Radioelectrochemistry at Well-Defined Electrodes - American

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Chapter 17

Radioelectrochemistry at Well-Defined Electrodes Andrzej Wieckowski Department of Chemistry, University of Illinois, Urbana, IL 61801 Radiochemical measurements of adsorption on single c r y s t a l , well-defined electrodes are reported and the method of the measurements i s described. Adsorption of acetic acid on Pt(111) surface was studied: the surface concentration data were correlated with voltammetric p r o f i l e s of the Pt(111) electrode i n perchloric acid e l e c t r o l y t e containing 0.5 mM of CH3COOH. I t i s concluded that acetic acid adsorption i s associative and occurs without a s i g n i f i c a n t charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(111) than on p o l y c r y s t a l l i n e platinum. A c l a s s i f i c a t i o n of adsorption processes on c a t a l y t i c electrodes and atmospheric methods of preparation of single c r y s t a l electrodes are discussed.

The methodology of surface electrochemistry i s at present s u f f i c i e n t l y broad to perform molecular-level research as required by the standards of modern surface science (.1). While ultra-high vacuum electron, atom, and ion spectroscopies connect electrochemistry and the state-of-the-art gas-phase surface science most d i r e c t l y (1-11), their application i s appropriate for systems which can be transferred from solution to the vacuum environment without desorption or rearrangement. That this usually occurs has been v e r i f i e d by several groups (see r e f . 11 for the recent discussion of this issue). However, for the characterization of weakly interacting i n t e r f a c i a l species, the vacuum methods may not be able to provide information d i r e c t l y relevant to the surface composition of electrodes i n contact with the e l e c t r o l y t e phase. In such a case, jln s i t u methods are preferred. Such techniques are also unique for the nonelectrochemical characterization of i n t e r f a c i a l k i n e t i c s and for the measurements of surface concentrations of reagents involved i n 0097-6156/88/0378-0245$06.00/0 ° 1988 American Chemical Society

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

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ELECTROCHEMICAL SURFACE SCIENCE

steady-state processes. Many iri s i t u t e c h n i q u e s have r e c e n t l y emerged and some o f them can p r o v i d e v a l u a b l e i n f o r m a t i o n as t o the c o m p o s i t i o n and s t r u c t u r e o f t h e "wet" s o l i d - l i q u i d i n t e r f a c e . V i b r a t i o n a l a n a l y s i s by SERS (12) and s u r f a c e IR (12,13)t i n s i t u X-ray d i f f r a c t i o n ( 1 2 ) , Raman second harmonic g e n e r a t i o n (IM, 15) and s c a n n i n g t u n n e l i n g microscopy (_16) a r e t h e best examples o f p r o b i n g t h e s o l i d / l i q u i d i n t e r f a c e w i t h o u t system emersion. We have r e c e n t l y m o d i f i e d one o f the s e v e r a l r a d i o c h e m i c a l methods (_18) which have been used f o r s u r f a c e e l e c t r o c h e m i s t r y i n v e s t i g a t i o n s i n order t o c h a r a c t e r i z e a d s o r p t i o n on w e l l - d e f i n e d , s i n g l e c r y s t a l e l e c t r o d e s . Below, we w i l l d e s c r i b e t h e t e c h n i q u e and i d e n t i f y some c h a l l e n g i n g i s s u e s which we w i l l be a b l e t o address. The proposed method i s s e n s i t i v e t o a few p e r c e n t o f a monolayer a t smooth s u r f a c e s , i s n o n d e s t r u c t i v e and s i m p l e t o use. The r a d i o c h e m i c a l measurements can be made w i t h a l l compounds which can be l a b e l l e d w i t h r e a s o n a b l y l o n g - l i v e d , p r e f e r a b l y 8~ e m i t t i n g r a d i o i s o t o p e s . We b e l i e v e t h i s t e c h n i q u e w i l l f u l f i l l the q u a n t i t a t i v e f u n c t i o n i n i n s i t u s u r f a c e a n a l y s i s as Auger s p e c t r o s c o p y c u r r e n t l y does i n vacuum, ex s i t u c h a r a c t e r i z a t i o n o f e l e c t r o d e s . The Method. The s k e t c h o f the r a d i o - e l e c t r o c h e m i c a l c e l l (17) used i n our l a b o r a t o r y i s shown i n F i g u r e 1. F i g u r e 2 i l l u s t r a t e s the e s s e n t i a l element o f t h e c e l l : the s i n g l e c r y s t a l e l e c t r o d e and t h e d e t e c t o r o f the n u c l e a r r a d i a t i o n , i . e . , t h e g l a s s s c i n t i l l a t o r . Two p o s i t i o n s o f t h e e l e c t r o d e a g a i n s t t h e s c i n t i l l a t o r are d e p i c t e d : i n p o s i t i o n A the e l e c t r o d e i s s i t u a t e d s u f f i c i e n t l y away from the d e t e c t o r t o a l l o w f o r a complete a t t e n u a t i o n , i n t h e bulk e l e c t r o l y t e , o f the r a d i a t i o n e m i t t e d from the e l e c t r o d e s u r f a c e . I n t h i s p o s i t i o n t h e c o u n t i n g e f f i c i e n c y o f t h e system i s d e t e r m i n e d . I n p o s i t i o n B, the e l e c t r o d e i s pressed a g a i n s t the g l a s s s c i n t i l l a t o r and t h e amount o f a d s o r p t i o n i s measured. The c o u n t i n g c o n t r i b u t i o n due t o the r a d i o a c t i v i t y i n t h e trapped f i l m can, i n some i n s t a n c e s , be n e g l i g i b l e , o r can e a s i l y be c a l i b r a t e d i n the "squeeze" (B) p o s i t i o n when the a d s o r p t i o n i s not o c c u r r i n g . A s u i t a b l e e l e c t r o d e p o t e n t i a l can v e r y o f t e n be found t o perform such calibration. S u r f a c e c o n c e n t r a t i o n and c o u n t i n g r a t e data are connected by E q u a t i o n 1: N

