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hydroquinone couple at platinum electrodes in aqueous solutions
3. Electroanal. Cheat, 185 (1985) 3 3 1 - 3 3 8 Elsevier Sequoia S:A., L a u s a n n e - Printed in T h e N e t h e r l a n d s
331
REACTION-MECHANISM OF THE BENZOQUINONE/HYDROQUINONE C O U P L E A T P L A T I N U M E L E C T R O D E S IN A Q U E O U S S O L U T I O N S R E T A R D A T I O N A N D E N H A N C E M E N T O F E L E C T R O D E K I N E T I C S BY S I N G L E CHE1VIISORBED L A Y E R S
J A M E S H. W H I T E , M A N U E L
P. S O R I A G A
~" a n d A R T H U R
T. HUBBAP..D *
Department of Chemistry, Unioersity of California, Santa Barbara, CA 93106 (U.S.A.) (Received 26th O c t o b e r 1984; in revised f o r m 22rid N o v e m b e r 1984)
ABSTRACT T h e electrochemical kinetics of the b e n z o q u i n 0 n e ( Q ) / h y d r o q u i n o n e ( H ~Q) r e d o x c o u p l e at p l a t i n u m electrodes it, a q u e o u s s o l u t i o n s has been f o u n d to be e x t r e m e l y sensitive to the n a t u r e o f species a d s o r b e d o n the ele. )de s u r f a c e at monolayer coverages. E x p e r i m e n t a l m e a s u r e m e n t s w e r e b a s e d o n thin-layer cyclic v o i t a m m e t r y ; the u s e o f thin-layer electrodes w a s dictated by the n ~ d to m i n i m i z e s u r f a c e • c o n t a m i n a t i o n . Bulky n e u t r a l o r a n i o n i c a r o m a t i c a d s o r b a t e s led to the familiar U - s h a p e d r a t e - v s . - p H curves; the rate m i n i m u m o c c u r r e d n e a r p H 4. Kinetic effects d u e to o r l e n t l l t:hanges o f c h e m i s o r b e d species were no~ed o n l y w h e n the rate w a s low. A d s o r b e d I a t o m s led to compztratively r a p i d reactivity (rate c o n s t a n t , k ° > 10 - 3 e m s - 1 ) a n d virtual i n d e p e n d e n c e of p H . P r o f o u n d r,:tardation resu!ted f r o m p r e t r e a t m e n t o f the s u r f a c e w i t h C N - a n d S C N - ; total h-reversibility ( k ° < 10 .-6 e m s - ~ ) w a s o b s e r v e d at p H 4, with a f u r t h e r decrease in rate at p H 7. In c o n t r a s t , w h e n the s u r f a c e c o n t a i n e d ,~ layer o f c h e m i s o r b e d p h e n y l t r i e t h y l a m m o u l u m cations, the electrode rate increased, w i t h i n c r e a s i n g p H . T h e results indicate that different reaction p a t h w a y s p r e d o m i n a t e w h e n different a d s o r b a t e s are p r e s e n t .
INTRODUCTION
The electrochemical kinetics of the b e n z o q u i n o n e ( Q ) / h y d r o q u i n o n e (HaQ) couple in aqueous solutions has been ix~vestigated extensively [1-10]; reviews of the earlier studies have appeared [11,12]. As is well-known, detai!ed treatment o f this redox couple is complicated, not o n l y due to the interrelation of reactants, intermediates and products by electron- ~.rtd proton-transfer reactions, but a!so by the influence, o f adsorbed species. O n Pt ,flectrodes, the redox mechanism proposed by Vetter D,2,11], in which th~ nature of the charge-transfer steps varies with pH, appears ;o have gained wide acceptance. At l o w pH, the order of electron and proton transfers for Q reduction was suggested l o be H + e - H + e - , while at higher pH, the order w a s e - H + e - H ~ - ; for a variety of quinones, a minimum in the * T o w h o m c o r r e s p o n d e n c e s h o u l d be a d d r e s s e d . 0022-0728/85/$03.30
"
© 1985 Elsevier S e q u o i a S.A.
332
exchange currents was found near p H --- 4 [12], the transition region between the two:. mechanisms. These conclusions were disputed by Losb_karev and T o m i l o v [3] who r ep o r ted that the kinetics of Q reduction was actually i n d e p e n d e n t of pH . A direct two-electron transfer step was thought to be rate-limiting, with the reaction sequence being e e H + H ÷. In a recent paper, Laviron [10] presented an analysis of Q / H 2 Q redox data based on a theory [13] which assumed equilibrium p r o t o n a t i o n reactions; at p H - 4 the reaction sequence for reduction was concluded to be e - H + H + e [10]. A few investigations have been reported of the influence of various adsorbed species [6-9]. It was generally found that exchange currents decreased as the a m o u n t of adsorbed material increased. T h e exchange currents were also noted to be p H - d e p e n d e n t , with a m i n i m u m at pI--I - 4 . At rather high concent rat i ons [ >/0.1 M], self-inhibition of H2Q electro-oxidation, presum ed to be a consequence o f adsorbed intermediates and products, was reported by Gileadi [9]. Ellipsometric work has indicated that such inlfibitors are polymeric films of thickness up to - 0.1 g m [14]. However, systematic studies using well characterized monolayer adsorbates apparently have not been previously pursued. In the present article, results (obtained in conj unct i on with systematic studies on the structure and reactivity of adsorbed intermediates [15-40]) are described which show, for the first time, that kinetic control (rate e n h a n c e m e n t or retardation) of the Q / H 2 Q couple at a given p H can be brought about by p r o p e r choice of single chemisorbed layers of charged or uncharged adsorbates. T h a t is, the reaction mechanism o f the Q / H 2 Q couple has been found to depend on the nature and composition o f species adsorbed at m o n o l a y e r coverages on the Pt surface. Th e influence of the following adsorbates on the electrode kinetics of the u n ad s o r bed Q / H 2 Q couple were studied: h y d r o q u i n o n e [H2Q; flat (~/6) and edge (r/2) orientations], h y d r o q u i n o n e sulfonate (7/6- and rlZ-HQS), pyri di ne [4,5-r/2-Py (from adsorption at p H = 0) and N-,/LPy (from adsorption at p H = 7)], phenyltrie t h y l a m m o n i u m perchlorate [r/6-PTEA and r/2-PTEA], iodine atoms, cyanide and thiocyanate ions. T h e adsorption characteristics of these c o m p o u n d s have been d o c u m e n t e d [15-39,41-43]. Electrochemical kinetic m easurem ent s were m a d e at p H = 0, 4 and 7. EXPERIMENTAL
Th e electrode kinetics of the Q / H 2 Q couple was studied using thin-layer cyclic voltammetry, along the lines described in the reviews [44]. Particular attention was given to m eas ur em e nt of (i) the separation in potential ( A E ) between the anodic (~Ep) and cathodic (¢Ep) peaks ( A E = a E p --aE¢), and (ii) the ratio between the anodic (aip) and cathodic ( t i p) peak currents from cyclic c u r r e n t - p o t e n t i a l curves: F o r totally irreversible reactions, the electrochemical rate constant k ° is related in a logarithmic fashion to the peak separation A E ; if the electron-transfer coefficient ,~ is equal to 0.5, the exact relationship is [17]: log k ° = -- ( 1 2 6 0 / T ) ( A E ) + log( Vb] r [ / 2 A R T )
