e 1. DgO TPD from R h ( l l l ) o f v a r y i n g e x p o s u r e s o f CO f o l lowed by 1 L DoO. (Reproduced w i t h p e r m i s s i o n from Ref. 5. Copyr i g h t 1988 E l s e v i e r . )

150

200 T

250

(K)

F i g u r e 2. HgO TPD from P t ( l l l ) o f v a r y i n g e x p o s u r e s o f CO f o l lowed by 1 L FLO. (Reproduced w i t h p e r m i s s i o n from R e f . 5. Copyr i g h t 1988 E l s e v i e r . )


WAGNER AND MOYLAN



5.

F i g u r e 3. R h ( l l l ) HREELS s p e c t r a of a.) 1 L D J ) , b. - f . ) v a r y i n g exposures o f CO f o l l o w e d by 1 L D 0. (Reproduced w i t h p e r m i s s i o n from Ref. 5. C o p y r i g h t 1988 E l s e v i e r . ) 2


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more s t r o n g l y - b o u n d CO m o l e c u l e t o a s i t e not p o p u l a t e d i n t h e absence of water. C o a d s o r p t i o n on R h ( l l l ) a l s o m o d i f i e s t h e HREELS f e a t u r e s due t o water. -As shown i n F i g u r e 3, a s u b s t a n t i a l s h a r p e n i n g and s h i f t t o 610 cm o f t h e pure D 0 band a t 580 cm" (due t o t h e superimposed D 0 l i b r a t i o n and Rh-0 s t r e t c h ) c o r r e l a t e s w i t h t h e growth of t h e ~I650 cm" CO mode. The f a c t t h a t t h e v i b r a t i o n a l s p e c t r a of b o t h s p e c i e s a r e m o d i f i e d by c o a d s o r p t i o n s u g g e s t s a d s o r p t i o n o f water and CO onto a d j a c e n t s i t e s , a form o f h y d r o p h i l i c c o a d s o r p t i o n . H 0 + C 0 / R h ( l l l ) produced f e a t u r e s analogous t o t h o s e shown f o r D 0 + C 0 / R h ( l l l ) . A p a r t i a l s u p e r p o s i t i o n o f t h e d o w n s h i f t e d CO s t r e t c h and t h e H-O-H s c i s s o r mode a t 1620 cm" makes t h e e f f e c t l e s s w e l l - r e s o l v e d f o r HgO than f o r DÔ, but we i n f a c t observed t h e CO d o w n s h i f t f i r s t w i t n HgO. 2

