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Z Electroanal. Chem., 351 (1993) 329-335Elsevier Sequoia S.A., LausanneJ E C 0 2 6 1 1S h o r t c o m m u n i c a t i o nIR spectroscopy as a probe of bisulfate absorption atplatinum electrodes under aqueous electrochemicalc o n d i t io n s : e l im i n a t io n o f b u l k i n t e r f e r e n c e b y u s i n g a t h i nlayer flow cellV.B . P au l i s s en an d C . K o r zen i ew s k i *Department of Chemistry, The University o f Michigan, An n Arbor, M I 48109 (USA )(Received 15 June 1992; in revised form 23 November 1992)

INTRODUCTION

I n r e c e n t y e a r s , s e v e ra l w o r k e r s h a v e e x a m i n e d t h e u t il it y o f I R s p e c t r o s c o p y t op r o b e i n s i t u t h e s p ec i f i c ad s o r p t i o n o f e l ec t r o l y t e an i o n s a t m e t a l e l e c t r o d es[ 1 -1 0 ] . T h e r e i s i n t e r e s t i n ex am i n i n g t h e b eh av i o r o f e l e c t r o l y t e an i o n s a t m e t a le l ec t r o d es t o g a i n an i n s i gh t i n to t h e s t r u c t u r e o f t h e e l ec t r i c a l d o u b l e l ay e r an d i tse f f ec t o n e l ec t r o ch e m i ca l r e ac ti v it y . A n ea r l y s e r i e s o f I R ex p e r i m en t s a i m e d a tp r o b i n g e l ec t r o l y t e ad s o r p t i o n ex am i n ed t h e i n t e r ac t i o n o f s u l f a t e an d b i s u l f a t e a tp o l y c r y s ta l l in e p l a t i n u m e l ec t r o d es u n d e r aq u eo u s e l ec t ro ch em i ca l co n d i t i o n s [ 3 - 6 ].T h e s e s t u d i e s e x p l o r e d e l e c t r o l y t e a d s o r p t i o n o v e r a w i d e r a n g e o f e l e c t r o d ep o t e n t i a l a n d s o l u t i o n p H v a l u e s . P o t e n t i a l - d e p e n d e n t a l t e r a t i o n s i n t h e p o s i t i o no f s u lf a t e an d b i s u l f a t e s p ec t r a l f e a t u r e s w e r e o b s e r v ed an d t h e v i b r a t io n a l b an d sw e r e a s s ig n e d t o m o d e s o f a d s o r b e d s u r fa c e s p e c i e s o n t h e b a s is o f t h e p e r t u r b a -t io n s o b s e r v e d [3 - 6 ]. M o r e r e c e n t I R e x p e r i m e n t s e x a m i n e d b i s u l fa t e a d s o r p t i o n a tP t ( l l l ) o v e r t h e d o u b l e l a y e r a n d h y d r o g e n a d s o r p t i o n p o t e n t ia l r e g i o n s a n dr ep o r t ed b i s u l f a t e s p ec t r a l p e r t u r b a t i o n s s i m i l a r t o t h o s e o b s e r y ed i n ex p e r i m en t st h a t em p l o y ed p o l y c r y s t a l l i n e p l a t i n u m [ 1 0 ] .

S t u d i e s w h i ch a i m e d t o d e f i n e t h e p r ac t i c a l an a l y t ic a l li m i ts o f t h e i n s i t u I Rt e c h n i q u e h a v e i n d i c a t e d t h e i m p o r t a n c e o f c o n si d e ri n g t h e e f f e c t o f p o t e n ti a l -d e -p e n d e n t a l t e ra t i o n s i n t h e c o m p o s i t io n o f t h e t h in l a y e r c av it y o n t h e a p p e a r a n c eo f t h e s u l f a t e an d b i s u l f a t e s p ec t r a l f e a t u r e s [ 7 - 9 ]. I n t h e t h i n l ay e r s p ec t r o e l ec t r o -

* To whom correspondence should be addressed.0022-0728/93/$06.00 1993 - Elsevier Sequoia S,A. All rights reserved