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where: N i s t h e s i g n a l measured i n t h e "squeezed" p o s i t i o n , F i g u r e 2-B, N i s t h e Avogadro c o n s t a n t , c i s t h e bulk c o n c e n t r a t i o n o f t h e a d s o r b a t e (M), N i s the counting r a t e measured i n t h e "open" p o s i t i o n , F i g u r e 2-A, y i s the a b s o r p t i o n c o e f f i c i e n t o f t h e B~ r a d i a t i o n i n s o l u t i o n , R i s the roughness f a c t o r , f i s t h e b a c k s c a t t e r i n g f a c t o r , and x i s t h e t h i c k n e s s of t h e t r a p p e d s o l u t i o n , F i g u r e 2-B. a d s

A

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17. W I E C K O W S K I

Radioekctrochemistry at Well-Defined Electrodes

JPMTJ F i g u r e 1. Diagram of T e f l o n c e l l : (1) p l a t i n u m e l e c t r o d e ; (2) g l a s s s c i n t i l l a t o r ; (3) Macor ceramic d i s k c e l l bottom; (H) T e f l o n O - r i n g ; (5) f l e x i b l e elbow (see i n s e r t ) ; (6) c e l l p o r t s ( s i x of them around c e l l body); (7) l i g h t p i p e . I n s e t shows the d e t a i l s of the f l e x i b l e elbow: (8) s t a i n l e s s s t e e l sphere; (9) concave T e f l o n s p a c e r ; (10) p l a t i n u m w i r e f o r e l e c t r i c a l c o n n e c t i o n a c r o s s elbow; (11) l o c k nut.

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F i g u r e 2. Diagram o f e l e c t r o d e i n a d s o r p t i o n p o s i t i o n and "squeezed" p o s i t i o n f o r a d s o r b a t e measurement: (1) smooth, p o l y c r y s t a l l i n e p l a t i n u m e l e c t r o d e ; (2) p o l i s h e d g l a s s s c i n t i l l a t o r d e t e c t o r ; (3) Macor ceramic d i s k , which forms c e l l bottom; (4) r e s i n ; (5) e l e c t r o d e s h a f t .


