Role of anode material on the electrochemical oxidation of methyl

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Jun 11, 2015 - ... Heidelberg 2015. Abstract The anodic oxidation of methyl orange (MO, 5-(4- ... chanical and chemical resistance even at high current density.
J Solid State Electrochem DOI 10.1007/s10008-015-2928-2

ORIGINAL PAPER

Role of anode material on the electrochemical oxidation of methyl orange Lazhar Labiadh 1 & Antonio Barbucci 2 & Giacomo Cerisola 2 & Abdellatif Gadri 1 & Salah Ammar 1,3 & Marco Panizza 2

Received: 21 May 2015 / Revised: 11 June 2015 / Accepted: 15 June 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract The anodic oxidation of methyl orange (MO, 5-(4nitrophenylazo)salicylic acid) has been studied by cyclic voltammetry and bulk electrolysis, using a range of electrode materials such as Ti–Ru–Sn ternary oxide, lead dioxide and boron-doped diamond (BDD), glassy carbon (GC) and gold anodes. The results of voltammetries show that with all the electrode materials, in the potential region before oxygen evolution, the oxidation of MO involves simple electrode transfer that produces a polymeric film that deactivates the electrode surface, as confirmed by Fourier Transform Infrared Reflection-Absorption Spectroscopy (FTIRRAS) analysis. A very different behaviour was observed among the electrodes in the region of water decomposition. While BDD and PbO2 regained their initial activity by simple polarisation at 2.3 V vs. saturated calomel electrode (SCE) due to the production of high amount of hydroxyl radicals that destroy the polymeric film, TiRuSnO 2 , GC and gold cannot be completely reactivated, because they have a low overpotential for oxygen evolution, and this secondary reaction is favoured over polymer mineralization. The results of bulk electrolysis showed that after 3 h of polarisation at 10 mA cm−2, complete colour and chemical oxygen demand (COD) removal were obtained only with BDD anode. Using PbO2 MO was oxidised but a

* Marco Panizza [email protected] 1

Département de Chimie, Faculté des Sciences de Gabès, Université de Gabès, Cité Erriadh, 6072 Gabès, Tunisia

2

Department of Civil, Chemical and Environmental Engineering, University of Genoa, P. le J.F. Kennedy 1, 16129 Genoa, Italy

3

Département de Chimie, Faculté des Sciences de Bizerte, Université de Carthage, 7021 Zarzouna, Tunisia

residual COD remains in the solution, while TiRuSnO2 permitted only a partial oxidation of MO. Keywords Electrocatalysis . Methyl orange . Voltammetry . Anode materials . Electrode fouling . Bulk electrolysis

Introduction Synthetic dyes are extensively used for colouring textiles, leather, paper, food, drinks, pharmaceuticals, cosmetics and inks. Azo dyes are widely used in textile and food industries in Mt quantity and represent about 70 % of the world dye production. These coloured compounds contain one or more azo bonds (–N=N–) as chromophore group linked to aromatic structures with functional groups such as –OH and –SO3H, among others [1, 2]. Effluents from textile and paper industries contain large quantities of organic compounds, inorganic salts, and reactive dyes. The release of coloured wastewater in the environment is a considerable source of non-aesthetic pollution and eutrophication. Therefore, proper treatment of these wastewaters has drawn increasing attention. Commonly employed methods for colour removal are adsorption [3], coagulation [4], chemical oxidation with ozone [5] or Fenton’s reagent [6] and advanced oxidation processes [7, 8]. However, these processes are quite expensive and involve several operational problems. For these reasons, there has been increasing interest in the use of new methods such as electrochemical oxidation [9–13]. Electrochemical oxidation treatment is a potential alternative method for eliminating pollutants with good efficiency. However, for the elimination of organic pollutants, it requires an anode with high oxygen overpotential, high electrical conductivity and suitable mechanical and electrochemical stability.

