Available online at www.sciencedirect.comJournal of

www.elsevier.com/locate/jelechem

Journal of Electroanalytical Chemistry 611 (2007) 126–132

ElectroanalyticalChemistry

On the role of the surfactant aliquat� 336 on the kineticsof oxygen reduction reaction and on the rate

of hydrogen peroxide electrosynthesis

Barbara de Oliveira, Rodnei Bertazzoli *

Departamento de Engenharia de Materiais, Faculdade de Engenharia Mecanica, Universidade Estadual de Campinas, C.P. 6122,

13083-970 Campinas-SP, Brazil

Received 14 June 2007; received in revised form 7 August 2007; accepted 8 August 2007Available online 17 August 2007

Abstract

This paper reports an investigation on the effect of the surfactant aliquat� 336 (A336) on the mass transport parameters of the oxygenreduction reaction and on H2O2 electro-generation rate. Results have shown that most of the positive effects of adding A336 to the elec-trolyte are observed at concentrations around the critical micelle concentration (CMC). Results analyzed in this paper have shown thatadding A336 at CMC shifted half-wave potential for O2 reduction by 80 mV more negative and, furthermore, extended limiting currentplateaus for the reaction O2!H2O2 over extra 100 mV more negative. Limiting currents for O2 reduction are higher in the presence ofA336, and the O2 diffusion coefficient increased from 2 · 10�6 cm2 s�1 to 9 · 10�6 cm2 s�1. All these findings were verified by controlledpotential electrolysis in a flow electrochemical reactor, where it was also found that the H2O2 electro-generation rate reached a maximumfor A336 concentrations close to the CMC.� 2007 Elsevier B.V. All rights reserved.

Keywords: Aliquat 336�; Oxygen reduction; Hydrogen peroxide

1. Introduction

Hydrogen peroxide synthesis based on oxygen electro-reduction has two important applications: cellulose pulpbleaching and degradation of organic pollutants in waste-water. The first of these requires the production of largeamounts of solution with a concentration above 4 wt%H2O2, and a NaOH:H2O2 ratio in the range of 0.3–1.2[1,2]. For the second, hydrogen peroxide can be synthesizedon site, in the aqueous stream itself, in neutral or mildlyacidic pH, in diluted solutions appropriate for the needsof water and effluent treatment [3].

Oloman’s research group has pioneered electro-reduc-tion of oxygen to hydrogen peroxide in the 1970s. Since

0022-0728/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2007.08.010

* Corresponding author. Tel.: +55 1935213311.E-mail address: rbertazzoli@fem.unicamp.br (R. Bertazzoli).

then, large scale O2 reduction to H2O2 in alkaline solutions,pH 13, has been a topic of investigation [4–6]. In the 1990s,several papers demonstrated that in situ electro-generatedH2O2 might also be successfully used for effluent treatment[7–16]. Graphite flat plates [7,8] and three dimensional elec-trodes (either reticulated vitreous carbon (RVC) [9–11] orgas diffusion electrodes (GDE) [12–16]) have been used toreduce oxygen to hydrogen peroxide at rates appropriateto the needs of effluent treatment. The knowledge gainedby those studies is still being applied to the degradationof hazardous pollutants contained in aqueous media eitherusing RVC cathodes [17,18] or GDE [19,20].

In order to optimize electro-generation rate, attentionhas been paid to the role of catalysts either adsorbed ontoelectrode surfaces or as an electrochemical reaction media-tor in the electrolytic medium. Quinones have proved effi-cient as catalysts when immobilized onto the surface ofcarbon electrodes. In some studies, carbon electrodes have

B. de Oliveira, R. Bertazzoli / Journal of Electroanalytical Chemistry 611 (2007) 126–132 127

been modified by organic catalysts. Several of these haveinvestigated the mechanism for oxygen reduction on qui-none modified glassy carbon surfaces [21–25]. The electro-chemical behavior of azobenzene and its derivatives insolution has also been extensively studied because of theinvolvement of proton transport, adsorption/desorption,and cis–trans isomerization [26–30]. Quinones and azoben-zene have also been incorporated into the graphitic massduring carbon GDE production [31,32]. Results haveshown a great improvement in the electro-generation rate,and a shifting of the O2 reduction potential to more posi-tive values, leading to a reduction in the cell potential.

