Indian Journal of Chemistry Vol. 42A, December 2003, pp. 2949-2953

Mediated electrocatalysis at the carbon paste electrode: The iron(lII)/hydrogen peroxide model

S Serradilla Razola, G Quarin & J-M Kauffmann-Universite Libre de Bruxelles, Pharmaceutical Institute, Campus Plaine , CP 205/6, 1050 Brussels, Belgium

Email: jmkauf@ulb.ac.be

Received 14 January 2003

Theoretical aspects of homogeneous mediated electrocataly sis have been studied and applied to the catalytic electroreduction of di ssolved Fe(IIl) at a carbon based electrode in the presence of hydrogen peroxide. The electrocatalytic reduction of Fe(IIJ) has been studied by cyclic voltammetry and the kinetic constant of the rate-determining step, i.e., the reaction between Fe(II) and hydrogen peroxide has been determined. The second order rate constant obtained (k 'f = 70 M' s· ') is in agreement with literature data at the dropping mercury electrode.

A catalytic current can be exemplified by a reaction scheme where a substrate 0 is transformed at the electrode to a product R (Eq. 1), and the former is regenerated by reaction of R with a compound Z (Eq.2),

kred

O+ne-• --R ... (1)

kox

k' f

R+Z. 0 ... (2)

kb

Reaction 1 is a heterogeneous process occurring at the electrode interface, while reaction 2 occurs homogeneously in solution . The term catalytic current is applicable to all types of currents in which the reactant 0 consumed in the electrochemical reaction (Eq.I) is regenerated by some chemical process (Eq. 2) involving a product R of the electrochemical reaction. Different situations can be considered depending on the reversible or irreversible character of the electrochemical reaction I. But in order to determine the k 'f rate constant, reaction 2 will always be considered as irreversible. In thi s work, the Fe(III) electro reduction at the carbon paste electrode, in the presence of hydrogen peroxide, is investigated as a model system as it has already been studi ed by Pospisi l at the dropping mercury electrode'.

Theory of homogeneous mediated electrocatalysis The electrochemical equations that characterize the

catalytic systems considered have been described initially for the stationary plane electrode using vOltammetry2. The reaction velocity can be extracted from the following general simplified equations:

... (3)

with v' = k'r Cz CR (4)

where 8C/8t is the concentration variation of the electroactive species with time, D is the diffusion coefficient and v' is the regeneration velocity of the electroactive species. The Nicholson & Shain theor/ developed in 1964 characterizes the morphology of the voltammetric curves for a reversible electron transfer as a function of the kinetic constant kr (where, kr= k 'rx Cz) and as a function of the parameter a (Eq.5):

nFv a=--

RT . .. (5)

where a is the inverse of a time constant and is directly proportional to the scan rate (v); the other parameters have thei r cllstomary meanings . This relation can be illustrated schematicall y as shown in Fi g. 1, by extracting several va lues of krla

2950 INDIAN J CHEM, SEC A, DECEMBER 2003

3.5

i: ......... _ ..... . 3 .0

2 .5

2.0 ~ ><

1.5 ~ 2 ____ ~ '.

1.0

0.5

-150 -100 -50 o 50 100 150

E(V)

Fig. I----S imulated current potential voltammetric curve shapes for different krl a ratios for a reversible catalytic process. [kr/ a for curve I = 3. I 6; curve 2 = 1.78; curve 3 = 1.0; curve 4 = 0.6; curve 5 = 0.4; curve 6 = 0.2; curve 7 = O. I; curve 8 = 0.04].

(dimensionless parameter), from Table XII of the Nicholson & Shain classical work2

.

As clearly shown in Fig.l, for a krla ratio of 0.4 and above (i.e. high Cz or high k'r), the curve exhibits a typical S shaped profile characteristic of a purely catalytic behavior. Below 0.4, the curves exhibit a mixed diffusionJcatalytic control which becomes purely diffusion control at a ratio of kr la = 0 (i.e., kr = o or a »kr).

