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Electrochimica Acta 54 (2009) 1491–1498

Contents lists available at ScienceDirect

Electrochimica Acta

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esign and electrochemical study of SnO2-based mixed oxide electrodes

rian Adams, Min Tian, Aicheng Chen ∗,1

epartment of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada

r t i c l e i n f o

rticle history:eceived 29 May 2008eceived in revised form2 September 2008ccepted 16 September 2008vailable online 26 September 2008

eywords:itrophenols

a b s t r a c t

For the electrochemical treatment of wastewater, it is critical to develop electrodes with a high activityfor the oxidation of pollutants, long lifetimes, and low cost. In the present study, we have fabricatedfour different SnO2-based electrodes (Ti/SnO2–Sb2O5, Ti/SnO2–Sb2O5–PtOx, Ti/SnO2–Sb2O5–RuO2 andTi/SnO2–Sb2O5–IrO2) using the thermal decomposition method and, for the first time, systemicallystudied their stability and electrocatalytic activity towards the degradation of 2-nitrophenol (2-NPh),3-nitrophenol (3-NPh) and 4-nitrophenol (4-NPh). Scanning electron microscope (SEM) and X-ray energydispersive spectrometry (EDS) were used to characterize the morphology and composition of the fourdifferent SnO2-based electrocatalysts. Lifetime tests show that doping IrO2 or RuO2 greatly improves

lectrochemical oxidationin oxideixed metal oxides

n situ UV/vis spectroscopy

the stability of the SnO2-based electrodes. The electrochemical activities of the prepared SnO2-basedelectrodes were characterized using the degradation of 2-NPh, 3-NPh and 4-NPh. In situ UV/vis spec-troscopy was used to monitor the concentration changes of the nitrophenols with time showing that therate constants for the electrochemical oxidation of the nitrophenols decrease in the order of: 2-NPh > 4-NPh > 3-NPh. The effect of the applied current densities and initial concentrations of nitrophenols have

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. Introduction

The establishment and enforcement of limits for the dischargend disposal of toxic and hazardous materials has required theevelopment of advanced technologies to effectively treat a varietyf gaseous and liquid effluents, solid waste and sludge. Electro-hemical oxidation is a promising approach to the destruction ofrganic pollutants and the remediation of wastewater due to itsase of operation and the fact that it uses a clean and cheap reagent,he electron [1–4]. It is well known that the electrode material is theey factor in the development of electrochemical oxidation tech-iques and the desired electrodes must fulfill three requirements:i) high efficiency in pollutant degradation; (ii) high stability undernodic polarization conditions; (iii) low production costs [5]. Thenodic evolution of oxygen in aqueous electrolytes has been stud-ed for decades in search of a stable anode material with low oxygenvolution over-potential. For industrial electrolysis processes, such

node materials generate drastic savings in energy costs. On thether hand, high oxygen evolution over-potential is desirable forhe electrochemical treatment of organic pollutants since anodicxygen evolution in aqueous electrolytes represents an unwanted

∗ Corresponding author. Tel.: +1 807 343 8318; fax: +1 807 346 7775.E-mail address: aicheng.chen@lakeheadu.ca (A. Chen).

1 ISE member.

dcllsoiiB

013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2008.09.034

y has shown that the fabricated Ti/SnO2–Sb2O5–IrO2 electrodes are verycal treatment of wastewater.

© 2008 Elsevier Ltd. All rights reserved.

eakage current, reducing the overall current efficiency and thevailable oxidation potential [6]. Typical electrodes of this typehich have been studied in the past include graphite [7], glassy-

arbon [8], platinum [9,10], boron-doped diamond (BDD) [11–14],nd various metal oxide electrodes including PbO2 [15–19], IrO220,21], RuO2 [22] and SnO2 [23–25]. Of these, the metal oxidelectrodes, especially Sb-doped SnO2 electrodes are thought to beuperior for the oxidation of organic compounds [6,24]. Pure SnO2s an n-type semiconductor with a band gap of about 3.5 eV andxhibits very high resistivity at room temperature; thus cannot besed as an electrode material directly [26]. By doping with Sb thelectrical conductivity is increased significantly. Grimm et al. [27]ompared the oxidation of phenol on Ti/SnO2–Sb2O5 and PbO2 andound that the former was more active. SnO2 has the additionaldvantage of being inexpensive compared to many other preciousetal oxides, Pt, and BDD. The main issue with the Ti/SnO2–Sb2O5

lectrode is its short lifetime. The known failure mechanisms ofimensionally stable anodes (DSA®) include selective loss of theatalyst to the electrolyte solution, the formation of a resistiveayer between the substrate and coating, and a non-conductingayer formed on the outer surface of the coating. Our previous

tudy has shown that the presence of a nanoscale gold thin filmn the Ti substrate can effectively prevent the growth of a TiO2nsulating layer between the substrate and the SnO2–Sb2O5 coat-ng, greatly prolonging the service life of the electrode [28]. Deattisti et al. [24] have also shown that an interlayer of IrO2 in