2

2


2

HREELS of CO p l u s water on P t ( l l l ) . F i g u r e 4 shows HREELS s p e c t r a f o r CO and a h a l f monolayer of water on P t ( l l l ) . The w a t e r - a l o n e spectrum ( F i g u r e 4 a ) , e x h i b i t s a l i b r a t i o n a t 690, s c i s s o r a t 1620, and OH s t r e t c h a t _ J 4 4 0 cm" , as w e l l as a d d i t i o n a l f e a t u r e s a t 970, 1020, and 1950 cm" p r e v i o u s l y a s c r i b e d t o a m i n o r i t y water s p e c i e s (4). On P t ( l l l ) metal c o a d s o r p t i o n produces not t h e s i n g l e low f r e quency t h r e e - f o l d peak seen f o r t h e s e CO and w a t e r . c o v e r a g e s on R h ( l l l ) , b u t r a t h e r two peaks a t 2080 and 1830 cm" , c o r r e s p o n d i n g t o the a t o p and t w o - f o l d CO s p e c i e s seen on both s u r f a c e s f o r h i g h e r c o v e r a g e s of CO a lone. I n c r e a s i n g water a d d i t i o n s t o a c o n s t a n t low coverage o f CO on P t ( l l l ) d e c r e a s e t h e a t o p / b r i d g e peak h e i g h t r a t i o i n t h e same way t h a t i n c r e a s i n g t h e coverage o f CO a l o n e would do. On P t ( l l l ) t h e HREELS f e a t u r e s due t o water a r e unchanged by t h e presence o f CO. These o b s e r v a t i o n s i n d i c a t e t h a t water and CO adsorb onto s e p a r a t e p a t c h e s on t h e s u r f a c e , i n a form o f hydrophobic coads o r p t i o n . Water condenses i n t o hydrogen-bonded i s l a n d s , as i n d i c a t e d by t h e low 0-H s t r e t c h i n g f r e q u e n c y . CO s p r e a d s t o c o v e r t h e r e s t o f s u r f a c e , g i v i n g a phase s i m i l a r t o t h a t f o r CO a l o n e , b u t w i t h a coverage n o r m a l i z e d t o t h e w a t e r - f r e e , not t o t a l , s u r f a c e a r e a . COCO r e p u l s i o n s , which have been w e l l documented on P t ( l l l ) ( 1 0 ) , produce a s u r f a c e p r e s s u r e w i t h i n t h e CO p a t c h e s which b e a r s upon t h e edges o f t h e water i s l a n d s . I t i s t h i s l a t e r a l p r e s s u r e which causes water t o desorb from P t ( l l l ) a t lower t e m p e r a t u r e s i n t h e p r e s e n c e o f coadsorbed CO. H y d r o p h i I i c v e r s u s hydrophobic c o a d s o r p t i o n . The c o n t r a s t between the h y d r o p h i l i c and hydrophobic c o a d s o r p t i o n seen on R h ( l l l ) and P t ( l l l ) , i f c o n f i r m e d under normal e l e c t r o c h e m i c a l c o n d i t i o n s , might be o f e l e c t r o c a t a l y t i c importance. On R h ( l l l ) , where n e t a t t r a c t i v e C0-H 0 i n t e r a c t i o n s produce a mixed phase i n which CO i s d i s p l a c e d t o a t h r e e - f o l d b i n d i n g s i t e which i s not o c c u p i e d i n t h e absence o f w a t e r , CO and water appear t o occupy a d j a c e n t b i n d i n g s i t e s . Such thorough m i x i n g o f t h e oxygen s o u r c e (water) and t h e i n t e r m e d i a t e [or p o i s o n ] (CO) s h o u l d improve e l e c t r o o x i d a t i o n r a t e s f o r C 0 H f u e l s (11). On P t ( l l l ) , where n e t r e p u l s i o n s cause c o n d e n s a t i o n ^ ? CO and water i n t o s e p a r a t e p a t c h e s , r e a c t i o n between t h e adsorbed s p e c i e s c o u l d o c c u r o n l y a t t h e b o u n d a r i e s between p a t c h e s , and one would expect slower k i n e t i c s . The d i f f e r e n t c o a d s o r p t i o n c h e m i s t r i e s on t h e s e two s t r u c t u r a l l y and c h e m i c a l l y s i m i l a r s u r f a c e s i l l u s t r a t e t h e importance of t h e 2

x


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T

1


1

1

1

1

1

i

i

r

Pt(111) 0.25 L C O + 1 L H 0 2


2110

400

1200

2000 -1

2800

3600

cm F i g u r e 4. P t ( l l l ) HREELS s p e c t r a f o r a.) 1 L HÔ a t 90 K (does not change upon a n n e a l i n g ) ; 0.25 L CO f o l l o w e d By 1 L HÔ: b.) a t 90 K, c.) a f t e r f l a s h t o 148 K ( n o t h i n g d e s o r b e d ) , d.) a f t e r f l a s h t o 323 K (water desorbed). (Reproduced w i t h p e r m i s s i o n from Ref. 5. C o p y r i g h t 1988 Elsevier.)


