Electrochemical Surface Characterization of Platinum Electrodes

Chapter 14

Downloaded by KTH ROYAL INST OF TECHNOLOGY on February 25, 2016 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch014

Electrochemical Surface Characterization of Platinum Electrodes Using Elementary Electrosorption Processes at Basal and Stepped Surfaces J. Clavilier Laboratoire d'Electrochimie Interfaciale du Centre National de la Recherche Scientifique, 1, Place A. Briand, 92195 Meudon Principal Cedex, France

An attempt has been made at using voltammetry of hydrogen adsorption-desorption on platinum as a tool for in-situ checking the crystalline surface structure of platinum at the atomic level. The behaviour of Pt(111), Pt(100) and stepped surfaces i s discussed in connection with the rôle of the long-range order on the unusual adsorption states. Application is made to the investigation of surface changes as the effect of annealing and characterization of platinum black crystallites. E l e c t r o c h e m i s t r y of surface processes a t s o l i d metal e l e c t r o d e s i s undergoing a profound change due t o t h emastery of t h eu s e o f metal s i n g l e c r y s t a l s as electrodes i n p a r a l l e l with t h eother techniques of s u r f a c e s c i e n c e . By c o m p a r i n g t h e e l e c t r o c h e m i c a l p r o p e r t i e s o f s e l e c t e d m e t a l s i n g l e c r y s t a l s with basal o r i e n t a t i o n s , and then i n t r o d u c i n g s u r f a c e d e f e c t s i n a w e l l - d e f i n e d manner l i k e p e r i o d i c a r r a y s o f monoatomic steps, t h eelectrochemical response of t h eelectrode surface gains a p h y s i c a l meaning which approaches t h es u r f a c e processes a t t h e atomic l e v e l o f t h e s u r f a c e . T h i s b e c o m e s t h e m a j o r way f o r t h e r e n e w a l o f s u r f a c e e l e c t r o c h e m i s t r y w h i c h i s now a p r o m i s i n g b r a n c h o f s u r face science. Among t h e m e t a l e l e c t r o d e s w i t h s u r f a c e s t r u c t u r e s e n s i t i v e p r o p e r t i e s , t h e f i r s t ones t o be considered a r ethose with e l e c t r o c a t a l y t i c a c t i v i t y , such a s p l a t i n u m group m e t a l s . Thus p l a t i n u m i s t h e most a t t r a c t i v e material because of i t s exceptional spectrum of c a t a l y t i c p r o p e r t i e s and i t s good s t a b i l i t y i n v a r i o u s e l e c t r o l y t i c media. A f e w y e a r s a g o p l a t i n u m was t h o u g h t t o b e a w e l l known s o l i d electrode and i t s adsorption and e l e c t r o c a t a l y t i c p r o p e r t i e s widely investigated, p a r t i c u l a r l y i t s adsorption properties f o r hydrogen and o x y g e n . Thus smooth p o l y c r y s t a l l i n e s u r f a c e s , f i n e l y d i v i d e d p a r t i c l e s , were c h a r a c t e r i z e d both b y t h e e x i s t e n c e o f two main electrochemical hydrogen adsorption s t a t e s i . e . weakly and s t r o n g l y bonded h y d r o g e n more e a s i l y o b s e r v a b l e w i t h t h e l a t t e r e l e c t r o d e s than with t h eformer. Extremely r e p r o d u c i b l e hydrogen adsorption c

0097-6156/88/0378-0202$06.00A) 1988 American Chemical Society

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

14.

CLAVILIER

Surface Characterization of Platinum Electrodes

203


s t a t e s are observed a f t e r a p p l i c a t i o n of the " a c t i v a t i o n " p r o c e d u r e , f i r s t a p p l i e d t o s m o o t h p l a t i n u m s a m p l e s by Hammett ( I K A r e v i e w of w o r k on p l a t i n u m a c t i v a t i o n was p u b l i s h e d i n 1967 b y J a m e s ( 2 ) w h i c h g i v e s a c o m p l e t e d i s c u s s i o n of the q u e s t i o n at t h a t time. T h i s p r o c e d u r e was a p p l i e d t o p l a t i n u m s i n g l e c r y s t a l e l e c t r o d e s b y W i l l (3) i n h i s p i o n n e r i n g work t o a s s i g n t h e w e a k l y and s t r o n g l y bonded hydrogen to s u r f a c e c r y s t a l l i n e h e t e r o g e n e i t y . Will's e x p e r i m e n t s h a v e s h o w n t h a t t h e w e a k l y b o n d e d h y d r o g e n was t h e m a j o r s p e c i e s on t h e P t ( 1 1 0 ) a c t i v a t e d e l e c t r o d e w h i l e t h e s t r o n g l y b o n d e d h y d r o g e n was t h e m a j o r o n e o n t h e a c t i v a t e d P t ( i O O ) s u r f a c e . He m i s i n t e r p r e t e d t h e s p e c i e s a d s o r b e d on t h e ( 1 1 1 ) o r i e n t a t i o n f o r t h e r e a s o n s we w i l l s e e b e l o w . I n t h i s w o r k t h e a c t i v a t i o n was f i r s t c o n s i d e r e d as a p r o c e d u r e f o r the e l e c t r o c h e m i c a l c l e a n i n g of the surface. Nevertheless the author concluded that a change in c r y s t a l l i n e s u r f a c e s t r u c t u r e was t h e c o n s e q u e n c e o f a c t i v a t i o n b y r e p e a t e d o x i d a t i o n and r e d u c t i o n of t h e o r i e n t e d p l a t i n u m surfaces. U n t i l r e c e n t l y the r e s u l t i n g s u r f a c e damage has been underestimated as w e l l as i t s e x a c t n a t u r e and e f f e c t . Clean