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492 B. Adams et al. / Electrochi

he Ti/IrO2/SnO2–Sb2O5 electrode strongly increases the serviceife of the anode. Another main cause for the deactivation of thenO2–Sb2O5 electrodes is the formation of a nonconductive tinydroxide in the outer layer of the oxide [6]. Recent studies indicatehat the incorporation of additive metal oxides into the SnO2–Sb2O5oating can avoid the formation of the outer passive layer andhus greatly increases the service life of SnO2–Sb2O5 electrodes4,5,29–32]. Thus, the objectives of this study are twofold: (i) toompare the stabilities of various SnO2-based electrodes; (ii) to sys-emically investigate their activities towards the electrochemicalxidation of nitrophenols.

Nitrophenols are a group of phenolic compounds which areresent in industrial effluents and have been detected in urbannd agricultural wastes. They are widely used in many indus-rial syntheses, such as those which produce pharmaceuticals,esticides, leather, and ammunitions [15]. Because of their tox-

city to humans, they are classified as high priority pollutantsy the United States Environmental Protection Agency [33], mak-ng it crucial that they are removed from both groundwater andastewater. The high stability and solubility of nitrophenols inater makes the pollution of drinking-water reservoirs and the

nvironment by these chemicals a dramatic problem [15]. Theurification of waters that contain these pollutants is very dif-cult since nitrophenols are resistant to most of the traditionalreatment techniques such as chemical oxidation [34], biodegra-ation [35], and adsorption [36]. The electron-withdrawing nitroroup and the electron-donating hydroxy group on the aromaticing of nitrophenols causes degradation to occur very slowly.ecent studies indicate that the advanced oxidation techniquesf photochemical degradation [37–40] and electrochemical oxi-ation [16,17,41,42] are promising for the degradation of organicollutants. In the present study, we have fabricated four differ-nt SnO2-based electrodes (Ti/SnO2–Sb2O5, Ti/SnO2–Sb2O5–PtOx,i/SnO2–Sb2O5–RuO2 and Ti/SnO2–Sb2O5–IrO2) and, for the firstime, systematically studied their stability and electrocatalyticctivity towards the degradation of 2-nitrophenol (2-NPh), 3-itrophenol (3-NPh) and 4-nitrophenol (4-NPh). The effects ofpplied current densities have been investigated and the kineticsf the electrochemical degradation of 2-NPh, 3-NPh and 4-NPh haseen examined.

. Experimental

.1. Materials and chemicals

Sodium sulphate (99%, Fischer Scientific), sulfuric acid (95–98%,ldrich), 2-NPh (98%, Aldrich), 3-NPh (ReagentPlusTM 99%,ldrich,), and 4-NPh (99+%, Aldrich,) were used to prepare solutionsf appropriate concentrations in pure water (18 M�) obtained fromNANOpure® DiamondTM UV ultrapure water purification sys-

em. The precursor solutions used in the electrode fabrication wererepared from an antimony doped tin oxide polymeric precursor30 wt% oxide, Alfa Aesar), IrCl3·3H2O, RuCl3·3H2O, and PtCl6·6H2OPressure Chemical Co.).

.2. Electrode preparation

Four kinds of SnO2-based electrodes were pre-ared using the thermal decomposition technique [28]:

i/SnO2–Sb2O5, Ti/SnO2–Sb2O5–PtOx, Ti/SnO2–Sb2O5–RuO2,nd Ti/SnO2–Sb2O5–IrO2 electrodes. The Ti substrates were cutnto strips (15 cm × 1.25 cm) and degreased in an ultrasonic bathf acetone for 10 min followed by 10 min in distilled water. Theubstrates were then etched in 18% HCl at approximately 85 ◦C