Electron transfer with reversible character ]n a general expression for reversible synems the

current can be expressed by Eq . 6,

... (6)

where A is the electroacti ve area and x(a, t) is a

parametric function depending on the potential as was introduced by Nicholson and Shain2

. Depending on the value of krla, the current may be controlled by a diffusion or by a catalytic process. At high krla values, the current depends on the catalytic process, while the diffusion contribution increases for lower krla ratios (Fig. 1).

The theory of Nicholson and Shain shows that two limiting cases are possible: when kr is small, voltammograms approximate those of a simple reversible electron transfer reaction, whereas if kr is

large. the current is directly proportional to jk; and

independent of the potential scan rate. These two situations are described below:

High kf value and catalytic current contribution The ilim is scan rate independent and the catalytic

current expression follows the general Eq. 7,

~im

l+exp[ n.F (E-EIIJ] R.T

.. . (7)

Note that at sufficiently negative potentials (Fig.!) the catalytic limiting current can be written in the form3

,

. .. (8)

F is in coulomb/M, A in cm2, Co and Cz in mol cm·3,

Do in cmz S-I and k'r in cm3 mor l

S· I. In this expression

the rate constant k' r is related to the veloci ty of the chemical reaction (Eq.2). Thus, the latter can be expressed as:

a b a v' = k' [" [R] .[Z] = kr. [R]

b where a=b= I and kr= k' r. [Z]

... (9)

From Eq. 8, it is clear that k' f determination requires the diffusion coefficient of species 0 (Do) and the electroactive area (A) determinations. The diffusion coefficient (Do), for the re\'ersible case, can be obtained experimentally by measuring the peak current (ip) by cyclic voltammetry according to the well known Randles-Sevcik equation4

.5

. . . (10)

where units are the same as for Eq. 8 and v =scan rate in Vs-I

.

Low /f-f value and catalytic current with diffusion current contribution

It is possible to study the phenomena using a rotating-disc electrode technique applying the Koutecky-Levich equation6

. The kr value may be also determined by exploiting a particular case included in the theory developed by Nicholson & Shain, by analyzing the data making use of the model curves in Fig. 14 of this theory. The kinetic parameter can be obtained by plotting krla versus the inverse of the scan rate2

. Under such conditions the current magnitude depends on the scan rate.

RAZOLA et al.: MEDIATED ELECTROCAT AL YTIC REDUCTION OF Fe(llI) 2951

Electron transfer with irreversible character For irreversible systems, the general current

expression is given by Eq. 11,

i=n.F.A.Co)n.Do·b.x(b,t) ... (11)

where b =_an-=a,-·F_. v--,(_E_-_Eo,,-"-) R.T '

with a being the electron transfer coefficient that characterizes an irreversible system. When the i lim is studied, this general expression can be reduced to Eq. 12 which has been developed for the peroxo­uranate system at the dropping mercury electrode?,

... (12)

where kJ =k;xCz From Eq. 12 it is possible to deduce that at low

k,~, [ ~ «kj J

the electronic transfer will essentially govern the response at the electrode. This current follows Eq.13,

icat=n.F.A.CO·kred ••• (13)

And for high values of k"", ( ;;0 »kj l the

current reaches a steady state value that can be expressed using Eq. 8, i.e., as for the catalytic current in a reversible system. As is indicated in Eq. 8, the determination of the k'f value requires also the calculation of the Do. The latter can be determined by two ways: In our work, the calculation of Df'e( llI ) is experimentally performed by measuring the peak current of Fe(IlI ) reduction (ip) from a cycl ic voltammogram realized in the absence of the catalyst (H20 2) and applying the equation for an irreversible process ( Eq.14),

ip =(2.99xI05 ) . Il~aHa .A.Co·JD;;.-Fv ... (14 )

where un its are the same as for Eq. 8.