(1)
333 w h e r e T is the t e m p e r a t u r e , V the t h i n - l a y e r ceil v o l u m e , F the F a r a d a y c o n s t a n t , r the sweep rate, A the e l e c t r o d e s u r f a c e area, a n d R the gas c o n s t a n t . T h e p a r a m e t e r ct is related to aip mid eip [44]: ¢~ = ~ i p / ( a i p - - e / p )
(2)
It m a y b e m e n t i o n e d that t h i n - l a y e r e l e c t r o c h e m i c a l t e c h n i q u e s are n o t p a r t i c u l a r l y suited for e x t r e m e l y fast electrode reactions. H o w e v e r , the n e e d to m i n i m i z e s u r f a c e c o n t a m i n a t i o n b y i m p u r i t i e s o t h e r t h a n the subject a d s o r b a t e s d i c t a t e d the use of t h i n - l a y e r e l e c t r o d e s ( T L E ) ; in a d d i t i o n , for the single p u r p o s e o f d e m o n s t r a t i n g relative changes in e i e c t r o d e kinetics b r o u g h t a b o u t b y c h a n g e s in s u r f a c e c o m p o s i tion, the T L E m e t h o d is m o r e t h a n a d e q u a t e . In this study, the p a r a m e t e r s k ° a n d ~ wel:e not a c t u a l l y c a l c u l a t e d since, as j u s t m e n t i o n e d , q u a l i t a t i v e t r e a t m e n t o f the d a t a is m o s t efficient for the p r e s e n t p u r p o s e ; instead, A E a n d aip/eip v a l u e s are t a b u l a t e d . U n d e r t h i n - l a y e r c o n d i t i o n s , for totally irreversible ( k ° < 10 - 6 c m s -1) r e d o x couples, A E is g r e a t e r t h a n 200 mV; for v e r y rapid ( k ° > 10 - 3 c m s -~) reactions, A E vanishes a n d ~ip/¢ip app r o a c h e s unity. T h e p e a k p o t e n t i a l s were c o r r e c t e d for i R d r o p b y c a l i b r a t i o n with reversible F o ( I I I ) / F e ( I I ) c o u p l e s in the s a m e electrolyte. V o l t a m m e t r i c c u r v e s were run at 2 m V / s to m i n i m i z e iR d r o p a n d to maintaAn true t h i n - l a y e r c o n d i t i o n s [44]. T h e p e a k c u r r e n t s were c o r r e c t e d f o r b a c k g r o u n d using v o l t a m m e t r i c c u r v e s obt a i n e d in the p r e s e n c e o f a d s o r b a t ¢ b u t in the a b s e n c e o f unadsorbed h y d r o q u i n o n e . T h e c o n c e n t r a t i o n o f h y d r o q u i n o n e used was limited to 0.1 m } l since, u n d e r these c o n d i t i o n s , e x c h a n g e r e a c t i o n s b e t w e e n a d s o r b e d a n d u n a d s o r b e d species d o not exist; s t r u c t u r a l transitions within the p r e a d s o r b e d l a y e r are likewise a b s e n t [31,351. S u r f a c e p r e t r e a t m e n t consisted o f e x p o s i n g the clean Pt t h i n - l a y e r e l e c t r o d e to a dilute a q u e o u s s o l u t i o n o f the selected a d s o r b a t e ; excess u n a d s o r b e d m a t e r i a l was rinsed a w a y p r i o r to v o l t a m m e t r i c e x p e r i m e n t s . Such p r e t r e a t m e n t f o r m e d stable, c l o s e - p a c k e d single ¢hernisorbed layers [ 1 5 - 3 9 , 4 1 - 4 3 ] . C o n c e n t r a t i o n a n d p H were e m p l o y e d to b r i n g a b o u t d i f f e r e n t a d s o r b a t e o r i e n t a t i o n s ( m o d e o f binding). F o r e x a m p l e , 7/6-HQ was o b t a i n e d b y H Q a d s o r p t i o n at c o n c e n t r a t i o n s b e l o w 0.1 m~,l, while 2,3-T/2-HQ was f o r m e d a b o v e 1 r a M ; N-~iLpyridine was f o r m e d b y a d s o r p t i o n at p H = 7, while 4,5-7/2-pyridine o c c u r r e d at p H = 0 [17,!8]. C l e a n e l e c t r o d e s were g e n e r a t e d b y e l e c t r o c h e m i c a l o x i d a t i o n at 1.2 V ( A g / A g / C I r e f e r e n c e ) a~d r e d u c tion at - 0 . 2 V in 1 M H 2 S O 4. S o l u t i o n s were m a d e with p y r o l y t i c a l l y distilled w a t e r [45]; p H = 0 solutions c o n t a i n e d I M H 2 S O 4, while those at p H = 4 a n d 7 c o n t a i n e d I M N a 2 S O 4 b u f f e r e d with N a H 2 P O 4 / H 2 S O 4 a n d N a H a P O a / N a O H , respectively [15,16]. RESULTS AND DISCUSSION F i g u r e 1 shows t h i n - l a y e r cyclic c u r r e n t - p o t e n t i a l c u r v e s for the e l e c t r o c h e m i c a l i n t e r c o n v e r s i o n o f b e n z o q u i n o n e (Q) a n d h y d r o q u i n o n e (H2Q), Q + 2 e - + 2 H + ~ H2Q, o n various p r e c o a t e d p l a t i n u m e l e c t r o d e s in a q u e o u s s o l u t i o n o f p H 4 w h e r e
334 the Q / H 2 Q e l e c t r o d e r e a c t i o n r a t e is s u p p o s e d to b e at a m i n i m u m [1,2,11]. T h e f o l l o w i n g f e a t u r e s a r e e v i d e n t : (i) T h e p e a k s e p a r a t i o n A E w a s a f f e c t e d o n l y slightly b y t h e o r i e n t a t i o n o f the a d s o r b e d H Q i n t e r m e d i a t e . A E ( c o r r e c t e d for i R d r o p ) = 80 m V with ~/6-HQ, 94 m V with r/2-HQ, t h a t is, the relatioe r a t e w a s l o w e r w h e n the s u r f a c e w a s p r e t r c a t e d w i t h e d g e w i s e - H Q t h a n with f l a t - H Q . A s will b e d i s c u s s e d b e l o w , these A E v a l u e s a r e higher ( t h e relative r a t e s a r e lower) t h a n at p H = 0 o r 7. (ii) T h e p e a k s e p a r a t i o n w h e n the e l e c t r o d e w a s p r e t r e a t e d with I a t o m s w a s close to z e r o ( A E = 5 mV), a n d i n d i c a t e s a p r o f o u n d e n h a n c e m e n t , to v i r t u a l l y c o m p l e t e reversibility ( k ° > ~ 10 -3 c m s - l ) , o f the Q / H , _ Q r e d o x c o u p l e . ( i i i ) T h e p e a k s e p a r a t i o n w h e n the e l e c t r o d e c o n t a i n e d c h e m i s o r b e d C N - , A E = 336 m V , w a s m u c h h i g h e r t h a n for the e d g e w i s e - H z Q - p r e t r e a t e d s u r f a c e , A E = 94 m V ; this reflects a d r a m a t i c r a t e r e t a r d a t i o n ( k ° ~ < 10 - 6 c m s - l ) o f the Q / H ~ Q c o u p l e b y a d s o r b e d C N - . T h u s . the r a t e o f the Q / H 2 Q c o u p l e at p H = 4 h a s b e e n v a r i e d f r o m r e v e r s i b l e to i r r e v e r s i b l e b y s u r f a c e p r e t r e a t m e n t ; in p a r t i c u l a r , it w a s m a d e , in essence, c o m p l e t e l y reversible b y I - p r e t r e a t m e n t , a n d totally i r r e v e r s i b l e b y C N - pretreatment. T a b l e 1 gives A E a n d ~ip/ci p values for the Q / H 2 Q c o u p l e at selected p H o n v a r i o u s p r e t r e a t e d Pt surfaces. (i) A s a l r e a d y s e e n in Fig. 1, the r a t e w a s o n l y w e a k l y
A
I
!