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s u b t l e b a l a n c e o f t h e many d i f f e r e n t l a t e r a l i n t e r a c t i o n s p r e s e n t i n muIticomponent a d s o r p t i o n systems. Both t r u e l a t e r a l i n t e r a c t i o n s and t h e s i t e - t o - s i t e v a r i a t i o n s i n m e t a I - a d s o r b a t e i n t e r a c t i o n s t r e n g t h s w i l l i n f l u e n c e t h e c o a d s o r p t i o n geometry. The v a l e n c e e l e c t r o n s i n P t have more d - c h a r a c t e r , and a r e t h e r e f o r e more l o c a l i z e d , t h a n t h o s e i n Rh. One would then e x p e c t a g r e a t e r s i t e - t o - s i t e v a r i a t i o n i n m e t a I - a d s o r b a t e i n t e r a c t i o n s on P t than on Rh, a s has been noted f o r NO a d s o r p t i o n ( 1 2 ) . Moving CO and rUO o f f o f t h e i r optimum one-component b i n d i n g s i t e s t o o p t i m i z e CO-RgO i n t e r a c t i o n s would t h u s e x a c t a g r e a t e r m e t a I - a d s o r b a t e f r e e energy p e n a l t y on P t than on Rh. There i s a l s o some e v i d e n c e t h a t t h e s t r e n g t h o f w a t e r water i n t e r a c t i o n s i s d i f f e r e n t on t h e s e two s u r f a c e s ( 4 ) . Thus, a l t h o u g h water i s weakly bound on s u r f a c e s , i t s p r e s e n c e can, b u t need b u t n o t i n a l l c a s e s , a l t e r t h e b a l a n c e o f f o r c e s which d e t e r m i n e s t h e a d s o r p t i o n s t a t e o f much more s t r o n g l y bound s p e c i e s . F u r t h e r work i s needed t o d e t e r m i n e whether t h e R h ( l l l ) o r t h e P t ( l l l ) c a s e i s more t y p i c a l , and a l s o t o d e t e r m i n e t h e e x t e n t t o which t h e c r y o g e n i c c o a d s o r p t i o n e x p e r i m e n t s a r e r e p r e s e n t a t i v e o f r e a l aqueous environments. I n t h e meantime, t h e e l e c t r o c h e m i s t seeking i n f o r m a t i o n from UHV gas-phase a d s o r p t i o n s t u d i e s t o h e l p i n t e r p r e t h i s r e s u l t s s h o u l d be aware t h a t t h e presence o f water can a l t e r a d s o r p t i o n c h e m i s t r y even f o r s t r o n g l y - b o u n d n e u t r a l m o l e c u l e s which are n o t good hydrogen-bond donors o r a c c e p t o r s . And t h e UHV s u r f a c e s c i e n t i s t hoping t o p r o v i d e g a s - a d s o r p t i o n d a t a r e l e v a n t t o e l e c t r o c h e m i s t r y s h o u l d c o n s i d e r d o i n g water c o a d s o r p t i o n e x p e r i m e n t s t o check f o r s u r p r i s i n g e f f e c t s such as t h o s e demonstrated here f o r Rh(lll). S t e p 2 - A d d i t i o n o f i o n i c s p e c i e s t o t h e UHV model: HF + HÔ A l l e l e c t r o c h e m i c a l t e c h n i q u e s measure charge t r a n s f e r r e d a c r o s s an i n t e r f a c e . S i n c e charge i s t h e measurable q u a n t i t y , i t i s n o t s u r p r i s i n g t h a t e l e c t r o c h e m i c a l t h e o r y has been founded on an e l e c t r o s t a t i c b a s i s , w i t h c h e m i c a l e f f e c t s added as a p e r t u r b a t i o n . I n t h e e l e c t r o s t a t i c l i m i t i o n s a r e t r e a t e d as f u l l y charged s p e c i e s w i t h some l e v e l o f s o l v a t i o n . I f we a r e t o use UHV models t o t e s t t h e o r i e s o f t h e d o u b l e l a y e r , we must be a b l e t o s t u d y i n UHV t h e weaklya d s o r b i n g systems where t h e s e i d e a l " e l e c t r o s t a t i c " i o n s c o u l d be p r e s e n t and where we would e x p e c t t h e e f f e c t s o f water t o be most dominant. To t h i s end, and t o a l l o w a p p l i c a t i o n o f UHV s p e c t r o s c o p i c methods t o t h e pH e f f e c t s which c o n t r o l s o much o f aqueous i n t e r f a c i a l c h e m i s t r y , we have s t u d i e d t h e c o a d s o r p t i o n o f water and anhydrous HF on P t ( l l l ) i n UHV ( 3 ) . S u r f a c e s p e c t r o s c o p i e s have a l l o w e d us t o f o l l o w t h e i o n i z a t i o n o f t h e a c i d and t o d e t e r m i n e t h e e x t e n t of s o l v a t i o n both i n t h e l a y e r a d j a c e n t t o t h e metal and i n subsequent l a y e r s . TPD o f HF a l o n e , and o f HF p l u s w a t e r , on P t ( l l l ) . Anhydrous HF, i n which hydrogen bonding i s e x t e n s i v e , s h a r e s many p r o p e r t i e s w i t h w a t e r , i n c l u d i n g a broad t e m p e r a t u r e range o f s t a b i l i t y o f t h e l i q u i d and t h e h i g h d i e l e c t r i c c o n s t a n t o f t h e l i q u i d . T h i s p a r a l l e l i s m o f behavior extends t o t h e i n t e r a c t i o n s with t h e P t ( l l l ) s u r f a c e . Like w a t e r , HF i s m o l e c u l a r l y adsorbed on P t ( l l l ) . Each s u b s t a n c e , a d s o r b e d a l o n e , g i v e s a s i n g l e d e s o r p t i o n peak f o r c o v e r a g e s up t o t h e monolayer and a second d e s o r p t i o n peak a t lower t e m p e r a t u r e f o r





