- Email: [email protected]
PTFE electrodes modified by 2-ethylanthraquinone
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 601 (2007) 63–67 www.elsevier.com/locate/jelechem
Electrochemical synthesis of hydrogen peroxide on oxygen-fed graphite/PTFE electrodes modified by 2-ethylanthraquinone J.C. Forti a, R.S. Rocha a, M.R.V. Lanza a
a,b
, R. Bertazzoli
a,*
Departamento de Engenharia de Materiais, Faculdade de Engenharia Mecaˆnica, Universidade Estadual de Campinas, CP 6122, cep: 13083-970, Campinas, SP, Brazil b Universidade Sa˜o Francisco, Av. Sa˜o Francisco de Assis, 218, cep: 12916-900, Braganc¸a Paulista, SP, Brazil Received 14 August 2006; received in revised form 5 October 2006; accepted 11 October 2006 Available online 15 November 2006
Abstract This paper reports an investigation on the performance of a catalyzed H2O2 electrogeneration process using a modified oxygen-fed graphite/PTFE electrodes in which the redox catalyst was incorporated into the graphitic mass. The organic redox catalyst chosen for the modification was 2-ethylanthraquinone (EAQ). The H2O2 electrogeneration rate was optimized relative to potential and catalyst concentration. Hydrogen peroxide formation rate on oxygen-fed graphite/PTFE was greatly improved by the presence of the organic redox catalyst and the overpotential for oxygen reduction was shifted more positive. During electrolysis, hydrogen peroxide electrogeneration reaction showed a pseudo-zero-order kinetics and apparent rate constants increased with EAQ concentration. For the electrodes containing 10% of EAQ, apparent rate constants were 30% higher at a potential 400 mV more positive when compared to the performance of a non catalyzed electrode. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen peroxide; Gas diffusion electrodes; 2-Ethylanthraquinone
1. Introduction The last two decades have witnessed an intense investigation on electrosynthesis of hydrogen peroxide, and several papers have demonstrated that in situ electrogenerated H2O2 may also be used successfully for the treatment of aqueous effluents containing organic pollutants [1–10]. Hydrogen peroxide is still one of the most popular nonselective oxidizing reactants for the oxidation of organic pollutants to carbon dioxide. Its reactivity is determined largely by the ratio of the concentration of H2O2 to substrate and by reaction conditions, particularly the presence of Fe ions and UV radiation that favors hydroxyl radical formation. Furthermore, hydrogen peroxide reactions leave no residuals in the reaction stream, and in concentrations such as those produced in electrolysis cells, reactions are *
Corresponding author. Fax: +55 19 3788 3311. E-mail address: [email protected] (R. Bertazzoli).
0022-0728/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.10.023
non-hazardous and carried out under moderate conditions. H2O2 electrosynthesis is also of interest because of the cost and risks associated with transportation, storing and handling of concentrated hydrogen peroxide. Graphite flat plates [1,2] and three-dimensional electrodes, made from reticulated vitreous carbon (RVC) [3–5] and gas diffusion electrodes (GDE) [6–12] have been used to reduce oxygen to hydrogen peroxide. Traditionally H2O2 is manufactured by reduction of O2 by H2. The reaction is mediated by anthraquinone and demands high availability of hydrogen [13,14]. Firstly, 2alkylanthraquinone is reduced to anthrahydroquinone in the presence of hydrogen. Then, self-oxidation of anthrahydroquinone promotes oxygen reduction to hydrogen peroxide. Taking the traditional H2O2 manufacturing process as an example, this paper reports an investigation on the performance of a catalyzed H2O2 electrogeneration process using modified O2-fed graphite/PTFE electrodes in which
64
J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
the catalyst had been incorporated into the graphitic mass. The organic catalyst chosen for the modification was 2-ethylanthraquinone (EAQ). The H2O2 electrogeneration rate was optimized relative to potential and catalyst concentration. Quinones proved to be efficient catalysts when immobilized on the surface of carbon electrodes. Several studies report carbon electrodes modified by organic catalysts [15–24]. A number of them have investigated the mechanism for oxygen reduction on anthraquinone-modified glassy carbon surfaces [15–17]. Rabinovich et al. [21] studied the performance of a carbon powder coated with a sol–gel derived carbon silicate composite modified with naphthoquinone. Potential for oxygen reduction has shifted by 200 mV more positive due to the presence of naphthoquinone. Glassy carbon electrodes were modified with physically adsorbed quinones derivatives compounds [23,24]. In these studies the modified electrodes were used for oxygen reduction and hydrogen peroxide formation, using ultrasonic effects. Presence of 9,10-phenanthraquinone has shifted potential for oxygen reduction to a less negative value [23] and the currents for hydrogen peroxide synthesis are greatly increased [24]. Some authors have investigated the electroreduction of O2 for large-scale H2O2 production in highly concentrated solutions. Huissoud and Tissot [18–20] used an aqueous/ organic two-phase alkaline emulsion as electrolyte. The 2ethylanthraquinone catalyst was dissolved in the organic phase, and reticulated vitreous carbon was the cathode. Gyeng and Oloman [25,26] used a similar emulsion in acidic pH aiming to meet the industrial feasibility requirements for on-site electrosynthesis of hydrogen peroxide for the bleaching and brightening of pulp and paper. By using quaternary ammonium ions as organic supporting electrolyte and a cationic surfactant, concentration of 0.6 M was reached at current density of 800 A m2. Taking a different approach, this paper reports on the electrosynthesis of hydrogen peroxide in dilute acidic solutions at rates appropriate to the needs of effluent treatment, and in mild conditions of current density and solution pH. Furthermore, larger-area modified gas diffusion electrode (MGDE) were produced and used for the optimization of H2O2 generation rate relative to potential and the catalyst concentration.
