Nitrogen and sulfur doped mesoporous carbon cathodes for water treatment

    Nitrogen and sulfur doped mesoporous carbon cathodes for water treatment Valentina Perazzolo, Christian Durante, Armando Gennaro PII: DOI: Reference:

S1572-6657(16)30564-1 doi: 10.1016/j.jelechem.2016.10.037 JEAC 2904

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

2 August 2016 3 October 2016 17 October 2016

Please cite this article as: Valentina Perazzolo, Christian Durante, Armando Gennaro, Nitrogen and sulfur doped mesoporous carbon cathodes for water treatment, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.10.037

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Nitrogen and Sulfur Doped Mesoporous Carbon

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Cathodes for Water Treatment

e-mail: [email protected], [email protected]

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Abstract

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In this paper, nitrogen and sulfur doped or co-doped meoporous carbons (N-MC, S-MC and N,S-

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MC) were prepared according to a hard template approach and employed for the in situ production of H2O2. N-MC and to a lesser extent S-MC showed catalytic activity towards oxygen reduction reaction with high selectivity, up to 80%, for the production of H2O2. The possible application of

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doped MCs for the in situ generation of H2O2 in water treatments was confirmed by the degradation of methyl orange, which is a benchmark for degradation of pollutants, in potential controlled electrolysis in an undivided electrochemical cell, resulting in the complete degradation of the organic dye.

Keywords: mesoporous carbon, nitrogen doping, hydrogen peroxide, dye degradation, ORR.

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Introduction

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H2O2 is a widespread bleaching agent, that is usually addressed as a green chemical since

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after reacting it generates essentially water and this has opened its utilization in

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environmentally friendly processes including water pollutants degradation [1]. At present, the anthraquinone process is the only economically feasible process for H2O2 production on

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an industrial scale, but it is expensive and generates hazardous waste [2]. Thus, novel, safer and cleaner methods for the production of H2O2 are being explored and particularly

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appealing is the development of electrolysers for the in situ generation of H2O2 [3]. In particular, electrolyser powered by renewable energy sources is one of the most promising

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self-sustainable technology for the future environmental challenges. Therefore, the

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discovering of catalysts that are stable, active and selective for the electroreduction of oxygen (ORR) to H2O2 is highly desirable [4]. ORR may proceed through a direct four-

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electron pathway leading to H2O (eq. 1); or a two bi-electronic step pathway where hydrogen peroxide is formed as an intermediate (eq. 2-3) [5]. O2 + 4H+ + 4e  H2O

(1)

O2 + 2H+ + 2e  H2O2

Eo = 0.69 V

(2)

H2O2 + 2H+ + 2e  H2O

Eo = 1.78 V

(3)

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Eo = 1.23 V

The ideal catalyst should boost the direct synthesis of H 2O2 avoiding the subsequent reduction to water. The most promising catalysts found for this reaction, are based on Fe and Co porphyrins complexes [6] or noble metals [4]. However, the macrocyclic ligands are subject to degradation by H2O2, resulting in rapid performance losses [7], whereas noble metals possesses adequate stability under the demanding reaction conditions, but are expensive and hardly disposable. It is widely documented that H2O2 electrogeneration, either for electro-Fenton application or for direct oxidation and disinfection, successfully occurs on 2

ACCEPTED MANUSCRIPT carbon-based cathodes due to their high overpotential toward water discharge. A comprehensive literature is available ranging over traditional electrode materials such as

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graphite [8–10], carbon felt [11–14], to less investigated cathodes such as glassy carbon

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[15], boron doped diamond (BDD) [16], activated carbon fiber [17], carbon nanotubes [18–

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20] and graphene [21–24]. Besides electrodic material, the electrogeneration of H2O2 was found to depend from several factors including cell geometry, electrode technology and operating parameters (O2 feeding, stirring rate or liquid flow rate, temperature, solution pH,

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electrolyte composition, and applied potential or current). A comprehensive comparative

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table can be found in Brillas’ work [25]. The most efficient configuration involves the employment of a divided cell with carbon-PTFE gas diffusion cathode, Pt mesh as anode and a Nafion membrane for dividing the two electrodic compartments. The electrolyte is

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usually a O2 saturated acidic solution with the addition of a supporting electrolyte, NaCl or Na2SO4. In such conditions concentrations of H2O2 higher than 1000 mg/L were detected

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with current efficiencies ranging between 70-100 % [26,27] Carbon materials show negligible catalysis for ORR, however their activity can be

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enhanced by pinning heteroatoms such as nitrogen, sulfur, boron, etc. on the carbon surface [28,29], obtaining de facto a metal-free and cost-effective electrocatalyst [30–34]. In particular, mesoporous carbons (MCs) are ideal materials for electrocatalysis since they can be easily doped, they have large surface area, uniform and adjustable pore size, mechanical stability, good conductivity [30,35] and, last but not least, they can be synthesized from a huge number of synthetic or natural precursors including biomasses [36,37]. Furthermore, the presence of mesopores allows a favourable mass transport; therefore the chemical intermediates can be released within a relatively short contact time, avoiding subsequent reactions which would lower the process selectivity. The enhanced activity of doped carbons versus ORR is widely reported in literature, especially for the tetra-electronic pathway, which is the desirable mechanism in fuel cell applications [38–49]. However, the tetra3

ACCEPTED MANUSCRIPT electronic pathway usually requires the presence, at least in traces, of transition metals such as Fe, Co or Ni [7,42,50–52], whereas metal free doped carbons preferentially are good

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cathodes for H2O2 production [53–55]. To the best of our knowledge, doped MCs were

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never tried for the direct degradation of pollutants. In the present study, we employ nitrogen

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and sulphur doped or co-doped MCs as electrode materials for the in situ generation of H2O2

Experimental.

