Electrocatalysis of cathodic oxygen reduction by metal phthalocyanines. Part IV. Iron phthalocyanine as electrocatalyst: mechanism

Electrocatalysis of cathodic oxygen reduction by metal

phthalocyanines. Part IV. Iron phthalocyanine as

electrocatalyst: mechanism

Citation for published version (APA):

Brink, van den, F. T. B. J., Visscher, W., & Barendrecht, E. (1984). Electrocatalysis of cathodic oxygen reduction

by metal phthalocyanines. Part IV. Iron phthalocyanine as electrocatalyst: mechanism. Journal of

Electroanalytical Chemistry, 175(1-2), 279-289. https://doi.org/10.1016/S0022-0728(84)80362-9

DOI:

10.1016/S0022-0728(84)80362-9

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Published: 01/01/1984

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E L E C T R O C A T A L Y S I S O F C A T H O D I C O X Y G E N R E D U C T I O N B Y M E T A L P H T H A L O C Y A N I N E S

P A R T IV. I R O N P H T H A L O C Y A N I N E A S E L E C T R O C A T A L Y S T : M E C H A N I S M

F. VAN DEN BRINK *, W. VISSCHER and E. BARENDRECHT

Laboratory of Electrochemistry, Eindhoven University of Technology, P. 0. Box 513, 5600 M B Eindhoven (The Netherlands)

(Received 23rd November 1983; in revised form 30th March 1984)

ABSTRACT

The kinetics of oxygen reduction on thick iron phthalocyanine films as reported in Part III of this series are explained in terms of a redox mechanism. In this mechanism, oxygen is adsorbed on a reduced Fe site, which transfers an electron to give a Fem-O~ - complex. This complex gives, in a series of fast steps, the reduction products OH and H O 2 . Such a process is very sensitive to the value of the F e m / F e u redox potential in the N4-complex. Arguments are given that the kinetic results lead to a value of E o = 0.67 V vs. RHE.

At high overpotential, an adsorption/desorption process occurs which "opens up" the film and makes an extra number of active sites available for oxygen reduction. This process is discussed in terms of the spin state of the Fe atom in in the N4-complex as a function of the external electric field.

INTRODUCTION AND SUMMARY OF KINETICS

I n P a r t I I I [1] w e p r e s e n t e d t h e r e s u l t s p e r t a i n i n g to t h e k i n e t i c s o f t h e e l e c t r o - c a t a l y s i s o f o x y g e n r e d u c t i o n b y i r o n p h t h a l o c y a n i n e ( F e P c ) . U s i n g k i n e t i c t e c h - n i q u e s ( R R D E ) , w e f o u n d t h e d e p e n d e n c e o f t h e p e r t i n e n t r a t e c o n s t a n t s o n e l e c t r o d e p o t e n t i a l . T h e k i n e t i c d e s c r i p t i o n w a s e x p a n d e d w i t h t h e r e s u l t s o f s u r f a c e - s e n s i t i v e t e c h n i q u e s , viz. a c - i m p e d a n c e m e a s u r e m e n t s a n d e l l i p s o m e t r y . I n t h i s p a p e r w e will t r y t o g i v e a m e c h a n i s t i c d e s c r i p t i o n a n d s h o w t h a t t h e r e s u l t s f o r F e P c c a n b e e x p l a i n e d b y a m e c h a n i s m w h i c h is v e r y s i m i l a r t o t h a t w h i c h w e p r o p o s e d f o r t h e e l e c t r o c a t a l y s i s o f o x y g e n r e d u c t i o n b y c o b a l t p h t h a l o c y a n i n e [2]. I t w a s s h o w n [1] t h a t , o n F e P c f i l m e l e c t r o d e s , i n I M K O H , o x y g e n is a l m o s t e x c l u s i v e l y r e d u c e d t o O H - , i.e. k l >> k2 f (see F i g . 1). O v e r t h e w h o l e p o t e n t i a l r a n g e i n v e s t i g a t e d , a f i r s t - o r d e r d e p e n d e n c e o f t h e r e a c t i o n r a t e i n t h e o x y g e n c o n c e n t r a - t i o n is f o u n d .