73

subsequent l a y e r s . However, HF d e s o r b s a t 50 K lower t e m p e r a t u r e s (130 and 126 K f o r t h e monolayer and m u l t i l a y e r , r e s p e c t i v e l y , v e r s u s 179 and 168 K f o r w a t e r ) , p r o b a b l y due t o t h e fewer hydrogen bonds per m o l e c u l e formed by HF. C o a d s o r p t i o n o f HF and water has no e f f e c t on t h e water des o r p t i o n peaks, but s t a b i l i z e s p a r t o r a l l of t h e HF t o h i g h e r t e m p e r a t u r e s , as shown by F i g u r e 5. As long as a t l e a s t 5 m o l e c u l e s of water per HF m o l e c u l e a r e added t o t h e s u r f a c e (up t o monolayer c o v e r a g e , o r 8 HÔ/HF f o r subsequent l a y e r s ) no HF d e s o r b s u n t i l water s t a r t s t o l e a v e t h e s u r f a c e around 170 K, p e a k i n g a t 180 K. As long as a t l e a s t 1 m o l e c u l e of water i s i n i t i a l l y p r e s e n t per HF, no HF d e s o r p t i o n w i l l o c c u r u n t i l 150 K, p e a k i n g a t 162 K. I f more HF t h a n HoO m o l e c u l e s a r e p r e s e n t i n i t i a l l y , some HF w i l l desorb i n a peak a t 136 K, near t h e t e m p e r a t u r e a t which HF a l o n e d e s o r b s . Coads o r p t i o n t h u s can y i e l d HF d e s o r p t i o n a t t h r e e peaks, one not s t a b i l i z e d v s . HF a l o n e , one s t a b i l i z e d by 30 K, and one s t a b i l i z e d by 50 K, i . e . , t o t h e water d e s o r p t i o n t e m p e r a t u r e . HREELS o f t h e HÔ + HF system. The n a t u r e o f t h e i n t e r a c t i o n s t a b i I i z i n g HF on t h e s u r f a c e i s made c l e a r by t h e HREELS s p e c t r a o f F i g ure 6. As t h e c o n c e n t r a t i o n of HF i n t h e water l a y e r i s i n c r e a s e d a new peak around 1150 cm" (and s e v e r a l s m a l l e r peaks) f i r s t i n c r e a s e s and t h e n , as t h e HF/HÔ r a t i o exceeds 1, d e c r e a s e s i n i n t e n s i t y . By a n a l o g y t o v i b r a t i o n a l s p e c t r a o f a c i d h y d r a t e s of known s t r u c t u r e (13-16), t h i s p e a k i s i d e n t i f i e d as t h e symmetric bending mode o f t h e p y r a m i d a l H»0 i o n . We have observed t h e same peak upon c o a d s o r p t i o n of water ana o t h e r , s t r o n g e r , m i n e r a l a c i d s . The r e a c t i o n +

H0 2

+ HF — >

H 0 3

+

+ F"

can proceed f u r t h e r t o t h e r i g h t i n t h e s e c r y o g e n i c e x p e r i m e n t s t h a n in room t e m p e r a t u r e aqueous s o l u t i o n because t h e low t e m p e r a t u r e d e c r e a s e s t h e importance of t h e u n f a v o r a b l e e n t r o p y o f s o l v a t i o n of the f l u o r i d e i o n ( 1 7 ) . S o l v a t i o n s t o i c h i o m t t r i e s . HREELS i s s e n s i t i v e t o i n t e r n a l modes o f ion c o r e s , but d e t a i l s of s o l v a t i o n would produce a t most secondary e f f e c t s on t h e v i b r a t i o n a l s p e c t r a . However, t h e i d e n t i f i c a t i o n o f the i o n i z a t i o n r e a c t i o n , c o u p l e d w i t h t h e TPD d a t a o u t l i n e d above, a l l o w us t o q u a n t i f y t h e s o l v a t i o n . The HF d e s o r p t i o n peak c o i n c i dent w i t h t h e water peak a t 180 K, seen f o r m o l e c u l a r r a t i o s o f HrtO/HF^ i n t h e monolayer and >8 i n subsequent l a y e r s , c o r r e s p o n d s t o trie r e c o m b i n a t i o n o f HÔ* and F" i o n s which had been f u l l y s o l v a t e d . Screened by s o l v a t i n g water m o l e c u l e s , t h e s e i o n s cannot combine t o form HF u n t i l t h e s o l v a t i o n water d e s o r b s . The f i g u r e o f 8 f o r l a y e r s subsequent t o t h e monolayer agrees w i t h t h e sum o f t h e p r i m a r y s o l v a t i o n numbers measured i n room t e m p e r a t u r e aqueous s o l u t i o n f o r the H [4] and F [4*1] i o n s ( 1 8 ) . T h i s agreement, i n c o m b i n a t i o n w i t h t h e f r o z e n e l e c t r o l y t e work of Stimining's group ( 2 ) , g i v e s hope t h a t t h e c r y o g e n i c aqueous environment a c c e s s i b l e i n UHV may be r e l e v a n t t o s t a n d a r d e l e c t r o c h e m i s t r y . The l e s s e r number of s o l v a t i o n w a t e r s seen i n t h e f i r s t monolayer s u g g e s t s t h a t even t h e s e c l a s s i c a l l y " n o n - s p e c i f i c a l l y adsorbed" i o n s l o s e some o f t h e i r s o l v a t i o n w a t e r s upon a d s o r p t i o n , a t l e a s t on P t ( l l l ) , a s u r f a c e w h i c h , when w e l l o r d e r e d , g i v e s n o t o r i o u s l y s t r a n g e voltammetry (19-20). I t a l s o