Ag/AgCl (KCl sat.) as reference electrode. The cyclic voltammogram was recorded in the aprotic medium with EAQ (60 mM), after N2 purging (30 min), at 100 mV s1. All experiments were controlled by a PGSTAT30 potentiostat/galvanostat connected to a BSTR-10A Current Booster (Autolab). 2.2. Electrode preparation The MGDE precursor mass was prepared from Degussa Printex 6L conductive carbon-black graphitic pigment. As hydrophobic binder, a 60% polytetrafluoroethylene dispersion (Dyneon TF 5035 PTFE) was used. The ratio of Printex to PTFE was 8/3.3, which is equivalent to 20% of PTFE. The mixture was homogenized in a 4:1 bi-distilled water:isopropanol solution. The selected amounts of 2-ethylanthraquinone, from 0.5% to 10%, relative to the carbon pigment, were incorporated into the MGDE precursor mass, which was dried at 110 °C during 24 h. A 200 mesh AISI 304 stainless steel screen current collector was placed at the bottom of a 60 mm diameter pressing tool which was then filled with 8 g of the precursor mass. Sintered 3 mm thick MGDE was obtained after 1.5 h at 310 °C, under load of 18 MPa. 2.3. MGDE behavior In a new series of experiments, the MGDE electrodes, prepared with different concentrations of EAQ (0.5%, 1%, 3%, 5% and 10%) were used as cathode. For the voltammetric and electrolysis experiments was used an electrochemical cell (one compartment, 250 mL) shown in Fig. 1.
2. Experimental 2.1. Ethylanthraquinone behavior Preliminary cyclic voltammetry experiments were performed to identify 2-ethylanthraquinone (EAQ) redox reactions in aprotic medium containing: vacuum distilled dimethylformamide (DMF) plus dried NaClO4 0.1 M (pH 7.0). For this, a three electrode arrangement was used in the electrochemical cell (one compartment, 100 mL) with glassy carbon (3 mm diameter) serving as the working electrode, a platinum foil used as the counter electrode and a
Fig. 1. Scheme of the electrochemical cell used in the voltammetric and electrolysis experiments. (A) Working electrode (GDE or MGDE); (B) reference electrode (Ag/AgCl, KCl sat.) and (C) counter electrode (platinum foil). Volume of solution: 250 mL.
J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
The MGDE was placed at the bottom of the cell with an exposed area of 19 cm2 and the electrode was oxygenback-fed. The reference was Ag/AgCl (KCl sat.) and a platinum foil was the counter electrode (A = 24 cm2). This electrochemical cell was thermostated at a constant temperature of 20 °C. The supporting electrolyte used was 0.1 M H2SO4 plus 0.1 M K2SO4, pH 1. Experiments were performed with mechanical agitation. The linear voltammetry was recorded from 0.2 V to 1.0 V vs. Ag/AgCl, at 20 mV s1. Before, the supporting electrolyte was saturated with nitrogen. Then, the i/E responses were also recorded in the presence of oxygen. Subsequently, controlled potential electrolysis was used for the optimization of H2O2 electrogeneration rate relative to the applied potential in the range 0.4 6 E 6 0.9 V vs. Ag/AgCl. During electrolysis an oxygen pressure of 0.16 bar was kept at the back side of the electrode. The electrolyte was sampled at intervals of 5 min for the first half hour and every 10 min after that. Hydrogen peroxide concentration was determined by UV–Vis spectrophotometer (Lambda 40, Perkin Elmer Instruments), recording the spectra over 200–500 nm. A solution of 2.4 mM (NH4)6Mo7O24 Æ 4H2O in 0.5 M H2SO4 was added to the samples resulting in a yellow color [27]. The absorbance was determined at 350 nm. Calibration plots based on Beer–Lambert’s law were established relating absorbance to concentration.