* Present address: DSM Central Laboratory, Department of Catalysis, P.O. Box 18, 6160 MD Geleen, The Netherlands.

k l

- - "" H 2 ~-- H

k 4

.2o2---,,.2o2

Fig. 1. Reactions involved in cathodic oxygen reduction on FePc. Superscripts refer to in bulk (s), in reaction plane (o) and at electrode surface (o).

T h e k i n e t i c results for F e P c films which have already b e e n polarized c a t h o d i c a l l y are s u m m a r i z e d i n Fig. 2. A t high p o t e n t i a l a Tafel slope of - 120 m V is f o u n d for all the rate c o n s t a n t s ( k l , k f, k3) , t h e n at low p o t e n t i a l log k b e c o m e s m o r e or less i n d e p e n d e n t of p o t e n t i a l . Between 0.3 a n d 0.5 V a t r a n s i t i o n occurs, where the k ' s start to increase again, with Tafel slopes of - 1 2 0 m V or m o r e negative, a n d finally level off again.

T h e t r a n s i t i o n m e n t i o n e d a b o v e is also reflected b y the two p o t e n t i a l regions of different a c t i v a t i o n energy, as f o u n d from the A r r h e n i u s plot of c u r r e n t d e n s i t y as a

-3 k/m s t -4 -5 - 6 -*,,,, i i 0.2 0.4 0.6 O. ERHE/V

Fig. 2. Oxygen reduction rate constants as functions of the potential for FePc films at 298 K: (O) 0.1× kl; (+)kf2; (zx)k 3.

function of temperature; for 0.6 V < E < 0.7 V, mna°ct = 38 kJ mo1-1 (0.39 eV) is found and for E ~< 0.4 V, AH~t = 52 kJ m o l - a (0.54 eV). Furthermore, this transi- tion is manifest in ellipsometric results where the optical changes at 0.4-0.5 V were attributed, using an effective medium model, to a change from a hydrophobic to a hydrophilic character; moreover, ac-impedance measurements reveal surface states at 0.82, 0.67 and at ca. 0.4 V vs. R H E .

In particular, ac-impedance measurements can be interpreted if the FePc films are regarded as classical p-type semiconductors, in which the transport of holes is very m u c h faster than that of electrons. In the potential range where oxygen reduction occurs, the main contribution to conduction is by the slow electrons, which leads to a considerable influence of ohmic resistance on the kinetics.

However, cathodic polarization of the film in 1 M K O H leads to a decrease of the ohmic resistance, which, from the ellipsometric data, could be explained by absorp- tion of the electrolyte solution in the film, giving a decrease of the film grain b o u n d a r y resistance. This process can also be seen from the impedance and conductivity data, which show a surface process as mentioned above. As for the slow surface processes at 0.82 and 0.67 V vs. R H E , the n u m b e r of surface states at 0.82 V

is estimated to be 1.5 × 10 - 6 mol m -2, implying that some 10% of the surface Pc

molecules are involved.

So, for a mechanistic description, we have the following kinetic features to explain:

(1) the surface states at 0.82, 0.67 and ca. 0.4 V vs. R H E as found with impedance measurements;

(2) the mechanistic transition between 0.3 and 0.5 V vs. R H E ; and

(3) the similar Tafel behaviour of the rate constants of the direct (02 --+ H 2 0 ) and the consecutive (02 ~ H202 --+ H 2 0 ) reaction pathways.

MECHANISM FOR OXYGEN REDUCTION ON FePc

The Tafel behaviour of the rate constants for oxygen and hydrogen peroxide reduction on FePc with the - 1 2 0 mV slope at low overpotential, followed by their levelling-off at higher overpotential is indicative of activation of oxygen b y a one-electron transfer on the active sites, the n u m b e r of which is rate-limiting. Furthermore, the parallelism between the kinetics and the surface processes observed indicates that the redox state of the surface is another important factor: at the potential where the Tafel slopes start to increase, a surface state is detected by capacitance measurements. Therefore, the situation is to some extent similar to that with CoPc [2]. There, our data could be explained by the assumption of a redox catalysis mechanism as originally proposed b y Beck [3,4]. Such a mechanism is also in accordance with other data, such as the correlation between the redox potential of the central metal a t o m in the Nn-complex and its electrocatalytic activity [5].