Some authors have investigated the electro-reduction ofO2 using an electrolytic multi-phase suspension with anorganic catalyst for large- scale H2O2 production in highlyconcentrated solutions. Huissoud and Tissot [33–35] haveused an aqueous/organic two-phase alkaline emulsion aselectrolyte. 2-Ethylanthraquinone catalyst was dissolvedin the organic phase, and reticulated vitreous carbon wasused as a cathode. More recently, Gyenge and Oloman[36,37] used a similar three-phase emulsion aqueousphase/organic phase/oxygen in acidic pH aiming to meetthe industrial feasibility requirements for the on site elec-trosynthesis of hydrogen peroxide. By using quaternaryammonium ions as organic supporting electrolyte and acationic surfactant, a concentration of 0.6 M was reachedat a current density of 800 A m�2.

Along with the catalysts, surfactants may play an impor-tant role in efforts to improve the electro-generation rateand inhibit H2O2 decomposition. Surfactant micelles maycause changes on the electrode surface regarding polaradsorption, electrical properties of the electrode/solutioninterface and O2transport parameters. The influence of sur-factants on the electro-reduction of oxygen to hydrogenperoxide has only been the subject of investigation morerecently. Pioneering once more, the Oloman’s group runa set of voltammetric and batch electrolysis experimentsin 0.1 mol L�1 Na2CO3 or in 0.1 mol L�1 H2SO4 solutions,in the presence of a cationic surfactant (tricaprylmethylam-monium chloride), aliquat 336� (or A336), a non ionic sur-factant (Triton X-100�) and an anionic surfactant (sodiumdodecyl sulfate, SDS) [38]. As Triton and SDS apparentlyblocked the access of O2 to the electrode surface by form-ing surface structures and hence increasing the overpoten-tial for the reduction reaction, most of the results wereanalyzed as a function of A336 concentration and its effectson reduction reaction kinetics and on mass transportparameters.

The benefits of a long chain quaternary ammonium saltare well established, and have been verified [36–38]. Mostof the positive or adverse effects are dependent of the sur-factant concentration in solution. Mechanisms of interfer-ence on the reaction rate are different depending on thesurfactant aggregation state, i.e. whether concentration isbelow or above the critical micelle concentration (CMC).In studies reported in the literature so far, surfactant con-centrations were above CMC, sometimes by as much as

one order of magnitude [36–38]. However, there is no needto increase concentration to values much above CMC,since most of the positive effects on the reaction rate areobserved around the CMC value.

This paper presents and discusses the influence of cat-ionic surfactants on the oxygen reduction reaction andon the stability of electro-generated hydrogen peroxide.Retarding the exchange of the last two electrons, andthereby inhibiting the transformation of hydrogen peroxidein water, is as important as increasing the rate of exchangeof the first two electrons. In order to investigate these twoeffects, the surfactant A336 (CH3(C8H17)3N+Cl�) was cho-sen. In the first experiment series, oxygen reduction reac-tion in the presence of the surfactant was studied byhydrodynamic voltammetry on the surface of a vitreouscarbon rotating disc. The goal was to obtain the potentialsof the electrochemical reactions involved, and the limitingcurrents as a function of surfactant concentration. Resultswere used as operating parameters for a flow-by electro-chemical reactor with an RVC cathode for hydrogen per-oxide generation and accumulation experiments.

2. Experimental

2.1. Surface tension measurements

In order to determine the value of critical micelle con-centration (CMC) of the surfactant A336 in sulfate solu-tion, measurements of surface tension were done with aSKV Instruments (Sigma model) that uses the du Nouyring method (platinum/iridium alloy ring). In this method,the ring is slowly lifted until the rupture of the film on thesolution surface. The force needed for the rupture allowsthe calculation of the surfactant solution surface. Measure-ments were performed in a 0.1 mol L�1 K2SO4 solution(pH 3.5) at 25 ± 1 �C.