65
in a potential range from 0.0 V to 1.8 V vs. Ag/AgCl. Two pairs of reversible peaks denoted by a and b are clearly observed. These peaks are relative to the EAQ reduction in two steps separated by almost 500 mV. Such a potential separation may give to an intermediate species, a quinone radical, as already hypothesised in the literature [15,25]. A short-live quinone radical is highly reactive, and the EAQ redox cycle involved in the oxygen reduction may not include the EAQ itself but the radical species, according to the scheme in Fig. 3. The resulting superoxide anion, as represented in Fig. 3, may undergo further reduction or disproportionation to hydrogen peroxide [15]. The MGDE electrodes, prepared with different concentrations of EAQ, were used as cathode in the aqueous medium used for the synthesis of hydrogen peroxide (0.1 M H2SO4 plus 0.1 M K2SO4, pH 1). Currents of EAQ reduction recorded in these experiments were proportional to EAQ concentration. Fig. 4A shows a linear potential scan for the MGDE in 250 mL of supporting electrolyte. Once the solution was thoroughly nitrogen-purged, currents recorded in this experiment were typically related to the reduction of EAQ.
O
3. Results and discussion O
3.1. Voltammetric experiments + 1e+ 1H+
Fig. 2 shows a cyclic voltammogram recorded on glassy carbon for the aprotic solution containing 60 mM of EAQ,
OH
0.8
a´
0.6
O
0.4
.
b´
O2
I / mA
0.2
+1e-
0.0
+ 1H+
-0.2 -0.4 -0.6
a
-0.8
OH
O2-
HO2- or H2O2
b
-1.0 -1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
E / V (Ag/AgCl) Fig. 2. Cyclic voltammogram on glassy carbon in: (- - -) Aprotic conditions (DMF + NaClO4 0.1 M) and (—) Aprotic conditions (DMF + NaClO4 0.1 M) + 2-ethylanthraquinone (60 mM). Scan rate of 100 mV s1, after N2 purging (30 min).
OH
Fig. 3. Overall reaction pathway for the oxygen reduction reaction mediated by EAQ redox catalyst.
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J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
A
800
0.0
-0.4 V -0.5 V -0.6 V -0.7 V -0.8 V -0.9 V
700 -0.2
600
-0.6 -0.8
-1
EAQ (0%) EAQ (0.5%) EAQ (1%) EAQ (3%) EAQ (5%) EAQ (10%)
H2O2 / mg L
I/A
-0.4
500 400 300 200
-1.0
100 -1.2 -1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
0 0
10
20
E / V (Ag/AgCl)
B
30
40
50
60
time / min. Fig. 5. H2O2 concentration as a function of electrolysis time for the MGDE containing 10% EAQ. Supporting electrolyte: 0.1 M H2SO4 + 0.1 M K2SO4. O2 pressure = 0.16 Bar.
0.0 -0.5 -1.0 -1.5 -2.0
of electrolysis time for the potential values shown, using a 10% MGDE. All the electrodes presented similar rising concentration profiles. The concentration curves in Fig. 5 show that slopes increase from 0.4 V to 0.6 V vs. Ag/ AgCl and decrease after that. More negative potential values stimulate water production in a four-electron reaction:
I/A
-2.5 -3.0
EAQ (0%) EAQ (0.5%) EAQ (1%) EAQ (3%) EAQ (5%) EAQ (10%)
-3.5 -4.0 -4.5 -5.0 -5.5 -0.8
-0.7
-0.6
-0.5
-0.4
-0.3
O2 þ 2Hþ þ 2e ! H2 O2 -0.2
E / V (Ag/AgCl) Fig. 4. Currents recorded for a linear potential scan on the MGDE surfaces with EAQ concentrations as shown. (A) Nitrogen-purged supporting electrolyte. (B) Oxygen-purged supporting electrolyte. Pressure = 0.16 Bar, scan rate of 20 mV s1.