So, for the oxygen reduction on FePc, we assume that the redox character originates from the F e 3 + / F e 2+ couple in the macrocyclic ligand; the oxygen molecule is expected to adsorb onto a Fe 2+ site and oxidize it to the Fe 3+ state. The

choice of this particular redox couple will be discussed later. T h e reaction sequence is then: 1 F e P c + + e - ~ F e P c (1) 2 F e P c + 0 2 ~ F e P c - O 2 (2) fast F e P c - O 2 ~ p r o d u c t s (3)

F o r such a scheme, the rate equation can be derived in a similar w a y to that given in Part II [2]:

i

/% [02

]

s

n----ff = 1 + exp f*/l + ( k 2 [O2

]/k? )

e %fnl (4) where the k ' s represent the rate constants of the elementary reactions, s is the n u m b e r o f active sites, and f = F / R T . T h e Tafel slope predicted b y this rate equation is

1 + exp f*/a

+(ka[O2]/k?)

exp

alf*/1

b / V

= - 0 . 0 5 9 (5)

exp f*/1 +

al(kz[O2]/k?)

exp

alf*/1

F o r small values of 1./11 and n o t too small values of the characteristic p a r a m e t e r

k2[O2]/k ~

(i.e. > 10), - 120 m V is f o u n d (for a 1 = 0.5), while for */1 << 0 the slope a p p r o a c h e s - ~ for all values of k z [ O z ] / k ~ . T h e reaction order in 02 is

p = d log

i/d

l o g [ 0 2 ] W i t h eqn. (4) this gives

p = 1

(kz/k?)[02]

exp

alf*/1

(6)

1 + exp

f*/1 + (k2/k?)

[O2 ] exp a, f*/1

Figure 3 gives a calculated plot for p as a function of */1 for a series of values of

k2[Oz]/k ~.

It is seen that if the ratio

k2102]/k ~

is large, a rather wide potential region a r o u n d */1 = 0 is predicted where the order in the oxygen c o n c e n t r a t i o n is considerably less than 1, but for

k2[O2]/k ~ <_

10, the order in 02 will be 1, i n d e p e n d e n t of */1.

So, our experimental result that the order in o x y g e n is 1, while the Tafel slope is - 1 2 0 m V for small

I*/I

a n d - o o for 71 << 0 means that

k2[O2]/k? ~

10, i.e. that, except for */1 > 0.2 V, the a d s o r p t i o n of oxygen on a reduced site is the rate-de- termining step. T h e transition f r o m b = - 120 m V to b ~ oo in the high potential region k I occurs at ca. 0.7 V. Also, a surface process was f o u n d at E = 0.67 V. Therefore, the s t a n d a r d potential of reaction (1) can be identified as E1 ° --- 0.67 V. Finally, we k n o w that ca. 90% of the p r o d u c t s are H 2 0 a n d ca. 10% H202. Thus,

P 0.8 1 0.6 0.4 0 . 2 0 I I | I | 1 i I I I I - 0 . 4 -0.2 0 0.2 0.4 q/v

Fig. 3. Reaction order ( p ) in oxygen concentration for kinetic equation (4) as a function of the overpotential for reaction (1). Parameter:

(k2[O2]/k ~ ).

if x is the f r a c t i o n o f the c o m p l e x F e P c - O 2 w h i c h is r e d u c e d t o H 2 0 , we f i n d

XkES

ka - (7a)

1 + exp

fTIl + (k2[O2]/k~)

exp % f * h

k2 f _ (1 -

x)k2s

( 7 b )

1 + exp fBa +

(kz[O2]/k ~ )

exp otlf~ h

K n o w i n g t h a t E~ ° - - 0 . 6 7 V a n d t h a t the c o r r e s p o n d i n g surface p r o c e s s has a c h a r a c t e r i s t i c f r e q u e n c y o f the o r d e r o f 1 0 3 s ] [1], we f i n d t h a t k 2 m u s t b e in the r a n g e 1 0 0 - 1 0 0 0 m 3 m o l - 1 s-1. F r o m the ( a l m o s t ) p o t e n t i a l - i n d e p e n d e n t p a r t s o f k~ a n d k l , at E = 0 . 6 V, w e f i n d f r o m Fig. 2 t h a t

xk2s

= 2.5 × 10 - 4 m s -1 a n d (1 -

x)k2s ---

1.6 × 10 -5 m s -1, so x = 0.94 a n d

k2s

= 2.7 × 10 - 4 m S - a .