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in potential controlled electrolyses aimed at the degradation of water pollutants.

The synthesis of doped mesoporous carbon employs commercial mesoporous silica (Sigma-

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Aldrich, 200 nm particle size, 4 nm pore size) as templating agent and 1,10 phenanthroline

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(Sigma-Aldrich, >99.5%), phenothiazine (Sigma-Aldrich, >98%), dibenzothiophene (Sigma-

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Aldrich, >98%) and phenanthrene (Sigma-Aldrich, >98%) as organic precursor. In the following, the resulting doped MCs are named according to the heteroatoms content as N-

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MC, N,S-MC and S-MC. The MC prepared from phenanthrene was referred as undoped mesoporous carbon (u-MC) and considered for comparison. The rotating ring (Pt) disk (Glassy Carbon - GC) electrode (RRDE) measurements as well as the ink preparation and the electrode cleaning and activation were carried out as previously reported [30,55]. Pt and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The RRDE experiments were carried out in 10 mM H2SO4 (Traceselect®, Aldrich) + 0.1 M Na2SO4 (Aldrich); the solutions were purged with Ar before each measurement, whereas for the ORR test, the solution was bubbled with either pure O2 or air mixture till saturation.

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

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The catalyst inks were prepared to obtain a carbon loading on the electrode of 0.6 mg/cm 2

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The procedure and the set-up used for the electrical conductivity measurements were carried out according to Celzard et al. [56]. Briefly, the doped MCs were placed in a glass tube

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between two metal plungers and the electrical conductivity at different compression was

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measured by a digital multimeter.

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Results and discussion

2.1

Doped MCs characterization.

Nitrogen and sulfur doped MCs were prepared according to a hard template approach employing P200 silica as inorganic template. The procedure consists in the dissolution of 1 g of silica and 1 g of organic precursor in 15 mL of acetone or ethanol depending on the precursor solubility in the medium. 200 L H2SO4 were also added to facilitate the oligomerization of the precursors during the impregnation process. The solution was dried in an oven for over 1 h at 100 °C and then heated 5

ACCEPTED MANUSCRIPT in a quartz tube, with a ramp of 5 °C min1 until the temperature reaches 750 °C, and kept constant for 2 h. Once cold, the crude material was treated with a liquid solution of 20 mL of 1 M NaOH and

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20 mL of ethanol for removing the inorganic template and recovered by vacuum filtration on a

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nylon nanometric filter (GVS, nylon 0.2 µm, 47 mm membrane diameter). More detailed

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information can be found elsewhere [55,57]. The prepared doped MCs show spherical shape carbon particles with a mean diameter of 200 nm (Figures 1a-b), surface area >850 m2g1, high pore volume (>600 cm3/g) and a pore size distributions centred at 3.7 nm (Table 1). The porous structure

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is easily visible from the SEM image reported in Figure 1b. The elemental analyses confirmed the

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presence of nitrogen in the samples N-MC and N,S-MC and sulfur in the samples N,S-MC and SMC (Table 1). XPS analysis confirmed that nitrogen is present mostly in the form of pyridinic,

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pyrrolic and N-graphitic groups (Figure 1c), whereas sulfur is present as thiophenic-like units

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(Figure 1d) [55]. The XRD pattern for all doped MCs after pyrolysis at 750 °C shows a diffraction peak at 2 < 5, which is characteristic of ordered mesostructured carbon materials [58]. A second

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peak centred at 2 = 24,8 is present, which is very similar to the (002) diffraction peak of the graphite structure (Figure 1e). The low intensity of the (002) peak indicates that the prepared doped

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carbons are for the most part amorphous materials with very small graphitic domains. This is in accordance with the moderate conductivity (  300 mS/m) determined for the doped MCs at a pressure of 185 kPa, which is more than 10 time lower with respect to a commercial graphitized MC ( = 3970 mS/m).

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Figure 1- (a-b) Representative SEM for N-MC. (c) N 1s and (d) S 2p XPS detailed study and

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deconvolution signals for N-MC and S-MC, respectively. (e) X-Ray diffraction patterns of N-MC.