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I n t h e squeezed p o s i t i o n , t h e 3~ r a d i a t i o n i s t r a p p e d i n t h e gap between t h e two s u r f a c e s and the e l e c t r o n s a r e b a c k s c a t t e r e d from both o f them (J_9). The l a r g e r c o n t r i b u t i o n comes from t h e s c a t t e r i n g from the h i g h d e n s i t y m a t e r i a l o f the metal e l e c t r o d e (due t o the Z-dependence o f t h e b a c k s c a t t e r i n g power). The f ^ c o r r e c t i o n which a c c o u n t s f o r t h e b a c k s c a t t e r i n g i s i n c l u d e d i n E q u a t i o n 1. A p o w e r f u l c h a r a c t e r i z a t i o n o f e l e c t r o d e s u r f a c e s can be a c h i e v e d by t h e combined r a d i o c h e m i s t r y and e l e c t r o c h e m i s t r y p r o c e d u r e s ; w i t h p o l y c r y s t a l l i n e m a t e r i a l s many measurements o f t h i s type have been r e p o r t e d (18,20,21). A s u c c e s s f u l study o f a d s o r p t i o n a t w e l l - d e f i n e d s u r f a c e s has r e c e n t l y been conducted i n our l a b o r a t o r y and i t i s r e p o r t e d below (22,23). Electrochemical Adsorption a t C a t a l y t i c Electrodes. A c l a s s i f i c a t i o n o f adsorption processes a t c a t a l y t i c e l e c t r o d e s , such as p l a t i n u m o r rhodium, f i r s t proposed by H o r a n y i (,24) and f u r t h e r developed by Wieckowski (21,25,26), c a t e g o r i z e s a d s o r p t i o n p r o c e s s e s i n t o t h r e e fundamental groups: 1) s u r f a c e complexing p r o c e s s e s ; 2) p r o c e s s e s l e a d i n g t o f o r m a t i o n o f CO-type p r o d u c t s ; 3) a d s o r p t i o n p r o c e s s e s o f weakly i n t e r a c t i n g s p e c i e s . Two f i r s t two groups c a n b e s t be u n d e r s t o o d i n terms o f c h e m i s o r p t i o n w i t h e n e r g e t i c s , i r r e v e r s i b i l i t y and s u r f a c e d i f f u s i o n c h a r a c t e r i s t i c s analogous t o those known from c h e m i s o r p t i o n r e s e a r c h i n the gas phase ( 2 ) . They encompass both d i s s o c i a t i v e and n o n d i s s o c i a t i v e a d s o r p t i o n p r o c e s s e s . The d i s c r i m i n a t i o n between the f i r s t and t h e second group was p o s s i b l e (21,25,26) due t o t h e i d e n t i f i c a t i o n o f two s e p a r a t e p o t e n t i a l ranges f o r t h e v o l t a m m e t r i c e l e c t r o o x i d a t i o n o f r e s p e c t i v e chemisorption products. This i s , i n turn, r e l a t e d t o the d i f f e r e n c e s i n a d s o r p t i o n e n e r g i e s : f o r t h e f i r s t group between 30 and 40 k c a l / m o l e and f o r the second group, h i g h e r than 40 k c a l / m o l e . The h i g h a d s o r p t i o n energy a s s o c i a t e d w i t h these s u r f a c e r e a c t i o n s makes t h e c h e m i c a l c o n t r i b u t i o n t o t h e t h e i r o v e r a l l e n e r g e t i c s much more pronounced than the e l e c t r i c a l c o n t r i b u t i o n . Such systems, i n t h e i r c o m p o s i t i o n a l and s t r u c t u r a l a s p e c t s , c a n s a f e l y be s t u d i e d by t h e vacuum methods: any d e s o r p t i o n (or rearrangement) from the s u r f a c e d u r i n g s o l u t i o n / v a c u u m t r a n s f e r a t room t e m p e r a t u r e c a n be r u l e d out from e n e r g e t i c r e a s o n s . The p r o c e s s e s c l a s s i f i e d i n t h e t h i r d group a r e o f primary importance i n e l u c i d a t i n g the s i g n i f i c a n c e o f e l e c t r i c v a r i a b l e s i n e l e c t r o s o r p t i o n and i n the double l a y e r s t r u c t u r e a t s o l i d e l e c t r o d e s . These p r o c e s s e s encompass i n t e r a c t i o n s o f i o n i c components o f s u p p o r t i n g e l e c t r o l y t e s w i t h e l e c t r o d e s u r f a c e s and a d s o r p t i o n o f some o r g a n i c m o l e c u l e s such as s a t u r a t e d c a r b o x y l i c a c i d s and t h e i r d e r i v a t i v e s ( e x c e p t f o r f o r m i c a c i d ) . The s p e c i e s t h a t a r e concerned here are weakly adsorbed on p l a t i n u m and rhodium e l e c t r o d e s and t h e i r heat o f a d s o r p t i o n i s w e l l below 20 k c a l / m o l e ( 2 5 ) . Due t o the r e v e r s i b i l i t y and s i g n i f i c a n t m o b i l i t y o f such weakly adsorbed i o n s o r m o l e c u l e s , t h e a p p l i c a t i o n o f t h e in s i t u methods f o r t h e s u r f a c e c o n c e n t r a t i o n measurements i s more a p p r o p r i a t e than t h a t o f the vacuum


17.