Fig. 4B shows the voltammetric response when the MGDE were fed with oxygen for different EAQ concentrations. Current values recorded in these experiments are probably a result of two simultaneous processes: oxygen direct reduction on the graphite surface, and reduction of EAQ. When the catalyst concentration was increased, an increasing current response was observed as a consequence of a reduction of the overpotential for the formation of hydrogen peroxide. 3.2. Electrolysis at controlled potential All the six electrodes with 0–10% of EAQ were used for the synthesis of hydrogen peroxide. They were oxygen-fed with 0.16 bar of pressure, and bulk electrolysis was carried out with potentials from 0.4 V to 0.9 V vs. Ag/AgCl. Hydrogen peroxide concentration was monitored during the 60 min of the experiments. Fig. 5 shows a representative plot of hydrogen peroxide concentration as a function
H2 O2 þ 2Hþ þ 2e ! 2H2 O
ðIÞ ðIIÞ
Data for hydrogen peroxide concentration, as in Fig. 5, were obtained for all MGDE (0–10% EAQ), and used for kinetic analysis. Reaction kinetics is not well defined for the first five minutes of electrolysis. After that point, the electrode inner channeling process completes, and the O2 flow rate reaches a steady state. Provided there are no limitations to reactant mass transfer, hydrogen peroxide formation proceeds in a pseudo-zero-order kinetics after five minutes of electrolysis. Fig. 6A shows values of pseudozero-order apparent rate constants (kapp) for hydrogen peroxide formation as a function of the potential values selected for the experiments, and compares the performance of electrodes containing different amounts of EAQ. As a common feature, all curves reach a maximum kapp value at potential of 0.6 V vs. Ag/AgCl and decrease for more negative electrolysis potential. The presence of EAQ in the MGDE shifts the optimal potential by 400 mV more positive as compared to non-catalyzed electrode, which has a maximum kapp value at 1.0 V vs. Ag/AgCl. This is an important figure if a scale-up of an electrolysis manufacturing process is contemplated. Fig. 6B also shows a plot of kapp values taken at 0.6 V vs. Ag/AgCl as a function of EAQ concentration. The rising portion of the curve shows an increasing gain for the elec-
J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
A
increased with EAQ concentration. For the electrodes containing 10% of EAQ, apparent rate constant were 30% higher at a potential 400 mV more positive when compared to the performance of a non catalyzed electrode.
0% EAQ 1% EAQ 5% EAQ 10% EAQ
12
8
Acknowledgement
6
The financial support of this work by FAPESP foundation is gratefully acknowledged.
-1
kapp / mg L min
-1
10
67
4
References 2 0 -1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
E / V (Ag/AgCl)
[5]
B 10
[6] [7]
8
[8] 6
[9]
-1
-1
kapp / mg L min (E = - 0.6V)
[1] [2] [3] [4]
4
[10] [11]
2
[12] [13]
0 0
1
2
3
4
5
6
7
8
9
10
% EAQ
[14]
Fig. 6. (A) Apparent rate constants for H2O2 electrogeneration as a function of applied potential. (B) Apparent rate constants at 0.6 V vs. Ag/AgCl as a function of EAQ concentration.
[15]
trogeneration rate that however is attenuated from 5% of EAQ, indicating 10% as an ideal EAQ concentration.
[17]
4. Conclusions Gas diffusion electrodes offer an alternative process for the manufacturing hydrogen peroxide in dilute solutions. Hydrogen peroxide formation rate on oxygen-fed graphite/PTFE may be greatly improved by the presence of an organic redox catalyst incorporated into the graphitic mass. The 2-ethylanthraquinone, when used as catalyst, improved the H2O2 formation rate and reduced the overpotential for oxygen reduction. During electrolysis, the hydrogen peroxide electrogeneration reaction displayed pseudo-zero-order kinetics, and apparent rate constants
[16]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