W i t h 100 ~<

k2,

k 3

<~

1000 m 3 mo1-1 s -1, we f i n d t h a t s = 1 0 - 7 - 1 0 - 6 m o l m 2. T h i s result is in k e e p i n g w i t h the f i n d i n g [1] t h a t the n u m b e r o f surface states r e s p o n s i b l e for the p r o c e s s at 0.67 V is o f the o r d e r Nss (0.67 V) ~< 2 × 10 - 7 m o l m -2. F o r the r a t e c o n s t a n t of the r e d u c t i o n o f h y d r o g e n p e r o x i d e , k 3 , a p o t e n t i a l d e p e n d e n c e a l m o s t i d e n t i c a l to t h a t o f k~ is f o u n d (cf. Fig. 2). I t t h e r e f o r e s e e m s r e a s o n a b l e to p o s t u l a t e a s i m i l a r m e c h a n i s m for the H O 2 r e d u c t i o n : F e P c + + e - ~ F e P c 3 F e P c + H O y ~ F e P c - H O ~ - fast F e P c - H O ~ ~ p r o d u c t s

(8)-

(9)

and so the overall rate c o n s t a n t k 3 is given by

k 3 = k 3 s ( 1 0 )

1 + e x p f , h

+(k3[HOZ]/k?)

exp

alfv h

The value of k 3 is, like that of k 2, of the order of 102-103 m 3 tool -1 s -1, as judged from the potential dependence, and again s = 10 .7 mol m -2 is found.

Summarizing, we have found that, for low overpotential, i.e. for E >/0.5 V vs. RHE, cathodic reduction of oxygen and of hydrogen peroxide on FePc films occurs by way of a heterogeneous redox catalysis mechanism. In this mechanism, which occurs on sites with a concentration of ca. 1 0 - 7 - 1 0 .6 mol m -2, i.e. involving less than 10% of the surface FePc molecules, the FePc molecule is reduced in a relatively fast reaction (kl ° - 10 3 s - 1 ) with a standard potential E1 ° --- 0.67 V. On the reduced electrocatalyst, the reactant (02, HO~-) is adsorbed, which leads to de-stabilization. The adsorption is the rate-determining step, with rate constants k2, k 3 of the order of ~ 102-103 m 3 mo1-1 s -1. Of the adsorbed oxygen, 94% is reduced to H 2 0 and 6% to HOE in a fast reaction, i.e. with a rate constant >> 1 0 3 s - 1 .

Between 0.3 and 0.4 V vs. RHE, a mechanistic transition occurs. The rate constants kl, k~ and k 3 s t a r t to increase again, with Tafel slopes of - 1 2 0 mV or more negative, but from E - - 0 . 3 V vs. R H E they become, again, independent of potential. From the surface-sensitive techniques employed [1], it was found that this transition may be due to a decrease of the grain boundary resistance of the film. In other words, due to some, as yet unexplained, phenomenon, the film "opens up" and the number of catalytic sites available increases. The activation enthalpy for this phenomenon is 0.54 eV. When the overpotential is negative enough to overcome this extra barrier, i.e. at a potential around 0.4 V vs. RHE, a mechanism similar to that described in eqns. (1)-(3) and (8) and (9) becomes active. The number of extra active sites can be estimated as follows. The surface process at 0.4 V is obviously connected with these sites. Since this process was found to have a characteristic frequency of the order of 1000 s-1, the rate constant for their reduction will be of the same order of magnitude. From capacitance data, the number of surface states around E = 0.4 V was estimated to be 1.5 × 10 - 6 mol m -2, i.e. the number of sites becoming active at low potential is slightly larger than that already active at high potential. The limiting values at low potential ( E - 0.2 V) of the rate constants k 1, k2 r and k 3 a r e

six- to ten-fold higher than those at E = 0.5-0.6 V, so, assuming expressions for them similar to eqns. (6), (7) and (10), the rates of adsorption of oxygen or HO2- , respectively, are slightly larger (i.e. less than ten-fold) on the low potential sites than on the high potential sites.