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Figure 2a reports the voltammetry recorded in an O2-saturated 0.01 M H2SO4 + 0.1 M Na2SO4 solution for the three differently doped MCs. N-MC shows an increased capacitance with respect to N,S-MC-1 and S-MC caused by the increased number of intrinsic charges and by the adsorption of proton and electrolyte ions in the electrical double layer. This effect can be correlated to acid-base properties of nitrogen functional groups, which involves ionic charges exchange from the surface to the electrode by proton transfer from or to pyridinic and pyrrolic groups. The same effect is absent in the case of thiophenic groups, so that lower capacitance is observed in the case of N,S-MC and even lower for S-MC. Cyclic voltammetries in Figure 2a show also an irreversible reduction peak not present in the background, responsible for the reduction of O2 molecule, thus attesting the capability of 7

ACCEPTED MANUSCRIPT these materials to reduce oxygen in the available potential window. A more detailed analysis of doped MC catalytic activity was carried out at RRDE electrode. Figure 2b reports the

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RRDE curves of N-MC, S-MC and N,S-MC in an O2 saturated 0.01 M H2SO4 + 0.1 M

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Na2SO4 electrolyte. Doped MCs catalyse the ORR since the reduction process starts at

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potential 440-570 mV more positive than at bare GC (Table 1). The catalytic activity expressed as ΔEonset= EonsetMCs – EonsetGC, where Eonset is the onset potential for ORR, decreases according to N-MC > N,S-MC > S-MC. In the case of S-MC the whole curve is

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shifted to more negative potential, attesting a lower catalytic activity for ORR with respect

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to both N-MC and N,S-MC. Furthermore, it may be observed an increased activity towards H2 evolution since the electrolyte discharge is anticipated of at least 300 mV with respect to GC.

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The ring electrode potential was set at 1.00 V for determining the H 2O2 produced at the disk electrode. In fact, the RRDE analysis allows a fast and precise determination of the number

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of transferred electrons (n) and therefore to establish the selectivity towards H 2O2 (

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[59], according to eq. 4 and eq. 5

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(4)

(5)

Where Id is the limiting disk current and it is proportional to the amount of O2 reduced, Ir is the ring current and it is proportional to the H2O2 generated at the disk electrode and oxidized at the ring electrode, and N is the collection efficiency (0.25 in our case), which is characteristic for each RRDE electrode. The number of transferred electrons was found to be close to 2 for all MCs, indicating a process preferentially leading to H2O2 with a selectivity higher than 80% (Table 1). At bare GC, O2 reduction involves the transfer of almost 3e ; which establishes a mixed tetra-/bi-electronic mechanism. Therefore, doped-MC modified 8

ACCEPTED MANUSCRIPT electrodes are not only active for ORR, but they selectively orient the O2 reduction toward

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H2O2.

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Figure 2- (a) Cyclic voltammetries at N-MC, S-MC and N,S-MC in O2 saturated 0.01 M H2SO4 + 0.1 M Na2SO4; v = 200 mVs1. (b) RRDE (Pt ring and GC disk) LSV at N-MC, S-MC and N,S-MC

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in O2 saturated 0.01 M H2SO4 + 0.1 M Na2SO4; v = 5 mVs1, rotation rate 1600 rpm, Ering = 1.0 V vs. SCE. (c) MO CV in 0.01 M H2SO4 + 0.1 M Na2SO4 at N-MC modified electrode, v = 200 mVs1.

2.2

Potential controlled electrolysis.

Advanced oxidation processes (AOP) involving the in situ generation of strong oxidants, such as hydroxyl radicals (OH), have emerged in the past decade as an important class of new technologies to accelerate the non-selective oxidation and removal of colorants and a variety of other organic structures found in wastewater [60]. OH radicals have very high 9

ACCEPTED MANUSCRIPT oxidant potential (eq. 6) and is formed as intermediate from water oxidation to O2 at the surface of a high O2-overvoltage anode such as BDD electrode [61] in the anodic oxidation process (AO). HO + H+ + e  H2O

Eo = 2.76 V



Eo = 1.50 V

HO2 + H+ + e  H2O2

Fe2+ + H2O2  Fe(OH)2+ + HO

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

(6) (7) (8)

The H2O2 electrogenerated at carbon cathode has a good oxidant power (Eo = 1.78 V) and

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can be fruitfully used for mineralizing organic pollutants [62]. Furthermore, when an

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undivided electrolytic cell is used, H2O2 can be oxidized to O2 at the anode with formation of hydroperoxyl radical (HO2) as intermediate, a much weaker oxidant than both H2O2 and HO (eq. 7) [63], in the so-called anodic oxidation with electrogenerated H2O2 (AO-H2O2).

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In acidic medium, the oxidizing power of H2O2 can be strongly enhanced using the electroFenton method (EF), where a small quantity of Fe 2+ is added as catalyst to the contaminated

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solution to generate HO and Fe3+ from Fenton’s reaction (eq. 8) [25]. Figure 3 reports the results of the electrochemical treatment of MO in O2 saturated

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0.01 M H2SO4 + 0.1 M Na2SO4 solution, where the working electrode was a doped MC modified GC electrode. The O2 solution saturation can be accomplished by insufflating either pure O2 or a standard air mixture. The degradation was carried out at pH = 2.4 that is very close to the optimum pH of 2.8 for Fenton’s reaction [64]. In fact, it is our intention to extend the employment of doped MCs also to electro-Fenton treatment, where acid pH are required for avoiding iron catalyst precipitation, and the process reaches its faster degradation rate [12][65]. The electrolysis was conducted at a fixed potential (E = 0.5 V vs. SCE) in order to generate H2O2 available for the dye degradation. When N-MC was used as cathode, the dye solution was completely decolourized after the passage of 150 C, with a mean current density observed of 0.783 mA cm2 (Figures 3a and b). Since the solution 10