J Solid State Electrochem

Due to their features of high surface area, excellent mechanical and chemical resistance even at high current density and in strongly acid media, dimensionally stable anodes (DSA®-type anode) have been also explored in the field of wastewater treatment [14–16]. For example, Johnson et al. [17] studied the incineration of benzoquinone using quaternary metal oxide anodes (Ti, Ru, Sn, Sb). They demonstrated that this type of electrode is stable and electrochemically active for the oxidation of organic compounds when it is used in the absence of a soluble supporting electrolyte, with a Nafion® membrane as solid-state electrolyte. Lead dioxide, with its high oxygen overpotential, is among the most commonly used anodes for the destruction of organics [18–20] because the rate of organic oxidation has proved to be higher than on other traditional anodes. Bonfatti et al. [21], comparing the oxidation of glucose on different electrode materials such as Ti/PbO2, Ti/Pt and Ti/Pt–SnO2, showed that the incineration of glucose and its oxidation intermediates (i.e. gluconic and glucaric acid) took place at a reasonable rate only at Ti/PbO2 electrodes. The electrochemical oxidation of phenol was thoroughly investigated under different experimental conditions by Belhadj and Savall [22]. Phenol and its intermediates (benzoquinone, maleic and fumaric acids) were completely eliminated at a pure Ta/ PbO2 anode through the intermediation of hydroxyl radicals adsorbed on the active site of the electrode. Recently, the electrochemistry of synthetic boron-doped diamond (BDD) films has received a great deal of attention. The electrochemical behaviour of such diamond thin films has also been studied with the goal of developing applications for the electrochemical oxidation of organics for wastewater treatment [23, 24]. It has been shown that using BDD electrodes, different organic compounds such as phenol [25], 4chlorophenols [26], drugs [27] and pesticides [28] can be mineralized with high current efficiency, even near 100 %. In this work, the catalytic activities of five anode materials (i.e. gold, carbon, boron-doped diamond, lead dioxide and Ti– Ru–Sn ternary oxide) on the direct electrochemical oxidation of synthetic dyes are investigated by potentiodynamic measurements and bulk electrolysis for boron-doped diamond, lead dioxide and Ti–Ru–Sn ternary oxide anodes. Methyl orange (MO) was selected as a model dye because it belongs to the azo dyes, the largest group among the synthetic colourants (60–70 %), and it is extensively used in textile dyeing and paper printing.

Experimental Chemicals The dyestuff solution was prepared dissolving different amount of MO (5-(4-nitrophenylazo)salicylic acid,

C13H9N3O5) used without further purification in bi-distilled water, using 0.05 M Na2SO4 (Carlo Erba Reagents) as supporting electrolyte because this salt is present in many textile wastewaters and gives good electrical conductivity. Electrode materials The Ti–Ru–Sn ternary oxide anodes, hereafter indicated as TiRuSnO2, have been prepared by coating titanium sheet by thermal decomposition of TiCl4, RuCl3·3H2O and SnCl4· 5H2O isopropanol solution [29]. The titanium plate with a thickness of 1.5 mm was sandblasted chemical etched in hydrochloric concentrated acid. The solution of the precursor was painted on the titanium and the solvent was evaporated in air at 80 °C. Then the sheets were fired at 450 °C for 1 h. Thereafter, this procedure was repeated until the coating thickness was 50 μm. The nominal composition of the ternary oxide was Ti/Ti0.5 Ru0.45 Sn0.05O2. The lead dioxide electrode (PbO2) was prepared by electrochemical approach of an aqueous solution of 0.1 M HNO3 containing 0.5 M Pb(NO3)2 and 0.05 M NaF on a titanium substrates (1.5-mm thick). The titanium plates were sandblasted and chemical etched in hydrochloric concentrated acid. The PbO2 coating was deposited at constant current density of 20 mA cm−2 for 30 min. In order to stabilise the electrode, a postdeposition treatment was applied to PbO2 films. The potential of the electrode was cycled in 0.5 M H2SO4 between the range 0 and 1.8 V until successive i-E curves were identical [29]. The PbO2 layer obtained after the post-treatment was continuous and crystalline with a thickness of about 200 μm. Boron-doped diamond (BDD) was purchased by Adamant technologies (Neuchatel). BDD films were synthesised by the hot filament chemical vapour deposition technique (HF CVD) on single crystal p-type Si wafers. The obtained diamond film thickness was about 1 μm with a resistivity of 10–30 mΩ cm. In order to stabilise the electrode surface and to obtain reproducible results, the diamond electrode was pre-treated by anodic polarisation in 1 M HClO4 at 10 mA cm−2 during 30 min. This treatment made the surface hydrophilic. Electrochemical measurements Cyclic voltammetries were carried out at 25 °C in a conventional three-electrode cell with a volume of 200 mL, using a computercontrolled Methom Autolab. TiRuSnO2, PbO2, BDD, Au and GC have been used as working electrode, a saturated calomel electrode (SCE) as a reference and Pt wire as a counter electrode (diameter 1 mm, length 10 cm). All the voltammetries were performed with a scan rate of 100 mV s−1. The exposed apparent area of the TiRuSnO2, PbO2 and BDD working electrodes was 1 cm2. The polycrystalline gold electrode and glassy carbon electrode (GC) were disks of about 0.07 cm2 exposed area and they were purchased by metrohm. These electrodes were first