2.2. Hydrodynamic voltammetry

Voltammetric experiments were performed in order tostudy the influence of the A336 cationic surfactant on thekinetics of the oxygen reduction reaction and on mass trans-fer parameters. Tests were done in a K2SO4 0.1 mol L�1

solution, pH 3.5, at 25 ± 1 �C, on a vitreous carbon (VC)rotating disc (area �0.12 cm2). The A336 (Cognis) concen-tration varied from 10�7 to 10�3 mol L�1. All reactantswere of analytical grade, and no previous purification wasdone. Voltammetric experiments were done with a potentio-stat/galvanostat PGSTAT 30 from Autolab. The singlecompartment electrolytic cell had three electrodes: theworking electrode, a platinum foil of 50 cm2 surface as thecounter electrode, and a reference electrode of calomel insaturated KCl (SCE). Oxygen was thoroughly bubbledthrough a sintered cylindrical glass frit of 12 mm diameterbefore each voltammetric run. As observed earlier, maxi-mum dissolved oxygen concentration in the supporting elec-trolyte at 25 �C reached 25 mg L�1 (7.4 · 10�4 mol L�1)

1E-5 1E-425

30

35

40

45

CMC = 4.0 x 10-5 mol L-1

γ (m

N/m

)

[A336] (mol L-1)

Fig. 1. Evolution of surface tension with A336 concentration in0.1 mol L�1 K2SO4, pH 3.5, 25 ± 1 �C.

128 B. de Oliveira, R. Bertazzoli / Journal of Electroanalytical Chemistry 611 (2007) 126–132

[17,18], which is less than the values reported for similarexperiments [38]. This value will be used later in the Levichequation for oxygen diffusion coefficient calculations. Vol-tammograms were obtained by linear potential scanningat a scan rate of 5 mV s�1, from 0.0 to �1.8 V vs. SCE forthe following working electrode rotation rates: 0, 400,900, 1600, 2500, 3600 and 4000 rpm.

2.3. Experiments in the flow-by electrochemical reactor

Electro-generation and hydrogen peroxide accumula-tion tests were carried out in a flow-by electrochemicalreactor of the filter press type, with a supporting electrolyteof 0.1 mol L�1 K2SO4 (20 mS cm�1), pH 3.5, and theabove-mentioned concentrations of A336. Determinationof hydrogen peroxide concentration was done with anautomatic potentiometric titrator Metrohm model702SM, and a standardized solution of potassium perman-ganate 2 · 10�3 mol L�1. During the batch recirculationexperiments, pH was kept at 3.5 at the catholyte and 12at the anolyte using H2SO4 and NaOH, respectively.Catholyte and anolyte volume was 3.5 L each. Oxygenwas bubbled at 0.1 L s�1, a flow sufficient to keep theconcentration of the electroactive species at 25 mg L�1.Constant potential electrolysis experiments were controlledby an Autolab PGSTAT30 potentiostat with a currentbooster BSTR10A.

The electrochemical reactor had the shape of a filterpress; an expanded view is shown elsewhere [18]. The cath-ode central plate was a 304 AISI stainless steel current fee-der in a PP frame (135 mm · 310 mm · 1.5 mm), ontowhich 80 ppi RVC (50 mm · 150 mm · 6 mm) plates werefixed on both sides with thermally curable Dylon PX GradeGraphpoxy conducting epoxy. Anolyte and catholyte flowswere separated by a N424 Nafion� membrane, and bothanodes were 1.5 mm thick titanium plates coated with(Ta2O5)0.6(IrO2)0.4 supplied by De Nora do Brasil Ltda.Total geometric anodic area was 150 cm2. End plates weremade from 15 mm thick PP. A rubber gasket was placedbetween each plate and membranes to prevent leakage.Inter-electrode gap was 5 mm including the thickness ofrubber gaskets and membrane. The recirculation systemcomprised flow meters, pumps and 5 L reservoirs. Duringthe experiments, the electrochemical reactor was operatedin batch recirculation mode at a flow rate of 500 L h�1, cor-responding to a linear velocity of 0.12 m s�1. Calculation ofsolution velocity took into account that RVC has a voidageof 93%.

A platinum tip inserted between the membrane and theRVC cathode was used as a pseudo-reference electrode. Byusing Pt as reference, a potential shift of 0.7 V more nega-tive was observed when compared to potentials referred toSCE. To prevent solution heating and improve oxygen sol-ubility, the temperature was kept at 25 �C. A 1/4 in. 304AISI stainless steel cooling coil was inserted into the cath-olyte reservoir and filled with a recirculating 25% ethyleneglycol solution. In keeping with previous experiments of

hydrogen peroxide generation rate optimization, all elec-trolyses were conducted at �1.6 V vs. Pt [17,18].