ACCEPTED MANUSCRIPT colour is pH dependent, the solution pH was monitored during the treatment confirming a slight pH change after the treatment, passing from 2.4 to 3.1 (Tab. 2). The bleaching of the

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solution was confirmed by the decrease of the absorption band centred at 507 nm associated

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with the –N=N– bond (Figure 3c), whereas the mineralization of the dye was evaluated by

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TOC analysis, which points out the incineration of the 67% of the organic fraction (Tab. 2). The degradation can be associated with the homogeneous oxidation of the dye by the H2O2 generated in situ. At the same time new absorption bands grow in the UV region,

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indicating the formation of lower molecular weight species, resulting from the degradation

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of MO. The cleavage of the azo bond is responsible of the chromophore characteristic loss and generates intermediates which, according to literature, absorb in the spectral range 260– 300 nm and can be referred as multisubstituted aromatic species as phenol, catechol or

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hydroquinone [66]. To verify whether the N-MC is effectively responsible for the production of the bleaching agent H2O2 and that the MO degradation is not the consequence

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of its direct reduction at the electrode surface or its direct and indirect oxidation at graphite counter electrode, two more experiments were implemented: one employing GC as working

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electrode (at E = 0.5 V vs. SCE) in the presence of O2, and a second one using a N-MC modified GC, but in the absence of oxygen. The results reported in Figure 3d clearly point out that the dye degradation is less effective when H2O2 is generated by O2 reduction at bare GC or when MO is degraded at the N-MC electrode, without the homogeneous mediation of H2O2. Furthermore, it may be observed that the sample was not mineralized since TOC values remains unchanged during the electrolysis (Tab. 2). In fact, at GC electrode the recorded current density is far lower than at N-MC (0.197 mA cm2), due to the scarce catalytic activity of GC with respect to the doped MC, so that also the production of H 2O2 is hindered. Furthermore, when N-MC modified GC is used and O2 is not purposely inflated in solution the only electrochemical process that may occur, at E = 0.5 V vs SCE, is the direct

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ACCEPTED MANUSCRIPT reduction of the dye at the working electrode, as confirmed by the voltammetric behaviour of MO reported in figure 2c. These outcomes allow to exclude the contribution of the direct

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oxidation of the dye at the graphite counter electrode (CE), whereas its indirect oxidation by

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oxidant species generated at CE can be considered not substantial. In fact, it is reasonable

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that if the CE has an active role in the dye oxidation or in the generation of HO2 radicals, this would occur notwithstanding the nature of the working electrode or the presence of O 2

oxidant power is lower than H2O2 itself.

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in solution. However, the generation of HO2 cannot be excluded at present, even though its

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The degradation tests were conducted also at S-MC and N,S-MC coated GC electrode; (Figure 3e). An undoped MC (u-MC), with similar surface area and morphology of N-MC,

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was also considered for sake of comparison. Figures 3e and f show the MO concentration

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and the current density variation during the electrolyses. It is clear from Figure 3e that u-MC coated GC electrode performs better than the naked GC, but worsen than N-MC. In fact, the

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final MO degradation is higher than 90%, but the TOC abatement is only slightly higher than 32% (Table 2). u-MC showed to improve the MO and TOC abatement with respect to

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the naked GC because of an higher surface area and a more intimate contact between O2, electrolyte and electrode surface. On the other hand, the superior activity of N-MC is easily appointable to a better catalytic activity towards ORR that nitrogen doped material showed with respect to u-MC [55]. In the case of S-MC the degradation efficiency is slightly lower than at N-MC (Figure 3e) and this may be referred to the lower catalytic activity at S-MC with respect to N-MC and to the sideway H2 evolution, which accounts for the higher mean current density recorded (0.867 mA/cm2). When N,S-MC is used, the degradation is initially faster than at both N-MC and S-MC (Figure 3e), but after two hours the degradation kinetics slows down as confirmed by the rapid slope change in the experimental point trend. This type of behaviour may be due to a loss of activity during the experiments possibly triggered by the same H2O2 generated at the electrode. However, deeper investigations on optimized 12

ACCEPTED MANUSCRIPT gas diffusion electrodes are needed for better addressing the material stability. The lower performances of S-MC and N,S-MC with respect to N-MC were confirmed also by TOC

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analysis which established a mineralization of 53% and 2% of the organic dye, respectively.

3 Conclusions

Nitrogen and sulfur doped MCs were successfully employed as electrodic materials for the in situ

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generation of H2O2 and the degradation of methyl orange dissolved in solution in an

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undevided electrochemical cell. The degradation of MO is faster and more efficient when NMC is employed as electrode material, affording the complete bleaching of the solution and the mineralization of 67% of the initial TOC. The results point out that the principal

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responsible for dye degradation is the H2O2 generated at doped MC modified cathode, even though the effect of hydroperoxyl radicals generated by H2O2 oxidation at graphite anode

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cannot be excluded. The reported results can be used as proof of concept for a future

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optimization of doped MCs in gas diffusion electrodes and electro-Fenton degradation.

