Electrochemistry: The Senior but Underused Area of Surface Science

Chapter 1

Electrochemistry: The Senior but Underused Area of Surface Science Manuel P. Soriaga

Downloaded by 185.89.101.64 on October 10, 2016 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch001

Department of Chemistry, Texas A&M University, College Station, TX 77843

Electrochemistry ought to be regarded as a vintage area of i n t e r f a c i a l science. It has long been known that the application of an external potential at a (conducting) s o l i d - s o l u t i o n interface enhances the r e a c t i v i t i e s of adjacent chemical species. Catalytic reactions activated by control of p o t e n t i a l at the electrode-solution interface thus serves as an alternative to activation by control of temperature at gas-solid interfaces. In fact, on the singular basis of c a t a l y t i c s e l e c t i v i t y , the electrochemical route towards synthesis and catalysis should be favored over the thermal approach because the application of elevated temperatures activates other reaction pathways. For a two-electron reaction, for example, a potential change of only half a volt can lead to a 10 -fold increase in reaction rate. Assuming an activation energy of 75 kJ m o l e -1, this enormous rate increase can be effected only at temperatures above 600 K. Such drastic conditions invariably lead to a wide spectrum of thermal excitations which could result in undesirable side-reactions such as indiscriminate bond-breaking. In addition to the universal concern for catalytic s e l e c t i v i t y , the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach: (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize c a t a l y t i c fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in e l e c t r o l y t i c solutions can be closely modelled after analogous wellcharacterized molecular or cluster complexes (1-8). (v) For hydrogenation (or oxidation) reactions, the aqueous solvent functions as a convenient source of hydrogen (or 8

0097H5156/88/0378-0001$06.00/0 ° 1988 American Chemical Society

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

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o x y g e n ) . ( v i ) B e c a u s e e l e c t r o c a t a l y s i s employs a r e d u c i n g and an oxidizing electrode, there is promise for implementation of the concept of p a i r e d e l e c t r o s y n t h e s i s