-'. ;I
I-Z
IIi
I Ii
i iii
i
~
•
!
-
/ ~
nO
1
i
I
i
0.2 0.4 O.G POTENTIAL/V Fig. 1. T h i n - l a y e r cyclic c u r r e n t - p o t e n t l a l c u r v e s for t h e r e a c t i o n Q + 2 - O.2
0
e-+2
H~--H2Q
o n Pt
electrodes pretreated with ( ) "q6-HQ, ( - - - - - - ) ~2-HQ, ( . . . . . ) I and ( . . . . . . ) CN- Aqueous solution, pH 4; electrode reaction rate is supposed to be at a minimum [1.2,11].
335
influenced by orientationaJ changes wittfin the adsorbed H2Q layer at pH = 4, the rates slightly lower with edge- than with flat-aromatic pretreatment. At p H 0 and 7, where the couple behaves quasi-reversibly, n o o r i e n t a t i o n a l effects were evident. (ii) Profound retardation (to total irreversibility) was observed when h y d r o q u i n o n e sulfonate (HQS), a bulky, aromatic anion, was present in the flat-orientation on the TABLE 1 Electrode kinetics of the b e n z o q u i n o n e / h y d r o q u i n o n e couple at precoated Pt elec~odes in aqueous solutions a Adsorbate b
pH during electrolysis
1 ~/°-Hydroquinone (flat H Q )
0 4
7 2 2,3-~/Z-Hydroquinone (edgewise HQ)
3 "06-Hyclrocluinone-sulfonate (flat HQS)
A E C/mV
aip/cip d
34 + 5 80 35
1.0 +_0.05 0.89 0.96
0 4 7 0 4 7
35 94 38 63 217 98
1.0 0.91 0.96 1.3 0.83 0.55
4
281
0.73
0 4 7
49 41 5
1.1 0.93 0.95
4
52
0.84
4
68
0.79
4
151
0.69
0
5
4
5
4 5,6-~2-hydroqulnone-sulfonate
5
(edgewise HQS) ~,6-Phenyltriethylammonium perchlorat.e (flat PTE&)
6 3,4-~/2-Phenyltriethylammonium perchlorate (edgewise P T E A ) 7 4,5-~z-Pyridine (edgewise lay from adsorption at p H = O) 8 .w1-N-pyridine (endwise Py from adsorption at p H = 7) 9 Iodine (I)
10 Cyanide ( C N - )
7
5
1.0 1.0 1.0
0 4
121 336 410
2.0 1.7 2.6
10
1.1
7
11
Thiocyanate(SCN-)
0
4 7 a T h e solutions contained 0.1 m M
hydroqulnone in I M
215 234 + 5
1.3 0.51:5:0.05
I-/zSO 4 ( p H = 0) or buffered 1 M" N a 2 S O 4
( p H = 4 a n d 7). b Preparation of adsorbate monolayer is desen'bed in the text. ¢ (AE--- a E p - - c E p ) peak potenti~l~ were corrected for i R drop, as described in the text; u n d e r thln-layer conditions. A E ----0 for a completely reversible redox couple. d Peak eta-rents were corrected for b a c k g r o u n d ctu-rent; ~ip ~alp for a rever~ble couple.
336
Pt surface at p H 4; a p p r e c i a b l e r e t a r d a t i o n was n o t e d even at p H 7 a n d zero. T h e 7/6 ~ r/2 r e o r i e n t a t i o n o f I-IQS c a u s e d gt~eater r e t a r d a t i o n ( A E C -- AEn, - 65 m V ) at p H 4, indicating that for this a d s o r b a t e , the Q / H 2 Q c o u p l e was m o r e sensitive to o r i e n t a t i o n a l changes. (iii) W h e n p h e n y l t r i e t h y l a m m o n i u m ( P T E A ) , a b u l k y a r o m a t i c cation, was used as the a d s o r b a t e , r a t e - e n h a n c e m e n t was n o t e d , which increased as the p H was increased (to com_p!ete reversibility at p H 7); since the rates were e n h a n c e d , o n l y a w e ~ d e p e n d e n c e o n o r i e n t a t i o n was o b s e r v e d . (iv) A d s o r p t i o n o f p y r i d i n e (lay) at p H 0, a p p a r e n t l y in the 4,5-r/2-orientation with the p r o t o n a t e d n i t r o g e n h e t e r o - a t o m directed a w a y f r o m the surface [17,18], slightly e n h a n c e d the Q / H 2 Q electrolysis rate at p H 4 (relative to the i n f l u e n c e o f 2,3-r/2-H2Q); a d s o r p tion at p H 7, which results in an N-r/~-Py layer, r e t a r d e d the rate c o n s i d e r a b l y . (v) S u r f a c e p r e t r e a t m e n t with iodine, which f o r m s a c l o s e - p a c k e d layer o f neutral I a t o m s [41,47], r e n d e r e d the e l e c t r o d e kinetics c o m p l e t e l y reversible a n d p H - i n d e p e n dent. (vi) A d s o r b e d cyanide, which f r o m studies at well-defined Pt single crystals is k n o w n to consist o f a d s o r b e d a n i o n s s u r r o u n d e d b y n e u t r a l molecules [46], led to d r a m a t i c r e t a r d a t i o n o f the rate, even at p H 0. T h e r e a c t i o n b e c a m e totally irreversible at p H 4, a n d the rate d e c r e a s e d still f u r t h e r at p H 7. (vii) C h e m i s o r p t i o n o f t h i o c y a n a t e , which has also b e e n s h o w n to f o r m a n i o n i c surface species [43], gave results similar to C N - in the sense that total irreversibility o f the Q / H 2 Q c o u p l e was o b s e r v e d at p H 4, with even slower rates at p H 7; however, the rate was e n h a n c e d to nearly c o m p l e t e reversibility at p H 0 by c h e m i s o r b e d S C N - . T h e foregoing results d e m o n s t r a t e that the r e a c t i o n m e c h a n i s m o f the Q / H 2 Q c o u p l e in a q u e o u s solutions is very sensitive to the n a t u r e a n d c o m p o s i t i o n o f species a d s o r b e d on the e l e c t r o d e surface at monolayer cooerages. F o u r d i f f e r e n t p a t t e r n s o f p H - d e p e n d e n c e have emerged, d e