WIECKOWSKI

Radioekctrochemistry at Well-Defined Electrodes

t e c h n i q u e s . The primary measurements t h a t can be done by the use of the r a d i o - e l e c t r o c h e m i s t r y r e g a r d i n g t h i s group o f processes are as f o l l o w s : - p o t e n t i a l dependence o f a d s o r p t i o n a t v a r i o u s metals and a t d i f f e r e n t c r y s t a l l o g r a p h i c planes o f a g i v e n m e t a l ; - a d s o r p t i o n isotherms and i s o t h e r m - r e l a t e d parameters such as f r e e energy o f a d s o r p t i o n and i n t e r a c t i o n parameters ( a g a i n related to surface crystallography); - dynamics o f t h e i n t e r f a c i a l processes i n c l u d i n g k i n e t i c s o f a d s o r p t i o n and d e s o r p t i o n and those o f m o l e c u l a r and atomic exchanges between the s p e c i e s i n the s u r f a c e a t t a c h e d and dissolved states; - steady-state c h a r a c t e r i s t i c s ; - m o l e c u l a r o r i e n t a t i o n o f molecules t h a t have more than one p o s s i b l e modes o f s u r f a c e attachment. To implement t h i s program o f measurements on w e l l - d e f i n e d e l e c t r o d e s , the method o f p r e p a r a t i o n o f c l e a n and w e l l - o r d e r e d s i n g l e c r y s t a l s u r f a c e s i s e s s e n t i a l . Such s u r f a c e s can be o b t a i n e d e i t h e r by the use o f u l t r a - h i g h vacuum o r "atmospheric" procedures, the l a t t e r methods w i l l b r i e f l y be d e s c r i b e d below. P r e p a r a t i o n o f W e l l - D e f i n e d E l e c t r o d e s f o r I n S i t u Measurements i n S u r f a c e E l e c t r o c h e m i s t r y . There are t h r e e methods o f p r e p a r a t i o n o f c a t a l y t i c , s i n g l e c r y s t a l e l e c t r o d e s t h a t do n o t r e q u i r e an u l t r a - h i g h vacuum environment: ( i ) c r y s t a l flame a n n e a l i n g f o l l o w e d by water quenching (27); ( i i ) c r y s t a l flame a n n e a l i n g f o l l o w e d by c o o l i n g i n gaseous hydrogen (£8) and ( i i i ) c r y s t a l e l e c t r i c a l a n n e a l i n g and c o o l i n g i n an i n e r t gas atmosphere c o n t a i n i n g i o d i n e vapor (29,30). To c r e a t e an i o d i n e - f r e e s u r f a c e , t h i s t h i r d method r e q u i r e s some a d d i t i o n a l s t e p s developed i n our l a b o r a t o r y : a replacement o f i o d i n e by carbon monoxide and anodic s t r i p p i n g o f chemisorbed CO has been proposed (3J_). C o n t r a r y t o the e l e c t r o o x i d a t i o n o f i o d i n e , t h e e l e c t r o o x i d a t i o n o f carbon monoxide does not cause s u r f a c e disorder. A voltammetric c h a r a c t e r i z a t i o n o f platinum s i n g l e c r y s t a l s u r f a c e s produced by these t h r e e methods shows t h a t a v e r y s i m i l a r s u r f a c e order and c l e a n l i n e s s are o b t a i n e d i n each case. The f e a t u r e s embodied by the t h i r d method which we use f o r t h e r a d i o - e l e c t r o c h e m i s t r y work, are as f o l l o w s : - up t o the stage o f I/CO r e p l a c e m e n t , the s i n g l e c r y s t a l p r e p a r a t i o n procedure was s t r i c t l y v e r i f i e d by LEED and Auger s p e c t r o s c o p y (29,30). A c l o s e correspondence between t h e s u r f a c e e l e c t r o c h e m i s t r y and the gas-phase s u r f a c e s c i e n c e o f p l a t i n u m has r e s u l t e d due t o t h i s e a r l i e r work. - slow c o o l i n g i n the i o d i n e atmosphere does n o t cause p o l y g o n i z a t i o n o f p l a t i n u m c r y s t a l r e p o r t e d by some a u t h o r s to t r o u b l e the quenching technique (32). F o r the a p p l i c a t i o n s r e q u i r i n g h i g h l e v e l o f s u r f a c e smoothness (as i n t h e case o f r a d i o - e l e c t r o c h e m i s t r y ) deformations o f the e l e c t r o d e m a t e r i a l must be a v o i d e d . - the c l e a n , i o d i n e - f r e e s i n g l e c r y s t a l s u r f a c e i s c r e a t e d i n the e l e c t r o l y t i c s o l u t i o n thus m i n i m i z i n g p o s s i b l e


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c o n t a m i n a t i o n s from the gaseous atmosphere b e f o r e the e l e c t r o c h e m i c a l steps are executed. - e x t e n s i o n o f the p r e p a r a t i o n o f s i n g l e c r y s t a l s u r f a c e s rhodium has become p o s s i b l e (J33). Aqueous quenching o f rhodium causes a f o r m a t i o n o f b l a c k s u r f a c e d e p o s i t and f u r t h e r w e l l - d e f i n e d work w i t h t h i s m e t a l d i f f i c u l t , i f impossible.