(A) It is true that electrochemistry has enjoyed tremendous successes i n the f i e l d s of chemical analysis ( s e n s o r s ) and e n e r g y c o n v e r s i o n (batteries). It i s also a fact that the use of electrochemistry has become w i d e s p r e a d i n modern-day i n o r g a n i c c h e m i s t r y where r e d o x p o t e n t i a l s o f newly s y n t h e s i z e d m a t e r i a l s are d e t e r m i n e d a l m o s t as r o u t i n e l y as s p e c t r o s c o p i c and c r y s t a l l o g r a p h i c d a t a . However, d e s p i t e i t s h i s t o r i c a l s i g n i f i c a n c e and despite a l l t h e a p p a r e n t advantages enumerated above, electrochemistry remains relegated to the background i n s o f a r as l a r g e - s c a l e c a t a l y t i c s y n t h e s i s i s c o n c e r n e d . T e c h n o l o g i c a l and e n g i n e e r i n g c o m p l e x i t i e s , s u c h as s l o w mass t r a n s p o r t i n s o l u t i o n and t h e c o s t o f e l e c t r i c i t y , are important f a c t o r s f o r the p e r s i s t e n t n e g l e c t of the e l e c t r o c h e m i c a l approach. Another e q u a l l y c r i t i c a l reason i s the absence of a t o m i c - l e v e l d e s c r i p t i o n s of processes which occur at the e l e c t r o d e - s o l u t i o n i n t e r f a c e . T h i s l a c k o f f u n d a m e n t a l i n f o r m a t i o n can be a t t r i b u t e d t o t h e f a c t that, until these past two decades, conventional electrochemical methods (10), s u c h as voltammetric, amperometric, coulometric, impedance, and transient m e a s u r e m e n t s , were t h e o n l y t o o l s r e a d i l y a v a i l a b l e f o r the study of e l e c t r o d e processes; these techniques are severely restricted i n the type of information they provide since the results obtained from them are m a n i f e s t a t i o n s of only the macroscopic p r o p e r t i e s of the electrode-solution interface. The recognition of the lack of molecular-level i n f o r m a t i o n on e l e c t r o c a t a l y t i c phenomena and t h e r e s o l v e t o have t h i s weakness r e c t i f i e d i s now f i r m l y e s t a b l i s h e d within the • e l e c t r o c h e m i c a l and surface science c o m m u n i t i e s . T h i s i s e v i d e n c e d by r e c e n t i n t e r n a t i o n a l conferences and workshops devoted solely to e l e c t r o c h e m i c a l problems (11-4). I t i s a l s o w e l l - r e a l i z e d t h a t e l e c t r o c h e m i s t r y cannot advance f u r t h e r u n l e s s the practitioners in this f i e l d begin t o view electrodes o l u t i o n p r o c e s s e s from a m o l e c u l a r - l e v e l p e r s p e c t i v e . The c e n t r a l i s s u e w h i c h has t o be a d d r e s s e d i n any c o m p r e h e n s i v e s t u d y o f e l e c t r o d e - s u r f a c e phenomena i s t h e determination of an unambiguous c o r r e l a t i o n between interfacial composition, interfacial structure, and interfacial reactivity. This p r i n c i p a l concern i s of course i d e n t i c a l to the goal of fundamental s t u d i e s i n h e t e r o g e n e o u s c a t a l y s i s a t g a s - s o l i d i n t e r f a c e s . However, e l e c t r o c h e m i c a l s y s t e m s a r e f a r more c o m p l i c a t e d since a f u l l t r e a t m e n t o f t h e e l e c t r o d e - s o l u t i o n i n t e r f a c e must i n c o r p o r a t e n o t o n l y t h e compact ( i n n e r ) l a y e r b u t a l s o the boundary (outer) l a y e r o f the e l e c t r i c a l d o u b l e - l a y e r . The e f f e c t o f t h e o u t e r l a y e r on e l e c t r o d e r e a c t i o n s has been n e g l e c t e d i n most s u r f a c e e l e c t r o c h e m i c a l s t u d i e s but i n c e r t a i n s i t u a t i o n s , s u c h as i n c o n d u c t i n g p o l y m e r s and