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Figure 3. MO solution before (a) and after (b) the electrochemical treatment. c) Evolution of the absorbance spectrum for MO during the electrolysis. (d) Dye degradation in 0.01 M H2SO4 + 0.1 M Na2SO4 at N-MC (circles and squares) and GC (triangles) electrodes in the presence (circles and triangles) and in the absence (squares) of O2. e) Dye degradation during the electrolysis at N-MC,

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intensity with the passed charge.

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Acknowledgements

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Funding from University of Padova (PRAT CPDA139814/13) is acknowledged.

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ACCEPTED MANUSCRIPT Tables Table 1- Physico-chemical and electrochemical data obtained from linear sweep voltammetry at

Eonsetb

(V)

(V)

0.489 0.087

GC N-MC S-MC N,S-MC

0.046 0.054

u-MC

0.021

d

nc (%)

--

2.93

0.576

2.31 2.29

0.443

85.5 81.5 74.5

2.49

(%)

Se

SBETf

(%)

(m2/g)

--

--

--

7.3

--

881

--

13.8

1103

4.5

4.1

855

--

--

953

Potentials are referred to SCE. bΔEonset= EonsetMCs – EonsetGC. cNumber of transferred electrons/molecule, calculated at E = 0.5 V vs. SCE. dSelectivity according to eq. 5. eElement weight content determined by elemental analysis. fSpecific surface areas [55].

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a

53.5 84.5

2.37

0.543 0.468

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Eonseta

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catalyst

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RRDE in O2 saturated 0.01 M H2SO4 + 0.1 M Na2SO4 solutions.

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Table 2- Electrolysis data in O2 saturated 0.01 M H2SO4 + 0.1 M Na2SO4 + 0.25 mM MO solution at different electrodes at Eapp = 0.5 V vs. SCE.

N-MC S-MC N,S-MC u-MC N-MCe GC

pHin

pHfin

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Electrode

2.4 2.4 2.4 2.4 2.4 2.4

3.1 3.1 3.0 2.9 2.5 2.6

ja

jb

Qtot

[MO]c

TOC

TOCd

(mA/cm2)

(mA/cm2)

(C)

(%)

mg/L

(%)

1.350 1.433 0.917 0.783 0.025 0.205

0.783 0.948 0.607 0.627 0.023 0.161

150 150 150 150 5.5 28.9

0.5 0.5 6.5 7.9 87.8 60.3

13.86 19.74 41.16 28.56 42.6 41.9

33 47 98 68 100 100 a b Starting current density with respect to the geometric surface area. Calculated mean current density. cMO concentration at the end of electrolysis (percentage of the initial one). dTOC at the end of electrolysis (percentage of the initial one). eIn the absence of O2.

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ACCEPTED MANUSCRIPT References J.M. Campos-Martin, G. Blanco-Brieva, J.L.G. Fierro, Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process, Angew. Chemie Int. Ed. 45 (2006) 6962–84.

[2]

J.K. Edwards, B. Solsona, E. Ntainjua N, A.F. Carley, A.A. Herzing, C.J. Kiely, G.J. Hutchings, Switching off hydrogen peroxide hydrogenation in the direct synthesis process, Science 323 (2009) 1037–1041.

[3]

J. Choi, S.H. Hwang, J. Jang, J. Yoon, High yield hydrogen peroxide production in a solid polymer electrolyte electrolyzer with a carbon fiber coated mesh substrate, Electrochem. Commun. 30 (2013) 95–98.

[4]

S. Siahrostami, A. Verdaguer-Casadevall, M. Karamad, D. Deiana, P. Malacrida, B. Wickman, M. Escudero-Escribano, E.A. Paoli, R. Frydendal, T.W. Hansen, Ib Chorkendorff, I.E.L. Stephens, J. Rossmeisl, Enabling direct H2O2 production through rational electrocatalyst design, Nat. Mater. 12 (2013) 1137–1143.

[5]

J.C. Byers, A.G. Gu, P.R. Unwin, A.G. Güell, P.R. Unwin, Nanoscale electrocatalysis: Visualizing oxygen reduction at pristine, kinked, and oxidized sites on individual carbon nanotubes, J. Am. Chem. Soc. 136 (2014) 11252–11255.

[6]

P. Gouérec, M. Savy, Oxygen reduction electrocatalysis: ageing of pyrolyzed cobalt macrocycles dispersed on an active carbon, Electrochim. Acta. 44 (1999) 2653–2661.

[7]

C.W.B. Bezerra, L. Zhang, K. Lee, H. Liu, A.L.B. Marques, E.P. Marques, E.P. Marquesb, H.Wanga, J. Zhang, A review of Fe-N/C and Co-N/C catalysts for the oxygen reduction reaction, Electrochim. Acta 53 (2008) 4937–4951.

[8]

A. Da Pozzo, L. Di Palma, C. Merli, E. Petrucci, An experimental comparison of a graphite electrode and a gas diffusion electrode for the cathodic production of hydrogen peroxide, J. Appl. Electrochem. 35 (2005) 413–419.

[9]

Z. Qiang, J.-H. Chang, C.-P. Huang, Electrochemical generation of hydrogen peroxide from dissolved oxygen in acidic solutions, Water Res. 36 (2002) 85–94.