J-S. Do, C-P. Chen, J. Electrochem. Soc. 140 (6) (1993) 1632. J-S. Do, C-P. Chen, J. Appl. Electrochem. 24 (9) (1994) 936. C.P. De Leon, D. Pletcher, J. Appl. Electrochem. 25 (4) (1995) 307. A. Alvarez-Gallegos, D. Pletcher, Electrochim. Acta 44 (5) (1998) 853. A. Alvarez-Gallegos, D. Pletcher, Electrochim. Acta 44 (14) (1999) 2483. E. Brillas, R.M. Bastida, E. Llosa, J. Casado, J. Electrochem. Soc. 142 (6) (1995) 1733. E. Brillas, E. Mur, J. Casado, J. Electrochem. Soc. 143 (3) (1996) L49. E. Brillas, R. Salueda, J. Casado, J. Electrochem. Soc. 144 (1997) 2374. E. Brillas, E. Mur, R. Salueda, L. Sanches, Appl. Catal. B 16 (1) (1998) 31. T. Harrington, D. Pletcher, J. Electrochem. Soc. 146 (8) (1999) 2983. E. Brillas, M.A. Banos, S. Camps, C. Arias, P.L. Cabot, J.A. Garrido, R.M. Rodrı´guez, New J. Chem. 28 (2) (2004) 314. E. Brillas, P.L. Cabot, R.A. Rodrı´guez, C. Arias, J.A. Garrido, R. Oliver, Appl. Catal. B 51 (2) (2004) 117. Ullmann’s, Encyclopedia of Industrial Chemistry, vol. A13, 1989, pp. 443–467. Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 13, 1981, pp. 12–38. K. Tammeveski, K. Kontturi, R.J. Nichols, R.J. Potter, D.J. Schiffrin, J. Electroanal. Chem. 515 (2001) 101. A. Sarapuu, K. Vaik, D.J. Schiffrin, K. Tammeveski, J. Electroanal. Chem. 541 (2003) 23. K. Vaik, D.J. Schiffrin, K. Tammeveski, Electrochem. Commun. 6 (2004) 1. A. Huissoud, P. Tissot, J. Appl. Electrochem. 28 (1998) 653. A. Huissoud, P. Tissot, J. Appl. Electrochem. 29 (1999) 11. A. Huissoud, P. Tissot, J. Appl. Electrochem. 29 (1999) 17. L. Rabinovich, V. Glezer, Z. Wu, O. Lev, J. Electroanal. Chem. 504 (2001) 146. R. Gopal, Electrochemical synthesis of hydrogen peroxide, U.S. PATENT 6, 712, 949 (2004). B. Sljukic, C.E. Banks, S. Mentus, R.G. Compton, Phys. Chem. Chem. Phys. 6 (2004) 992. A. Salimi, C.E. Banks, R.G. Compton, Phys. Chem. Chem. Phys. 5 (2003) 3988. E.L. Gyenge, C.W. Oloman, J. Appl. Electrochem. 33 (2003) 655. E.L. Gyenge, C.W. Oloman, J. Appl. Electrochem. 33 (2003) 665. X.-S. Chai, Q.X. Hou, Q. Luo, J.Y. Zhu, Anal. Chim. Acta 507 (2004) 281.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 601 (2007) 63–67 www.elsevier.com/locate/jelechem
Electrochemical synthesis of hydrogen peroxide on oxygen-fed graphite/PTFE electrodes modified by 2-ethylanthraquinone J.C. Forti a, R.S. Rocha a, M.R.V. Lanza a
a,b
, R. Bertazzoli
a,*
Departamento de Engenharia de Materiais, Faculdade de Engenharia Mecaˆnica, Universidade Estadual de Campinas, CP 6122, cep: 13083-970, Campinas, SP, Brazil b Universidade Sa˜o Francisco, Av. Sa˜o Francisco de Assis, 218, cep: 12916-900, Braganc¸a Paulista, SP, Brazil Received 14 August 2006; received in revised form 5 October 2006; accepted 11 October 2006 Available online 15 November 2006
Abstract This paper reports an investigation on the performance of a catalyzed H2O2 electrogeneration process using a modified oxygen-fed graphite/PTFE electrodes in which the redox catalyst was incorporated into the graphitic mass. The organic redox catalyst chosen for the modification was 2-ethylanthraquinone (EAQ). The H2O2 electrogeneration rate was optimized relative to potential and catalyst concentration. Hydrogen peroxide formation rate on oxygen-fed graphite/PTFE was greatly improved by the presence of the organic redox catalyst and the overpotential for oxygen reduction was shifted more positive. During electrolysis, hydrogen peroxide electrogeneration reaction showed a pseudo-zero-order kinetics and apparent rate constants increased with EAQ concentration. For the electrodes containing 10% of EAQ, apparent rate constants were 30% higher at a potential 400 mV more positive when compared to the performance of a non catalyzed electrode. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen peroxide; Gas diffusion electrodes; 2-Ethylanthraquinone
1. Introduction The last two decades have witnessed an intense investigation on electrosynthesis of hydrogen peroxide, and several papers have demonstrated that in situ electrogenerated H2O2 may also be used successfully for the treatment of aqueous effluents containing organic pollutants [1–10]. Hydrogen peroxide is still one of the most popular nonselective oxidizing reactants for the oxidation of organic pollutants to carbon dioxide. Its reactivity is determined largely by the ratio of the concentration of H2O2 to substrate and by reaction conditions, particularly the presence of Fe ions and UV radiation that favors hydroxyl radical formation. Furthermore, hydrogen peroxide reactions leave no residuals in the reaction stream, and in concentrations such as those produced in electrolysis cells, reactions are *
Corresponding author. Fax: +55 19 3788 3311. E-mail address: [email protected] (R. Bertazzoli).