and

Well-Ordered

Platinum

Surfaces

for Electrochemical

Purpose

a) I n i t i a l l y c l e a n s u r f a c e s . I n o r d e r t o a v o i d t h e d i f f i c u l t i e s r e l a t e d to the a c t i v a t i o n procedure for o b t a i n i n g e f f i c i e n t adsorbing platinum surfaces, platinum s i n g l e c r y s t a l s with oriented s u r f a c e s w e r e c l e a n e d and c h a r a c t e r i z e d by u s i n g u l t r a h i g h v a c u u m t e c h n i q u e (UHV) t h e n t r a n s f e r r e d i n t o a n e l e c t r o c h e m i c a l c e l l (4). The use of a t h i n l a y e r c e l l r e d u c e d t h e amount of a d s o r b a b l e sol u t i o n i m p u r i t i e s (5) i n t e r f e r i n g w i t h h y d r o g e n a d s o r p t i o n . E x p e r i m e n t s c a r r i e d o u t i n t h i s way c o n f i r m e d W i l l ' s c o n c l u s i o n s o n t h e o r i g i n o f t h e two h y d r o g e n a d s o r p t i o n s t a t e s and s h o w e d d e f i n i t e l y t h a t e l e c t r o s o r b e d h y d r o g e n c o u l d be u s e d as a s u r f a c e sensitive probe.Nevertheless i f we c o n s i d e r t h e e l e c t r o c h e m i c a l r e s u l t s obtained with clean platinum oriented surfaces prepared d u r i n g UHV e x p e r i m e n t s , s t u d i e d i n t h e r a n g e o f p o t e n t i a l o f t h e r e v e r s i b l e a d s o r p t i o n - d e s o r p t i o n of h y d r o g e n , the amount of a d s o r b e d h y d r o g e n was n o t t h e s a m e w i t h t h e v a r i o u s e x p e r i m e n t a l d e v i c e s u s e d . T h i s q u a n t i t y was r e m a r k a b l y d i f f e r e n t f o r t h e ( 1 1 1 ) o r i e n t a t i o n , v a r y i n g f r o m 0 . 0 3 t o 0.4 o f a m o n o l a y e r ( 6 ) , ( 7 ) . When s u c h a n e l e c t r o d e was a c t i v a t e d , a f t e r f e w c y c l e s o f 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 and d e s o r p t i o n of o x y g e n , t h e amount of h y d r o g e n a d s o r b e d was i n c r e a s e d a n d t h e v o l t a m m e t r i c p r o f i l e o f h y d r o g e n a d s o r p t i o n d e s o r p t i o n was c h a n g e d ( 8 ) . T h e g e n e r a l b e h a v i o u r o f t h e (111) o r i e n t a t i o n o f p l a t i n u m was i n d i c a t i v e t h a t t h e t e c h n i q u e u s e d d i d not succeed i n p r e p a r i n g c l e a n s u r f a c e s a f t e r the c o n t a c t with the e l e c t r o l y t e was e s t a b l i s h e d . A n o t h e r r o u t e f o r c l e a n s u r f a c e p r e p a r a t i o n , c l e a n t r a n s f e r of t h e s a m p l e and c l e a n c o n t a c t w i t h t h e e l e c t r o l y t e was required. b> C l e a n p l a t i n u m s u r f a c e s i n c o n t a c t w i t h an e l e c t r o l y t i c s o l u t i o n . The t e c h n i q u e w h i c h h a s b e e n d e v e l o p e d s i n c e 1980 c o n s i s t s i n u s i n g the c a t a l y t i c p r o p e r t i e s of p l a t i n u m f o r o x i d a t i o n of c a r b o n a c e o u s and s u l p h u r s u r f a c e compounds w h i c h a r e t h e m a j o r c o n t a m i n a n t s of the s u r f a c e of p l a t i n u m samples exposed t o the atmospheric



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ELECTROCHEMICAL SURFACE SCIENCE

e n v i r o n m e n t and w h i c h r e a d s o r b a t t h e s u r f a c e a f t e r a p p l i c a t i o n o f c l e a n i n g p r o c e d u r e i f n o s p e c i a l c a r e i s t a k e n . The r e m o v a l o f i m p u r i t i e s was a c h i e v e d b y h e a t i n g t h e p l a t i n u m i n a g a s - o x y g e n f l a m e and q u e n c h i n g t h e s a m p l e i n t o a s t r e a m of u l t r a p u r e w a t e r during c o o l i n g i n a i r . A d r o p l e t o fwater stayed attached to the o r i e n t e d s u r f a c e of the c r y s t a l p r o t e c t i n g i t a g a i n s t back c o n t a m i n a t i o n b y d e c r e a s i n g t o a g r e a t e x t e n t the r a t e of t r a n s f e r of i m p u r i t i e s from atmosphere to the s u r f a c e (9). T h e p l a t i n u m s i n g l e c r y s t a l s t r e a t e d i n t h i s way a r e s m a l l s p h e r i c a l p l a t i n u m b e a d s w i t h a d i a m e t e r r a n g i n g f r o m 1.5 t o 1.9 mm o b t a i n e d by m e l t i n g o f a w i r e . T h e y a r e o r i e n t e d , c u t and p o l i s h e d a c c o r d i n g t o t h e t e c h n i q u e d e s c r i b e d i n (10.) w i t h a n a c c u r a c y w i t h i n 3 m i n u t e s of the nominal o r i e n t a t i o n . A f t e r p o l i s h i n g , the samples are annealed at 1300°-1500°C to e l i m i n a t e the perturbed s u r f a c e l a y e r . In t h e i n i t i a l e x p e r i m e n t s r e p o r t e d i n r e f . ( 9 ) t h e t h e r m a l decontamination t r e a t m e n t was c a r r i e d o u t a t h i g h t e m p e r a t u r e (t > 9 0 0 ° C ) . T h i s t e c h n i q u e has been improved i n o r d e r t o m a i n t a i n the q u a l i t y of t h e c r y s t a l f o r r e p e a t e d t r e a t m e n t s . W i t h t h e new procedure the s i n g l e c r y s t a l i s not put i n d i r e c t c o n t a c t with the flame but heated i n d i r e c t l y through the thermal c o n d u c t i o n of the p l a t i n u m w i r e . The t e m p e r a t u r e o f a n n e a l i n g i s n e a r l y 5 0 0 ° C f o r few s e c o n d s , t h e n t h e c r y s t a l i s c o o l e d i n a i r and q u e n c h e d when i t s temperature i s n e a r l y 2 0 0 C i . e . when c a t a l y t i c o x i d a t i o n o f i m p u r i t i e s coming i n t o contact with the s u r f a c e ceases to be e f f i c i e n t . This thermal treatment a p p l i e d for surface decontamination must b e d i s t i n g u i s h e d from the treatment f o r s u r f a c e r e - o r d e r i n g which r e q u i r e s higher annealing temperatures a s an example w i l l demonstrate below. P l a t i n u m s u r f a c e s w i t h (111) and (100) o r i e n t a t i o n s t r e a t e d i n t h i s way h a v e b e e n c h e c k e d b y u s i n g L E E D c h a r a c t e r i z a t i o n o n a s r e c e i v e d " s a m p l e s , both showed the c h a r a c t e r i s t i c LEED p a t t e r n with t h e i r r e s p e c t i v e ( l x l ) s u r f a c e s y m m e t r y . The non o b s e r v a t i o n of t h e ( 5 x 2 0 ) s y m m e t r y f o r t h e ( 1 0 0 ) o r i e n t a t i o n was d u e t o t h e p r e s e n c e o f r e s i d u a l adsorbed i m p u r i t i e s at the s u r f a c e of "as r e c e i v e d " s a m p l e s . Simply t h i s c o n f i r m s the c r y s t a l l i n e s u r f a c e q u a l i t y of the p l a t i n u m samples prepared according to t h i s technique (10). 9