The nature of the surface state located at E = 0.82 V vs. R H E remains to be explained. In the case of oxygen reduction on CoPc films, we found a surface state at the same potential [2] and explained its origin as reduction of adsorbed oxygen, i.e. as the standard potential of reaction (3). However, in the case of FePc, this state was found in deoxygenated as well as in oxygenated solution. Furthermore, from the kinetics we have concluded that reaction (3) is relatively fast, while ac-impedance

measurements showed that the surface state has a characteristic frequency not exceeding 10 3 s -1, i.e. the rate is comparable to that of reactions (1) and (2).

On the other hand, it is well known that oxygen can be adsorbed in Pc films [6,7], and that its presence has a profound influence on, among other things, the film's electrocatalytic behaviour, as witnessed, for example by our own kinetic results [1]. The influence of absorbed oxygen on the conductivity behaviour of FePc-films has been explained by a mechanism, where charge is conducted along chains such as [8,9]

- - F e - O - O - F e - O - O - F e - -

where excited oxygen can be ionized: O ~ ' - F e P c + - O ~ - ~ O ~ - F e P c + - O ~ '

Possibly, the surface state at 0.82 V is connected with charge injection in this type of chains. However, the precise nature of the process remains rather obscure.

Summarizing: the oxygen reduction on FePc films occurs on two types of sites, one of which becomes active at low overpotential ( E >/0.8 V vs. RHE). The other type does not take part in the reduction at potentials above ca. 0.4 V vs. RHE. At this potential it becomes available for oxygen reduction due to some process which "opens up" the film. Apart from the potential where it occurs, oxygen reduction on both types of sites follows quite similar mechanisms. Slow reduction of the site is followed by equally slow adsorption of oxygen (or hydrogen peroxide). The adsorp- tion de-stabilizes the adsorbed species (02 or H O 2 ) to such an extent that its reduction occurs in a (series of) fast step(s).

DISCUSSION

An interesting point remains: the nature of the process occurring at ca. 0.4 V vs. RHE, where the second high overpotential type of electrocatalytic sites are activated. To discuss it we must, however, first discuss the value of the redox potential of the surface F e 3 + / F e z+ couple, which we postulated to be ca. 0.67 V. The reasoning for this is similar to that given for the redox potential of the C o P c + / C o P c couple [2], which was found to be ca. 0.80 V vs. RHE. The F e 3 + / F e 2+ redox potential is expected to be somewhat lower because the redox potential of "free" F e 3 + / F e 2+ is lower than that of "free" C o 3 + / C o 2÷ in aqueous solution. Also, FeIIpc is expected to form a complex with oxygen, contrary to F e n l p c [10].

Iron can be in redox state I in Pc and porphyrins, but the F e n / F e ~ transition lies at least 1 V more negative than the F e n I / F e n transition [11], so the process at - 0.4 V, generating the second high overpotential type of sites, cannot be ascribed to a reduction of Fe n. Furthermore, in Part I I I [1] we described a probable phtha- locyanine ring reduction at l o w potentials ( E ~< 0.1 V). Ring reductions have been reported in this potential range [3,11], so it is improbable that the process at E -- 0.4 V is a ring reduction as well. Surface states for thin-film metal-free and zinc phthalocyanine, centred at ca. 0 . 4 V have been reported [12], but no explanation as to the nature of the surface states was offered.