3. Results and discussion

3.1. CMC determination

Fig. 1 shows the variation in surface tension in the duNouy ring as a function of A336 concentration. As the con-centration of the surfactant increases, surface tensiondecreases until it reaches a constant value. The discontinu-ity between the curves is identified as the CMC, the pointfrom which the micelle formation begins to take place. Inspite of the dispersion observed in the points of Fig. 1,the CMC value can be assumed to be in the range of 3.0-4.0 · 10�5 mol L�1 at 298 K. This CMC value found forA336 in a 0.1 mol L�1 K2SO4 solution (pH 3.5) is smallerthan the value in pure water (1.4 · 10�4 mol L�1 at298 K) taken as a reference in the experiments reportedin the literature [38]. As discussed further, the choice ofconcentrations above CMC prevents the main benefits ofadding a cationic surfactant from taking place.

3.2. Hydrodynamic voltammetry experiments

Fig. 2a presents the i/E couples obtained without addingsurfactant, and Fig. 2b with an A336 concentration equalto the CMC; the latter is representative of a series of vol-tammograms with increasing surfactant concentrations.Solutions were saturated with oxygen, and potential sweep-ing started with 0.0 V vs. SCE toward more negative values.In Fig. 2a two subsequent reduction processes can be seen.The first plateau relates to the exchange of 2e� of the reac-tion O2! H2O2. The second wave, characterized by a cur-rent peak between �1.4 and �1.6 V vs. SCE, concerns thereaction H2O2! H2O with the exchange of another 2e�.The influence of mass transport in the reduction processes

-2.0 -1.6 -1.2 -0.8 -0.4 0.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5aI /

mA

E vs SCE/V

400 rpm 900 rpm

1600 rpm 2500 rpm 3600 rpm 4000 rpm

-2.0 -1.6 -1.2 -0.8 -0.4 0.0

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

b

I / m

A

E vs SCE/V

[A336] = CMC

400 rpm 900 rpm

1600 rpm 2500 rpm 3600 rpm 4000 rpm

-1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4

-1.5

-1.0

-0.5

0.0c

I / m

A

E vs SCE/V

-1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4-1.0

-0.5

0.0

[A336] = CMC

d

I / m

A

E vs SCE/V

[A336] = 0 mol L-1

[A336] = 0 mol L-1

Fig. 2. i/E curves for O2 reduction on vitreous carbon disc for rotation rates as shown. (a) supporting electrolyte; (b) with CMC of A336; (c and d)comparative expansion of both voltammograms. Supporting electrolyte of O2 saturated 0.1 mol L�1 K2SO4, pH 3.5, 25 ± 1 �C. Scan rate of 5 mV s�1.

1E-7 1E-6 1E-5 1E-4 1E-3

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

I L /

mA

[A336] / mol L-1

400 rpm 900 rpm

1600 rpm 2500 rpm 3600 rpm 4000 rpm

Fig. 3. Limiting current values, taken at �0.95 V vs. SCE, as a function ofsurfactant concentration for rotation rates as shown. Data taken fromvoltammograms similar to those of Fig. 2 and obtained under the sameconditions.

B. de Oliveira, R. Bertazzoli / Journal of Electroanalytical Chemistry 611 (2007) 126–132 129

is well characterized by the increase of current values as theelectrode rotation rate increases. On the other hand, vol-tammograms obtained in the presence of A336 (Fig. 2b)are marked by the absence of the second reduction plateau.In this potential range (�1.4 to �1.6 V), current values aresmaller than those of Fig. 2a. The presence of A336 in solu-tion also displaces the half-wave potential by 80 mVtowards more negative values. In the absence of A336,E1/2 = �0.50 V, whereas in its presence, E1/2 = �0.58 V.The comparative expansion of voltammograms (Fig. 2cand d) shows that the limiting current plateaus of theO2 ! HO�2 reaction, which extended for about 350 mV inthe absence of A336, increased to 450 m V in its presence.This behavior can be an important factor in the stabiliza-tion of hydrogen peroxide, by making the second electronexchange happen in more negative overpotentials.