J Solid State Electrochem

Analysis Fourier Transform Infrared Reflection-Absorption Spectroscopy (FTIRRAS) spectra were recorded by a Nicolet Magna 750 Fourier transform spectrometer (4 cm−1 resolution), using a Specac 19700 specular reflectance accessory (angle of incidence 26.5° from normal). The colour removal was monitored by measuring the decrease in the absorbance using a JascoV-570 spectrophotometer in 1 cm path-length cells and the chemical oxygen demand (COD) of the solution was monitored using a Dr. Lange LASA50 system. The current efficiency (CE) for the anodic oxidation of methyl orange was calculated from the values of the COD using the relationship [12] CEð%Þ ¼

ðCOD0 −CODt Þ F  V  100 8I t

ð1Þ

where COD0 and CODt are the chemical oxygen demands at times t = 0 (initial) and t (in gO2 dm−3), respectively, I the current (A), F the Faraday constant (96,500 C mol−1), V the volume of the electrolyte (dm3) and 8 is the oxygen equivalent mass (gequiv.−1).

0.8

0 ppm

0.7

i (mA cm-2)

polished with 0.05 μm alumina slurry and then washed ultrasonically in triply distilled water and ethanol for a few minutes, respectively before transfer to the cell. The bulk electrolyses were performed in a onecompartment electrolytic flow cell with parallel plate electrodes applying a constant current of 500 mA, using an AMEL 2055 potentiostat/galvanostat. BDD, PbO2 and TiRuSnO2 were used as anodes and stainless steel AISI 304 as the cathode. Both electrodes were circular with a geometric area of 50 cm2 each and an interelectrode gap of 1 cm. The electrolyte was stored in a 0.35 L thermo-regulated glass reservoir (20 °C) and circulated through the electrolytic cell by a centrifugal pump with a recirculation flow rate of 300 dm3 h−1.

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0.4 0.3 0.2 0.1 0.0 -0.1

0

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1

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E (V vs SCE)

Fig. 1 Cyclic voltamogramms recorded with BDD anode with different MO concentrations in 0.05 M Na2SO4 solutions at a scan rate of 100 mV s −1 . Conditions: T = 25 °C, electrode area = 1 cm 2 , V = 200 mL. The dash line represents the background curve

that the current density at a given potential in the region of water decomposition increases with organic concentration, and this indicates that in this potential region, organic compounds are oxidised by water decomposition intermediates (hydroxyl radicals), which are only available in conditions of oxygen evolution (Eq. 2): 2H2 O→2 OH þ 2Hþ þ 2e−

ð2Þ

However, during continuous potential cycling (Fig. 2), the anodic current peak decreases as the number of cycles increases until a steady state is reached after about 3 cycles. This fact can be explained by the formation of an organic film that covered the electrode surface. Similar deactivation of BDD electrodes during the voltammetry with aromatic compounds such as phenol [25], naphthol [29] and 4-CP [26] has already been reported in the literature. The presence of an organic film on the BDD surface was confirmed by FTIR analysis (Fig. 3). The spectrum referring to that obtained with a new electrode 0.8 (1)