The value of (ana) is extracted from the slope of

the experimental curve by plotting the Ln i against the potential E at the foot of the wave, i.e., where there is no diffusion IColx=o=IColx-too and by applying Eq. 15:

R.T dLni a=-.--

nF dE ... (15)

The second way to calculate the diffusion coefficient is based on the application of the Cottrell equation. This second method can only be applied when diffusion is the process that controls the reaction, i.e., in the diffusion control mode, the plot i against t - 112 must exhibit a linear relationship.

Materials and Methods All the electrochemical measurements were

realized using a three-electrode cell with a working volume of 10 mJ. A solid carbon paste electrode (sCPE) served as working electrode (dia.=3 mm). The working electrode8

•9 is made of solid paraffin (40%

wlw) and graphite (60% wlw) . The electrochemical area (0.070 cm2) of the sCPE was determined by chronoamperometry with the hexacyanoferrate(lVIII) system5

. The reference Ag/AgCl 3M NaCI electrode and a platinum wire as counter electrode, completed the cell. A potentiostat BAS CV27 connected to a PC served for the electrochemical experiments and signal recordings. All assays were realized at 298.0 K (thermostated cell) and a N2 current was passed over the solutions. All solutions were prepared in high purity water (Milli-Q).

Hydrogen peroxide (30% wlv) was supplied from Vel (Belgium). Stock solution of H20 2 (1 M) was prepared daily in acetate buffer (0.1 M, pH 4.5) and protected from the light during storage. A ferric ammonium sulfate FeNH4(S04h solution (1 mM) was prepared in acidic media (H2S04, 0.1 M) using ferric ammonium sulfate dodecahydrate (Merck, Belgium). All reagents were of analytical grade.

Results and Discussion The Fe(lTI)/(Il)-H20 2 electrocatalytic system may

be expressed by Eqs 16 and 17,

kred electrode 2Fe (llI) + 2 e' ~ 2 Fe (II) ... (1 6)

k'f --~. 2Fe (UI) + 2 H20

.. . (17)

2952 INDIAN J C HEM , SEC A, DECEMBER 2003

The first examples of catalytic currents involving hydrogen peroxide were described by Brdicka and coworkers for the reduction of various complexes of Fc(IIl)3. Pospisil applied the Koutecky's treatment for the dropping mercury electrode and obtained values of the order of 80 MI S·I for k'f at 297 K by the ~olarographic method l

,3. The hydrogen peroxide r ~duction occurs irreversibly in acidic medium at the ~ e PE at negative potentials lower than - 1.0 V, as ~hown by cyclic voltammetry (Fig. 2 curve band Fig. 3 curve b). Thi s sluggish behavior of hydrogen peroxide at the sCPE (in contras t to its behavior at pl atinum , gold or glassy carbon electrodes 10) is of particular interest since it is not occurring within the potenti al domains of Fe(IIl) reduction (i.e . no overlapping of the catalyst and substrate reduction waves). The latter (l x lO·3 M) has a typical

irreversible behav ior with a reduction to Fe(II) at -0.1 V and the reoxidation peak at + 1.0 V (Fig. 2 curve c). [n the presence of 5x l0·2 M H20 2 . a cl ear catalytic current (S shaped curve) is observed during Fe(Hl) electroreduction (Figs 2 & 3, curve d).

A sli ght raise of the steady state current (instead of a stab le pl ateau ) is noted however (Fig. 2 and Fig. 3 curve d) and may be attributed to the H20 2 residual current contribution. By subtracting the H20 2

contri bution to the catalytic curve a typical catalytic curve profi Ie is obtai ned that reaches a constant current plateau (Fig. 3 curve e).

Determination of electron tramIer coefficient (ex) and Do

The part of the curve analyzed must correspond to a process governed on ly by the electron transfer, i.e. the foot of the reduction wave of Fe([)[) at the sCPE. The process is irreversible and Eq . 15 is applied. By plotting the resulting Ln i vs E, the following regression line is obtained:

y=-6.7963x+O.072 (R2=O.99) allowing U determi­nation from the slope of the regression: u =O.17 RSD=O.02 (n=3).