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c h e m i c a l c e l l s e m p l o y e d f o r t h e i n s i t u I R m e a su r e m e n t s , t h e wo r k i n g e l e c t r o d e i sp o s i t i o n e d f i rm l y a g ai n s t a w i n d o w o f I R t r a n sp a r e n t m a t e r i a l, w i t h th ee l e c t r o d e - w i n d o w d i s t a n c e b e i n g o f t h e o r d e r o f a f e w m i c r o m e t e r s [1 ,1 1,1 2]. T h een t rap pe d so lu t ion l aye r is su f f i c ien t ly th in tha t , fo r man y spec ie s , t r ansp or tb e t w e e n t h e t h i n l a y e r ca v it y a n d t h e b u l k so l u t io n i s s lo w o n t h e t i m e s c a l e o f th eI R e x p e r i m e n t . A s a c o n se q u e n c e , t h e c o m p o s i t io n o f th e so l u t io n w i th i n t h e t h i nl a y e r c a v it y i s a l t e r e d b y c h a n g e s i n t h e e l e c t r o d e p o t e n t ia l . I R s t u d i e s a i m e d a tp r o b i n g a n i o n i c a d so r p t i o n h a v e n o t e d c o n d i t i o n s u n d e r wh i c h p o t e n t i a l - d e p e n -d e n t a l t e ra t i o n s i n t h e p H v a l u e a n d i o n i c c o m p o s i t i o n o f t h e t h i n l a y e r c a v it y a r eo b se r v e d [ 2 ,7 - 9 ]. S p e c t r a l f e a t u r e s a s so c i a t e d w i t h t h e se so l u t i o n sp e c i e s a p p e a r i np o t e n t i a l - d i f f e r e n c e I R sp e c t r a a n d c a n m a k e i t d i f f i c u l t t o d i s c e r n f e a t u r e s o fspec ie s in the in t e r rac i a l r eg ion . Indeed , t hese s tud ie s have re fu ted the va l id i ty o fa s s i g n i n g I R b a n d s t o a d so r b e d su l f a t e a n d b i su l f a t e , a n d h a v e c a l l e d f o r e x p e r i -m ent s w hich u t i l ize a t h in l aye r f low ce i l [13 ,14] wh ich pe rm i t s t h e s o lu t ion in thet h i n l a y e r c av i ty t o b e r e p l e n i sh e d c o n t in u o u s l y , t h e r e b y k e e p i n g t h e c o m p o s i t io nof the so lu t ion l aye r cons tan t [9 ] .

I n t h is c o m m u n i c a t i o n , w e r e p o r t t h e f ir s t I R sp e c t r a o f b i su lf a t e a t a p l a ti n u me l e c t r o d e u n d e r a q u e o u s 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 o b t a i n e d u s i n g a t h i n l a y e rspec t roe lec t ro chem ica l f low ce l l, wh ich a l lows d i rec t con t ro l o f t he so lu t ion com-pos i t ion w i th in the th in l aye r cav i ty by fo rc ing so lu t ion th roug h a sma l l open in g int h e c e n t e r o f t h e I R t r a n sp a r e n t w i n d o w [ 1 4] . T h e p o t e n t i a l - d i ff e r e n c e I R sp e c t r ash o w a l t e r a t i o n s i n t h e p o s i t i o n a n d i n t e g r a t e d i n t e n s i t y o f t h e b i su l f a t e b a n d ,wh i c h i s c o n s i s t e n t w i t h t h e e a r l y s t u d i e s wh i c h p r o b e d b i su l f a t e a d so r p t i o n a tpo lyc rys t a l li ne [3 -6 ] an d mo nocrys t a l l ine [10] p l a t inum e lec t rodes . The re su l t s o ft h e p r e se n t e x p e r i m e n t s i n d i c a t e th a t sp e c t r a f r o m e a r l i e r s t u d i e s o f b i su l f a tea d so r p t i o n [ 3 - 6 ,1 0 ] c o n t a i n i m p o r t a n t i n f o r m a t i o n r e g a r d i n g t h e c o m p o s i t i o n a n db o n d i n g o f e le c t r o l y te a n i o n s a t t h e i n te r f a ce .EXPERIMENTAL