0.7

Potentiodynamic measurements Figure 1 shows the cyclic voltammograms for background electrolyte (i.e. Na2SO4 0.05 M) and for MO-containing electrolyte at BDD anode, recorded with a scan rate of 100 mV s−1. As can be seen on BDD anode the oxygen evolution commenced at about 1.8 V, confirming that BDD has high oxygen evolution overpotential. Addition of MO to the electrolyte resulted in an anodic peak current at 1.1 V vs. SCE and a broad peak at 1.5 V vs. SCE that is partially overlapped by oxygen evolution. This indicates that MO can be directly oxidised on BDD anode in the region before oxygen evolution. It is also interesting to observe

i (mA cm-2)

Results and discussion

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(2)

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(3)=(4)

0.4 0.3 0.2 0.1 0.0 -0.1 -0.2

0

0.5

1

1.5

2

E (V vs SCE)

Fig. 2 Consecutive cyclic voltammograms on BDD recorded in MO 100 ppm in 50 mM Na2SO4 at a scan rate of 100 mV s−1. Conditions: T = 25 °C, electrode area = 1 cm2, V = 200 mL. The dash line represents the voltammetry after reactivation for 1 min at 2.3 V vs. SCE

Absorbance / a.u.

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3000

2500

2000 Wavenumber /

1500

1000

cm-1

Fig. 3 IR spectrum of the BDD electrode recorded after five consecutive cyclic voltammograms of 100 ppm in 50 mM Na2SO4. The spectrum is referenced to that obtained with a new electrode

clearly confirms that an organic substrate covered the electrode surface. In fact, IR spectra show the presence of the adsorption bands at 2300/2400 cm−1 characteristic of C=O, 1400/1600 cm −1 characteristic of C–H and a band at 1200 cm−1 characteristic of CHO. It has been found (Fig. 2, dotted line) that the polymeric film on the BDD surface can be destroyed by simple polarisation at high anodic potential for 1 min at E = 2.3 V vs. SCE in the region of water decomposition. In fact, it was demonstrated [30] that high potential BDD anodes produce high amount of hydroxyl radicals (Eq. 2) that oxidise the polymeric film regaining the initial activity (Eq. 3): Organic film þ OH→CO2 þ H2 O

ð3Þ

Figure 4 shows cyclic voltammogram obtained with PbO2 in the potential region between 0 and 2.2 V vs. SCE. The

voltammogram recorded in the potential region below oxygen evolution is nearly featureless and display no significant peaks in presence of MO with respect to the voltammogram of the supporting electrolyte, but the current is larger in the presence of MO than that in blank solution. During the following scans, the anodic oxidation peak substantially diminished and only a small current flowed due to the electrode fouling by a polymeric film, as in the case of BDD electrode. Moreover, the current density at a given potential in the region of supporting electrolyte decomposition increases in the solution containing MO. It is reasonable to assume that MO oxidation involves water decomposition intermediates, mainly hydroxyl radicals (Eq. 2), which are only available in conditions of oxygen evolution as in the case of BDD anode. In fact, PbO2 surface can restore its initial activity and the polymeric film can be destroyed by simple polarisation at high anodic potential (E = 2.3 V vs. SCE) for 1 min in the region of water decomposition. The voltammogram obtained with a TiRuSnO2 in Na2SO4 0.05 M (Fig. 5) presented the typical behaviour of ternary thermally prepared oxide layer. In fact, in the region 0.5– 1.0 V, it is possible to observe a peak typical of the Ru(III)/ Ru(IV) transition, but it is broad and not well defined because in the ternary electrodes there is a large heterogeneity in the surface site and superposition of the redox processes for the transition lower metal oxide/higher metal oxide [31]. In the presence of Na2SO4, oxygen evolution commence at about 1.4 V vs. SCE meaning that this electrode has low oxygen evolution overpotential. On the other hand, during the first cycle in the solution containing 100 ppm of MO (Fig. 5), no significant differences were observed with respect to the voltammogram of the supporting electrolyte in the region before oxygen evolution. However, the current density at a given potential in the region of supporting electrolyte decomposition decreased, 4.0

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i (mA cm-2)

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1.5

2

2.5

-0.2

2.0 1.5 1.0 0.5 0.0 0.000 -0.5

-0.4

-0.6 E (V vs SCE)

Fig. 4 Consecutive cyclic voltammograms on PbO2 recorded in MO 100 ppm in 50 mM Na2SO4 at a scan rate of 100 mV s−1. Conditions: T = 25 °C, electrode area = 1 cm2, V = 200 mL. The dash line represents the background curve