p e n d i n g o n e l e c t r o d e surface s t r u c t u r e . a n d c o m position: (i) fast reactions at p H 0 and 7, but slow at p H 4, in the p r e s e n c e o f a r e n e a d s o r b a t e s such as H Q a n d H Q S ; (ii) rates which d e c r e a s e d with increasing p H in the p r e s e n c e o f a d s o r b e d small a n i o n s such as S C N - and C N - ; (iii) rates which increased with increasing p H [the reverse o f (ii)] as for e l e c t r o d e s p r e t r e a t e d with the a r e n e q u a t e r n a r y a m m o n i u m cation, P T E A ; (iv) very fast kinetics, i n d e p e n d e n t o f p H , in the p r e s e n c e o f a d s o r b e d I atoms. Case (i) is c o n s i s t e n t with the Vetter m e c h a n i s m [1,2,11], that is; H ÷ e - H + e - at pH <4and e-H+e-H+atpH >7. T h e r e t a r d a t i o n o f the r e a c t i o n rate by n e g a t i v e l y - c h a r g e d c h e m i s o r b e d species, case (ii), a p p e a r s to result f r o m the influence o f the electrical d o u b l e - l a y e r . Evidently, the reaction s e q u e n c e in the r e d u c t i o n o f b e n z o q u i n o n e involves e l e c t r o n t r a n s f e r to a negatively-charged q u i n o n o i d i n t e r m e d i a t e ( L o s h k a r e v - T o m i l o v mecha n i s m [3]); at p H 4 on a C N --coated Pt s,wface, the r e a c t i o n m o s t p r o b a b l y involves r e d u c t i o n o f the Q - radical a n i o n as the r a t e - d e t e r m i n i n g step: Q+eQ'-+e-
~
CN -/Pt
Q-
s] o~.,
~
(3a)
Q2-
(3b)
CN -/Pt
Q-'-+ 2 H +
~
CN -/Pt
H.Q -
(3c)
337
Excess positive charge in the double-layer would e n h a n c e reaction (3b) [44]; -accordingly, the rate e n h a n c e m e n t brought a b o u t by the bulky a d s o r b e d cation, P TEA, case (Hi), is also consistent with the e - e - H : - H + sequence shown above. Rapidity of the Q / H 2 Q electrode reactions at Pt surfaces cont ai ni ng adsorbed I atoms, case (iv), is attributable to the c o m b i n e d effects of the small thickness of the I-atom adsorbed layer ( c o m p a r e d with molecular layers such as of aromatics) and exclusion of adsorbed anions by the layer of neutral a d s o r b e d I atoms. R a t e e n h a n c e m e n t by I atoms constitutes an exception to the generalization [6-8,12] that higher coverages of adsorbed material lead to lower rates. T o the contrary, the close-packed layer of I atoms [41-43] enhanced reactivity to an extent a p p r o a c h i n g c o m p l e t e reversibility a t any pH . A similar effect ,vas report ed earlier [48] on the electrode rates of P t ( I I / I V ) complexes. ACKNOWLEDGEMENTS Acknowledgment is m a d e t o t h e A i r F o r c e O f f i c e o f S c i e n t i f i c R e s e a r c h a n d t h e Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We thank Professor Roger Parsons for helpful comments. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
I,. -tter, Z. Elektrochem.. 56 (1952) "197. J.l. 1ale a n d R. Parsons, Trans. Faraday Soc., 59 (1963) 1429. M.A. Loshkarev a n d B.I. Tornilov, Z.h. Fiz. Khim., 34 (1960) 1735; 36 (1962) 132. R. Rosenthal, A.E. Lorch and L.P. H a m m e t t , J. Am. Chem. Soe., 59 (19~7) 1795. W.R. T u r n e r and P J . Elving, J. Electrochem. Soe., 112 (1965) 1215. B.I. Tornilov and M.A. Loshkarev, Zh. Fiz. Khim., 36 (1962) 1902. Y. Lu-an, Yu.B. Vasiliev a n d V.S. Bagotskii, Zh. Fiz. Khim., 38 (1964) 205; Elektrokhimiya, i (1965) 170. Y. Lu-an, V.E. Kazatinov, Yu.B. Vasiliev a n d V.S. Bagotskii, Elektrokhirrfiya, 1 (1965) 176; Dold. Akad. Nauk SSSR, 151 (1963) 151. E. Zeigerson and E. Gileadi, J. Electroanal. Chem., 28 (1970) 421. F__ Laviron, J. Eleetroanal. Chem., 164 (1984) 213. K.J. Vetter, Electrochemical K.ineties, Academic Press, New Y:,rk, 1967. J.Q. C h a m b e r s in S. Patai (Ed.), Chemistry of Q u i n o n o i d C o m p o u n d s , Wiley. New York, 1974. Ch. 14. E. Laviron, J. Electroanal. Chem., 146 (1983) 15. M. Babai and S. Got~.esfeld, Surf. Sci., 96 (1980) 461. M.P. Soriaga and A.T. H u b b a r d , J. Am. Chem. Soe., 104 (1982) 2735. M.P. Soriaga and A.T. H u b b a r d , J. Am. Chem. Soe., 104 (1982) 2742. M.P. Soriaga and A.T. H u b b a r d , J. Am. Chem. Soe., 104 (1982) 3937. M.P. Soriaga, P.H. Wilson, A.T. H u b b a r d a n d C.S. Benton. J. Electroanal. Chem., 142 (1982) 317. V.K.F. Chia, M.P. Soriaga, A.T. H u b b a r d a n d S.E. Anderson, J. Phys. Chem., 87 (1983) 232. M.P. Soriaga, J.H. White a n d A.T. H u b b a r d , J. Phys. Chem., 87 (1983) 3048. J.L. Stickney, M.P. Soriaga, A.T. H u b b a r d a n d S.E. Anderson. J. Eleetroanal. Chem., 125 (1981) 73. M.P. Sorlaga, J.L. Stiekney and A.T. H u b b a r d , J. Electroanat. Chem., 144 (1983) 207. M.P. Soriaga, J . L Stickney and A.T. H u b b a r d , J. Mol. Catal., 21 (1983) 211. M.P. Soriaga and A.T. H u b b a r d , J. Electroanal. Chem., 159 (1983) 101. M.P. Soriaga and A.T. H u b b a r d , J. Phys. Chem., 88 (1984) 1089.