[10]

E. Petrucci, A. Da Pozzo, L. Di Palma, On the ability to electrogenerate hydrogen peroxide and to regenerate ferrous ions of three selected carbon-based cathodes for electro-Fenton processes, Chem. Eng. J. 283 (2016) 750–758.

[11]

M. Pimentel, N. Oturan, M. Dezotti, M.A. Oturan, Phenol degradation by advanced electrochemical oxidation process electro-Fenton using a carbon felt cathode, Appl. Catal. B Environ. 83 (2008) 140–149.

[12]

M. Panizza, G. Cerisola, Electro-Fenton degradation of synthetic dyes, Water Res. 43 (2009) 339–344.

[13]

J. Miao, H. Zhu, Y. Tang, Y. Chen, P. Wan, Graphite felt electrochemically modified in H2SO4 solution used as a cathode to produce H2O2 for pre-oxidation of drinking water, Chem. Eng. J. 250 (2014) 312–318.

[14]

J.A. Bañuelos, O. García-Rodríguez, A. El-Ghenymy, F.J. Rodríguez-Valadez, L.A. Godínez, E. Brillas, Advanced oxidation treatment of malachite green dye using a low cost carbon-felt air-diffusion cathode, J. Environ. Chem. Eng. 4 (2016) 2066–2075.

[15]

A. Alvarez-Gallegos, D. Pletcher, The removal of low level organics via hydrogen peroxide formed in a reticulated vitreous carbon cathode cell, Part 1. The electrosynthesis of hydrogen peroxide in aqueous acidic solutions, Electrochim. Acta 44 (1998) 853–861.

[16]

E. Peralta, R. Vatividad, G. Roa, R. Marin, R. Romero, T. Pavon, A comparative study on the electrochemical production of H2O2 between BDD and graphite cathodes, Sustain.

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CE P

TE

D

MA

NU

SC R

IP

T

[1]

17

ACCEPTED MANUSCRIPT Environ. Res. 23 (2013) 259–266. A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Mineralization of an azo dye Acid Red 14 by electroFenton’s reagent using an activated carbon fiber cathode, Dye. Pigment 65 (2005) 227–233.

[18]

I. Kruusenberg, N. Alexeyeva, K. Tammeveski, The pH-dependence of oxygen reduction on multi-walled carbon nanotube modified glassy carbon electrodes, Carbon 47 (2009) 651–658.

[19]

Q. Tang, D. Wang, D.M. Yao, C.W. Yang, Y.C. Sun, Highly efficient electro-generation of hydrogen peroxide using NCNT/NF/CNT air diffusion electrode for electro- Fenton degradation of p -nitrophenol, Water Sci. Technol. 73 (2016) 1652–1658.

[20]

G. Gao, Q. Zhang, Z. Hao, C.D. Vecitis, Carbon nanotube membrane stack for flow-through sequential regenerative electro-Fenton, Environ. Sci. Technol. 49 (2015) 2375–2383.

[21]

E.P. Randviir, C.E. Banks, The oxygen reduction reaction at graphene modified electrodes, Electroanalysis 26 (2014) 76–83.

[22]

C.-Y. Chen, C. Tang, H.-F. Wang, C.-M. Chen, X. Zhang, X. Huang, Q. Zhang, Oxygen Reduction Reaction on Graphene in an Electro-Fenton System: In Situ Generation of H2O2 for the Oxidation of Organic Compounds, ChemSusChem 9 (2016) 1194–1199.

[23]

T.X.H. Le, M. Bechelany, S. Lacour, N. Oturan, M.A. Oturan, M. Cretin, High removal efficiency of dye pollutants by electron-Fenton process using a graphene based cathode, Carbon 94 (2015) 1003–1011.

[24]

F. Zhao, L. Liu, F. Yang, N. Ren, E-Fenton degradation of MB during filtration with Gr/PPy modified membrane cathode, Chem. Eng. J. 230 (2013) 491–498.

[25]

E. rillas, I. ir s, M.A. Oturan, Electro-Fenton Process and Related Electrochemical Technologies ased on Fenton’s Reaction Chemistry, Chem. Rev. 109 (2009) 6570–6631.

[26]

G.R. Agladze, G.S. Tsurtsumia, B.-I. Jung, J.-S. Kim, G. Gorelishvili, Comparative study of hydrogen peroxide electro-generation on gas-diffusion electrodes in undivided and membrane cells, J. Appl. Electrochem. 37 (2007) 375–383.

[27]

A. Da Pozzo, E. Petrucci, C. Merli, Electrogeneration of hydrogen peroxide in seawater and application to disinfection, J. Appl. Electrochem. 38 (2008) 997–1003.

[28]

L. Perini, C. Durante, M. Favaro, S. Agnoli, G. Granozzi, A. Gennaro, Electrocatalysis at palladium nanoparticles: Effect of the support nitrogen doping on the catalytic activation of carbonhalogen bond, Appl. Catal. B Environ. 144 (2014) 300–307.

[29]

W. Ju, M. Favaro, C. Durante, L. Perini, S. Agnoli, O. Schneider, U. Stimming, G. Granozzi, Pd Nanoparticles deposited on nitrogen-doped HOPG: New Insights into the Pdcatalyzed Oxygen Reduction Reaction, Electrochim. Acta. 141 (2014) 89–101.