-1.0

0.500

1.000

1.500

2.000

E (V vs SCE)

Fig. 5 Consecutive cyclic voltammograms on TiRuSnO2 recorded in MO 100 ppm in 50 mM Na 2SO 4 at a scan rate of 100 mV s −1. Conditions: T = 25 °C, electrode area = 1 cm2, V = 200 mL. The dash line represents the background curve

J Solid State Electrochem

þ

H2 O→§O2 þ 2H þ 2e



5.0 4.0 3.0 i (mA cm-2)

meaning the inhibition for the oxygen evolution reaction due to the partial deactivation of the active sites on the TiRuSnO2 surface. Similar deactivation of mixed oxide electrodes in presence of aromatic organic substrates such as phenol, naphthol [29] and safrole [32] has already been reported in the literature. We observed that an anodic polarisation at high potential in the region of water discharge cannot reactivate the electrode. In fact, on a TiRuSnO2 anode, which has a low overpotential for oxygen evolution, this secondary reaction is favoured over organic mineralization (Eq. 4):

2.0 1.0 0.0 0

ð4Þ -1.0

The voltammograms obtained with the GC is shown in Fig. 6. During the first scan in presence of MO, a broad anodic current peak corresponding to the direct oxidation of MO is observed at about +1.2 V vs. SCE. However, in case of continuous potential cycling (curves 2 and 3), it has been found that the anodic current peak decreases as the number of cycles increases until a steady state is reached after about 4 cycles. This decrease in electrode activity appears to be due to deposition of aromatic polymeric products on the electrode surfaces. Polarisation at high anodic potential cannot completely reactivate the GC electrode because it has a slightly low overpotential for oxygen evolution and consequently this secondary reaction is in competition with polymer oxidation. Figure 7 shows the results obtained with Au electrode. In Na2SO4 50 mM we obtained, the characteristic single sharp gold oxide reduction peak is located at about 0.92 V vs. SCE, and a broad oxidation peak at about 1.26 V vs. SCE. In the presence of 100 ppm of MO, during the first scan, there was an increase of the anodic peak and an enhancement of the current density at a given potential in the region of water decomposition. On the reverse scan, the reduction peak increased and it

0.5

1

1.5

2

E (V vs SCE)

Fig. 7 Consecutive cyclic voltammograms on Au recorded in MO 100 ppm in 50 mM Na2SO4 at a scan rate of 100 mV s−1. Conditions: T = 25 °C, electrode area = 0.07 cm2, V = 200 mL. The dotted line represents the first cycle; the dash line represents the background curve

was also shifted to more positive potentials. However, from the second cycle, there was a rapid decrease of both peaks due to the adsorption of organic compounds on the Au surface and consequently the electrode was highly passivated. To confirm this fact, the current density at a given potential in the region of supporting electrolyte decomposition decreased, meaning the inhibition for the oxygen evolution reaction due to the partial deactivation of the active sites. We also observed that an anodic polarisation at high potential in the region of water discharge cannot reactivate the electrode. However, during polarisation at high anodic potentials, both GC and Au electrodes are not stable and they are subjected to corrosion phenomena and thus these electrodes are not used for the bulk electrolysis. Bulk electrolysis

3.0 2.5

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2.0 1.5 1.0 0.5 0.0 0 -0.5

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E (V vs SCE)

Fig. 6 Consecutive cyclic voltammograms on GC recorded in MO 100 ppm in 50 mM Na2SO4 at a scan rate of 100 mV s−1. Conditions: T = 25 °C, electrode area = 0.07 cm2, V = 200 mL. The dash line represents the background curve

The degradation of MO using TiRuSnO2, PbO2 and BDD electrodes was also studied by bulk electrolysis of a solution containing 100 ppm of MO at 500 mA, and the progress of oxidation was monitored by UV/vis and COD measurements. The absorption spectra of MO at room temperature present an absorption band at about 373 nm which is directly related to the colour of the solution and a peak at about 273 nm related to aromatic ring. Figure 8 shows the evolution of the MO spectra during the electrolysis and the evolution of the absorbance at 373 nm. With BDD (Fig. 8a) and PbO2 (Fig. 8b), all the peaks decreased until disappearance, indicating that the electrochemical oxidation can remove the colour of MO and can open the ring of the aromatic compounds present in MO. In fact, using these electrodes, MO is oxidised by the reaction with the hydroxyl radicals electrogenerated at high potentials (Eq. 2). Moreover, the MO removal rate is almost the same with the two electrodes (Fig. 8d). On the contrary, using the