Since the reaction of reduction of O2 to hydrogen perox-ide is a mass transport controlled process, the examinationof limiting current values can be an important factor inassessing the effect of the surfactant on the H2O2 genera-tion rate. Fig. 3 shows the values of the limiting current

130 B. de Oliveira, R. Bertazzoli / Journal of Electroanalytical Chemistry 611 (2007) 126–132

taken at �0.95 V vs. SCE for all A336 concentrations andfor different rotation rates. It is noteworthy that the limit-ing current maxima are found in values near the CMC. Asa general behavior, current increases up to values near theCMC, and then decreases and returns to the initial value. Itfollows that concentrations above the A336 may not harm,but they do not help either.

In experiments using rotating disc electrode, Levich’sequation is a suitable tool to decide whether or not an elec-trochemical reaction is fully mass transfer controlled. Datafrom Fig. 2 were organized in an IL vs. x0.5 plot as shownin Fig. 4a. The slopes (s) are related to oxygen diffusioncoefficients according to:

s ¼ 0:62nFD0:67m�0:17CO2ð1Þ

where n is the number of electrons exchanged in the reac-tion (2 in this case), m is the electrolyte cinematic viscosity(10�5 cm2 s�1); F is the Faraday constant; D is the diffusioncoefficient, and C is the oxygen bulk concentration(25 mg L�1/7.4 mmol L�1).

Thus, for different A336 concentration, diffusion coeffi-cient values were obtained, as shown in Fig. 4b. The valueof D increases dramatically around the CMC, highlightingthe importance of A336 in transporting oxygen to the elec-trode surface. Results in Fig. 4b make clear that the pres-ence of the cationic surfactant A336 has a direct actionon oxygen transport parameters, notably on the diffusioncoefficient. In the concentration range chosen for the pres-ent study, this seems to be the main contribution of the sur-factant, along with a local pH increasing and the formationof surfactant structural micelle aggregates, as reported inthe literature [38]. For concentrations much above theCMC, micelles can display a lamellar arrangement perpen-dicular to the electrode surface [39] which, in view of theresults obtained, is less efficient in the oxygen transport.

6 8 10 12 14 16 18 20

-5

-4

-3

-2

-1Limiting currents at -0.95 V

I L (

mA

cm

-2)

ω0.5 / s0.5

[A336] / mol L-1

1.0 × 10-7

1.0 × 10-6

1.0 × 10-5

3.9 × 10-5

1.0 × 10-4

1.0 × 10-3

a

Fig. 4. (a) Levich’s plot using limiting current values taken from i/E curves simLevich’s straight line slopes, as a function of A336 concentration.

3.3. Electro-generation of hydrogen peroxide in the flow

reactor

The electrochemical reactor was operated at �1.6 V vs.Pt, with a flow rate of 500 L h�1 and total recirculating vol-ume of 3.5 L. Temperature was kept at 17 ± 1 �C, and oxy-gen flow was 6 L min�1. As in voltammetric experiments,the hydrogen peroxide electro-generation and accumula-tion experiments were done for different A336 concentra-tions. Each run lasted for 2 h, and samples for analyseswere taken at 15 min-intervals. Fig. 5a presents the hydro-gen peroxide concentration profiles as a function of elec-trolysis time for the A336 concentrations shown. In theright hand side Fig. 5b shows a characteristic current pro-file during electrolysis. Curves in Fig. 5a show that H2O2

concentration increases linearly with slopes depending onthe A336 concentration. Closer observation reveals thatslopes increase up to the concentration of 10�5 mol L�1,and decrease beyond that. It is worth noting that the oxy-gen reduction reaction is not mass transfer limited in theoperating conditions reported here. Apparently, currentsresulting from the controlled potential experiments arebelow the limiting current. As a consequence, hydrogenperoxide synthesis followed a pseudo-zero order kineticsas seen in Fig. 5a. Hence, the solution of the differentialequation resulting from the mass balance is:

½H2O2�ðtÞ ¼ kappt ð2Þ

where kapp is the zero-order apparent rate constant forhydrogen peroxide generation.