Our results on the ellipsometric behaviour of FePc films indicate that some a d s o r p t i o n / d e s o r p t i o n process m a y provide the explanation, in particular that of

O H of H 2 0 . Q u a n t u m mechanical calculations [13] indicate that spin state

transitions can occur in the d s system of F e m P c in the D4h, C4,, or C2~ symmetries. Such transitions can be brought about by changes in the crystal field parameters. These parameters are mainly determined by the properties of the ligands surround- ing iron and by the electric field strength over the interphase, so that a change in potential m a y well bring about a spin state transition. A similar situation m a y exist

for FenPc, where, in the D4h ,

Car

or Czv symmetries low spin (S = 0) and high spin

(S = 2) states are possible. Also, intermediate spin states seem to be possible in phthalocyanines, depending on the crystal field parameters [7,14,15]. So, the spin state m a y be profoundly influenced by potential for surface iron phthalocyanine molecules, and it is reasonable to assume that the spin state of the active site will determine its adsorption behaviour. Therefore, we suggest the following explanation for the observed phenomena. At E = 0.67 V, F e m p c is reduced to FeIIpc; due to the effect of underlying phthalocyanine layers and oxygen a n d / o r water molecules incorporated in the surface layer, the resultant FeHPc is distributed over two spin states; one of these leads to adsorption of O H - or H 2 0 , while the other leaves the active Fe n site free. On the free sites, oxygen reduction occurs at all potentials in the range investigated, while the blocked sites only become available for oxygen reduc- tion below E - - 0 . 4 V, where desorption of the adsorbed species occurs. Another possibility is that oxygen adsorbs on sites in all possible spin states, but that reduction of adsorbed O 2 occurs only on sites in certain states. In any case, we submit that the electrode potential influences the crystal field parameters and thus the spin state of the electrocatalytic sites. The spin state, in turn, determines the adsorption behaviour of the site and the electronic distribution of the adsorbed species. This m a y also explain the different activation enthalpies in the low and high overpotential r e g i o n s - - d i f f e r e n t spin states lead to different activation processes. It must be admitted that this explanation is somewhat tentative, but it is, at least, plausible, since it explains the observed dependence of the kinetics on electrode potential and it is in keeping with the phenomenological description of the state of the electrode surface.

A few detailed mechanisms have been published hitherto, pertaining to oxygen

reduction to H 2 0 on FePc [3,4,6,8,16]. Based upon an observed Tafel slope of - 120

mV, it was proposed by Zagal and co-workers [16] that the rate-determining step of oxygen reduction to H 2 0 on FePc at high overpotentials is the reduction of the charge transfer complex F e m - O 2 :

Fe n + 0 2 ~- F e m - O ~ fast

F e l I I - o 2 - + e - ~- F e t I - O ~ rds

F e n - O 2 - --+ products

In the first place, this mechanism predicts a Tafel slope of - 120 mV at potentials more negative than the standard potential of the second reaction only if its intrinsic

rate is orders of magnitude smaller than that of the chemisorption step. We have demonstrated that this is an improbable proposition. Further, the + 3 oxidation state is not expected to be very stable, even when it is brought about by oxygen. So, the standard potential for the r.d.s, in the mechanism given above is certainly not expected to be very low, i.e. below the range of potentials where oxygen reduction is investigated. Therefore, the second reaction given here is expected to be rate-de- termining over, at most, part of the potential range, and beyond its standard potential the chemisorption step is certainly expected to become rate-determining, which predicts an infinite Tafel slope. However, Zagal and co-workers [16] found b = - 120 mV at high overpotential for FePc. Such behaviour would, in our view, be better explained by assuming that the reduction of the central metal ion from its + 3 state to its + 2 state is rate-determining, where it is most able to form the charge transfer complex with oxygen. In that proposition, the thermodynamics of the problem, reflected in the value of the standard potential of the reduction of F e l l I - o 2- to FeU-O~ -, are reasonably satisfactory, and the kinetics take their rightful place.

For low overpotential, a mechanism for oxygen reduction to H 2 0 or O H - on FePc has been proposed by Zagal et al. [16], which is essentially the same as that proposed here:

F e l I I O H + e - ~ Fe ll + O H - fast

Fe II + 02 ~- F e l I I - o 2 - fast

F e l l l - o 2 - + e - --~ products rds

F o r this mechanism a rate equation is derived in which the implicit assumption is m a d e that only a small part of the available Fe II sites is occupied by oxygen.