338 26 M.P. Soriaga and A.T. H u b b a r d , J. Phys. Chem., 88 (1984) 1758. 27 M.P. Soriaga, V.K.F. Chia, J.H. White, D. Song a n d A.T. H u b b a r d , J. Electroanal. Chem., 162 (1984) 143. 28 M.P. Soriaga and A.T. H u b b a r d , J. Electroanal. Chem., 167 (1984) "79. 29 V.K.F. Chla, M.P. Sorlaga and A.T. H u b b a r d , J. Eltmtroanal. Chem., 167 (1984) 97. 30 M.P. Sofiaga, J.H. White, D. Song and A.T. H u b b a r d , J. Electroanal. Chem., 171 (1984) 359. 31 M.P. Soriaga, J.H. White, D. Song a n d A.T. H u b b a r d , J. Phys. Chem., gg (!984) 2285. 32 M.P. Soriaga, E. Binamira-Soriaga, A.T. H u b b a r d , J.B. Benziger a n d K.-W.P. Pang. Inorg. Chem., in press. 33 M.P. Soriaga, J.H. White, D. Song, V.K.F. Chia, P.O. Arrhenius a n d A.T. H u b b a r d , Lnorg. Chem., in press. 34 V.K.F. Chia, J.L. Stickney, M.P. Soriaga, S.D. Rosasco, G.N. Salaita, A.T. H u b b a r d , J.B. Bermiger and K.-W.P. Pang, J. EIectroanal. Chem., 163 (1984) 407. 35 J.H. White, M.P. Soriaga ~.and A.T. H u b b a r d , J. Electroanal. Chem., 177 (1984) 89. 36 M.P. Soriaga, D. Song and A.T. H u b b a r d , J. Phys. Chem., in press. 37 M.P. Soriaga, D. Song, D. Zapien a n d A.T. H u b b a r d , Langmuir, in press. 38 K_-W.P. Pang, J.B. Benziger, M.P. Soriaga and A.T. H u b b a r d , J. Phys. Chem., 88 (1984) 4583. 39 D. Song, M.P. Soriaga, K-L. Vieira a n d A.T. H u b b a r d , J. Electroanal. Chem., 184 (1985) 171. 40 A.T. H u b b a r d , J.L. Stiekney, M.P. Soriaga, V.K.F. Chia, S.D. Rosasco, B.C. SchardL, T. Solomun, O. Song. J.H. White a n d A. Wieckowski, J. Electroanal. Chem., 168 0 9 8 4 ) 43. 41 T.E. Pelter a n d A.T. H u b b a r d , J: Electroan~l. Chem., 100 (1979) 473; G.A_ Oarwood a n d A.T. H u b b a r d , Surf. Sol., 92 (1980) 617; 42 R_F. Lane and A.T. H u b b a r d , J. Phys. Chem., 79 (1975) 808; 43 J.L. Stickney, S.D. Rosasco, G.N. Salaita a n d A.T. H u b b a r d , Langmuir, in press. 44 A.T. H u b b a r d and F.C. Anson, J. Eleetroanal. Chem_, 4 (1970) 129; A.T. H u b b a r d , Crit. Rev. Anal. Chem., 3 (1973) 201; J. Electroanal_ Chem., 22 (1969) 165. 45 B.E. Conway, H. Angerstein-Kozlowka, W.B.A. Sharp a n d E.E. Criddle, Anal. Chem., 45 (1973) 1331. 46 S.D. Rosasco, J.L. Stickney, G.N. Salaita, D.G. Frank; J.Y. Katekaru, B.C. Schardt, M.P. Soriaga, D.A. Stern a n d A.T. H u b b a r d , Langmuir, in press. 47 R.F. Lane and A.T. H u b b a r d , J. Phys. Chem., 77 (1973) 1412. 48 A.L.Y. Lau and A.T. H u b b a r d , J. ElectroanaL Chem., 24 (1970) 237; 33 (1971) 77.
331
REACTION-MECHANISM OF THE BENZOQUINONE/HYDROQUINONE C O U P L E A T P L A T I N U M E L E C T R O D E S IN A Q U E O U S S O L U T I O N S R E T A R D A T I O N A N D E N H A N C E M E N T O F E L E C T R O D E K I N E T I C S BY S I N G L E CHE1VIISORBED L A Y E R S
J A M E S H. W H I T E , M A N U E L
P. S O R I A G A
~" a n d A R T H U R
T. HUBBAP..D *
Department of Chemistry, Unioersity of California, Santa Barbara, CA 93106 (U.S.A.) (Received 26th O c t o b e r 1984; in revised f o r m 22rid N o v e m b e r 1984)
ABSTRACT T h e electrochemical kinetics of the b e n z o q u i n 0 n e ( Q ) / h y d r o q u i n o n e ( H ~Q) r e d o x c o u p l e at p l a t i n u m electrodes it, a q u e o u s s o l u t i o n s has been f o u n d to be e x t r e m e l y sensitive to the n a t u r e o f species a d s o r b e d o n the ele. )de s u r f a c e at monolayer coverages. E x p e r i m e n t a l m e a s u r e m e n t s w e r e b a s e d o n thin-layer cyclic v o i t a m m e t r y ; the u s e o f thin-layer electrodes w a s dictated by the n ~ d to m i n i m i z e s u r f a c e • c o n t a m i n a t i o n . Bulky n e u t r a l o r a n i o n i c a r o m a t i c a d s o r b a t e s led to the familiar U - s h a p e d r a t e - v s . - p H curves; the rate m i n i m u m o c c u r r e d n e a r p H 4. Kinetic effects d u e to o r l e n t l l t:hanges o f c h e m i s o r b e d species were no~ed o n l y w h e n the rate w a s low. A d s o r b e d I a t o m s led to compztratively r a p i d reactivity (rate c o n s t a n t , k ° > 10 - 3 e m s - 1 ) a n d virtual i n d e p e n d e n c e of p H . P r o f o u n d r,:tardation resu!ted f r o m p r e t r e a t m e n t o f the s u r f a c e w i t h C N - a n d S C N - ; total h-reversibility ( k ° < 10 .-6 e m s - ~ ) w a s o b s e r v e d at p H 4, with a f u r t h e r decrease in rate at p H 7. In c o n t r a s t , w h e n the s u r f a c e c o n t a i n e d ,~ layer o f c h e m i s o r b e d p h e n y l t r i e t h y l a m m o u l u m cations, the electrode rate increased, w i t h i n c r e a s i n g p H . T h e results indicate that different reaction p a t h w a y s p r e d o m i n a t e w h e n different a d s o r b a t e s are p r e s e n t .
INTRODUCTION
The electrochemical kinetics of the b e n z o q u i n o n e ( Q ) / h y d r o q u i n o n e (HaQ) couple in aqueous solutions has been ix~vestigated extensively [1-10]; reviews of the earlier studies have appeared [11,12]. As is well-known, detai!ed treatment o f this redox couple is complicated, not o n l y due to the interrelation of reactants, intermediates and products by electron- ~.rtd proton-transfer reactions, but a!so by the influence, o f adsorbed species. O n Pt ,flectrodes, the redox mechanism proposed by Vetter D,2,11], in which th~ nature of the charge-transfer steps varies with pH, appears ;o have gained wide acceptance. At l o w pH, the order of electron and proton transfers for Q reduction was suggested l o be H + e - H + e - , while at higher pH, the order w a s e - H + e - H ~ - ; for a variety of quinones, a minimum in the * T o w h o m c o r r e s p o n d e n c e s h o u l d be a d d r e s s e d . 0022-0728/85/$03.30
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© 1985 Elsevier S e q u o i a S.A.