to hot makes not

Case S t u d i e s ; A d s o r p t i o n o f A c e t i c A c i d on P t ( 1 1 1 ) S i n g l e C r y s t a l E l e c t r o d e s . A c e t i c a c i d i s one o f a very few o r g a n i c compounds which i s r e v e r s i b l y adsorbed on p l a t i n u m a t room temperature (20,25). We r e p o r t below our r a d i o c h e m i c a l r e s u l t s on a d s o r p t i o n of t h i s compound on Pt(111) and on p o l y c r y s t a l l i n e Pt. The Pt(111) e l e c t r o d e was c h a r a c t e r i z e d by the low e l e c t r o n energy d i f f r a c t i o n i n the u l t r a - h i g h vacuum i n s t r u m e n t t o be d e s c r i b e d s h o r t l y . A proper hexagonal LEED p a t t e r n and i t s l o w - i n t e n s i t y background ( F i g u r e 3) i n d i c a t e a h i g h degree o f surface order. The c r y s t a l was d i s c o n n e c t e d from the UHV chamber and t r a n s f e r r e d v i a a i r t o the r a d i o c h e m i c a l c e l l , F i g u r e 1 (17). The c l e a n i n g and o r d e r i n g o f the s u r f a c e was performed a c c o r d i n g to the I/CO method d e s c r i b e d above and i n r e f . 31. Addition of 0.5 mM a c e t i c a c i d t o the p e r c h l o r i c a c i d s o l u t i o n produced the voltammogram shown i n F i g u r e 4. The f o r m a t i o n o f some new v o l t a m m e t r i c f e a t u r e s between 0.1 and 0.4 V and the s u p p r e s s i o n of the peaks between 0.3 and 0.6 V, c h a r a c t e r i s t i c f o r c l e a n p e r c h l o r i c a c i d e l e c t r o l y t e s h o u l d be n o t e d . S i m i l a r voltammetr i c behavior o f the P t ( l l l ) e l e c t r o d e has p r e v i o u s l y been observed a f t e r a d d i t i o n t o p e r c h l o r i c a c i d e l e c t r o l y t e of s u l f u r i c a c i d i n the m i l l i m o l a r range o f i t s bulk c o n c e n t r a t i o n (27).

The r a d i o c h e m i c a l r e s u l t s o b t a i n e d w i t h the P t ( l l l ) e l e c t r o d e i n the e l e c t r o l y t e c o n t a i n i n g C-14 l a b e l e d a c e t i c a c i d a r e shown in Figure 4 ( c i r c l e s ) . The a d s o r p t i o n i s n e g l i g i b l e at 0.15 V, s m a l l a t more n e g a t i v e p o t e n t i a l s and v e r y pronounced a t p o t e n t i a l s more p o s i t i v e than 0.15 V. I n the p o t e n t i a l range between 0.3 and 0.85 V, the r a d i o c h e m i c a l s i g n a l from the s u r f a c e , i . e . , the s u r f a c e c o n c e n t r a t i o n o f a c e t i c a c i d , e q u a t i o n 1, i s p o t e n t i a l independent. T h i s r e s u l t i s i n t e r e s t i n g s i n c e v e r y s t r o n g p o t e n t i a l e f f e c t s i n t h i s p o t e n t i a l range were found i n the case o f CH^COOH a d s o r p t i o n on p o l y c r y s t a l l i n e p l a t i n u m (see r e f s . 20, 25, 34 and F i g u r e 5 o f t h i s w o r k ) . I f the p r o d u c t s on both e l e c t r o d e s are the same, t h i s o b s e r v a t i o n would be i n d i c a t i v e o f an i n c r e a s e d s t a b i l i t y o f adsorbed a c e t i c a c i d on the w e l l - o r d e r e d , s i n g l e c r y s t a l e l e c t r o d e v s . the p o l y c r y s t a l l i n e s u b s t r a t e . I n g e n e r a l , such b e h a v i o r r e f l e c t s the p r o p e r t i e s o f i n t e r f a c i a l atoms and m o l e c u l e s which, when adsorbed on w e l l - d e f i n e d s u b s t r a t e s , tend t o c r e a t e s t a b l e and d i s t i n c t phases o f t w o - d i m e n s i o n a l p e r i o d i c i t y and order ( 3 5 ) . As shown i n F i g u r e 4, the i n c r e a s e i n a d s o r p t i o n o f a c e t i c a c i d on P t ( l l l ) o c c u r s i n the p o t e n t i a l range o f 0.15 t o 0.3 V, w h i c h r o u g h l y c o i n c i d e s w i t h p o s i t i o n o f the c u r r e n t - p o t e n t i a l peak of the voltammogram r e c o r d e d i n the same s o l u t i o n . An i n t e r d e p e n d e n c e can thus be sought between a c e t i c a c i d a d s o r p t i o n


WIECKOWSKI

Radioelectrochemistry at Well-Defined Electrodes

F i g u r e 3. LEED p a t t e r n o f the P t ( l l l ) e l e c t r o d e used f o r t h e r e p o r t e d r e s e a r c h . The beam energy was 65 eV.