0022-0728/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.10.023
non-hazardous and carried out under moderate conditions. H2O2 electrosynthesis is also of interest because of the cost and risks associated with transportation, storing and handling of concentrated hydrogen peroxide. Graphite flat plates [1,2] and three-dimensional electrodes, made from reticulated vitreous carbon (RVC) [3–5] and gas diffusion electrodes (GDE) [6–12] have been used to reduce oxygen to hydrogen peroxide. Traditionally H2O2 is manufactured by reduction of O2 by H2. The reaction is mediated by anthraquinone and demands high availability of hydrogen [13,14]. Firstly, 2alkylanthraquinone is reduced to anthrahydroquinone in the presence of hydrogen. Then, self-oxidation of anthrahydroquinone promotes oxygen reduction to hydrogen peroxide. Taking the traditional H2O2 manufacturing process as an example, this paper reports an investigation on the performance of a catalyzed H2O2 electrogeneration process using modified O2-fed graphite/PTFE electrodes in which
64
J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
the catalyst had been incorporated into the graphitic mass. The organic catalyst chosen for the modification was 2-ethylanthraquinone (EAQ). The H2O2 electrogeneration rate was optimized relative to potential and catalyst concentration. Quinones proved to be efficient catalysts when immobilized on the surface of carbon electrodes. Several studies report carbon electrodes modified by organic catalysts [15–24]. A number of them have investigated the mechanism for oxygen reduction on anthraquinone-modified glassy carbon surfaces [15–17]. Rabinovich et al. [21] studied the performance of a carbon powder coated with a sol–gel derived carbon silicate composite modified with naphthoquinone. Potential for oxygen reduction has shifted by 200 mV more positive due to the presence of naphthoquinone. Glassy carbon electrodes were modified with physically adsorbed quinones derivatives compounds [23,24]. In these studies the modified electrodes were used for oxygen reduction and hydrogen peroxide formation, using ultrasonic effects. Presence of 9,10-phenanthraquinone has shifted potential for oxygen reduction to a less negative value [23] and the currents for hydrogen peroxide synthesis are greatly increased [24]. Some authors have investigated the electroreduction of O2 for large-scale H2O2 production in highly concentrated solutions. Huissoud and Tissot [18–20] used an aqueous/ organic two-phase alkaline emulsion as electrolyte. The 2ethylanthraquinone catalyst was dissolved in the organic phase, and reticulated vitreous carbon was the cathode. Gyeng and Oloman [25,26] used a similar emulsion in acidic pH aiming to meet the industrial feasibility requirements for on-site electrosynthesis of hydrogen peroxide for the bleaching and brightening of pulp and paper. By using quaternary ammonium ions as organic supporting electrolyte and a cationic surfactant, concentration of 0.6 M was reached at current density of 800 A m2. Taking a different approach, this paper reports on the electrosynthesis of hydrogen peroxide in dilute acidic solutions at rates appropriate to the needs of effluent treatment, and in mild conditions of current density and solution pH. Furthermore, larger-area modified gas diffusion electrode (MGDE) were produced and used for the optimization of H2O2 generation rate relative to potential and the catalyst concentration.
Ag/AgCl (KCl sat.) as reference electrode. The cyclic voltammogram was recorded in the aprotic medium with EAQ (60 mM), after N2 purging (30 min), at 100 mV s1. All experiments were controlled by a PGSTAT30 potentiostat/galvanostat connected to a BSTR-10A Current Booster (Autolab). 2.2. Electrode preparation The MGDE precursor mass was prepared from Degussa Printex 6L conductive carbon-black graphitic pigment. As hydrophobic binder, a 60% polytetrafluoroethylene dispersion (Dyneon TF 5035 PTFE) was used. The ratio of Printex to PTFE was 8/3.3, which is equivalent to 20% of PTFE. The mixture was homogenized in a 4:1 bi-distilled water:isopropanol solution. The selected amounts of 2-ethylanthraquinone, from 0.5% to 10%, relative to the carbon pigment, were incorporated into the MGDE precursor mass, which was dried at 110 °C during 24 h. A 200 mesh AISI 304 stainless steel screen current collector was placed at the bottom of a 60 mm diameter pressing tool which was then filled with 8 g of the precursor mass. Sintered 3 mm thick MGDE was obtained after 1.5 h at 310 °C, under load of 18 MPa. 2.3. MGDE behavior In a new series of experiments, the MGDE electrodes, prepared with different concentrations of EAQ (0.5%, 1%, 3%, 5% and 10%) were used as cathode. For the voltammetric and electrolysis experiments was used an electrochemical cell (one compartment, 250 mL) shown in Fig. 1.
2. Experimental 2.1. Ethylanthraquinone behavior Preliminary cyclic voltammetry experiments were performed to identify 2-ethylanthraquinone (EAQ) redox reactions in aprotic medium containing: vacuum distilled dimethylformamide (DMF) plus dried NaClO4 0.1 M (pH 7.0). For this, a three electrode arrangement was used in the electrochemical cell (one compartment, 100 mL) with glassy carbon (3 mm diameter) serving as the working electrode, a platinum foil used as the counter electrode and a
Fig. 1. Scheme of the electrochemical cell used in the voltammetric and electrolysis experiments. (A) Working electrode (GDE or MGDE); (B) reference electrode (Ag/AgCl, KCl sat.) and (C) counter electrode (platinum foil). Volume of solution: 250 mL.