332
exchange currents was found near p H --- 4 [12], the transition region between the two:. mechanisms. These conclusions were disputed by Losb_karev and T o m i l o v [3] who r ep o r ted that the kinetics of Q reduction was actually i n d e p e n d e n t of pH . A direct two-electron transfer step was thought to be rate-limiting, with the reaction sequence being e e H + H ÷. In a recent paper, Laviron [10] presented an analysis of Q / H 2 Q redox data based on a theory [13] which assumed equilibrium p r o t o n a t i o n reactions; at p H - 4 the reaction sequence for reduction was concluded to be e - H + H + e [10]. A few investigations have been reported of the influence of various adsorbed species [6-9]. It was generally found that exchange currents decreased as the a m o u n t of adsorbed material increased. T h e exchange currents were also noted to be p H - d e p e n d e n t , with a m i n i m u m at pI--I - 4 . At rather high concent rat i ons [ >/0.1 M], self-inhibition of H2Q electro-oxidation, presum ed to be a consequence o f adsorbed intermediates and products, was reported by Gileadi [9]. Ellipsometric work has indicated that such inlfibitors are polymeric films of thickness up to - 0.1 g m [14]. However, systematic studies using well characterized monolayer adsorbates apparently have not been previously pursued. In the present article, results (obtained in conj unct i on with systematic studies on the structure and reactivity of adsorbed intermediates [15-40]) are described which show, for the first time, that kinetic control (rate e n h a n c e m e n t or retardation) of the Q / H 2 Q couple at a given p H can be brought about by p r o p e r choice of single chemisorbed layers of charged or uncharged adsorbates. T h a t is, the reaction mechanism o f the Q / H 2 Q couple has been found to depend on the nature and composition o f species adsorbed at m o n o l a y e r coverages on the Pt surface. Th e influence of the following adsorbates on the electrode kinetics of the u n ad s o r bed Q / H 2 Q couple were studied: h y d r o q u i n o n e [H2Q; flat (~/6) and edge (r/2) orientations], h y d r o q u i n o n e sulfonate (7/6- and rlZ-HQS), pyri di ne [4,5-r/2-Py (from adsorption at p H = 0) and N-,/LPy (from adsorption at p H = 7)], phenyltrie t h y l a m m o n i u m perchlorate [r/6-PTEA and r/2-PTEA], iodine atoms, cyanide and thiocyanate ions. T h e adsorption characteristics of these c o m p o u n d s have been d o c u m e n t e d [15-39,41-43]. Electrochemical kinetic m easurem ent s were m a d e at p H = 0, 4 and 7. EXPERIMENTAL
Th e electrode kinetics of the Q / H 2 Q couple was studied using thin-layer cyclic voltammetry, along the lines described in the reviews [44]. Particular attention was given to m eas ur em e nt of (i) the separation in potential ( A E ) between the anodic (~Ep) and cathodic (¢Ep) peaks ( A E = a E p --aE¢), and (ii) the ratio between the anodic (aip) and cathodic ( t i p) peak currents from cyclic c u r r e n t - p o t e n t i a l curves: F o r totally irreversible reactions, the electrochemical rate constant k ° is related in a logarithmic fashion to the peak separation A E ; if the electron-transfer coefficient ,~ is equal to 0.5, the exact relationship is [17]: log k ° = -- ( 1 2 6 0 / T ) ( A E ) + log( Vb] r [ / 2 A R T )
(1)
333 w h e r e T is the t e m p e r a t u r e , V the t h i n - l a y e r ceil v o l u m e , F the F a r a d a y c o n s t a n t , r the sweep rate, A the e l e c t r o d e s u r f a c e area, a n d R the gas c o n s t a n t . T h e p a r a m e t e r ct is related to aip mid eip [44]: ¢~ = ~ i p / ( a i p - - e / p )
(2)
It m a y b e m e n t i o n e d that t h i n - l a y e r e l e c t r o c h e m i c a l t e c h n i q u e s are n o t p a r t i c u l a r l y suited for e x t r e m e l y fast electrode reactions. H o w e v e r , the n e e d to m i n i m i z e s u r f a c e c o n t a m i n a t i o n b y i m p u r i t i e s o t h e r t h a n the subject a d s o r b a t e s d i c t a t e d the use of t h i n - l a y e r e l e c t r o d e s ( T L E ) ; in a d d i t i o n , for the single p u r p o s e o f d e m o n s t r a t i n g relative changes in e i e c t r o d e kinetics b r o u g h t a b o u t b y c h a n g e s in s u r f a c e c o m p o s i tion, the T L E m e t h o d is m o r e t h a n a d e q u a t e . In this study, the p a r a m e t e r s k ° a n d ~ wel:e not a c t u a l l y c a l c u l a t e d since, as j u s t m e n t i o n e d , q u a l i t a t i v e t r e a t m e n t o f the d a t a is m o s t efficient for the p r e s e n t p u r p o s e ; instead, A E a n d aip/eip v a l u e s are t a b u l a t e d . U n d e r t h i n - l a y e r c o n d i t i o n s , for totally irreversible ( k ° < 10 - 6 c m s -1) r e d o x couples, A E is g r e a t e r t h a n 200 mV; for v e r y rapid ( k ° > 10 - 3 c m s -~) reactions, A E vanishes a n d ~ip/¢ip app r o a c h e s unity. T h e p e a k p o t e n t i a l s were c o r r e c t e d for i R d r o p b y c a l i b r a t i o n with reversible F o ( I I I ) / F e ( I I ) c o u p l e s in the s a m e electrolyte. V o l t a m m e t r i c c u r v e s were run at 2 m V / s to m i n i m i z e iR d r o p a n d to maintaAn true t h i n - l a y e r c o n d i t i o n s [44]. T h e p e a k c u r r e n t s were c o r r e c t e d f o r b a c k g r o u n d using v o l t a m m e t r i c c u r v e s obt a i n e d in the p r e s e n c e o f a d s o r b a t ¢ b u t in the a b s e n c e o f unadsorbed h y d r o q u i n o n e . T h e c o n c e n t r a t i o n o f h y d r o q u i n o n e used was limited to 0.1 m } l since, u n d e r these c o n d i t i o n s , e x c h a n g e r e a c t i o n s b e t w e e n a d s o r b e d a n d u n a d s o r b e d species d o not exist; s t r u c t u r a l transitions within the p r e a d s o r b e d l a y e r are likewise a b s e n t [31,351. S u r f a c e p r e t r e a t m e n t consisted o f e x p o s i n g the clean Pt t h i n - l a y e r e l e c t r o d e to a dilute a q u e o u s s o