252


-i

1

1

1

r

1

_2_}o.5mM CH3COOH

'S

2

o E

'O X

0.2 0.4 E/V (Ag/AgCI)

Figure 4. A combined figure representing the following r e l a t i o n s h i p s : c y c l i c voltammogram of the P t ( l l l ) electrode i n 0.1 M HCIO^ ( s o l i d l i n e ) ; c y c l i c voltammogram following addition of 0.5. mM CH3COOH to the 0.1 M HC10 e l e c t r o l y t e (broken l i n e ) ; surface concentration of adsorbed acetic acid plotted as a function of the electrode p o t e n t i a l ( s o l i d l i n e and c i r c l e s ) . (Reprinted with permission from r e f . 23. Copyright 1988 Elsevier.) 4


17. WIECKOWSKI


0.0

—i

0.4

0.8

1

1

_°-}

I

I

I

0.4

E/V

vs.

r

ImM CH3COOH

I

0.0

L2

—•

0,8

U

1.2

SCE

Figure 5. A combined figure representing the following r e l a t i o n s h i p s : c y c l i c voltammogram of the p o l y c r y s t a l l i n e Pt electrode i n 0.1 M HCIO^ ( s o l i d l i n e ) ; c y c l i c voltammogram following addition of 1 mM CH C00H to the 0.1 M HCIO^ e l e c t r o l y t e (broken l i n e ) ; surface concentration of adsorbed acetic acid plotted as a function of the electrode p o t e n t i a l ( s o l i d l i n e and c i r c l e s ) . (Reprinted with permission from r e f . 23. Copyright 1988 E l s e v i e r . ) 3


253

254


and t h e r e s p e c t i v e c u r r e n t - g e n e r a t i n g p r o c e s s . The s i m p l e s t explanation i s that adsorption of a c e t i c a c i d i s d i s s o c i a t i v e (36) and f o l l o w e d by t h e e l e c t r o n charge t r a n s f e r p r o c e s s , a c c o r d i n g t o e q u a t i o n 2: CH COOH 3

>

CH COO 3

ads

+ H

+

+ e"

(2)