The expe r iment s invo lve a spec t roe lec t rochemica l f low- th rough ce l l s imi l a r i nd e s i g n t o o n e p r e v i o u s ly d e sc r i b e d b y R o t h a n d W e a v e r [ 14 ] . T h e c e ll w i n d o w w a sa c a lc i u m f l u o r i d e t r a p e z o i d w i th a sm a l l ( 0 .9 m m ) h o l e d r i ll e d t h r o u g h t h e c e n t e r( So l o n T e c h n o l o g i e s , So l o n , OH) i n t o wh i c h a p o l y e t h y l e n e t u b e wa s e p o x i e d . T h eth in l aye r cav i ty was pu rge d co n t inuou s ly wi th an e l ec t ro ly t e so lu t ion o f 0 .05 MH 2 S O 4 ( B a k e r U l t r e x H 2 S O 4 w i t h B a r n s t e a d N a n o p u r e w a t e r ) a t a f lo w r a te o f 0 .4m l m i n - 1 . T h e wo r k i n g e l e c t r o d e w a s a p o l y c r y s ta l li n e p l a ti n u m d i sk 6 m m i nd i a m e t e r s e a l e d i n a g l a s s t u b e a n d p o l i sh e d w i t h a l u m i n a ( w i t h p a r t i c l e s i z e s o f1 .0 , 0 .3 and 0 .05 /zm) to a mi r ro r f in i sh . In the expe r iment s , a sa tu ra t ed ca lomelr e f e r e n c e e l e c t r o d e ( S C E ) a n d a p l a t i n u m w i r e c o u n t e r e l e c t r o d e w e r e u s e d . T h eI R sp e c t r a we r e o b t a i n e d u s i n g a D i g i l a b FT S- 4 0 I R sp e c t r o m e t e r . A t o t a l o f 1 0 2 4i n t e r f e r o g r a m s we r e c o l l e c t e d a t e a c h p o t e n t i a l a n d t h e p o t e n t i a l wa s sw i t c h e db e t w e e n t h e r e f e r e n c e a n d s a m p l e e v e r y 6 4 s c an s . T h e r e f e r e n c e p o t e n t ia l w a s- 0 . 2 5 V . A l l t h e p o t e n ti a ls a r e r e p o r t e d w i t h r e s p e c t t o t h e S C E .

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Fi g u r e 1 sh o ws a c y c li c v o l t a m m o g r a m o f t h e p o l y c r y s ta l li n e p la t i n u m e l e c t r o d ei m m e r se d i n 0 .0 5 M H 2 S O 4 in t h e t h i n l a y e r f lo w c e ll w i th t h e e l e c t r o d ep o s i t io n e d a w a y f r o m t h e I R w i n d ow . T h e v o l ta m m o g r a m s w e r e r e c o r d e d j u s tp r i o r t o c o l le c t io n o f t h e sp e c t r a l d a t a . T h e p o t e n t i a l - d i ff e r e n c e I R sp e c t r ao b t a i n e d u n d e r c o n s t a n t f l o w c o n d i t i o n s a n d u n d e r s t a g n a n t t h i n l a y e r c o n d i t io n sa re shown in F igs . 2 and 3 re spec t ive ly . Cons i s t en t wi th ea r l i e r s tud ie s [3 -6 ,10] , ap o t e n t i a l - d e p e n d e n t sp e c t r a l f e a t u r e is o b se r v e d n e a r 1 20 0 c m - 1 a n d i s a s s i g n e d t ot h e a s y m m e t ri c S - O s t re t ch i n g m o d e o f a d s o r b e d b i s u lf a te . I n t h e s p e c t r a o b -t a i n e d u n d e r f l o w c o n d i t io n s , t h e a d s o r b e d b i su l f a te sp e c t ra l f e a t u r e s a p p e a r t o b es l i g h t l y n a r r o we r a n d a r e p o s i t i o n e d a t a h i g h e r e n e r g y l e v e l c o m p a r e d w i t h t h eb i su l f a t e sp e c t r a l f e a t u r e s o b t a i n e d u n d e r s t a g n a n t t h i n l a y e r c o n d i t i o n s . T h e sed i f fe r e n c e s a r e m o r e a p p a r e n t w h e n t h e s p e c t r a o b t a in e d u n d e r t h e t w o d i ff e r e n tc o n d i t i o n s a r e p l o t t e d t o g e t h e r , a s sh o wn i n F i g . 4 . T h e d i f f e r e n c e s a r e a t t r i b u t e dt o p o t e n t i a l - d e p e n d e n t a l t e ra t i o n s i n t h e t h i n l a y e r c a vi ty e le c t r o l y te c o m p o s i t i o n ,wh i c h o c c u r s u n d e r s t a g n a n t c o n d i t i o n s , an d d e m o n s t r a t e s t h e sp e c t r a l p e r t u r b a -t io n s e n c o u n t e r e d wh e n t h e c o m p o s i t i o n o f t h e t h i n l a y e r c a v it y is n o t s t ri c tl ym a i n t a i n e d .