Granting this condition, a Tafel slope of - l o g

eRT/(1 + a ) F

is predicted. To

explain the observed b = - 35 mV, a value a = 0.71 must be adopted, for which no explanation is offered. M a y b e such an explanation could be found in the orientation of the (in this case reversibly adsorbed) phthalocyanine molecule on the graphite surface used as substrate in this work [17]. On the other hand, the first step of the mechanism given above gives an explanation for the observed order of the reaction rate in hydroxyl ions of - 1 [8,16].

FINAL REMARKS

The principal source of uncertainty in the determination of the mechanism of oxygen reduction on FePc, and, for that matter, on any macrocyclic N4-chelate transition metal complex, seems, in our opinion, to be the value of the redox potential of the M m P c / M n P c couple.

We have assumed for thick FePc layers that this value is E = 0.67 V vs. R H E . This proposition is also supported by inductive reasoning from our results for the redox couple C o m P c / C o I I p c . F o r irreversibly adsorbed FeTSP on the basal plane of pyrolytic graphite (where TSP stands for tetrasulphonated phthalocyanine, which is

s o l u b l e i n a q u e o u s solution), a s o m e w h a t higher value has b e e n r e p o r t e d [14], which is u n d o u b t e d l y d u e to differences i n the actual e n v i r o n m e n t of the c e n t r a l m e t a l atom. A n even higher value was inferred f r o m i n - s i t u M r s s b a u e r spectroscopy m e a s u r e m e n t s o n F e P c s u p p o r t e d o n high-surface-area c a r b o n [18]. A l t h o u g h i n this last case the f o r m a t i o n of crystalline F e P c was claimed, which w o u l d c o n t r a d i c t o u r p r o p o s i t i o n s , some i n t e r e s t i n g parallels c a n b e d r a w n . Firstly, the F e P c c o n t e n t is n o t so high as to exclude a s t r o n g i n t e r f e r e n c e of surface g r o u p s of the c a r b o n with F e P c molecules (a physical surface area of 2000 m 2 per 7.5 c m 2 of geometric surface area c o n t a i n i n g ca. 0.5 g of F e P c is given, which c o r r e s p o n d s to four to five F e P c layers o n the carbon). So, we see that the value of the F e m / F e H redox p o t e n t i a l , which is crucial to the reaction p a t h w a y , is very sensitive to the e n v i r o n m e n t of the i r o n atom, especially the fifth a n d sixth ligands.

Secondly, it was r e p o r t e d b y Scherson et al. [18] that a c o n s i d e r a b l e c h a n g e i n the M r s s b a u e r q u a d r u p o l e splitting of the c e n t r a l i r o n a t o m occurs w h e n the F e P c - o n - c a r b o n s a m p l e is i m m e r s e d i n a n a q u e o u s a l k a l i n e solution, which is e x p l a i n e d b y the occurrence of a n axial C - O - F e - O H complex, where the other l i g a n d s i n the o c t a h e d r a l c o m p l e x are the four p h t h a l o c y a n i n e nitrogens. This a s s u m p t i o n is quite s i m i l a r to t h a t m a d e here for the e x p l a n a t i o n of the process o c c u r r i n g i n thick F e P c films below ca. 0.5 V vs. R H E .

I n conclusion, o u r kinetic a n d surface c h a r a c t e r i z a t i o n d a t a c a n b e e x p l a i n e d r a t h e r satisfactorily b y a s s u m i n g a redox catalysis m e c h a n i s m . Such a m e c h a n i s m m a y lead to a variety of kinetic e q u a t i o n s , d e p e n d i n g on, firstly, the p o t e n t i a l where the electrocatalyst is r e d u c e d and, secondly, o n the relative rates of the e l e m e n t a r y r e a c t i o n s involved. These quantities, however, are f o u n d to b e strongly i n f l u e n c e d b y the precise g e o m e t r y of the electrocatalytic site.

ACKNOWLEDGEMENTS

T h e p r e s e n t i n v e s t i g a t i o n s were carried o u t with the s u p p o r t of the N e t h e r l a n d s F o u n d a t i o n for C h e m i c a l Research (S.O.N.) a n d with f i n a n c i a l aid f r o m the N e t h e r - l a n d s O r g a n i z a t i o n for the A d v a n c e m e n t of Pure Research (Z.W.O.).

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