74


a

100

150

200

250

T (K) F i g u r e 5. HF thermal d e s o r p t i o n s i g n a l s , n o r m a l i z e d t o HÔ c o v e r age, f o r HF/H 0 monolayer r a t i o s o f : a.) 0.12, b.) 0.15, c.) 0.20, d.) 0.42, e.)^0.56, f . ) 0.75, and g.) 1.28. (Reproduced w i t h p e r m i s s i o n from Ref. 3. C o p y r i g h t 1987 E l s e v i e r . ) 9


5. WAGNER AND M O Y I A N

Aqueous-Metal Interface in Ultrahigh Vacuum Loss Energy (1/cm) 1200 2000 2800

400

3600


770

0

100

200 300 Loss Energy (mV)

400

500

F i g u r e 6. HREELS s p e c t r a f o r HJ) and HF a d s o r p t i o n and c o a d s o r p t i o n on P t ( l l l ) a t 100 K, HF/HJJ r a t i o i n c r e a s i n g upwards, a.) 1 monolayer (ML) H 0 , xlOO; b.) iML H 0 + 0.05ML HF, xlOO; c.) 1ML H 0 * 0.12 ML HF; xlOO; d.) 1 ML HJ) + 0.5 ML HF, xlOO; e.) 1 ML HÔ • 1 ML HF; f . ) 1 ML HF + 0.25 ML H 0 , xlOO; g.) 1 ML HF, x l 6 . 5 and x 110. (Reproduced w i t h p e r m i s s i o n from R e f . 3. C o p y r i g h t 1987 E l s e v i e r . ) 9

9

o

2


75


76

draws i n t o q u e s t i o n t h e c o n c e p t o f a h y d r a t i o n s h e a t h s e p a r a t i n g t h e s u r f a c e from t h e s o l v a t e d i o n s ( 2 1 ) , which would y i e l d a h i g h e r number o f w a t e r s p e r HF i n t h e f i r s t i o n i c l a y e r than i n subsequent layers. The HF d e s o r p t i o n peak around 160 K c o r r e s p o n d s t o t h e f r a c t i o n a l d i s t i l l a t i o n o f HF from t h e monohydrate phase [HJ) ] [ F ] , which has been w e l l e s t a b l i s h e d i n b u l k s t u d i e s ( 2 2 ) . The b u l k work has not i d e n t i f i e d any w a t e r - r i c h d i s t i n c t phases. The 136 K HF des o r p t i o n peak seen f o r HF/H Q>1 i s due t o t h e l o s s o f e x c e s s m o l e c u l a r HF from t h e l a y e r . C o a d s o r p t i o n o f HF and HgO has shown t h a t we can grow i n UHV j m , i f n o t y e t demonstrably t h e , i o n i c aqueous environment w i t h c o n t r o l l e d pH and c o m p o s i t i o n . 2


Step 3 - Control o f e f f e c t i v e e l e c t r o d e p o t e n t i a l :