J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
The MGDE was placed at the bottom of the cell with an exposed area of 19 cm2 and the electrode was oxygenback-fed. The reference was Ag/AgCl (KCl sat.) and a platinum foil was the counter electrode (A = 24 cm2). This electrochemical cell was thermostated at a constant temperature of 20 °C. The supporting electrolyte used was 0.1 M H2SO4 plus 0.1 M K2SO4, pH 1. Experiments were performed with mechanical agitation. The linear voltammetry was recorded from 0.2 V to 1.0 V vs. Ag/AgCl, at 20 mV s1. Before, the supporting electrolyte was saturated with nitrogen. Then, the i/E responses were also recorded in the presence of oxygen. Subsequently, controlled potential electrolysis was used for the optimization of H2O2 electrogeneration rate relative to the applied potential in the range 0.4 6 E 6 0.9 V vs. Ag/AgCl. During electrolysis an oxygen pressure of 0.16 bar was kept at the back side of the electrode. The electrolyte was sampled at intervals of 5 min for the first half hour and every 10 min after that. Hydrogen peroxide concentration was determined by UV–Vis spectrophotometer (Lambda 40, Perkin Elmer Instruments), recording the spectra over 200–500 nm. A solution of 2.4 mM (NH4)6Mo7O24 Æ 4H2O in 0.5 M H2SO4 was added to the samples resulting in a yellow color [27]. The absorbance was determined at 350 nm. Calibration plots based on Beer–Lambert’s law were established relating absorbance to concentration.
65
in a potential range from 0.0 V to 1.8 V vs. Ag/AgCl. Two pairs of reversible peaks denoted by a and b are clearly observed. These peaks are relative to the EAQ reduction in two steps separated by almost 500 mV. Such a potential separation may give to an intermediate species, a quinone radical, as already hypothesised in the literature [15,25]. A short-live quinone radical is highly reactive, and the EAQ redox cycle involved in the oxygen reduction may not include the EAQ itself but the radical species, according to the scheme in Fig. 3. The resulting superoxide anion, as represented in Fig. 3, may undergo further reduction or disproportionation to hydrogen peroxide [15]. The MGDE electrodes, prepared with different concentrations of EAQ, were used as cathode in the aqueous medium used for the synthesis of hydrogen peroxide (0.1 M H2SO4 plus 0.1 M K2SO4, pH 1). Currents of EAQ reduction recorded in these experiments were proportional to EAQ concentration. Fig. 4A shows a linear potential scan for the MGDE in 250 mL of supporting electrolyte. Once the solution was thoroughly nitrogen-purged, currents recorded in this experiment were typically related to the reduction of EAQ.
O
3. Results and discussion O
3.1. Voltammetric experiments + 1e+ 1H+
Fig. 2 shows a cyclic voltammogram recorded on glassy carbon for the aprotic solution containing 60 mM of EAQ,
OH
0.8
a´
0.6
O
0.4
.
b´
O2
I / mA
0.2
+1e-
0.0
+ 1H+
-0.2 -0.4 -0.6
a
-0.8
OH
O2-
HO2- or H2O2
b
-1.0 -1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
E / V (Ag/AgCl) Fig. 2. Cyclic voltammogram on glassy carbon in: (- - -) Aprotic conditions (DMF + NaClO4 0.1 M) and (—) Aprotic conditions (DMF + NaClO4 0.1 M) + 2-ethylanthraquinone (60 mM). Scan rate of 100 mV s1, after N2 purging (30 min).
OH
Fig. 3. Overall reaction pathway for the oxygen reduction reaction mediated by EAQ redox catalyst.
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J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
A
800
0.0
-0.4 V -0.5 V -0.6 V -0.7 V -0.8 V -0.9 V
700 -0.2
600
-0.6 -0.8
-1
EAQ (0%) EAQ (0.5%) EAQ (1%) EAQ (3%) EAQ (5%) EAQ (10%)
H2O2 / mg L
I/A
-0.4
500 400 300 200
-1.0
100 -1.2 -1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
0 0
10
20
E / V (Ag/AgCl)
B
30
40
50
60
time / min. Fig. 5. H2O2 concentration as a function of electrolysis time for the MGDE containing 10% EAQ. Supporting electrolyte: 0.1 M H2SO4 + 0.1 M K2SO4. O2 pressure = 0.16 Bar.