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F i g . 1 . C y c l ic v o l t a m m o g r a m o f a p o l y c ry s t a ll in e p l a t i n u m e l e c t r o d e i m m e r s e d i n 0 . 0 5 M H 2 S O 4 j u s tp r i o r t o c o l le c t i o n o f s p e c t r a l d a t a . A s c a n r a t e o f 1 0 0 m V s - 1 w a s e m p l o y e d .

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(a )

0 . 0 V0 . 2 V

0.;3 V0 . 4 V

0 . 5 V

0.6V0 . 7 V

0 . 8 V

1 5 0 0 1 2 0 0W a v e n u m b e nt ! c m - 1

0 .0 V' : o . 2 v

/ 0 . 4 VI ~ 0 . 5 V0.6V0 . 7 V

0 . 8 V

b) 1300 12'00W ~ v e nu m b e m / c m - 1Fig. 2. Potential difference spectra of platinum in 0 .05 M H2SO 4 obtained with a flow rate of 0.4 mlm in -i. (a) unsm oothed spectra; (b) spectra shown in part (a) sm oothed with an 11-pointSavitsky-Golayfunction. The background spectrum was collected at -0.2 5 V. Spectra are displayed in units of A R /R .

F i g u r e 5 sh o w s t h e p o t e n t i a l d e p e n d e n c e o f th e b a n d p o s i t io n a n d i n t e g r a t e di n t e n s i t y f o r t h e s p e c t r a o b t a i n e d w i t h a c o n s t a n t e l e c t r o l y t e f lo w . A t p o t e n t i a l sb e t w e e n 0 .0 a n d 0 . 5 V , t h e p o s i t i o n o f th e b i s u l f a t e b a n d c o m p a r e s f a v o r a b l y w i t ht h e s p e c t r a l f e a t u r e s o b s e r v e d i n s t u d i e s w h i c h p r o b e d b i s u l f a t e a d s o r p t i o n f r o md i l u t e s u l fu r i c a c id e l e c t r o l y t e s o l u t io n s a t p o l y c r y s t a ll i n e a n d m o n o c r y s t a I l i n ep l a t i n u m [4 ,1 0 ] u n d e r s t a g n a n t t h i n l a y e r c o n d i ti o n s . I n t h e p r e s e n t s t u d y , t h ep o s i t i o n o f t h e b i s u l f a t e b a n d b e g i n s t o s h i ft b a c k t o w a r d i t s b u l k s o l u t io n v a l u e a t0 .6 V , c o i n c i d e n t w i th t h e o n s e t o f s u r f a c e o x i d a t i o n . I n e a r l i e r p o l y c r y s t a ll i n e [4 ]a n d m o n o c r y s t a l l i n e [ 10 ] s t u d i es , t h i s s h if t c o m m e n c e s a t h i g h e r p o s i ti v e p o t e n t ia l s .T h e t u n i n g r a t e w a s d e t e r m i n e d t o b e 8 7 c m - 1 V - 1 i n t h e p r e s e n t s tu d y , a sc o m p a r e d w i th t h e v a lu e s o f 10 0 c m - 1 V - 1 a n d 1 08 c m - 1 V - 1 r e p o r t e d i n e ar l i e rp o l y c r y s t a l l i n e [ 4 ] a n d m o n o c r y s t a U i n e [ 1 0 ] b i s u l f a t e a d s o r p t i o n s t u d i e s r e s p e c -t ive ly .

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~ 0 .0 V

0.2 v

0 .4 v

~ O . S v0 .6 v

I O 0 .8 v. 1 0 %

1.~)0 12 '00Wavenumbers cm 1

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iI 1 2 4 4 - - - J :I 1 2 2 9

i ' o o 1 2 ' o oW o v e n um b e rs I e m - 1

Fig. 3. Potential difference spectra of p latinum in 0.05 M H2SO4,obtained under stagn ant thin layerconditions. The background spectrum was collected at -0.2 5 V. Sp ectra are displayed in units ofAR/R.Fig. 4. P otential difference spe ctra from Figs. 2 and 3 at +0.4V (smoothed usin g the ll-poin tSavitsky-Golayfunction).