Once the electron transfer coefficient is known, it is possible to determine the diffusion coefficient applying Eq. 14 where n=l , u=0. I7, A=O.070 cm2

,

Co=l x IO·(i mol cm"" and v=5x I0·2 V S· I. The value of Do for Fe( llI ) under the experimental cond itions is DFe(III )=7.7XIO·6 cm2 S· I.

1.0 1-____ ...:.I,;::.5_...:-1,;::.0_...::.O,;:.5_...::0;.:;,.II_...::0l.:.5_....:Il.:.O_ 0.5

0.0

-0.5

~ -1.0 ......-or.~ -1.5

::, -2.0 c:--2.5

-3.0

-3.5

-4.0

-4.5

d E(V)

Fig. 2-Cycl ic voltammetry of FeIlINH4(S04.i2 a t sCPE (vs Ag/

AgCl). [Curve (a) O.IM H2S04; (b) SxlO·2M H~02 ; (cl l.Ox I0·3M FeIlINH4(S04)2 and (d) l.Ox l ()· ~M FeIiINI-l4(S 0 4)2+Sx I0·2 M H20 2. Scan rate SOmV/ sand T= 298 KJ .

0.5 -1.5 -1.0 -0.5 0.0 1.5 1.0 0.0 ~_~~;""":':':;';"'-;';;;';;""'~~"" __ oe:...

-0.5 b

....... -1.0

~ -1.5 or. ....... ::, -2.11

::- -2.5

-3.0

-3.5 e _~..,.~

-4.0 d

-4.5 E (V)

Fig. 3-Corrected CV for the reduction of Fe Il INH4(SO-l)2 in the presence of H20 2 (curve e). Experimental conc itions as in Fig. 2.

Second order rate (k'f) constant determination From the above data, it is possible to calculate the

second order rate constant of the catalytic current using Eq. 8. It is worth recalling that the application of thi s equation is restricted to the cases where the CUlTent is purely catalytic, i.e., no dIffusion control. The calculated second order rale constant is k' f = 70 kr' s· ' . This value is close to the literature value of 80 M·I S·I obtained at the dropping mercury electrode' . The Fe(III)/Fe(rI)-H20 2 system appears to be a perfect exampl e of a pure electrocatalytic process, where the only limiting factor for the reaction is the rate constant calculated abo\'e. There is no diffusion control in this process as inferred from Fig. 3 (curve e) profile. To confirm this assertion, the system was studied as a function of scan rate and as a function of the hydrogen peroxide concentration .

It may also be pointed out that at sufficicntly negative potentials all the curves tend to a limiting value i'im independent of thc scan rate v. This limiting

RAZOLA et al.: MEDIATED ELECTROCAT AL YTIC REDUCTION OF Fe(llI) 2953

-1 .25 -0.75 -0.25 0.25 0.75 1.25

0.7

0.00

~ -0.70

Ir>~

::, -1.40 ~

-2.10

-2.110

6 -3.50 E(V)

Fig. 4--Cyclic voltammetry of FeIlINH4(S04)2 as a function of H?O? concentration [( I) 1.0 x 1O-3M FeUlNH4(S04)2; (2) as (I) + 5_0xlO-3M HzOz; (3) as (l)+1.0xI0-zM H20 2; (4) as (1)+ 2.0xlO-2M H20 2; (5) as (I) +3.0xlO-2M HP2; (6) as ( 1)+ 5.0x I0-2M H20 2- Other experimental conditions as in Fig_ 2].

current con-esponds to the situation where the rate removal of oxidant (Fe(IlI)) by electrolysis is exactly compensated by the rate of reformation of the oxidant by the chemical reaction. The effect of scan rate on the voltammograms of FeIlINH4(S04h reduction in the presence of H20 2 was studied between 100 m V /s and 10m V Is. the voltammograms showed no change at different scan rates studied.