The graph in Fig. 6 is a plot of the curve slopes in Fig. 5aas a function of A336 concentrations. Increasing values ofkapp are observed up to the concentration of 10�5 mol L�1,a value near the CMC. Beyond that, a decrease of kapp

1E-7 1E-6 1E-5 1E-4 1E-3

1

2

3

4

5

6

7

8

9

10

D 1

0-6/ c

m2 s

-1

[A336] mol L-1

b

ilar to those of Fig. 2. (b) Oxygen diffusion coefficients, calculated from the

0 20 40 60 80 100 1200

100

200

300

400

500

[H2O

2] /

mg

L-1

Time / min

[A336] / mol L-1

no surfactant 1.0 × 10-7

1.0 × 10-6

1.0 × 10-5

4.0 × 10-5

1.0 × 10-4

1.0 × 10-3

0 2000 4000 6000 8000-2.8

-2.6

-2.4

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

i / A

Time / s

a b

Fig. 5. (a) Concentration of electrogenerated H2O2 as a function of electrolysis time. (b) Current profile during the electrolysis for A336 concentration of10�6 mol L�1. Solution of 0.1 mol L�1 of K2SO4, pH 3.5. Oxygen flow rate of 6 L min�1. Temperature 17 ± 1 �C. Recirculation flow rate 500 L h�1.Applied potential �1.6 V vs. Pt.

1E-7 1E-6 1E-5 1E-4 1E-3

2.8

3.2

3.6

4.0

4.4

r2 = 0.999

r2 = 0.996

r2 = 0.999

r2 = 0.999

r2 = 0.999

r2 = 0.998

k app

/ m

g L-1

min

-1

[A336] / mol L-1

Fig. 6. Zero-order apparent rate constants for the electrogeneration ofH2O2 as a function of A336 concentration. Values calculated from theslopes of the straight lines from Fig. 5.

0.00 0.08 0.96 0.98 1.0050

60

70

80

90

1000.00 0.08 0.96 0.98 1.00

0

1

2

3

4

5

Cur

rent

effi

cien

cy /

%

[A336] 10-3 / mol L-1

Ene

rgy

cons

umpt

ion

/ kW

h kg

-1

Fig. 7. Current efficiency and energy consumption as a function of A336concentration. See Fig. 5 for operational parameters.

B. de Oliveira, R. Bertazzoli / Journal of Electroanalytical Chemistry 611 (2007) 126–132 131

values takes place. These results verify those obtained involtammetric experiments, and, apparently, confirm theelectrocatalyst role of A336 in concentrations near theCMC, regardless of the reaction rate control mode.

Current efficiency and energy consumption profiles arealso dependent on A336 concentration. Fig. 7 shows thebehavior of both as a function of A336 concentration.Current efficiency increases up the concentration of10�5 mol L�1, and remains practically constant beyondthat. At the same time, and as a consequence, energyconsumption decreases up to the same concentration, andremains around 3.5 kWh kg�1 of generated hydrogenperoxide, which is lower than the energy consumptionof the electrochemical technology marketed by H-D Tech[2].

4. Conclusions

Results discussed in this paper have shown that oxygentransport parameters are strongly affected by the A336concentration, especially the limiting current for the oxy-gen reduction reaction and thus, the diffusion coefficient.Most of the benefits due to the presence of the surfactantare observed around the critical micelle concentration inwhich transport parameters were maximized.

In the experiments of hydrogen peroxide electro-genera-tion and accumulation, maximum electro-generation rate isalso reached for A336 concentrations near the CMC. Inthese experiments, current efficiency increased and energyconsumption decreased with increasing values of A336concentration until CMC, beyond which they remainedapproximately constant.

132 B. de Oliveira, R. Bertazzoli / Journal of Electroanalytical Chemistry 611 (2007) 126–132

References

[1] J.R. Anderson, B. Amini, in: C.W. Dence, D.W. Reeve (Eds.), PulpBleaching: Principles and Practice, Tappi Press, Atlanta, GA, 1998, p.413.

[2] C. Oloman, Electrochemical processing for the pulp and paperindustry, p. 143, The Electrochemical Consultancy, Romsey, UK,1996.

[3] D. Pletcher, Watts New 4 (1) (1999), electrosynthesis.com, acess 03/03/2007.

[4] C. Oloman, A.P. Watkinson, Can. J. Chem. Eng. 53 (1975) 268–273.[5] C. Oloman, A.P. Watkinson, Can. J. Chem. Eng. 54 (1976) 312–318.[6] C. Oloman, A.P. Watkinson, J. Appl. Electrochem. 9 (1979) 117–123.[7] J-S. Do, C-P. Chen, J. Electrochem. Soc. 140 (1993) 1632–1637.[8] J-S. Do, C-P. Chen, J. Appl. Electrochem. 24 (1994) 936–942.[9] C.P. De Leon, D. Pletcher, J. Appl. Electrochem. 25 (1995) 307–314.