T h e p o t e n t i a l d e p e n d e n c e o f t h e i n t e g r a t e d i n t e n s it y o f th e b i s u l fa t e b a n d ss h o w n i n F i g . 2 is q u a l it a t iv e l y s i m i la r t o f i n d i n g s r e p o r t e d i n t h e e a r l i e r p o l y c r y s -t a l l i n e s t u d i e s [ 4 ] , i n c r e a s i n g s h a r p l y a t p o t e n t i a l s c o r r e s p o n d i n g t o t h e o n s e t o fs u r f a c e o x id a t io n . F u r t h e r m o r e , i n th e e x p e r i m e n t s to e x a m i n e t h e e f f e c t o f th es o l u t i o n fl o w ra t e o n t h e i n t e n s i t y o f th e b i s u l f a t e b a n d a t p o t e n t i a l s i n th e o x i d er e g io n , t h e b a n d i n te n s i ty w a s o b s e r v e d t o r e m a i n c o n s t a n t f o r s o l u t io n f l o w r a te sa t a n d i n e x c e s s o f 0 .4 m l m i n - 1 . S t u d i e s a i m e d a t d e f i n i n g t h e p r a c t i c a l a n a l y t ic a ll im i t s o f t h e i n si tu I R t e c h n i q u e s u g g e s t t h a t t h e i n c r e a s e i n i n t e n s i t y o f t h e a n i o nb a n d s a t p o t e n t i a l s i n t h e o x i d e r e g i o n c a n b e a t t r i b u t e d , i n p a r t , t o t h e m i g r a t i o no f a n i o n s i n t o t h e t h i n l a y e r c a v it y t o c o m p e n s a t e f o r t h e e x c e s s p o s i t iv e c h a r g ew h i c h a c c u m u l a t e s w h e n p r o t o n s a r e f o r m e d d u r i n g s u r f a c e o x i d a t i o n [ 2 , 7 - 9 ] .C h a r g e c o m p e n s a t i o n o f t h is s o rt s h o u l d n o t b e r e q u i r e d w h e n a c o n s t a n t e le c -t r o l y t e fl o w r a t e is e m p l o y e d . A t a f l o w r a t e o f 0 . 4 m l r a in - 1 , t h e a v e r a g e s o l u t io n

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33 5[1 6 ] . Hen ce , i n s i t u IR t ech n i q u es a r e ex p ec t ed t o s am p l e t h e ad s o rb ed b i s u l f a t el ay e r an d o b s e rv e s p ec t r a l f e a t u re s a s s i g n ab l e t o ad s o rb ed s p ec i e s . I n v i ew o f t h es p ec t r a l i n t e r f e r e n ce s a s s o c ia t ed w i t h b u l k s o l u t io n a l t e r a t i o n s [2 , 7 -9 ] an d o p t ica ld i s t o rt i o n s [1 7 ] , o b s e rv i n g p o t e n t i a l -d ep e n d e n t a l t e r a t i o n s i n v i b ra t io n a l b a n dp o s i t io n is an i m p o r t an t c r i t e r i o n fo r a s si g n in g v i b ra ti o n a l b an d s t o m o d es o fa d s o r b e d s p e c i e s .

I n s u m m a r y , p o t e n t i a l - d e p e n d e n t a l t e r a ti o n s i n t h e s p e c tr a l f e a t u r e s o f b i s u lf a t ea d s o r b e d a t p l a t i n u m w e r e o b s e r v e d u n d e r a q u e o u s e l e c t r o c h e m i c a l c o n d i ti o n s b yu s i n g a s p e c t ro e l ec t ro c h em i ca l f lo w ce l l wh i ch e l i m i n a t ed p o t en t i a l - i n d u ce d a l t e r -a t i o n s in t h e co m p o s i t io n o f t h e t h i n l ay e r c av ity . T h e p o t en t i a l -d e p en d en t s p ec t ra lf ea t u re s a r e co n s i s t en t w i t h t h o s e o b s e rv ed i n ea r l i e r s t u d i e s o f s u l f a t e an db i s u l f a te ad s o rp t i o n a t p o l y c ry s ta l li n e an d m o n o c ry s t a l l i n e p l a t i n u m m e t a l e l ec -t ro d es , wh i ch s u g g es t s t h a t t h e s e ea r l y s t u d ie s p ro v i d e i m p o r t an t i n si g h ts i n t o t h ei n t e r ac t i o n o f b i s u l f a te w i t h p l a t i n u m e l ec t ro d es .A C K N O W L E D G M E N T

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