u

Voltammetry

for Surface

A n a l y s i s and

Structural Characterization

It may b e s e e n i n f i g . l a n e x a m p l e of s u r f a c e a n a l y s i s b y u s i n g voltammetry. Inthese experiments the thermally t r e a t e d samples, have been put i n c o n t a c t with the s o l u t i o n a t a c o n t r o l l e d p o t e n t i a l c h o s e n i n s u c h a way t h a t n o o r n e g l i g i b l e c u r r e n t t r a n s i e n t s w e r e d e t e c t e d when t h e s a m p l e s c o n t a c t e d t h e s o l u t i o n . T h i s a l l o w s t h e a n a l y s i s of i n i t i a l s u r f a c e c o n d i t i o n s ( l U . T h e oxygen a d s o r b e d d u r i n g t h e t h e r m a l t r e a t m e n t i s r e d u c e d i n t h e f i r s t n e g a t i v e sweep i n t h e p o t e n t i a l r a n g e 0.9-0.6 V/R.H.E. The amount o f e l e c t r i c c h a r g e f o r t h i s r e d u c t i o n i s a c h a r a c t e r i s t i c q u a n t i t y and c o r r e s p o n d s t o t h e maximum c o v e r a g e w h i c h i s f o r m e d o n e a c h o r i e n t a t i o n by the thermal treatment i n a i r . In t h e s u b s e q u e n t p r o c e s s , w h i c h i s h y d r o g e n a d s o r p t i o n , e a c h o r i e n t a t i o n h a s i t s maximum a d s o r p t i o n a c t i v i t y . H y d r o g e n i s a d s o r b e d i n a n amount c o r r e s p o n d i n g t o one H adatom per s u r f a c e P t atom o f t h e (111) and (100) o r i e n t a t i o n s and s i g n i f i c a n t l y more f o r




CLAVILIER

F i g u r e 1. F i r s t v o l t a m m o g r a m s o f f l a m e - t r e a t e d p l a t i n u m , basal o r i e n t a t i o n s . Standard experimental conditions f o r a l l f i g u r e s s e l e c t r o l y t e 0 . 5 M H S 0 « ; s w e e p r a t e : 5 0 mV.s~ 2



206


P t ( l l O ) due t o i t e n d e n c y f o r t h i s o r i e n t a t i o n t o u n d e r g o t h e (1x2) s u r f a c e r e c o n s t r u c t i o n . E v i d e n t l y both t h e r m a l l y a d s o r b e d o x y g e n and e l e c t r o c h e m i c a l l y adsorbed hydrogen are r e l a t e d through the surface d e n s i t y of p l a t i n u m atoms. T h i s s e t of i n i t i a l d a t a i s i n d i c a t i v e o f t h e c l e a n l i n e s s of s a m p l e s p r e p a r e d and t r a n s f e r r e d b y t h i s m e t h o d . The a v a i l a b i l i t y of c l e a n p l a t i n u m s u r f a c e s w i t h d e f i n e d o r i e n t a t i o n in contact with the e l e c t r o l y t i c s o l u t i o n allows the separation o f the various processes o c c u r r i n g during the electrochemical a c t i v a t i o n a p a r t from the o x i d a t i o n of i m p u r i t i e s . However t h e more i m p o r t a n t q u e s t i o n which can b e s o l v e d w i t h such samples i s r e l a t e d t o c h a r a c t e r i z a t i o n of s u r f a c e s i t e s because each b a s a l o r i e n t a t i o n shows w e l l - c h a r a c t e r i z e d hydrogen a d s o r p t i o n d e s o r p t i o n peaks which might b e h e l p f u l l y used f o r t h i s purpose. The main f e a t u r e of e l e c t r o s o r b e d h y d r o g e n a t b a s a l o r i e n t a t i o n i s the presence of s e v e r a l peaks or a d s o r p t i o n s t a t e s f o r s u r f a c e s c o n t a i n i n g a s i n g l e t y p e of s i t e l i k e P t ( l l l ) and P t ( l O O ) . T h i s means t h a t the i n f o r m a t i o n a voltammogram c o n t a i n s , c o n c e r n s more t h a n t h e s i m p l e i n t e r a c t i o n b e t w e e n a d s o r b e d h y d r o g e n and t h e s u r f a c e s i t e (short range i n t e r a c t i o n ) . Other s u r f a c e i n f o r m a t i o n must b e found p a r t i c u l a r l y b y u s i n g s t e p p e d s u r f a c e s which a l l o w s a m o d u l a t i o n o f l o n g r a n g e i n t e r a c t i o n s when t h e y a r e s u s p e c t e d t o b e at t h e o r i g i n of some a d s o r p t i o n s t a t e s . This p o s s i b l e e f f e c t o f long range i n t e r a c t i o n s on the hydrogen a d s o r p t i o n s t a t e s i s suggested by the f a c t that platinum e l e c t r o d e s w i t h (111) and (100) o r i e n t a t i o n s , a f t e r t h e r m a l t r e a t m e n t , showed a d s o r p t i o n s t a t e s at higher p o t e n t i a l than those observed p r e v i o u s l y with a l l t y p e s of p l a t i n u m e l e c t r o d e s which were s u b j e c t e d t o a c t i v a t i o n o r were i n s u f f i c i e n t l y c l e a n . T h i s p u z z l i n g d i f f e r e n c e s i n c e has been s t u d i e d e x t e n s i v e l y e i t h e r b y u s i n g p u r e e l e c t r o c h e m i c a l t e c h n i q u e ( 1 0 ) , ( 1 2 , 1 3 ) o r UHV s u r f a c e s c i e n c e methods (14-17). I n d e p e n d e n t l y of the t e c h n i q u e used to i n v e s t i g a t e the p r i m e c a u s e of the e x i s t e n c e of the a d s o r p t i o n s t a t e s at the h i g h e r p o t e n t i a l s on P t ( l l l ) and P t ( l O O ) , t h e c o n c l u s i o n i s t h e y r e s u l t f r o m t h e e x i s t e n c e of a t o m i c a l l y f l a t e x t e n d e d s u r f a c e d o m a i n s w i t h t h e r e s p e c t i v e o r i e n t a t i o n , i . e . e x i s t e n c e of long range s u r f a c e o r d e r . T h e s e a d s o r p t i o n s t a t e s a p p e a r a s a n i n t r i g u i n g and a u n i q u e p r o p e r t y of w e l l - o r d e r e d P t ( l l l ) and P t ( l O O ) s u r f a c e s i n c o n t a c t with various e l e c t r o l y t e s o l u t i o n s because no equivalent e f f e c t s are known w i t h t h e same o r i e n t a t i o n s i n gas p h a s e e x p e r i m e n t s . Inthis s e n s e , t h e s e p r o p e r t i e s may b e c o n s i d e r e d a s a s p e c i f i c a s p e c t o f the e l e c t r o c h e m i s t r y o f surface processes. The q u e s t i o n of t h e s p e c i e s i n v o l v e d i n t h e s e u n u s u a l adsorption s t a t e s i s s t i l l c o n t r o v e r s i a l . No d i r e c t i n f o r m a t i o n ( s u c h a s s p e c t r o s c o p i c d a t a ) o n the n a t u r e of the s p e c i e s i s a v a i l a b l e . Only e l e c t r o c h e m i c a l d a t a h a v e been o b t a i n e d and t h e s e r e q u i r e s e v e r a l c r o s s d e t e r m i n a t i o n s and c a r e f u l d i s c u s s i o n t a k i n g i n t o a c c o u n t the w h o l e e x p e r i m e n t a l d a t a s e t and t h e p a r a l l e l b e h a v i o u r o f t h e (100) and (111) o r i e n t a t i o n s . With t h i s i n mind the s p e c i e s which f i t s the best with the experimental data f o r the anomalous a d s o r p t i o n s t a t e s would b e a f u l l y r e v e r s i b l y d i s c h a r g e d s p e c i e s i n c l u d i n g t r a n s f e r o f one e l e c t r o n . It could b e an adsorbed hydrogen-like s p e c i e s under f u l l c o n t r o l o f the s p e c i f i c a d s o r p t i o n of a n i o n s of the base e l e c t r o l y t e


14.