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b i o l o g i c a l systems, processes w i t h i n t h e boundary l a y e r can e x e r t n o n - t r i v i a l i n f l u e n c e s . In the surface electrochemist s pursuit of fundamental s t r u c t u r e - c o m p o s i t i o n - r e a c t i v i t y c o r r e l a t i o n s , l e s s o n s c a n be a n d have been l e a r n e d f r o m t h e s u c c e s s e s a c h i e v e d i n b o t h h e t e r o g e n e o u s a n d homogeneous c a t a l y s i s . F o r example, e x t e n s i v e work on f u e l c e l l e l e c t r o d e s ( 1 5 2A) h a s a c c o m p l i s h e d much t o show t h a t c o n c e p t s d e v e l o p e d i n t h e s t u d y o f oxygen r e d u c t i o n by P t p a r t i c l e s i n g a s s o l i d reactions are applicable t o the electrochemistry of H2 a n d O2 . T h e e s s e n t i a l i n g r e d i e n t s in experimental s t r a t e g i e s w h i c h have c o n t r i b u t e d t o t h e m o l e c u l a r - l e v e l u n d e r s t a n d i n g o f a wide v a r i e t y o f homogeneous c a t a l y t i c processes i n c l u d e (25-6) : ( i ) t h e u s e o f p u r e a n d w e l l characterized starting materials, ( i i ) s t r u c t u r a l and compositional a n a l y s i s o f homogeneous c a t a l y s t s , ( i i i ) structural and compositional analysis o f important intermediates, (iv) detailed kinetic measurements i n c l u d i n g t h e c o n c e n t r a t i o n o f c a t a l y s t p r e c u r s o r s , and (v) quantitative analysis of reaction product distributions. T h e s e e l e m e n t s , w h i c h h a v e now become r o u t i n e i n model s t u d i e s o f h e t e r o g e n e o u s c a t a l y s i s a t g a s - s o l i d i n t e r f a c e s , a r e j u s t b e g i n n i n g t o be a p p r e c i a t e d in surface electrochemical investigations. S t u d i e s o f t h e p h y s i c s and c h e m i s t r y a t t h e g a s - s o l i d interface have e n j o y e d tremendous advances from t h e systematic a p p l i c a t i o n o f modern s u r f a c e spectroscopic methods (27.) , s u c h a s l o w - e n e r g y e l e c t r o n d i f f r a c t i o n f o r surface c r y s t a l l o g r a p h i c determinations, Auger e l e c t r o n spectroscopy f o r surface elemental analysis, X-ray photoelectron spectroscopy f o r surface bonding studies, high-resolution electron-energy loss spectroscopy f o r s u r f a c e v i b r a t i o n a l i n f o r m a t i o n , and t h e r m a l desorption mass s p e c t r o m e t r y f o r a d s o r p t i o n e n t h a l p y measurements. T h e s e p o w e r f u l t e c h n i q u e s have p r o v i d e d r e v e a l i n g g l i m p s e s of g a s - s o l i d i n t e r f a c i a l processes at t h e atomic l e v e l . Unfortunately, they c a n n o t be e m p l o y e d t o p r o b e t h e e l e c t r o d e - e l e c t r o l y t e i n t e r f a c e under r e a c t i o n c o n d i t i o n s s i n c e s u r f a c e c h a r a c t e r i z a t i o n h a s t o be p e r f o r m e d o u t s i d e the electrochemical cell. Doubts have been raised concerning the validity of correlating structural i n f o r m a t i o n a c q u i r e d i n t h e a b s e n c e o f an e l e c t r o c h e m i c a l environment with r e a c t i v i t y data o b t a i n e d under p o t e n t i a l c o n t r o l . To h e l p m o l l i f y t h e s e c o n c e r n s , i t has been pointed out that fruitful structure-reactivity c o r r e l a t i o n s have been a c c o m p l i s h e d i n homogeneous systems (25-6) d e s p i t e t h e f a c t t h a t s t r u c t u r a l d e t e r m i n a t i o n s a r e done i n t h e s o l i d s t a t e w h i l e t h e r e a c t i v i t y measurements a r e p e r f o r m e d i n s o l u t i o n . R e s u l t s from r e c e n t s t u d i e s o f emersed e l e c t r o d e s (12,28-9) h a v e a l s o d i s p e l l e d most o f t h e e a r l i e r doubts. A r e c e n t workshop o r g a n i z e d by t h e N a t i o n a l R e s e a r c h C o u n c i l h a s recommended t h a t t h e u s e o f h i g h - v a c u u m s u r f a c e p h y s i c s methods s h o u l d b e c o n t i n u e d (12.) . The u t i l i z a t i o n o f such techniques i n surface electrochemistry was i n i t i a l l y restricted t o only a