[30]

L. Perini, C. Durante, M. Favaro, V. Perazzolo, S. Agnoli, O. Schneider, G. Granozzi, A. Gennaro, Metal − upport Interaction in Platinum and Palladium Nanoparticles Loaded on Nitrogen-Doped Mesoporous Carbon for Oxygen Reduction Reaction, ACS Appl. Mater. Interfaces 7 (2015) 1170–1179.

[31]

M. Favaro, L. Perini, S. Agnoli, C. Durante, G. Granozzi, A. Gennaro, Electrochemical behavior of N and Ar implanted highly oriented pyrolytic graphite substrates and activity toward oxygen reduction reaction, Electrochim. Acta 88 (2013) 477–487.

[32]

D.S. Su, S. Perathoner, G. Centi, Nanocarbons for the development of advanced catalysts, Chem. Rev. 113 (2013) 5782–5816.

[33]

M. Favaro, L. Ferrighi, G. Fazio, L. Colazzo, C. Di Valentin, C. Durante, F. Sedona, A. Gennaro, S. Agnoli, G. Granozzi, Single and Multiple Doping in Graphene Quantum Dots: Unraveling the Origin of Selectivity in the Oxygen Reduction Reaction, ACS Catal. 5 (2015)

AC

CE P

TE

D

MA

NU

SC R

IP

T

[17]

18

ACCEPTED MANUSCRIPT 129–144. F. Haschè, M. Oezaslan, P. Strasser, T.P. Fellinger, Electrocatalytic hydrogen peroxide formation on mesoporous non-metal nitrogen-doped carbon catalyst, J. Energy Chem. 25 (2016) 251–257.

[35]

M.-M. Titirici, R.J. White, N. Brun, V.L. Budarin, D.S. Su, F. del Monte, J.H. Clark, M.J. MacLachlan, Sustainable carbon materials, Chem. Soc. Rev. 44 (2015) 250–290.

[36]

X. Liu, Y. Zhou, W. Zhou, L. Li, S. Huang, S. Chen, Biomass-derived nitrogen self-doped porous carbon as effective metal-free catalysts for oxygen reduction reaction, Nanoscale. 7 (2015) 6136–6142.

[37]

J. Gong, J. Liu, Z. Jiang, X. Chen, X. Wen, E. Mijowska, T. Tang, Converting mixed plastics into mesoporous hollow carbon spheres with controllable diameter, Appl. Catal. B Environ. 152–153 (2014) 289–299.

[38]

N. Alexeyeva, E. Shulga, V. Kisand, I. Kink, K. Tammeveski, Electroreduction of oxygen on nitrogen-doped carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions, J. Electroanal. Chem. 648 (2010) 169–175.

[39]

Z. Liu, H. Nie, Z. Yang, J. Zhang, Z. Jin, Y. Lu, Z. Xiao, S. Huang, Sulfur-nitrogen co-doped three-dimensional carbon foams with hierarchical pore structures as efficient metal-free electrocatalysts for oxygen reduction reactions., Nanoscale 5 (2013) 3283–3288.

[40]

Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X, Chen, S. Huang, Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction, ACS Nano 6 (2012) 205–211.

[41]

C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Sneyder, D. Li, J.A. Herron, M. Mavrikakis, M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P.Yang, V.R. Stamenkovic, Highly crystalline multimetallic nanoframes with three-dimensional electrocatalytic surfaces, Science 343 (2014) 1339–43.

[42]

D. Yang, M.Y. Song, K.P. Singh, J. Yu, The role of iron in the preparation and oxygen reduction reaction activity of nitrogen-doped carbon, Chem. Commun. 51 (2015) 2450–2453.

[43]

K.N. Wood, R. O’Hayre, . Pylypenko, Recent progress on nitrogen/carbon structures designed for use in energy and sustainability applications, Energy Environ. Sci. 7 (2014) 1212–1249.

[44]

W.Y. Wong, W.R.W. Daud, A.B. Mohamad, A.A.H. Kadhum, K.S. Loh, E.H. Majlan, Recent progress in nitrogen-doped carbon and its composites as electrocatalysts for fuel cell applications, Int. J. Hydrogen Energy 38 (2013) 9370–9386.

[45]

Z. Yang, H. Nie, X.X. Chen, X.X. Chen, S. Huang, Recent progress in doped carbon nanomaterials as effective cathode catalysts for fuel cell oxygen reduction reaction, J. Power Sources 236 (2013) 238–249.

[46]

J. Yan, H. Meng, W. Yu, X. Yuan, W. Lin, W. Ouyang, D. Yuan, Preparation of nitrogendoped graphitic carboncages as electrocatalyst for oxygen reduction reaction, Electrochim. Acta 129 (2014) 196–202.

[47]

Z. Liu, C. Zhang, L. Luo, Z. Chang, X. Sun, One-pot synthesis and catalyst support application of mesoporous N-doped carbonaceous materials, J. Mater. Chem. 22 (2012) 12149-12154.

[48]

X. Liu, H. Zhu, X. Yang, One-step synthesis of dopamine-derived micro/mesoporous nitrogen-doped carbon materials for highly efficient oxygen-reduction catalysts, J. Power Sources 262 (2014) 414–420.