CLAVILIER

207

Surface Characterization ofPlatinum Electrodes

( 1 8 ) . T h i s c o u l d a c c o u n t f o r t h e a b s e n c e o f a n y pH d e p e n d e n c e o f t h e unusual a d s o r p t i o n s t a t e s i n presence of s p e c i f i c a d s o r p t i o n of s u l p h u r i c a n i o n (13.). R e c e n t r e s u l t s ( K r a u s k o p f , E . K.; R i c e , L . H . j W i e c k o w s k i , A. J . E l e c t r o a n a l . C h e * . i n p r e s s ) o b t a i n e d w i t h l a b e l e d s u l p h u r i c anions are i n agrement with the hydrogen i n t e r p r e t a t i o n . The m a j o r d i f f i c u l t y w i t h t h e h y d r o g e n h y p o t h e s i s i s t h e p o t e n t i a l of the u n u s u a l a d s o r b e d h y d r o g e n i n a b s e n c e of a n i o n s p e c i f i c a d s o r p t i o n c a n n o t be c o r r e l a t e d w i t h e n e r g y f o r h y d r o g e n a d s o r bed f r o m gas p h a s e a t t h e same o r i e n t a t i o n . T h e r e i s t o o l a r g e a d i f f e r e n c e i n t h e h y d r o g e n b i n d i n g e n e r g y v a l u e s ( 1 5 ) . In t h i s r e s p e c t i t was f o u n d i n g a s p h a s e f o r P t m ( l l l ) x ( l l l ) t h a t h y d r o g e n i s m o r e s t r o n g l y b o n d e d on s t e p t h a n o n t e r r a c e (19.). I t w i l l b e s h o w n b e l o w that the p o t e n t i a l sequence for the r e s p e c t i v e hydrogen s t a t e s gives the inverse order. This conclusion i s v a l i d for stepped surfaces w i t h (100) t e r r a c e s . Thus a d i r e c t c o m p a r i s o n b e t w e e n t h e gas p h a s e b i n d i n g e n e r g y and t h e a d s o r p t i o n p o t e n t i a l d o e s n o t seem p o s s i b l e . Other h y p o t h e s e s i n t r o d u c e d i f f i c u l t i e s t o o , the most a p p a r e n t being e i t h e r a l a r g e d i f f e r e n c e i n the s u r f a c e c h e m i s t r y of adsorbed a n i o n s a t P t ( l l l ) and P t ( l O O ) , or t h e c h a n g e of s p e c i e s i n t h e p r e sence or absence of s p e c i f i c a d s o r p t i o n . Whatever the s p e c i e s i n v o l v e d t h e s e a n o m a l o u s s t a t e s o b l i g e us t o r e - e x a m i n e c a r e f u l l y , c l a s s i c a l i n t e r f a c i a l c o n c e p t s . An e x a m p l e i s g i v e n w i t h t h e new concept of the r o l e of s u r f a c e long range order i n a d s o r p t i o n p r o p e r t i e s w h i c h i s now c l e a r l y e s t a b l i s h e d .


T

As an (111)

i l l u s t r a t i o n of the above comments, r e s u l t s o b t a i n e d and (100) o r i e n t a t i o n s a r e d i s c u s s e d i n p a r a l l e l .

Surface

processes

r e l a t e d to electrochemical

with

activation

The u n d e r s t a n d i n g o f a c t i v a t i o n e f f e c t s on t h e e l e c t r o c h e m i c a l b e h a v i o u r of o r d e r e d p l a t i n u m s u r f a c e s w i l l a l l o w the p o s s i b i l i t y of c o r r e l a t i n g the c l a s s i c a l knowledge of p l a t i n u m a d s o r p t i o n behaviour w i t h t h e new observations. a/ H i g h o x y g e n c o v e r a g e . L e t us c o n s i d e r f i r s t t h e e f f e c t of a h i g h o x y g e n c o v e r a g e by e l e c t r o s o r b e d o x y g e n a t a c l e a n P t ( l l l ) / 0 . 5 M H2SO4 i n t e r f a c e . The i n i t i a l voltammogram i s shown i n f i g . 1 . A t such a s u r f a c e o x y g e n a d s o r p t i o n o c c u r s a b o v e 1.25 V / R . H . E . a s s h o w n i n f i g . 2 A c u r v e 1. I t a m o u n t s t o n e a r l y 5 0 0 p C . c m " a n d t h e b a l a n c e o f charge i s exact for desorption. This i s a requirement in agreement with surface c l e a n l i n e s s . Subsequent, hydrogen adsorption-desorption l o o k s s i m i l a r t o t h a t a t a p o l y c r y s t a l l i n e s u r f a c e and t h e u n u s u a l b o n d e d s t a t e s h a v e d e c r e a s e d and s h i f t e d p o s i t i v e l y . T h i s e f f e c t i s r e i n f o r c e d i n the second c y c l e where oxygen a d s o r p t i o n p o t e n t i a l i s i s decreased. A f t e r t h i r t y c y c l e s between 0.05-1.45 V the r e v e r s i b l e p e a k s a t 0.110 V/R.H.E. a r e w e l l d e v e l o p e d , and t h e u n u s u a l s t a t e s a r e s p r e a d i n a w i d e r a n g e of p o t e n t i a l w i t h a low amplitude. The same t y p e of v o l t a m m o g r a m has been o b t a i n e d w i t h a P t ( l l l ) e l e c t r o d e a f t e r i t s o r d e r e d s u r f a c e was s u b j e c t e d t o a r g o n i o n b o m b a r d ment, i n t r o d u c i n g s t r u c t u r a l d e f e c t s l i k e randomly d i s t r i b u t e d s t e p s (14.). T h e s i m i l a r e f f e c t s o f o x y g e n e l e c t r o s o r p t i o n a n d i o n b o m b a r d ment show c l e a r l y t h a t t h e f o r m e r p e r t u r b s t h e s u r f a c e o r d e r . 2

The f i r s t c o n s e q u e n c e of b o t h t r e a t m e n t s i s t h e c o n s i d e r a b l e d e c r e a s e of t h e a n o m a l o u s a d s o r p t i o n s t a t e s . T h i s i s one of t h e



208


F i g u r e 2 B. Low o x y g e n c o v e r a g e o n P t ( l l i ) , (f) l a s t voltammograois.

(i) initial,

F i g u r e 2 C. E f f e c t o f a n n e a l i n g o n t h e r e o r d e r i n g o f p o l i s h e d P t ( l l l ) , a n n e a l i n g t i m e : ( l ) n o a n n e a l i n g j ( 2 ) 1.5 mn ; ( 3 ) 6.5am


14.