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h a n d f u l o f l a b o r a t o r i e s ( 3 0 - 2 ) b u t t h e number h a s now i n c r e a s e d . I t i s p a r t i c u l a r l y encouraging t o note t h a t s u r f a c e s c i e n c e l a b o r a t o r i e s which p r e v i o u s l y devoted a l l e f f o r t s t o g a s - s o l i d i n t e r f a c e s now a p p o r t i o n s i g n i f i c a n t t i m e t o t h e s t u d y e l e c t r o c a t a l y t i c phenomena (33-4). O p t i c a l s p e c t r o s c o p i c methods n o t r e s t r i c t e d t o h i g h vacuum c o n d i t i o n s , s u c h a s i n f r a r e d r e f l e c t i o n - a b s o r p t i o n s p e c t r o s c o p y (25.) , e l l i p s o m e t r y (2£) , and s u r f a c e - e n h a n c e d Raman s p e c t r o s c o p y ( 3 7 - 8 ) , a r e a v a i l a b l e f o r e x a m i n i n g i n t e r m e d i a t e s a t t h e 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 under r e a c t i o n c o n d i t i o n s . However, none o f t h e s e c a n d e t e r m i n e the complete structure and composition of the electrocatalyst-adsorbate interfacial aggregate. The c a p a b i l i t i e s o f l a s e r [as i n s e c o n d - h a r m o n i c g e n e r a t i o n ( 3 9 - 4 0 ) ] a n d s y n c h r o t r o n r a d i a t i o n [as i n n e a r - e d g e a n d extended X-ray absorption fine structure (4_L) ] i n e l u c i d a t i n g i n t e r f a c i a l s t r u c t u r e s under 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 have r e c e n t l y been demonstrated; research i n these areas have been i n t e n s i f i e d . Present vigorous activities i n s u r f a c e e l e c t r o c h e m i c a l s t u d i e s have i n c l u d e d t h e a d a p t a t i o n o f new t e c h n o l o g i e s , s u c h a s scanning t u n n e l i n g microscopy (42-3), and t h e r e v i v a l o f c o n v e n t i o n a l methods, s u c h as M o s s b a u e r s p e c t r o s c o p y (££), n u c l e a r m a g n e t i c r e s o n a n c e , and X - r a y d i f f r a c t i o n ( 4 5 ) . In v i e w o f t h e c o m p l e x i t y o f h e t e r o g e n e o u s systems, none o f t h e above t e c h n i q u e s w i l l be a b l e t o s u p p l y , by i t s e l f , a complete atomic-level description of surface phenomena. A m u l t i - t e c h n i q u e a p p r o a c h h a s b e e n p e r c e i v e d by many a s most a p p r o p r i a t e f o r f u n d a m e n t a l studies i n 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 ( 3 0 - 2 ) . S i n c e none o f t h e existing electrochemical laboratories are adequately equipped t o perform a comprehensive experimental study, c o l l a b o r a t i v e e f f o r t s between r e s e a r c h g r o u p s o f d i f f e r e n t expertise are burgeoning. E a s i e r access t o n a t i o n a l or central facilities are also being contemplated f o r experiments which cannot be p e r f o r m e d elsewhere. The judicious combination of the available methods i n conjunction with the appropriate electrochemical measurements a r e p e r m i t t i n g s t u d i e s o f e l e c t r o c a t a l y s t s u r f a c e phenomena u n p a r a l l e l e d i n m o l e c u l a r d e t a i l . I n a n y s c i e n t i f i c e n d e a v o r , mere a c c u m u l a t i o n o f e m p i r i c a l i n f o r m a t i o n i s n o t enough t o g u a r a n t e e major advances. Efforts have t o be e x e r t e d t o f o r m u l a t e f u n d a m e n t a l c o n c e p t s from t h e a v a i l a b l e d a t a . F o r example, i t i s no l o n g e r s u f f i c i e n t t o d e s c r i b e how an a d s o r b e d s p e c i e s i s bound t o t h e s u r f a c e s i m p l y i n terms o f i t s i n t e r f a c i a l o r i e n t a t i o n . I t i s e s s e n t i a l t o p r o c e e d beyond such phenomenological i l l u s t r a t i o n s a n d a c t u a l l y examine t h e n a t u r e o f t h e s u r f a c e - a d s o r b a t e c h e m i c a l bond. I t i s i n t h i s a s p e c t t h a t c o n c e p t s from t h e r e l a t e d a r e a s o f c o o r d i n a t i o n a n d o r g a n o m e t a l l i c c h e m i s t r y h a v e b e e n most m e a n i n g f u l . F o r example, i n t h e s o - c a l l e d s u r f a c e - c l u s t e r analogy (1-4) , b o n d i n g i n chemisorption systems i s modelled a f t e r bonding i n m o l e c u l a r o r c l u s t e r complexes. S k e p t i c i s m t o t h e s u r f a c e - c l u s t e r analogy has been r a i s e d