0.0 -0.5 -1.0 -1.5 -2.0
of electrolysis time for the potential values shown, using a 10% MGDE. All the electrodes presented similar rising concentration profiles. The concentration curves in Fig. 5 show that slopes increase from 0.4 V to 0.6 V vs. Ag/ AgCl and decrease after that. More negative potential values stimulate water production in a four-electron reaction:
I/A
-2.5 -3.0
EAQ (0%) EAQ (0.5%) EAQ (1%) EAQ (3%) EAQ (5%) EAQ (10%)
-3.5 -4.0 -4.5 -5.0 -5.5 -0.8
-0.7
-0.6
-0.5
-0.4
-0.3
O2 þ 2Hþ þ 2e ! H2 O2 -0.2
E / V (Ag/AgCl) Fig. 4. Currents recorded for a linear potential scan on the MGDE surfaces with EAQ concentrations as shown. (A) Nitrogen-purged supporting electrolyte. (B) Oxygen-purged supporting electrolyte. Pressure = 0.16 Bar, scan rate of 20 mV s1.
Fig. 4B shows the voltammetric response when the MGDE were fed with oxygen for different EAQ concentrations. Current values recorded in these experiments are probably a result of two simultaneous processes: oxygen direct reduction on the graphite surface, and reduction of EAQ. When the catalyst concentration was increased, an increasing current response was observed as a consequence of a reduction of the overpotential for the formation of hydrogen peroxide. 3.2. Electrolysis at controlled potential All the six electrodes with 0–10% of EAQ were used for the synthesis of hydrogen peroxide. They were oxygen-fed with 0.16 bar of pressure, and bulk electrolysis was carried out with potentials from 0.4 V to 0.9 V vs. Ag/AgCl. Hydrogen peroxide concentration was monitored during the 60 min of the experiments. Fig. 5 shows a representative plot of hydrogen peroxide concentration as a function
H2 O2 þ 2Hþ þ 2e ! 2H2 O
ðIÞ ðIIÞ
Data for hydrogen peroxide concentration, as in Fig. 5, were obtained for all MGDE (0–10% EAQ), and used for kinetic analysis. Reaction kinetics is not well defined for the first five minutes of electrolysis. After that point, the electrode inner channeling process completes, and the O2 flow rate reaches a steady state. Provided there are no limitations to reactant mass transfer, hydrogen peroxide formation proceeds in a pseudo-zero-order kinetics after five minutes of electrolysis. Fig. 6A shows values of pseudozero-order apparent rate constants (kapp) for hydrogen peroxide formation as a function of the potential values selected for the experiments, and compares the performance of electrodes containing different amounts of EAQ. As a common feature, all curves reach a maximum kapp value at potential of 0.6 V vs. Ag/AgCl and decrease for more negative electrolysis potential. The presence of EAQ in the MGDE shifts the optimal potential by 400 mV more positive as compared to non-catalyzed electrode, which has a maximum kapp value at 1.0 V vs. Ag/AgCl. This is an important figure if a scale-up of an electrolysis manufacturing process is contemplated. Fig. 6B also shows a plot of kapp values taken at 0.6 V vs. Ag/AgCl as a function of EAQ concentration. The rising portion of the curve shows an increasing gain for the elec-
J.C. Forti et al. / Journal of Electroanalytical Chemistry 601 (2007) 63–67
A
increased with EAQ concentration. For the electrodes containing 10% of EAQ, apparent rate constant were 30% higher at a potential 400 mV more positive when compared to the performance of a non catalyzed electrode.
0% EAQ 1% EAQ 5% EAQ 10% EAQ
12
8
Acknowledgement
6
The financial support of this work by FAPESP foundation is gratefully acknowledged.
-1
kapp / mg L min
-1
10
67
4
References 2 0 -1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
E / V (Ag/AgCl)
[5]
B 10
[6] [7]
8
[8] 6
[9]
-1
-1
kapp / mg L min (E = - 0.6V)
[1] [2] [3] [4]
4
[10] [11]
2
[12] [13]
0 0
1
2
3
4
5
6
7
8
9
10
% EAQ
[14]
Fig. 6. (A) Apparent rate constants for H2O2 electrogeneration as a function of applied potential. (B) Apparent rate constants at 0.6 V vs. Ag/AgCl as a function of EAQ concentration.
[15]
trogeneration rate that however is attenuated from 5% of EAQ, indicating 10% as an ideal EAQ concentration.
[17]
4. Conclusions Gas diffusion electrodes offer an alternative process for the manufacturing hydrogen peroxide in dilute solutions. Hydrogen peroxide formation rate on oxygen-fed graphite/PTFE may be greatly improved by the presence of an organic redox catalyst incorporated into the graphitic mass. The 2-ethylanthraquinone, when used as catalyst, improved the H2O2 formation rate and reduced the overpotential for oxygen reduction. During electrolysis, the hydrogen peroxide electrogeneration reaction displayed pseudo-zero-order kinetics, and apparent rate constants
[16]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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