CLAVILIER

209

Surface Characterization of Platinum

p i e c e s of e v i d e n c e e x i s t e n c e of t h e s e

f o r the r e s p o n s i b i l i t y of states.

long-range

order

in

the


A s e c o n d c o n s e q u e n c e o f b o t h t r e a t m e n t s i s t h a t a m a j o r i t y o f new s i t e s a p p e a r s g i v i n g r i s e t o p e a k s a t 0.110 V t h e e x a c t p o t e n t i a l o f the main peaks observed with Pt(110) f i g . l c o n s e q u e n t l y they are a s s u m e d of t h e same n a t u r e . The h y d r o g e n a d s o r b e d a t t h i s p o t e n t i a l c o r r e s p o n d s t o t h e w e a k l y b o n d e d h y d r o g e n on a p o l y o r i e n t e d s u r f a c e . R e v e r s i b l e peaks corresponding to the c l a s s i c a l s t r o n g l y bonded h y d r o g e n grow a t t h e f i r s t c y c l e and d i s a p p e a r p r o g r e s s i v e l y w i t h c y c l i n g m e a n i n g t h a t (100) s i t e s r e a c h a r a t h e r low s u r f a c e d e n s i t y . The t h i r d c o n s e q u e n c e i s t h a t t h e t r a n s f o r m a t i o n of t h e v o l t a m m o g r a m i n f i g . 2 A o c c u r s w i t h a c o n s t a n t amount of e l e c t r i c c h a r g e when t h e c h a r g e i s summed o v e r a l l t h e r e v e r s i b l e a d s o r p t i o n s t a t e s i n c l u d i n g t h e u n u s u a l o n e s . The d e c r e a s e i n c h a r g e f o r u n u s u a l s t a t e s i s c o m p e n s a t e d by t h e i n c r e a s e i n c h a r g e f o r t h e n o r m a l s t a t e s ( 1 2 ) ( 2 0 ) . Such b e h a v i o u r i s c o h e r e n t w i t h the a s s u m p t i o n of h y d r o g e n as t h e adsorbed species for a l l the r e v e r s i b l e adsorption s t a t e s . The same b e h a v i o u r i s o b s e r v e d i n p e r c h l o r i c or h y d r o f l u o r i c a c i d s o l u t i o n s . U n f o r t u n a t e l y a q u a n t i t a t i v e a n a l y s i s c a n n o t be c a r r i e d o u t because the unusual adsorption s t a t e current i s merging i n t o the curr e n t of oxygen a d s o r p t i o n at the s u r f a c e d e f e c t s , the former decreases while the l a t t e r i n c r e a s e s with s u r f a c e d e f e c t s d e n s i t y (17). In v i e w of t h e l a r g e c o n t r i b u t i o n of (110) s i t e s t o t h e h y d r o g e n a d s o r p t i o n a t an e l e c t r o c h e m i c a l l y p e r t u r b e d ( 1 1 1 ) o r i e n t a t i o n , s t e p p e d s u r f a c e s w i t h (110) s t e p s and (111) t e r r a c e s h a v e b e e n used f o r the modeling of the main v o l t a m m e t r i c features of p e r t u r b e d P t ( l l l ) . F i g . 5 shows the voltammogram of a f l a m e t r e a t e d Pt(332) e l e c t r o d e otherwise noted 6(111)x(111) or 5 ( l l l ) x ( 1 1 0 ) b e c a u s e t h e s t e p s i t e s h a v e t h e (110) c o n f i g u r a t i o n . As e x p e c t e d t h e v o l t a m m o g r a m s h o w s t h e r e v e r s i b l e p e a k s a t 0.110 V c h a r a c t e r i s t i c of h y d r o g e n a d s o r p t i o n - d e s o r p t i o n a t P t ( l l O ) . The o t h e r s t a t e s a r e due t o t h e h y d r o g e n a d s o r p t i o n - d e s o r p t i o n at t e r r a c e s . T h i s voltammogram compared t o t h a t of P t ( l l l ) f i g . l shows c l e a r l y a s m a l l e r amount of the unusual a d s o r p t i o n s t a t e at a more p o s i t i v e p o t e n t i a l much l i k e a f t e r t h e f i r s t o x y g e n a d s o r p t i o n d e s o r p t i o n a t h i g h c o v e r a g e . On t h e o t h e r h a n d , t h e v o l t a m m o g r a m o f P t ( 3 3 2 ) i n f i g . 5 i s another c o n f i r m a t i o n of the e f f e c t of long-range o r d e r on t h e u n u s u a l a d s o r p t i o n s t a t e s . On t h i s v o l t a m m o g r a m t h e h y d r o g e n a d s o r b e d a t s t e p s i t e s c o r r e s p o n d s to a s t a t e near the lower l i m i t of the p o t e n t i a l r a n g e i . e . to the weakly bonded s t a t e , a c o n c l u s i o n at v a r i a n c e with gas p h a s e r e s u l t s (19.). b / Low o x y g e n c o v e r a g e . L e t u s now c o n s i d e r t h e e f f e c t o f l o w o x y g e n c o v e r a g e f o r t h e o b s e r v a t i o n of t h e f i r s t s t e p s i n t h e P t ( l l l ) s u r f a c e t r a n s f o r m a t i o n . The p r o g r e s s i v e t r a n s f o r m a t i o n o b s e r v e d i n f i g . 2 B i s o b t a i n e d by a p p l y i n g s u c c e s s i v e l y f a s t r e p e t i t i v e s e q u e n c e s o f c y c l i n g b e t w e e n 0.05 - 1.2 V. I t c a n b e s e e n that the f o l l o w i n g changes are simultaneous : i n c r e a s e of the peaks a t 0 . 1 1 0 V, d e c r e a s e o f t h e u n u s u a l a d s o r p t i o n s t a t e s , i n c r e a s e o f t h e amount of adsorbed oxygen. These e f f e c t s a l l o w the d e f i n i t i o n