+ HÔ

The unique a s p e c t o f e l e c t r o c h e m i s t r y l i e s i n t h e a b i l i t y t o change t h e e l e c t r o d e p o t e n t i a l and t h u s c o n c e n t r a t e an a p p l i e d p e r t u r b a t i o n r i g h t a t t h e i n t e r f a c e . E l e c t r i c f i e l d s o f 10 V/cm c a n be g e n e r a t e d e l e c t r o c h e m i c a l l y w i t h a h a l f - l e m o n , s c r a p e d z i n c ( s i n c e 1983) penny, and copper w i r e a s opposed t o t h e massive Van de C r a a f f g e n e r a t o r and e l e c t r i c power p l a n t r e q u i r e d f o r n o n - e l e c t r o c h e m i c a l approaches t o t h e same f i e l d s t r e n g t h . I f UHV models a r e t o p r o v i d e u s e f u l molecul a r - s c a l e i n s i g h t i n t o e l e c t r o c h e m i s t r y , some means o f c o n t r o l l i n g t h e e f f e c t i v e e l e c t r o d e p o t e n t i a l o f t h e models must be d e v e l o p e d . One approach t o p o t e n t i a l c o n t r o l i n UHV l i e s i n chemical p o i s i n g , r o u g h l y analogous t o p i n n i n g an i n e r t b u t c a t a l y t i c e l e c t r o d e a t a g i v e n p o t e n t i a l by immersing i t i n a s o l u t i o n c o n t a i n i n g c o n t r o l l e d c o n c e n t r a t i o n s o f both members o f a redox c o u p l e . To a p p l y t h i s approach i n UHV we s u p p l y both members o f t h e redox c o u p l e a s s p e c i e s a d s o r b e d , i n c o n t r o l l e d q u a n t i t i e s , from t h e gas phase. We then a l l o w e q u i l i b r a t i o n t o occur. The c l o s e r e l a t i o n between t h e work f u n c t i o n , measurable i n UHV, and t h e e l e c t r o c h e m i c a l p o t e n t i a l has been e x p e r i m e n t a l l y demons t r a t e d by Hansen and K o l b (23-24). Bange e t a l . (25) have p r e s e n t e d a d i r e c t comparison between UHV and e l e c t r o c h e m i c a l d a t a ( t h e l a t t e r c o r r e c t e d f o r d i f f u s e l a y e r e f f e c t s ) on t h e Br +H 0/Ag(110) system. They found t h a t c o a d s o r p t i o n o f water w i t h B r Brought t h e UHV d a t a i n t o good agreement w i t h e l e c t r o c h e m i c a l measurements. We c o n s i d e r here t h e H + H 0 / P t ( l l l ) system ( i n which m o l e c u l a r H i s never p r e s e n t a s an adsorbed s p e c i e s ) . Because o f t h e extreme r e v e r s i b i l i t y of hydrogen e l e c t r o c h e m i s t r y on P t , t h e e f f e c t i v e e l e c t r o d e p o t e n t i a l s h o u l d be a s i n g l e - v a l u e d f u n c t i o n o f t h e amount o f hydrogen added t o t h e water l a y e r . Our attgmpts t o measure work f u n c t i o n s were f r u s t r a t e d by a p e r s i s t a n t 10 fl path t o ground on o u r sample m a n i p u l a t o r a t t h e t i m e . I n s t e a d , we w i l l i n f e r t h e e l e c t r o d e p o t e n t i a l , r e l a t i v e t o t h e p o t e n t i a l o f z e r o c h a r g e , from s p e c t r o s c o p i c e x a m i n a t i o n of t h e t y p e s o f s p e c i e s p r e s e n t on t h e s u r f a c e a f t e r e q u i l i b r a t i o n . F i g u r e 7 compares e l e c t r o c h e m i c a l and UHV data f o r hydrogen a d s o r p t i o n on P t ( l l l ) . The s o l i d c u r v e , a, i s t h e c a t h o d i c sweep from a c y c l i c voltammogram i n 0.3 M HF (20) a t 25 mV/s, w i t h a c o n s t a n t d o u b l e - l a y e r c h a r g i n g c u r r e n t s u b t r a c t e d . The voltammogram p l o t s c u r r e n t I (charge/time) v e r s u s p o t e n t i a l V f o r a c o n s t a n t negat i v e sweep r a t e (dV/dt=-ICI). The c u r r e n t i n t h i s p o t e n t i a l r e g i o n i s g e n e r a l l y a s c r i b e d t o t h e d i s c h a r g e o f hydronium i o n s t o form 2

2

2

2

2





V (mV 100

RHE) 30C

-200

0 (mV)

A

Electrochemical Surface Science - American Chemical Society

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