[10] A. Alvarez-Gallegos, D. Pletcher, Electrochim. Acta 44 (1998) 853–861.

[11] A. Alvarez-Gallegos, D. Pletcher, Electrochim. Acta 44 (1999) 2483–2492.

[12] E. Brillas, R.M. Bastida, E. Llosa, J. Casado, J. Electrochem. Soc.142 (1995) 1733–1741.

[13] E. Brillas, E. Mur, J. Casado, J. Electrochem. Soc. 143 (1996) L49–L53.

[14] E. Brillas, R. Salueda, J. Casado, J. Electrochem. Soc. 144 (1997)2374–2379.

[15] E. Brillas, E. Mur, R. Salueda, L. Sanches, J. Peral, X. Domenech, J.Casado, Appl. Cat. B-Environ 16 (1998) 31–42.

[16] T. Harrington, D. Pletcher, J. Electrochem. Soc. 146 (1999) 2983–2989.

[17] C. Badellino, C.A. Rodrigues, R. Bertazzoli, J. Hazard. Mat. B 137(2006) 856–864.

[18] C. Badellino, C.A. Rodrigues, R. Bertazzoli, J. Appl. Electrochem. 37(2007) 451–459.

[19] E. Brillas, M.A. Banos, S. Camps, C. Arias, P.L. Cabot, J.A. Garrido,R.M. Rodrıguez, New J. Chem. 28 (2004) 314–322.

[20] E. Brillas, P.L. Cabot, R.A. Rodrıguez, C. Arias, J.A. Garrido, R.Oliver, Appl. Cat. B-Environ 51 (2004) 117–127.

[21] K. Tammeveski, K. Kontturi, R.J. Nichols, R.J. Potter, D.J.Schiffrin, J. Electroanal. Chem. 515 (2001) 101–112.

[22] A. Sarapuu, K. Vaik, D.J. Schiffrin, K. Tammeveski, J. Electroanal.Chem. 541 (2003) 23–29.

[23] K. Vaik, D.J. Schiffrin, K. Tammeveski, Electrochem. Commun. 6(2004) 1–5.

[24] B. Sljukic, C.E. Banks, S. Mentus, R.G. Compton, Phys. Chem.Chem. Phys. 6 (2004) 992–997.

[25] A. Salimi, C.E. Banks, R.G. Compton, Phys. Chem. Chem. Phys. 5(2003) 3988–3993.

[26] S. Wawzonek, J.D. Fredrickso, J. Am. Chem. Soc. 77 (1955) 3985–3995.

[27] J.L. Salder, A.J. Bard, J. Am. Chem. Soc. 90 (1967) 1979–1989.[28] E. Laviron, Y. Mungnier, J. Electroanal. Chem. 111 (1980) 337–344.[29] E. Laviron, J. Electroanal. Chem. 169 (1984) 29–46.[30] L. Flamigni, S. Monti, J. Phys. Chem. 89 (1985) 3702–3707.[31] J.C. Forti, R.S. Rocha, M.R.V. Lanza, R. Bertazzoli, J. Electroanal.

Chem. 601 (2007) 63–67.[32] J.C. Forti, J.A. Nunes, M.R.V. Lanza, R. Bertazzoli, J. Appl.

Electrochem. 37 (2007) 527–532.[33] A. Huissoud, P. Tissot, J. Appl. Electrochem. 28 (1998) 653–657.[34] A. Huissoud, P. Tissot, J. Appl. Electrochem. 29 (1999) 11–16.[35] A. Huissoud, P. Tissot, J. Appl. Electrochem. 29 (1999) 17–25.[36] E.L. Gyenge, C.W. Oloman, J. Appl. Electrochem. 33 (2003) 655–

663.[37] E.L. Gyenge, C.W. Oloman, J. Appl. Electrochem. 33 (2003) 665–

674.[38] E.L. Gyenge, C.W. Oloman, J. Appl. Electrochem. 31 (2001) 233–

243.[39] J.F. Rusling, Acc. Chem. Res. 24 (1991) 75–81.