210



of c r i t e r i a f o r t h e e v a l u a t i o n of the q u a l i t y of a P t ( l l l ) s u r f a c e . It i s e v i d e n t t h a t t h e s e c h a n g e s a r e due t o a n i n c r e a s e i n s t r u c t u r a l s u r f a c e d e f e c t d e n s i t y , and o x y g e n a d s o r p t i o n o c c u r s p a r t i c u l a r l y a t t h e s e d e f e c t s w h i l e s i m u l t a n e o u s l y (110) s i t e s may be d e t e c t e d b y h y d r o g e n a d s o r p t i o n . Thermal r e o r d e r i n g of P t ( l l l ) a f t e r p o l i s h i n g T h e s e c r i t e r i a a r e shown t o b e u s e f u l t o i n v e s t i g a t e t h e r e o r g a n i s a t i o n o f t h e s u r f a c e l a y e r s a f t e r t h e p o l i s h i n g o f a (111) p l a t i n u m s u r f a c e . F i g . 2 C shows the voltammograms of a f r e s h l y p o l i s h e d P t ( l l l ) s a m p l e . T h e f i r s t c u r v e was r e c o r d e d w i t h t h e s a m p l e t r e a t e d few s e c o n d s i n t h e r a n g e 4 0 0 - 5 0 0 ° C i n p r e s e n c e o f a i r f o r s u r f a c e c l e a n i n g . The f i r s t v o l t a m m o g r a m shows t h e s u r f a c e c l e a n l i n e s s . I t s h o w s c o m m o n f e a t u r e s w i t h t h e c u r v e 1, f i g . 2 A , w e l l d e v e l o p e d p e a k s f o r t h e h y d r o g e n a d s o r p t i o n a t (110) s i t e s a t 0.110 V s m a l l r e v e r s i b l e p e a k s a t 0.26 V c o r r e s p o n d i n g t o s t r o n g l y b o n d e d h y d r o g e n o n (100) s i t e s and a non n e g l i g i b l e c o n t r i b u t i o n of the unusual a d s o r p t i o n s t a t e s . T h i s i s c o r r e l a t e d with oxygen a d s o r p t i o n a t s u r f a c e s t r u c t u r a l d e f e c t s . The p r e s e n c e of a non n e g l i g i b l e amount of unusual a d s o r p t i o n s t a t e s means t h a t wide d o m a i n s w i t h (111) o r i e n t a t i o n a r e p r e s e n t a t t h e s u r f a c e a f t e r p o l i s h i n g . A f t e r t h a t t h e e l e c t r o d e was a n n e a l e d f o r l . S mn a t 1 3 0 0 - 1 4 0 0 ° C . T h e n t h e s u b s e q u e n t v o l t a m m o g r a m was r e c o r d e d , c u r v e 2 , f i g . 2 C . A s u b s t a n t i a l d e c r e a s e of t h e p e a k s a t 0.110 V i s o b s e r v e d , due t o t h e e l i m i n a t i o n of t h e (110) s i t e s w h i l e t h e u n u s u a l a d s o r p t i o n s t a t e s a r e e n h a n c e d . A t t h e same t i m e o x y g e n i s a d s o r bed t o a l e s s e r e x t e n t . A f t e r new a n n e a l i n g i n t h e same t e m p e r a t u r e r a n g e f o r 5 mn t h e v o l t a m m o g r a m f o r a w e l l o r d e r e d ( 1 1 1 ) P t s u r f a c e i s p r a c t i c a l l y a c h i e v e d . P e a k s a t 0.110 V h a v e d i s a p p e a r e d , the unusual adsorption s t a t e s are well developed with the sharp s p i k e at 0.440 V, a d s o r p t i o n o f oxygen has p r a c t i c a l l y disappeared too. F i g . 3 shows the voltammogram of a P t ( l l l ) e l e c t r o d e with r e o r d e r e d s u r f a c e b y a n n e a l i n g at h i g h t e m p e r a t u r e w i t h the minimum amount o f s u r f a c e d e f e c t s . Simple model f o r P t ( l l l ) p e r t u r b e d b y oxygen e l e c t r o s o r p t i o n . It may b e assumed t h a t o x y g e n a d s o r p t i o n a s a r e s u l t o f p l a c e e x c h a n g e m e c h a n i s m l e a v e s a f t e r d e s o r p t i o n some p l a t i n u m a t o m s i n a p o s i t i o n of a d a t o m s a t t h e (111) o r d e r e d s u r f a c e . I f we c o n s i d e r i n a h a r d s p h e r e m o d e l o f f e e c r y s t a l s , a d e n s e r o w o f a d a t o m s o n t h e (111) s u r f a c e , o n e a c h s i d e o f t h e row new s i t e s a r e c r e a t e d a t t h e s u r f a c e i n same a m o u n t , (110) o n one s i d e ( f i g . 4a) and (100) ( f i g . 4 b ) o n t h e o t h e r s i d e . The e x p e r i m e n t a l r e s u l t s show t h a t t h e m a j o r i t y of t h e new s i t e s h a v e t h e (110) symmetry t h i s i m p l i e s t h a t s u c h a row i s n o t a s t a b l e c o n f i g u r a t i o n f o r t h e adatoms. More s t a b l e s m a l l t r i a n g u l a r i s l a n d s whose edge s i t e s h a v e t h e (110) symmetry seem t o b e f o r m e d , f i g . 4 c p o s s i b l y (100) k i n k s may a p p e a r , f i g . 4 d . T h i s t y p e o f s u r f a c e a r r a n g e m e n t i s n o t u n u s u a l a t a (111) o r i e n t a t i o n and p r o b a b l y i s a t t h e o r i g i n o f t h e s h a p e of c o r r o s i o n p i t s o b s e r v e d w i t h o n t h e (111) f a c e o f many f e e m e t a l s . The mean w i d t h s of (111) o r d e r e d d o m a i n s may b e e s t i m a t e d from t h e s h a p e and p o s i t i o n of t h e u n u s u a l adsorption state observed with m ( l l l ) x ( l l l ) surfaces.


CLAVILIER 1/ lA.cm"

Surface Characterization of Platinum Electrodes 2

- -50


0

1

0.1

211

I

I

I

\ J

/1).5

^

^—

1.0

E/V

rhe

-

F i g u r e 3. V o l t a m m o g r a m o f P t ( l l l ) w i t h t h e l o w e s t s u r f a c e d e f e c t c o n t r i b u t i o n t o h y d r o g e n and oxygen a d s o r p t i o n .

F i g u r e 4. H a r d s p h e r e m o d e l f o r p e r t u r b e d P t ( l l l ) . ( a ) d e n s e row o f P t a d a t o m s f o r m i n g (110) s i t e s , f r o n t v i e w , (b) r e a r v i e w s h o w i n g (100) s i t e s . ( c ) i s l a n d f o r m i n g (110) edge s i t e s , (d) a (100) k i n k s i t e .



212 Pt(lOO)

o r i e n t a t i o n and

(100)

s i t e s at stepped

surfaces.