It is worth pointing out that the theory developed by Nicholson and Shain2, i.e., analyzing the data making use of Fig. 14 of thi s theory and calculating the kinetic parameter by plotting klla versus the inverse of scan rate, is valid only when the process studied is partly controlled by diffusion, i.e. , a non-pure catalysis phenomena. Such a si tuation is exemplified below by varying the hydrogen peroxide concentration.

The experimental conditions used till now were adequate to obtain a system controlled exclusively by a purely catalytic process. This was possible because the hydrogen peroxide concentration employed was fifty-fold higher than the concentration of Fe(IJI). This permitted maintaining an excess of hydrogen peroxide in the diffusion layer to ensure that the Fe(II) formed at the electrode/solution interface is rapidly regenerated to Fe(lII). No diffusion limitation was observed on the global kinetic system under such experimental conditions, indeed . As is show n in the Fig. 4, vanatlon of the hydrogen peroxide concentration influences considerably the magnitude of the limiting CUITent magnitude and shape of the investigated system. At low concentration of hydrogen peroxide the kinetic process progressively becomes controlled by diffusion (peak shaped curve).

The hydrogen peroxide concentration range studied was between 5x 10-3 M and 5x lO-2 M. This effect is clearly reported in the literature4 (see Fig. 1). The electrochemical reoxidation peak of Fe(Il) in the absence of H20 2 at Ep =+ 1.0 V may also be noted in curve 1 of Fig. 4.

Digital simulation The knowledge of the descriptive mathematical

equations for the kinetic behavior of the Fe(III)/Fe(II)-H202 system, allows a simulation of experimental curves. The theoretical curves were matched below with the experimental curves obtained to validate the model. The mathematical model and experimental curves overlap to a large extent at k'f = 70 M - IS- I, a = 0.17 and Do = 7.7 x 1O-6cm2

S-I.

Such mathematical equations may also be used to obtain an overview of the shape of the catalytic plot as a function of the magnitude of the different parameters.

It was observed that there is a significant influence of the electron transfer rate (a) on the shape of the catalytic curve. Thi s highlights the importance of determining a as precisely as possible in order to generate correct kinetic data. The marked influence of Do on the intensity of the catalytic response was also seen when Do was changed from 7.7x lO-7 cm2

S-I to 7.7 x 1O-5 cm2 S-I .

The curves plotted for different k'f values (150 MI S- I and 25 MI S-I) showed significantly different magnitudes compared to the studied system (k'f = 70 MI S- I); however, the curves still exhibited a catalytic shape profile. Depending on the catalytic rate constant, however, the magnitude of the limiting current may be significantly lowered without exhibiting the typical S shaped response (see Fig. 1).

References I Pospi si I Z, Coli Czech Chem Comml/n, 18 (1953) 337. 2 Nicholson R S & Shain I, Allal Chem. 36 ( 1964) 706. 3 Delahay P & Stiehl G J , J Am chem Soc, 74 ( 1952) 3500. 4 Bard A & Faulkner L R in Electrochemical methods: Fllllda­

IIlelltals and applications (Marcel Dekker, New York) 1980. 5 Ki ssinger P T & Heine man W R, in Laboratory techlliqlles ill

eleClroallalytical chelllistry, (Marcel Dekker, New York) 1984. 6 Levich V G. Acta Physicochilll , 17 ( 1942) 257. 7 Buess P, Contributioll if l'etl/de eleclrochimiqlle des ions

pe roXO - II/'(/1I ate, Ph.D Thesi s" UIlIversite Libre de Bruxelles. 1978.

8 Petit C, Gonzales-Cortes A & Kauffmann J- M, Talallta, 42 ( 1995) In3.

9 Delahay P, in New instmmel11almethods ill electrochemistry. (I ntersc ience Pub, New York) 1954.

10 Awad M I, Harnoode C, Tok uda K & Ohsaka T, Anal Chel1l, 73 (200 1) 1839.