l u t i o n o f the selected a d s o r b a t e ; excess u n a d s o r b e d m a t e r i a l was rinsed a w a y p r i o r to v o l t a m m e t r i c e x p e r i m e n t s . Such p r e t r e a t m e n t f o r m e d stable, c l o s e - p a c k e d single ¢hernisorbed layers [ 1 5 - 3 9 , 4 1 - 4 3 ] . C o n c e n t r a t i o n a n d p H were e m p l o y e d to b r i n g a b o u t d i f f e r e n t a d s o r b a t e o r i e n t a t i o n s ( m o d e o f binding). F o r e x a m p l e , 7/6-HQ was o b t a i n e d b y H Q a d s o r p t i o n at c o n c e n t r a t i o n s b e l o w 0.1 m~,l, while 2,3-T/2-HQ was f o r m e d a b o v e 1 r a M ; N-~iLpyridine was f o r m e d b y a d s o r p t i o n at p H = 7, while 4,5-7/2-pyridine o c c u r r e d at p H = 0 [17,!8]. C l e a n e l e c t r o d e s were g e n e r a t e d b y e l e c t r o c h e m i c a l o x i d a t i o n at 1.2 V ( A g / A g / C I r e f e r e n c e ) a~d r e d u c tion at - 0 . 2 V in 1 M H 2 S O 4. S o l u t i o n s were m a d e with p y r o l y t i c a l l y distilled w a t e r [45]; p H = 0 solutions c o n t a i n e d I M H 2 S O 4, while those at p H = 4 a n d 7 c o n t a i n e d I M N a 2 S O 4 b u f f e r e d with N a H 2 P O 4 / H 2 S O 4 a n d N a H a P O a / N a O H , respectively [15,16]. RESULTS AND DISCUSSION F i g u r e 1 shows t h i n - l a y e r cyclic c u r r e n t - p o t e n t i a l c u r v e s for the e l e c t r o c h e m i c a l i n t e r c o n v e r s i o n o f b e n z o q u i n o n e (Q) a n d h y d r o q u i n o n e (H2Q), Q + 2 e - + 2 H + ~ H2Q, o n various p r e c o a t e d p l a t i n u m e l e c t r o d e s in a q u e o u s s o l u t i o n o f p H 4 w h e r e
334 the Q / H 2 Q e l e c t r o d e r e a c t i o n r a t e is s u p p o s e d to b e at a m i n i m u m [1,2,11]. T h e f o l l o w i n g f e a t u r e s a r e e v i d e n t : (i) T h e p e a k s e p a r a t i o n A E w a s a f f e c t e d o n l y slightly b y t h e o r i e n t a t i o n o f the a d s o r b e d H Q i n t e r m e d i a t e . A E ( c o r r e c t e d for i R d r o p ) = 80 m V with ~/6-HQ, 94 m V with r/2-HQ, t h a t is, the relatioe r a t e w a s l o w e r w h e n the s u r f a c e w a s p r e t r c a t e d w i t h e d g e w i s e - H Q t h a n with f l a t - H Q . A s will b e d i s c u s s e d b e l o w , these A E v a l u e s a r e higher ( t h e relative r a t e s a r e lower) t h a n at p H = 0 o r 7. (ii) T h e p e a k s e p a r a t i o n w h e n the e l e c t r o d e w a s p r e t r e a t e d with I a t o m s w a s close to z e r o ( A E = 5 mV), a n d i n d i c a t e s a p r o f o u n d e n h a n c e m e n t , to v i r t u a l l y c o m p l e t e reversibility ( k ° > ~ 10 -3 c m s - l ) , o f the Q / H , _ Q r e d o x c o u p l e . ( i i i ) T h e p e a k s e p a r a t i o n w h e n the e l e c t r o d e c o n t a i n e d c h e m i s o r b e d C N - , A E = 336 m V , w a s m u c h h i g h e r t h a n for the e d g e w i s e - H z Q - p r e t r e a t e d s u r f a c e , A E = 94 m V ; this reflects a d r a m a t i c r a t e r e t a r d a t i o n ( k ° ~ < 10 - 6 c m s - l ) o f the Q / H ~ Q c o u p l e b y a d s o r b e d C N - . T h u s . the r a t e o f the Q / H 2 Q c o u p l e at p H = 4 h a s b e e n v a r i e d f r o m r e v e r s i b l e to i r r e v e r s i b l e b y s u r f a c e p r e t r e a t m e n t ; in p a r t i c u l a r , it w a s m a d e , in essence, c o m p l e t e l y reversible b y I - p r e t r e a t m e n t , a n d totally i r r e v e r s i b l e b y C N - pretreatment. T a b l e 1 gives A E a n d ~ip/ci p values for the Q / H 2 Q c o u p l e at selected p H o n v a r i o u s p r e t r e a t e d Pt surfaces. (i) A s a l r e a d y s e e n in Fig. 1, the r a t e w a s o n l y w e a k l y
A
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i iii
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0.2 0.4 O.G POTENTIAL/V Fig. 1. T h i n - l a y e r cyclic c u r r e n t - p o t e n t l a l c u r v e s for t h e r e a c t i o n Q + 2 - O.2
0
e-+2
H~--H2Q
o n Pt
electrodes pretreated with ( ) "q6-HQ, ( - - - - - - ) ~2-HQ, ( . . . . . ) I and ( . . . . . . ) CN- Aqueous solution, pH 4; electrode reaction rate is supposed to be at a minimum [1.2,11].
335
influenced by orientationaJ changes wittfin the adsorbed H2Q layer at pH = 4, the rates slightly lower with edge- than with flat-aromatic pretreatment. At p H 0 and 7, where the couple behaves quasi-reversibly, n o o r i e n t a t i o n a l effects were evident. (ii) Profound retardation (to total irreversibility) was observed when h y d r o q u i n o n e sulfonate (HQS), a bulky, aromatic anion, was present in the flat-orientation on the TABLE 1 Electrode kinetics of the b e n z o q u i n o n e / h y d r o q u i n o n e couple at precoated Pt elec~odes in aqueous solutions a Adsorbate b
pH during electrolysis
1 ~/°-Hydroquinone (flat H Q )
0 4
7 2 2,3-~/Z-Hydroquinone (edgewise HQ)
3 "06-Hyclrocluinone-sulfonate (flat HQS)
A E C/mV
aip/cip d
34 + 5 80 35
1.0 +_0.05 0.89 0.96
0 4 7 0 4 7
35 94 38 63 217 98
1.0 0.91 0.96 1.3 0.83 0.55
4
281
0.73
0 4 7
49 41 5
1.1 0.93 0.95
4
52
0.84
4
68
0.79
4
151
0.69
0
5
4
5
4 5,6-~2-hydroqulnone-sulfonate
5
(edgewise HQS) ~,6-Phenyltriethylammonium perchlorat.e (flat PTE&)
6 3,4-~/2-Phenyltriethylammonium perchlorate (edgewise P T E A ) 7 4,5-~z-Pyridine (edgewise lay from adsorption at p H = O) 8 .w1-N-pyridine (endwise Py from adsorption at p H = 7) 9 Iodine (I)
10 Cyanide ( C N - )
7
5
1.0 1.0 1.0
0 4
121 336 410
2.0 1.7 2.6
10
1.1
7
11
Thiocyanate(SCN-)
0
4 7 a T h e solutions contained 0.1 m M
hydroqulnone in I M
215 234 + 5
1.3 0.51:5:0.05
I-/zSO 4 ( p H = 0) or buffered 1 M" N a 2 S O 4
( p H = 4 a n d 7). b Preparation of adsorbate monolayer is desen'bed in the text. ¢ (AE--- a E p - - c E p ) peak potenti~l~ were corrected for i R drop, as described in the text; u n d e r thln-layer conditions. A E ----0 for a completely reversible redox couple. d Peak eta-rents were corrected for b a c k g r o u n d ctu-rent; ~ip ~alp for a rever~ble couple.