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because o f t h e w i d e s p r e a d knowledge t h a t w h i l e metal c o m p l e x e s c a n be d e s c r i b e d b y d i s c r e t e o r b i t a l s , m e t a l s u r f a c e s a r e u s u a l l y c h a r a c t e r i z e d by band s t r u c t u r e . However, what i s n o t known t o a s many i s t h e f a c t t h a t t h e s u r f a c e b a n d s t r u c t u r e c a n a c t u a l l y be d e c o n v o l u t e d into components o f g i v e n symmetries w i t h r e s p e c t t o a g i v e n s i t e ( 3 - 4 ) . I n t h i s manner, t h e s u r f a c e - a d s o r b a t e b o n d c a n be d e s c r i b e d i n t e r m s o f t h e o v e r l a p o f s y m m e t r y - a d a p t e d s u r f a c e bands and a d s o r b a t e o r b i t a l s ; t h i s d e s c r i p t i o n i s e x a c t l y t h e same a s t h a t o f t h e m e t a l - l i g a n d b o n d i n monometal o r s m a l l c l u s t e r complexes. With t h e experimental and t h e o r e t i c a l s t r a t e g i e s a v a i l a b l e today, research i n electrochemical surface s c i e n c e has been r e v i v e d . There i s g r e a t o p t i m i s m t h a t much o f t h e m y s t e r i e s s u r r o u n d i n g e l e c t r o c a t a l y s i s w i l l soon be unravelled i n molecular detail hitherto unachievable. Literature Cited

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20. Stonehart, P.; Kinoshita, K . ; Bett, J.S. In Electrocatalysis; Breiter, M.W., E d . ; The Electrochemical Society: Princeton, NJ, 1974. 21. Blurton, K . F . ; Greenberg, P . ; Oswin, H . G . ; Rutt, D.R. J . Electrochem. Soc. 1972, 119, 559. 22. Bett, J . S . ; Lundquist, J.; Washington, E.; Stonehart, P. Electrochim. Acta. 1979, 18, 343. 23. Bregoli, L.J. Electrochim. Acta. 1978, 23, 489. 24. Peuckert, M . ; Yoneda, T . ; Dalla Betta, R . A . ; Boudart, M. J . Electrochem. Soc. 1986, 133, 944. 25. Schrauzer, G.N. Transition Metals i n Homogeneous Catalysis; Marcel Dekker: New York, 1971. 26. Cotton, F . A . ; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1980. 27. Somorjai, G.A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981. 28. Hansen, W.N. J. Electroanal. Chem. 1983, 150, 133. 29. Stickney, J.L.; Rosasco, S.D.; Salaita, G.N.; Hubbard, A.T. Langmuir. 1985, 1, 66. 30. Hubbard, A . T . Accts. Chem. Res. 1980, 13, 177. 31. Ross, P.N. In Chemistry and Physics of Solid Surfaces; Vaneslow, R.; Howe, R., Eds.; Springer-Verlag: NY, 1982.

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41. Gordon, J.G.; Melroy, O.R.; Borges, G . L . ; Reisner, D . L . ; Abruña, H . D . ; Chandrasekhar, P . ; A l b a r e l l i , M . J . ; Blum, L . J. E l e c t r o a n a l . Chem. 1986, 210, 311. 42. L i n , W.; Fan, F . - R . F ; Bard, A.J. J. Electrochem. Soc. 1987, 134, 1038. 43. Sonnefield, R.; Hansma, P.K. Science. 1986, 232, 211. 44. Scherson, D . A . ; Yao, S.B.; Yeager, E.; Eldridge, J.; Kordesch, Μ.Ε.; Hoffman, R.W. J. Electroanal. Chem. 1983, 150, 535. 45. Fleischman, M . ; Graves, P . ; Hill, I . ; O l i v e r , Α . ; Robinson. J . J. Electroanal. Chem. 1983, 150, 33.


R E C E I V E D June 27, 1988


Electrochemistry: The Senior but Underused Area of Surface Science

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