C o n t r a r y t o t h e s i n g l e - c r y s t a l r e s u l t s d i s c u s s e d above, t h e e f f e c t of a d d i t i o n of a c e t i c acid t o the p e r c h l o r i c acid e l e c t r o l y t e on t h e voltammograms o f the p o l y c r y s t a l l i n e Pt (34) was i n s i g n i f i c a n t ( F i g u r e 5 ) . Only a s m a l l e x t r a charge i n t h e hydrogen range and an i n i t i a l s u p p r e s s i o n o f p l a t i n u m o x i d a t i o n were observed ( d o t t e d c u r v e ) v e r s u s t h e base voltammogram o b t a i n e d i n pure HClOj^ e l e c t r o l y t e ( s o l i d voltammogram). N o t i c e a b l y , t h e c u r r e n t - p o t e n t i a l curve i n t h e double l a y e r r e g i o n (from 0.1 t o 0.4 V) was u n a f f e c t e d by a c e t i c a c i d p r e s e n t in the e l e c t r o l y t e . I n t h e same p o t e n t i a l range, t h e s u r f a c e c o n c e n t r a t i o n o f a c e t i c a c i d was m o n o t o n i c a l l y i n c r e a s i n g , as shown by the r a d i o c h e m i c a l measurements ( c i r c l e s ) . We may t h e r e f o r e conclude that accumulation o f a c e t i c a c i d molecules a t the e l e c t r o d e s u r f a c e between 0.1 and 0.4 V was n o t a s s o c i a t e d w i t h a measurable e l e c t r o n charge t r a n s f e r p r o c e s s a c r o s s t h e interface. I n o t h e r words, a d s o r p t i o n o f a c e t i c a c i d on t h e p o l y c r y s t a l l i n e substrate i s associative. Comparison o f R e s u l t s O b t a i n e d w i t h P t ( l l l ) and P o l y c r y s t a l l i n e E l e c t r o d e s . We have shown above t h a t p o t e n t i a l dependence o f a d s o r p t i o n o f a c e t i c a c i d on P t ( l l l ) and on p o l y c r y s t a l l i n e P t i s d i f f e r e n t . However, t h e c e n t r a l q u e s t i o n whether a d s o r p t i o n o f CH^COOH can be d i s s o c i a t i v e on P t ( l l l ) ( e q u a t i o n 2) and a s s o c i a t i v e on t h e p o l y c r y s t a l l i n e Pt c a n n o t unambiguously be answered by t h e r a d i o c h e m i c a l and e l e c t r o c h e m i c a l s t u d i e s a l o n e (23). To a d d r e s s t h e i s s u e o f t h e "anomalous" s i n g l e - c r y s t a l voltammetry (£7), we have r e c e n t l y s t u d i e d a d s o r p t i o n o f s u l f a t e ( b i s u l f a t e ) a n i o n s on P t ( l l l ) (37). We found t h a t t h e s p e c i f i c a d s o r p t i o n model o f t h e anomalous b e h a v i o r (_38) would r e q u i r e an exchange o f t h r e e e l e c t r o n s between adsorbed a n i o n and the m e t a l . S i n c e t h i s appeared u n l i k e l y , we concluded t h a t t h e anomalous charge i s due t o a d s o r p t i o n o f h i g h - e n e r g y hydrogen, as o r i g i n a l l y proposed by C l a v i l i e r (2J). We have a l s o i n d i c a t e d the p o s s i b i l i t y t h a t t h e p o t e n t i a l range o f a d s o r p t i o n / d e s o r p t i o n o f t h i s h i g h - e n e r g y hydrogen can be c o n t r o l l e d by t h e e x t e n t o f c o a d s o r p t i o n w i t h hydrogen o f a n i o n s o r o t h e r e l e c t r o n - r i c h molecules. S i n c e t h e voltammetry o f the P t ( l l l ) e l e c t r o d e i n s o l u t i o n s c o n t a i n i n g a c e t i c a c i d and s u l f u r i c a c i d i s v e r y s i m i l a r , we c o n s e q u e n t l y c o n c l u d e t h a t d e s o r p t i o n o f h i g h - e n e r g y hydrogen i s a more p r o b a b l e s o u r c e o f e l e c t r i c c u r r e n t than t h e a c e t i c a c i d d i s c h a r g e r e p r e s e n t e d by e q u a t i o n 2. Whether such s t r o n g l y - b o u n d hydrogen i s p r e s e n t on t h e P t ( 1 1 1 ) s u r f a c e has been a s u b j e c t o f debate f o r some time. By a n a l y z i n g then e x i s t i n g d a t a on a d s o r p t i o n energy o f gas phase hydrogen on P t ( 1 1 1 ) , Wagner and Ross (39) concluded t h a t e l e c t r o c h e m i c a l a d s o r p t i o n o f hydrogen i s n o t e n e r g e t i c a l l y a l l o w e d t o occur i n t h e h i g h p o t e n t i a l range c l o s e l y p r e c e d i n g


17. W I E C K O W S K I


p l a t i n u m e l e c t r o o x i d a t i o n . However, some newer data i n d i c a t e t h a t a t l e a s t 10 - 15% o f a monolayer o f hydrogen desorbs a t a temperature o f 100 Κ h i g h e r than t h a t where most o f t h e monolayer hydrogen desorbs (^0). We b e l i e v e t h a t t h i s o b s e r v a t i o n g i v e s an a d d i t i o n a l support t o t h e o r i g i n a l i n t e r p r e t a t i o n o f t h e P t ( l l l ) voltammetry i n a c i d i c media ( 2 7 ) . We would a l s o l i k e t o note t h a t more d a t a r e g a r d i n g a d s o r p t i o n o f hydrogen on s i n g l e - c r y s t a l s u r f a c e s may soon be p r o v i d e d by t h e i n s i t u s p e c t r o s c o p i e s o f e l e c t r o c h e m i c a l s u r f a c e s c i e n c e . Recent p r o g r e s s i n s u r f a c e IR (13,41,42) and i n Raman second harmonic g e n e r a t i o n (15,43) i s p a r t i c u l a r l y p r o m i s i n g here. Acknowledgments T h i s work was supported by t h e N a t i o n a l S c i e n c e Foundation under Grant NSF DMR-86-12860 ( a d m i n i s t e r e d by the M a t e r i a l s Research L a b o r a t o r y o f the U n i v e r s i t y o f I l l i n o i s ) and by Dow Chemical U.S.A. Literature Cited