336
Pt surface at p H 4; a p p r e c i a b l e r e t a r d a t i o n was n o t e d even at p H 7 a n d zero. T h e 7/6 ~ r/2 r e o r i e n t a t i o n o f I-IQS c a u s e d gt~eater r e t a r d a t i o n ( A E C -- AEn, - 65 m V ) at p H 4, indicating that for this a d s o r b a t e , the Q / H 2 Q c o u p l e was m o r e sensitive to o r i e n t a t i o n a l changes. (iii) W h e n p h e n y l t r i e t h y l a m m o n i u m ( P T E A ) , a b u l k y a r o m a t i c cation, was used as the a d s o r b a t e , r a t e - e n h a n c e m e n t was n o t e d , which increased as the p H was increased (to com_p!ete reversibility at p H 7); since the rates were e n h a n c e d , o n l y a w e ~ d e p e n d e n c e o n o r i e n t a t i o n was o b s e r v e d . (iv) A d s o r p t i o n o f p y r i d i n e (lay) at p H 0, a p p a r e n t l y in the 4,5-r/2-orientation with the p r o t o n a t e d n i t r o g e n h e t e r o - a t o m directed a w a y f r o m the surface [17,18], slightly e n h a n c e d the Q / H 2 Q electrolysis rate at p H 4 (relative to the i n f l u e n c e o f 2,3-r/2-H2Q); a d s o r p tion at p H 7, which results in an N-r/~-Py layer, r e t a r d e d the rate c o n s i d e r a b l y . (v) S u r f a c e p r e t r e a t m e n t with iodine, which f o r m s a c l o s e - p a c k e d layer o f neutral I a t o m s [41,47], r e n d e r e d the e l e c t r o d e kinetics c o m p l e t e l y reversible a n d p H - i n d e p e n dent. (vi) A d s o r b e d cyanide, which f r o m studies at well-defined Pt single crystals is k n o w n to consist o f a d s o r b e d a n i o n s s u r r o u n d e d b y n e u t r a l molecules [46], led to d r a m a t i c r e t a r d a t i o n o f the rate, even at p H 0. T h e r e a c t i o n b e c a m e totally irreversible at p H 4, a n d the rate d e c r e a s e d still f u r t h e r at p H 7. (vii) C h e m i s o r p t i o n o f t h i o c y a n a t e , which has also b e e n s h o w n to f o r m a n i o n i c surface species [43], gave results similar to C N - in the sense that total irreversibility o f the Q / H 2 Q c o u p l e was o b s e r v e d at p H 4, with even slower rates at p H 7; however, the rate was e n h a n c e d to nearly c o m p l e t e reversibility at p H 0 by c h e m i s o r b e d S C N - . T h e foregoing results d e m o n s t r a t e that the r e a c t i o n m e c h a n i s m o f the Q / H 2 Q c o u p l e in a q u e o u s solutions is very sensitive to the n a t u r e a n d c o m p o s i t i o n o f species a d s o r b e d on the e l e c t r o d e surface at monolayer cooerages. F o u r d i f f e r e n t p a t t e r n s o f p H - d e p e n d e n c e have emerged, d e p e n d i n g o n e l e c t r o d e surface s t r u c t u r e . a n d c o m position: (i) fast reactions at p H 0 and 7, but slow at p H 4, in the p r e s e n c e o f a r e n e a d s o r b a t e s such as H Q a n d H Q S ; (ii) rates which d e c r e a s e d with increasing p H in the p r e s e n c e o f a d s o r b e d small a n i o n s such as S C N - and C N - ; (iii) rates which increased with increasing p H [the reverse o f (ii)] as for e l e c t r o d e s p r e t r e a t e d with the a r e n e q u a t e r n a r y a m m o n i u m cation, P T E A ; (iv) very fast kinetics, i n d e p e n d e n t o f p H , in the p r e s e n c e o f a d s o r b e d I atoms. Case (i) is c o n s i s t e n t with the Vetter m e c h a n i s m [1,2,11], that is; H ÷ e - H + e - at pH <4and e-H+e-H+atpH >7. T h e r e t a r d a t i o n o f the r e a c t i o n rate by n e g a t i v e l y - c h a r g e d c h e m i s o r b e d species, case (ii), a p p e a r s to result f r o m the influence o f the electrical d o u b l e - l a y e r . Evidently, the reaction s e q u e n c e in the r e d u c t i o n o f b e n z o q u i n o n e involves e l e c t r o n t r a n s f e r to a negatively-charged q u i n o n o i d i n t e r m e d i a t e ( L o s h k a r e v - T o m i l o v mecha n i s m [3]); at p H 4 on a C N --coated Pt s,wface, the r e a c t i o n m o s t p r o b a b l y involves r e d u c t i o n o f the Q - radical a n i o n as the r a t e - d e t e r m i n i n g step: Q+eQ'-+e-
~
CN -/Pt
Q-
s] o~.,
~
(3a)
Q2-
(3b)
CN -/Pt
Q-'-+ 2 H +
~
CN -/Pt
H.Q -
(3c)
337
Excess positive charge in the double-layer would e n h a n c e reaction (3b) [44]; -accordingly, the rate e n h a n c e m e n t brought a b o u t by the bulky a d s o r b e d cation, P TEA, case (Hi), is also consistent with the e - e - H : - H + sequence shown above. Rapidity of the Q / H 2 Q electrode reactions at Pt surfaces cont ai ni ng adsorbed I atoms, case (iv), is attributable to the c o m b i n e d effects of the small thickness of the I-atom adsorbed layer ( c o m p a r e d with molecular layers such as of aromatics) and exclusion of adsorbed anions by the layer of neutral a d s o r b e d I atoms. R a t e e n h a n c e m e n t by I atoms constitutes an exception to the generalization [6-8,12] that higher coverages of adsorbed material lead to lower rates. T o the contrary, the close-packed layer of I atoms [41-43] enhanced reactivity to an extent a p p r o a c h i n g c o m p l e t e reversibility a t any pH . A similar effect ,vas report ed earlier [48] on the electrode rates of P t ( I I / I V ) complexes. ACKNOWLEDGEMENTS Acknowledgment is m a d e t o t h e A i r F o r c e O f f i c e o f S c i e n t i f i c R e s e a r c h a n d t h e Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We thank Professor Roger Parsons for helpful comments. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
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