AC

CE P

TE

D

MA

NU

SC R

IP

T

[34]

19

ACCEPTED MANUSCRIPT Y.J. Wang, D.P. Wilkinson, J. Zhang, Noncarbon Support Materials for Polymer Electrolyte Membrane Fuel Cell Electrocatalysts, Chem. Rev. 111 (2011) 7625–7651.

[50]

S. Liu, C. Deng, L. Yao, H. Zhong, H. Zhang, The key role of metal dopants in nitrogendoped carbon xerogel for oxygen reduction reaction, J. Power Sources. 269 (2014) 225–235.

[51]

H. Liang, W. Wei, Z. Wu, X. Feng, Mesoporous Metal − Nitrogen-Doped Carbon Electrocatalysts for Highly Efficient Oxygen Reduction Reaction, J. Am. Chem. Soc. 135 (2013) 16002−16005.

[52]

G. Wu, P. Zelenay, Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction, Acc. Chem. Res. 46 (2013) 1878–1889.

[53]

T.P. Fellinger, F. Hasché, P. Strasser, M. Antonietti, Mesoporous nitrogen-doped carbon for the electrocatalytic synthesis of hydrogen peroxide, J. Am. Chem. Soc. 134 (2012) 4072– 4075.

[54]

H.W. Lee, H.J. Cho, J.-H. Yim, J.M. Kim, J.-K. Jeon, J.M. Sohn, K.-S. Yoo, S.-S. Kim, Y.K. Park, Removal of Cu(II)-ion over amine-functionalized mesoporous silica materials, J. Ind. Eng. Chem. 17 (2011) 504–509.

[55]

V. Perazzolo, C. Durante, R. Pilot, A. Paduano, J. Zheng, G.A. Rizzi, A. Martucci, G. Granozzi, A. Gennaro, Nitrogen and sulfur doped mesoporous carbon as metal-free electrocatalysts for the in situ production of hydrogen peroxide, Carbon 95 (2015) 949–963.

[56]

A. Celzard, J.F. Marêché, F. Payot, G. Furdin, Electrical conductivity of carbonaceous powders, Carbon . 40 (2002) 2801–2815.

[57]

V. Perazzolo, E. Grądzka, C. Durante, R. Pilot, N. Vicentini, G.A. Rizzi, G. Granozzi, A. Gennaro, Chemical and Electrochemical Stability of Nitrogen and Sulphur Doped Mesoporous Carbons, Electrochim. Acta 197 (2016) 251–262.

[58]

T. Kim, I. Park, R. Ryoo, A Synthetic Route to Ordered Mesoporous Carbon Materials with Graphitic Pore Walls, Angew. Chemie 115 (2003) 4511–4515.

[59]

J. Park, Y. Nabae, T. Hayakawa, M. Kakimoto, Highly Selective Two-Electron Oxygen Reduction Catalyzed by Mesoporous Nitrogen-Doped Carbon, ACS Catal. 4 (2014) 3749– 3754.

[60]

C. Durante, M. Cuscov, A.A. Isse, G. Sandonà, A. Gennaro, Advanced oxidation processes coupled with electrocoagulation for the exhaustive abatement of Cr-EDTA, Water Res. 45 (2011) 2122–2130.

[61]

M. Panizza, G. Cerisola, Application of diamond electrodes to electrochemical processes, Electrochim. Acta 51 (2005) 191–199.

[62]

M. Giomo, A. Buso, P. Fier, G. Sandonà, B. Boye, G. Farnia, A small-scale pilot plant using an oxygen-reducing gas-diffusion electrode for hydrogen peroxide electrosynthesis, Electrochim. Acta 54 (2008) 808–815.

[63]

E. Brillas, J.C. Calpe, J. Casado, Mineralization of 2,4-D by Advanced Electrochemical Oxidation Processes, Water Res. 34 (2000) 2253–2262.

AC

CE P

TE

D

MA

NU

SC R

IP

T

[49]

[64] Y. Sun, J.J. Pignatello, Photochemical Reactions Involved in the Total Mineralization of 2,4D by Fe3+/H202/UV, Environ. Sci. Technol. 27 (1993) 304–310. [65]

E. Brillas, B. Boye, M.M. Dieng, Peroxi-Coagulation and Photoperoxi-Coagulation Treatments of the Herbicide 4-Chlorophenoxyacetic Acid in Aqueous Medium Using an Oxygen-Diffusion Cathode, J. Electrochem. Soc. 150 (2003) E148–E154.

[66]

E. Guivarch, S. Trevin, C. Lahitte, M.A. Oturan, Degradation of azo dyes in water by Electro-Fenton process, Environ. Chem. Lett. 1 (2003) 38–44. 20

ACCEPTED MANUSCRIPT

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights 1. N and S doped mesoporous carbon cathodes were employed in H2O2 generation.

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2. The electrogenerated H2O2 is the main responsible for methyl orange degradation.

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3. Nitrogen doped mesoporous carbon is more active than sulfur doped one.

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4. High selectivity up to 85% for H2O2 was observed at N and S doped carbon.

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5. The mineralization of 67% of the initial TOC content was obtained.

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