J Solid State Electrochem 2

2

1.5

1.5 a.u. / -

a.u. / -

Fig. 8 Time evolution of the UV/ visible spectra of the MO solutions during the electrolysis of 100 ppm of MO. Conditions: I = 500 mA, T = 20 °C, V = 0.35 L, flow rate = 300 dm3 h−1. Anodes: a BDD; b PbO2 and c TiRuSnO2. Panel d reports the decrease of the absorbance obtained with (triangle) BDD; (circle) PbO2 and (square) TiRuSnO2

1 0.5 0 200

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400 500 Wavelength / nm

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300 400 500 Wavelength / nm

(a)

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100 80 [MO] / mg/L

a.u. / -

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80

COD / mg/L

CE / %

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(d)

complete COD removal was only obtained with BDD anode, indicating that MO and all the organic intermediates are incinerated by the reaction with •OH. Using PbO2, a residual COD remains in the solution after 3 h and more electrolysis time is necessary for the complete mineralization of the MO. This behaviour is probably due to the fact that on PbO 2 , the electrogenerated •OH are strongly adsorbed and consequently less reactive than those produced on BDD surface. At TiRuSnO2 ternary oxide, as expected from the evolution of MO concentration, only a small COD depletion was obtained. According with the evolution of COD, the CE calculated with Eq. 1 (Fig. 9, inset) follows the following order BDD>PbO2>TiRuSnO2, confirming the higher oxidation ability of BDD anode.

Conclusions 0

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t / min

60 40 20

0 50

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100

0

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0 300 400 500 Wavelength / nm

TiRuSnO2 anode (Fig. 8c), the absorption peaks at 273 and 373.5 nm are only partially reduced meaning that this electrode is not able to remove completely the colour and the aromatic compounds. In fact, as shown by the potentiodynamic measurements, TiRuSnO2 had low oxygen evolution overpotential and consequently it favoured the secondary reaction of oxygen evolution in comparison with methyl orange oxidation (Eq. 4). Figure 9 shows the comparison of the evolution of COD and CE during the electrolysis of a solution containing 100 ppm of MO at 500 mA. As can be seen after 3 h of treatment, the

120

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(c)

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100 t / min

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Fig. 9 Comparison of the trend of COD during the oxidation of 100 ppm MO in 0.5 M Na2SO4. Conditions: I = 500 mA, T = 20 °C, V = 0.35 L, flow rate = 300 dm3 h−1. Anodes: (triangle) BDD; (circle) PbO2; (square) TiRuSnO2. The insets present the evolution of the CE

In this paper, the catalytic activities of five electrode materials (i.e. TiRuSnO2, Au, PbO2, GC and BDD) for the anodic oxidation of MO have been investigated. The voltammetric measurements have shown that with all the electrode materials, in the potential region before oxygen evolution, the oxidation of MO involves simple electrode transfer. However, polymeric adhesive films, which cause rapid electrode fouling, were also formed in this potential region. However, the initial activity of BDD and PbO2 was restored by an anodic polarisation in the potential region of water discharge because using these high oxygen-

J Solid State Electrochem

overvoltage electrodes, the water discharge involves the production of hydroxyl radicals that oxidise the polymeric film on the surface. On the contrary, the TiRuSnO2, Au and GC anodes were not completely reactivated by anodic polarisation because they have low overpotential for oxygen evolution and therefore this secondary reaction is favoured over organic mineralization. The bulk electrolysis confirmed the results of the voltammetries; in fact, the complete MO oxidation was only obtained using PbO2 and BDD due to the electrogeneration of hydroxyl radicals from water discharge. In particular, faster mineralization and decolourization were achieved using BDD. TiRuSnO2 only allows a partial oxidation of methyl orange, but not the complete mineralization, due to the accumulation of oxidation intermediates which are quite stable against further attack at these electrodes.

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