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Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177. Somorjai, G.A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, 1981. 3. Ishikawa, R. M.; Hubbard, A. T.; J. Electroanal. Chem. 1976, 69, 317. 4. Ross, R. Ν. J . Electroanal. Chem. 1977, 78, 139. 5. Yeager, E . ; O'Grady, E. W.; Woo, M. Y. C.; Hagans, P. J. Electrochem. Soc. 1978, 125, 346. 6. Hubbard, A. T. J . Vac. S c i . Technol. 1980, 17, 49. 7. Yeager, E. J . Electrochem. Soc. 1981, 128, 160c. 8. Ross, P. Ν.; Wagner, F. T. Adv. in Electrochem. and Electrochem. Eng; Gerisher, H . , Ed.; Wiley-Interscience, New York, 1984; Vol. 31, p. 69. 9. D'Agostino, A. T.; Hansen, W. N. Surf. S c i . 1986, 165, 268. 10. Kotz, E. R.; Neff, H.; Muller, K. J . Electroanal. Chem. 1986, 215, 331. 11. Kolb, D. M. Z. Phys. Chem. 1987, 154, 179. 12. Fleischmann, M. Advances in Electrochemistry; The Robert A. Welch Foundation Conferences on Chemical Research, 1986; No. 3-5, p. 91. 13. Bewick, Α.; Pons, S. Adv. in Infrared, Raman Spectr.; Clark, R. J. H.; Hester, R. E . , Ed.; Wiley Heyden: New York, 1985; Vol. 12, p. 1. 14. Chen, C. K.; Heinz, T. F.; Ricard, D; Shen, Y. R. Phys. Rev. Β 1983, 27, 1965. 15. Richmond, G. L. Langmuir 1986, 2, 132. 16. Sonnenfeld, R.; Hansma, P. K. Science 1986, 232, 211. 17. Krauskopf, Ε. K.; Chan, K.; Wieckowski, A. J . Phys. Chem. 1987, 91, 2327. 18. Kazarinov, V. E.; Andreev, V. N. Comprehensive Treatise of Electrochemistry; Yeager, E.; Bockris, J. O'M.; Conway, B. E.; Sarangapani, S., Ed.; Plenum: New York, 1984; Vol. 9, p. 393.


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Price, W. J. McGraw-Hill Series in Nuclear Engineering; Zinn, W. H . ; Luntz, J. D., Ed.; McGraw-Hill: New York, 1958; p. 131. 20. Horanyi, G. Electrochimica Acta 1980, 25, 43. 21. Krauskopf, E. K.; Wieckowski, A. Modern Aspects of Electrochemistry; submitted. 22. Wieckowski, A.; Krauskopf, E. K. Presented at the 194th National Meeting of the ACS, August 30-September 4, 1987, New Orleans, LA. 23. Rice, L . ; Krauskopf, E. K.; Wieckowski, A. J . Electroanal. Chem. in press. 24. Horanyi, G. J . Electroanal. Chem. 1974, 51, 163. 25. Wieckowski, A. Electrochimica Acta 1981, 26, 1121. Wieckowski, A.; Sobkowski, J.; Zelenay, P.; Franaszczuk, K. Electrochimica Acta 1981, 26, 1111. 26. Wieckowski, A. The Analysis of Electrochemical Adsorption Processes on Platinum; P. W. U . , Warsaw, Poland, 1984. 27. Clavilier, J . J . Electroanal. Chem. 1980, 107, 211. 28. Motoo, S.; Furuya, N. J . Electroanal. Chem. 1984, 172, 339. 29. Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Stickney, J . L . ; Hubbard, A. T. Inorg. Chem. 1984, 23, 565. 30. Wieckowski, A.; Schardt, B.C.; Rosasco, S. D.; Stickney, J . L . ; Hubbard, A. T. Surf. S c i . 1984, 146, 115. 31. Zurawski, D.; Rice, L . ; Hourani, M.; Wieckowski, A. J . Electroanal. Chem. 1987, 230, 221. 32. Aberdam, D.; Durand, R.; Faure, R.; El-Omar, F. Surf. S c i . 1986, 171, 303. 33. Hourani, M.; Wieckowski, A. J . Electroanal. Chem. 1987, 227, 259. 34. Corrigan, D. S.; Krauskopf, E. K.; Rice, L. M.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. in press. 35. Roelofs, L. D.; Estrup, P. J. Surf. S c i . 1983, 125, 51. 36. Avery, N. R. J . Vac. S c i . Technol. 1982, 20, 3. 37. Krauskopf, E. K., Rice, L. M., and Wieckowski, A. J . Electroanal. Chem., in press. 38. Al Jaaf-Golze, K.; Kolb, D. M.; Scherson, D. J . Electroanal. Chem. 1986, 200, 353. 39. Wagner, F. T.; Ross, P. N . , Jr. J . Electroanal. Chem. 1983, 150, 141. 40. Zhou, X . - L . ; White, J. M. Surf. S c i . 1987, 185, 450. 41. Leung, L-W. H. and Weaver, M. J. J . Electroanal. Chem., 1988, 240, 341. 42. Sun, S. G., Clavilier, J., and Bewick, A. J . Electroanal. Chem., 1988, 240, 147. 43. Campbell, D. J. and Corn, R. M. J . Phys. Chem., 1987, 91, 4668. R E C E I V E D May

17, 1988


Radioelectrochemistry at Well-Defined Electrodes - American

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