The P t ( l O O ) o r i e n t a t i o n has a more c o m p l e x e l e c t r o c h e m i c a l b e h a v i o u r . The v o l t a m m o g r a m o n f i g . l c o r r e s p o n d s t o a s u r f a c e c o o l e d i n a i r . C o o l i n g i n t h e p r e s e n c e of h y d r o g e n t h e same a d s o r p t i o n s t a t e s a r e o b s e r v e d but w i t h a d i f f e r e n t d i s t r i b u t i o n , the most p o s i t i v e b e i n g t h e h i g h e s t one ( 1 U , ( 2 L ) . A s i m i l a r d i s t r i b u t i o n i s o b t a i n e d i f t h e e l e c t r o d e i s s u b j e c t e d t o f e w c y c l e s i n a IM H C 1 s o l u t i o n p r i o r t o t h e v o l t a m m o g r a m r e c o r d i n g i n a 0.5M H S 0 « s o l u t i o n (22). F i n a l l y the enhancement o f the r e v e r s i b l e peaks at h i g h e s t p o t e n t i a l may b e o b t a i n e d b y a f a s t c y c l i n g i n a 0.5M H2SQ4 s o l u t i o n a s s t u d i e d i n ( 1 2 ) . I t i s shown i n f i g . 5 t h e two l i m i t i n g v o l t a m m e t r i c p r o f i l e s f o r a (100) o r i e n t a t i o n . E x p e r i m e n t a l res u l t s i n d i c a t e t h a t (100) s u r f a c e w i t h a w e l l - d e v e l o p e d peak a t high p o t e n t i a l i s probably that with the lowest surface defect c o n t e n t i n view of the lowest c u r r e n t i n the r e g i o n 0.05-0.2 V a s a c o n c l u s i o n of the r e s u l t s o b t a i n e d with stepped s u r f a c e s (see below). Because of i t s a b s e n c e i n normal v o l t a m m e t r y of a c t i v a t e d p l a t i n u m the r e v e r s i b l e s t a t e at 0.365 V i s a n unusual s t a t e t o o . When t h e P t ( l O O ) o r i e n t a t i o n i s s u b j e c t e d t o o x y g e n a d s o r p t i o n d e s o r p t i o n at high coverage, t h i s s t a t e d i s a p p e a r s p r o g r e s s i v e l y l e s s r a p i d l y than i n the case of P t ( l l l ) w h i l e the a d s o r p t i o n d e s o r p t i o n c u r r e n t i n t h e r a n g e 0.05 - 0 . 2 V i n c r e a s e s ( f i g . 5 ) without appearance of d e f i n e d a d s o r p t i o n - d e s o r p t i o n s t a t e s l i k e on P t ( 1 1 1 ) . C o n t r a r y to the l a t t e r o r i e n t a t i o n , even a f t e r a l a r g e p e r t u r b a t i o n o f t h e (100) s u r f a c e , t h e u n u s u a l s t a t e may b e r e s t o r e d b y f a s t c y c l i n g (12.). T h e d i s a p p e a r a n c e o f t h e u n u s u a l s t a t e with extensive a d s o r p t i o n - d e s o r p t i o n o foxygen suggests that surface long-range order i s responsible for i t s presence too.


2

Stepped surfaces with

(100)

sites

( 1 0 0 ) t e r r a c e s i t e s . Me c o n s i d e r f i r s t s u r f a c e s w i t h (100) t e r r a c e s . Two t y p e s of s t e p may b e o b t a i n e d e i t h e r w i t h (110) s i t e s f o r s u r f a c e s c h o s e n i n t h e C0013 z o n e o r w i t h (111) s i t e s f o r s u r f a c e s chosen i n the [011] zone. In f i g . 5 t h e v o l t a m m o g r a m of t h e P t ( 3 1 0 ) and P t ( 5 1 1 ) o r i e n t a t i o n s b o t h c o n t a i n i n g (100) t e r r a c e s w i t h t h r e e a t o m i c rows and (110) o r (111) s t e p s r e s p e c t i v e l y a r e r e p r e s e n t e d . T h e i r s i m i l a r a d s o r p t i o n s t a t e s a b o v e 0.2 V may b e u n a m b i g u o u s l y a s c r i b e d t o t h e t e r r a c e s i t e c o n t r i b u t i o n , while c h a r a c t e r i s t i c s t a t e s at lower p o t e n t i a l s which are not p r e s e n t on the voltammogram of an o r d e r e d Pt(lOO) s u r f a c e may b e a s c r i b e d t o s t e p c o n t r i b u t i o n o n b o t h s u r f a c e s . C o n s i d e r i n g t h e s t a t e s r e l a t e d t o t h e t e r r a c e c o n t r i b u t i o n i t may be n o t e d t h e a b s e n c e o f t h e r e v e r s i b l e p e a k a t 0.365 V w h i c h c o n f i r m s i t s assignment to the bidimensional long-range order. A l l t h e s t e p c o n t r i b u t i o n s a r e l o c a t e d i n t h e low p o t e n t i a l r a n g e b e l o w 0.2 V. B o t h (110) and (111) s t e p s i t e s h a v e t h e i r c o n t r i b u t i o n t o h y d r o g e n a d s o r p t i o n i n t h e same r a n g e of p o t e n t i a l , t h i s remark w i l l be u s e f u l i n the i n t e r p r e t a t i o n o fthe weakly bonded hydrogen p a r t i c u l a r l y on a c t i v a t e d platinum s u r f a c e s . Here the step c o n t r i b u t i o n s correspond to the s t a t e s i n the lower p o t e n t i a l range i . e . to the weakly bonded hydrogen.



14.

CLAVILIER


213

(100) s t e p s i t e s . T h e a d s o r p t i o n o f h y d r o g e n on (100) s t e p s i t e s •ay be c h e c k e d by u s i n g s u r f a c e s c h o s e n i n t h e C0113 z o n e . T h i s i s the case f o r t h e Pt(211) o r i e n t a t i o n which c o n t a i n s (111) t e r r a c e s with t h r e e atomic rows. F i g . 5 shows t h a t t h e (100) s i t e s c o n t r i b u t i o n i s U n i t e d t o a s i n g l e r e v e r s i b l e peak a t t h e p o t e n t i a l of the l e a s t s t r o n g l y bonded hydrogen on P t ( l O O ) , P M 3 1 0 ) and P t (511), w h i l e t h e t e r r a c e c o n t r i b u t i o n g i v e s r i s e t o t h e broad peak i n t h e low p o t e n t i a l r e g i o n . C o n s e q u e n t l y i t may b e c o n c l u d e d f r o * t h e r e s u l t s o b t a i n e d with t h e t h r e e o r i e n t a t i o n s t h a t (111) s i t e s i n c l u ded i n t o s u r f a c e d o « a i n s w i t h no b i d i « e n s i o n a l l o n g - r a n g e order (steps, terraces) give a c o n t r i b u t i o n to hydrogen adsorption i n the low p o t e n t i a l r e g i o n which i s superposed with t h e (110) s i t e n a r r o w c o n t r i b u t i o n a t 0 . 1 1 0 V. N e v e r t h e l e s s t h e p r e s e n c e o f ( 1 1 1 ) t e r r a c e s * a y be d e t e c t e d by t h e p r e s e n c e o f r e s i d u a l u n u s u a l s t a t e s a b o v e 0.4 V . Volta**etric

surface analysis of p l a t i n i z e d o l a t i n u *

R e c e n t l y i n t h i s l a b o r a t o r y we h a v e r e - i n v e s t i g a t e d o n p l a t i n i z e d p l a t i n u * surfaces the conditions f o r the observation of the var i o u s adsorption s t a t e s as a consequence of the existence of o r dered do«ains with basal o r i e n t a t i o n s at t h e surface of the c r y s t a l l i t e s . N h e n we c o n s i d e r c l a s s i c a l r e s u l t s w i t h p l a t i n i z e d p l a t i n u * e l e c t r o d e a v o l t a « * o g r a « s i Hilar t o t h a t r e p o r t e d on F i g . 6a i s o b s e r v e d . T h i s t y p e o f v o l t a « * o g r a * was o b t a i n e d a f t e r c y c l i n g t h e e l e c t r o d e b e t w e e n 0 . 0 2 - 1.6 V, i n o t h e r w o r d s f o r p e r t u r b e d c r y s t a l l i t e s . It i s i n t e r e s t i n g to investigate these electrodes f r o * the point of view of the unavoidable presence of s * a l l ordered domains at the surface of the unperturbed c r y s t a l l i t e s . If s u c h d o m a i n s e x i s t t h e y h a v e t o y i e l d a d s o r p t i o n s t a t e s w h i c h were not o b s e r v e on t h e c o n v e n t i o n a l voltammogram o f p l a t i n i z e d p l a t i num. T h e i r o b s e r v a t i o n r e q u i r e s t h e p r e p a r a t i o n o f c l e a n s a m p l e s a v o i d i n g t h e u s e o f c y c l i n g u p t o 1.5 V. D e p o s i t s o f p l a t i n u * b l a c k w i t h a s u r f a c e e x p a n s i o n c o e f f i c i e n t o f a b o u t 110 w e r e p r e pared and s t u d i e d i n a narrow range of p o t e n t i a l . A t y p i c a l v o l tammogram i s g i v e n i n f i g . 6 b . T h i s voltammogram shows t h e p r e s e n c e o f a d s o r p t i o n s t a t e s i n t h e r a n g e 0 . 3 - 0.4 V c o r r e s p o n d i n g t o h y drogen adsorbed a t (100) t e r r a c e s with v a r i o u s widths and t h e low a n d w i d e s t a t e f r o m 0.4 t o 0 . 6 V w h i c h i s c h a r a c t e r i s t i c o f ( 1 1 1 ) narrow t e r r a c e s . Contrary t o the conventional d i s t r i b u t i o n of hydrogen at a c t i v a t e d p l a t i n u m t h e p e a k a t 0 . 2 6 V i s h i g h e r t h a n t h e p e a k a t 0 . 1 1 0 V. The r e g i o n o f t h e w e a k l y b o n d e d h y d r o g e n s h o w s t h e n a r r o w peak corresponding t o (110) s i t e s superposed t o a broader adsorption s t a t e as expected from (111) s t e p s and t e r r a c e s c o n t r i b u t i o n s . A f t e r a p p l y i n g c y c l e s b e t w e e n 0 . 0 2 a n d 1.6 V t h e v o l t a m m o g r a m i s changed t o that reported i n f i g . 6 a , the surface expansion decreas e s down t o 86 a n d t h e u n u s u a l s t a t e s d i s a p p e a r w i t h a d r a m a t i c d e c r e a s e o f t h e c u r r e n t i n t h e d o u b l e l a y e r r e g i o n a b o v e 0.4 V. These r e s u l t s with p l a t i n u * black f i t well with data f r o * stepped s u r f a c e s and c o n f i r m t h a t v o l t a m m e t r y o f a d s o r b e d h y d r o g e n c a n be used a s a t o o l l e a d i n g t o an i n t e r e s t i n g l e v e l o f d e s c r i p t i o n of the c r y s t a l l i n e surface s t r u c t u r e of a c a t a l y s t l i k e platinum.



214


F i g u r e 5. V o l t a m m o g r a m s o f P t ( l O O ) T h e s i s ) and s t e p p e d s u r f a c e s .

(two p r o f i l e s ,

S.6.

Sun,

F i g u r e 6. V o l t a m m o g r a m s o f p l a t i n u m b l a c k ( a ) a f t e r c y c l e s b e t w e e n 0 . 0 2 - 1.6 V ( b ) f r e s h l y e l e c t r o d e p o s i t e d p l a t i n u m b l a c k c y c l e d w i t h o u t o x y g e n a d s o r p t i o n ( a f t e r Sun and Clavilier).


14. CLAVILIER


215

The u s e o f a n u m e r i c a l a n a l y s i s o f v o l t a m m o g r a m s w i t h a p r e c i s e deconvolution technique of t h e adsorption s t a t e s (23 24) andt h e comparison of r e s u l t i n g data with hard sphere models oft h e s t e p p e d s u r f a c e s i n number o f c a s e s a l l o w s an u n d e r s t a n d i n g o f t h e i n t e r f a c i a l processes a t t h e atomic level of t h e working s u r f a c e , in t h e presence of t h e e l e c t r o l y t e . T


Acknowledgments The a u t h o r i s g r a t e f u l t o D r s O.Armand, M . P e t i t , S.6.Sun, T. V a n d e r n o o t a n d times A . P l a z a a n d Y. R o d i e r f o r t h e i r a s s i s t a n c e i n v a r i o u s ways d u r i n g t h e p r e p a r a t i o n o f t h i s m a n u s c r i p t .

Literature Cited

1. Hammett, V. W. J.Am.Chem.Soc 1924, 46, 7. 2. James, S. D. J.Electrochem.Soc. 1967, 114, 1113. 3. Will, F. G. J.Electrochem.Soc. 1965, 112, 451. 4. Ishikawa, R. M.; Hubbard, A. T. J.Electroanal.Chem. 1976, 69, 317. 5. O'Grady, W. E.; Woo, M. Y. C . ; Hagans, P. L.; Yeager, E. J.Vac. Sci.Technol. 1977, 14, 365. 6. Yeager, E; O'Grady, W. E.; Woo, M. Y. C . ; Hagans, P. L. J. Electrochem.Soc. 1978, 125, 348. 7. Yamamoto, K.; Kolb, D.M.; Kotz, R; Lehmpfuhl, G. J.Electroanal. Chem. 1979, 96, 233. 8. Hubbard, A. T.; Ishikawa, R. M.; Katekaru, J . J. Electroanal. Chem.1978, 86, 271. 9. C l a v i l i e r , J.; Faure, R.; Guinet G.; Durand, R. J.Electroanal. Chem. 1980, 107, 205. 10. C l a v i l i e r , J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. 11. C l a v i l i e r , J.; Armand, D.; Wu, B. L. J.Electroanal.Chem. 1982, 135,159. 12. Clavilier, J.; Armand, D. J.Electroanal.Chem. 1986, 199, 187. 13. Al-Jaaf-Golze, K.; Kolb, D.; Scherson, D. J.Electroanal. Chem. 1986, 200, 353. 14. Aberdam, D.; Corotte, C.; Dufayard, D.; Durand, R.; Faure R.; Guinet, G. Proceedings of the 4 Conference on Solid Surfaces, September 22-26(1980), Cannes, France, Supplément à "Le Vide, Les Couches Minces" 1980, 201, 622. 15. Wagner, F. T.; Ross, P. N. J r . J.Electroanal.Chem. 1985, 150, 141. 16. Wagner, F. T.; Ross, P. N. J r . Surface Science 1985, 160, 305. 17. Aberdam, D.; Durand, R.; Faure, R.; El-Omar, F. Surface Science 1986, 171, 303. 18. C l a v i l i e r , J. J.Electroanal.Chem. 1980, 107, 211. 19. Christmann, K.; Ertl, G. Surface Science 1976, 60, 365. 20. Furuya, N.; Motoo, S.; Kunimatsu, K. J.Electroanal.Chem. 1988, 239, 347. 21. Motoo, S.; Furuya, N. J.Electroanal.Chem. 1984, 167, 309. 22. Scortichini, C. L.; Reilley, C. N. J.Electroanal.Chem. 1982, 139, 233. 23. Armand, D., Clavilier, J. J.Electroanal.Chem. 1987, 225, 205. 24. Armand, D., C l a v i l i e r , J . J.Electroanal.Chem. 1987, 233, 251. th

R E C E I V E D May 17,

1988


Electrochemical Surface Characterization of Platinum Electrodes

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