F2fot

cta 54 (2009) 1491–1498

or 30 min. Coating solutions were prepared by dissolving 2.31 mLf the SnO2–Sb2O5 precursor in 23.1 mL of isopropanol, 0.69 gf IrCl3·3H2O in 10 mL of ethanol, 0.86 g of RuCl3·3H2O in 10 mLsopropanol, and 1.00 g of PtCl6·6H2O in isopropanol. The desiredompositions of the coating solutions for the ternary metal oxidelectrodes were achieved by mixing various proportions of thetarting solutions such that the end product would be 90 wt.%nO2–Sb2O5 and 10 wt.% additive metal oxide. The coating solu-ions were painted onto the pretreated Ti substrates using a brushechnique, with excess solvent being evaporated by hot air at about0 ◦C and followed by calcinations at 450 ◦C for 10 min betweenach coat. This process was repeated until an oxide coating loadf approximately 30 g/m2 was reached (usually 12 coats). A finalnnealing was then done at 450 ◦C for 1 h to complete the fabrica-ion process. After the Ti strips were coated completely, they wereut into smaller pieces (1.0 cm × 1.25 cm) and used for the surfacenalysis and electrochemical experiments.

.3. Electrode characterization

Following the electrode preparation, the surface morphologynd composition of the coatings were characterized using a JEOL900LV scanning electron microscope (SEM) and X-ray energy dis-ersive spectrometry (EDS).

The electrochemical performance of the electrodes was charac-erized using cyclic voltammetry (CV), chronopotentiometry (CP),nd electrochemical impedance spectroscopy (EIS). For this, ahree-electrode cell system and a Solartron analytical SI 1287 sys-em were used. The counter electrode used was a Pt wire coil10 cm2) and this was cleaned before each experiment by flamennealing and quenching with pure water. The reference electrodeas a saturated calomel electrode connected to the cell through a

alt bridge. The electrolyte for these characterization studies was.5 M NaOH, and the solution was deaerated with ultrapure argonas prior to measurements to remove any dissolved oxygen. Argonas was continually passed over the solution throughout the experi-entation. EIS data was acquired using a Solartron analytical 1252A

requency response analyzer in combination with the 1287 system.he data for EIS was recorded using Zplot software.

.4. Lifetime test

The lifetime test was performed using two-electrode cell sys-ems and an Arbin Instruments MSTAT 12-channel potentiostat.he fabricated electrodes were used as the working anodes andhe counter cathodes were Pt wire coils (10 cm2). The electrolytesed was 0.5 M NaOH, and a current of 200 mA was applied. Theell potential and test time for each cell were monitored with Arbinnstruments MITS Pro software. The electrodes were deemed to beeactivated once the cell voltage increased by 2 V with respect tohe overall cell voltage.

.5. Optical absorbance measurement and electrochemicalxidation studies

For the oxidation studies, a Cary 50 UV/vis spectrophotome-er coupled with a fiber optic dip probe was used to monitorhe absorbance change of the nitrophenols. In all cases, the elec-rolyte used was 0.5 M Na2SO4, adjusted to pH 2 using sulfuriccid. The UV spectra were recorded over the range of 210–550 nm.

or 2-NPh and 3-NPh, two strong bands were observed in the10–550 nm range. The bands were centered at 279 nm and 350 nmor 2-NPh, and at 273 nm and 331 nm for 3-NPh. The appearancef the UV spectra for 4-NPh was distinct compared to that ofhe 2-NPh and 3-NPh with a very large band at 320 nm and a

B. Adams et al. / Electrochimica Acta 54 (2009) 1491–1498 1493

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tcTrecorded in 0.5 M NaOH at a scan rate of 50 mV/s. As expected,the CV of Ti/SnO2–Sb2O5 electrode does not exhibit any oxidationor reduction peaks in the applied potential range. The CVs of theTi/SnO2–Sb2O5–RuO2 and Ti/SnO2–Sb2O5–IrO2 electrodes have

Fig. 1. SEM images of the coating surfaces at 7500×: (a) SnO2–Sb2O5

mall band at 224 nm [43,44]. Calibration was carried out usinghe procedure described in our previous study [42]. The linearynamic range of 2-NPh was 0.0–0.30 mM with a linear regressionquation of A = 2.21 × c(mM) + 0.048 and a correlation coefficientf 0.9997 for the 350 nm peak used to examine the kinetics ofegradation of this compound. The dynamic linear range of 3-NPhas 0.0–0.20 mM with a linear equation for the 273 nm peak of= 6.33 × c(mM) + 0.061 and a correlation coefficient of 0.9998. 4-Ph was found to have a dynamic linear range of 0.0–0.18 mMnd had linear regression equations of A = 11.04 × c(mM) + 0.030ith a correlation coefficient of 0.9996 for the broad band at

20 nm. These linear regression equations were later used to con-ert absorbance readings into concentrations.

. Results and discussion

.1. Characterization of the prepared Ti/SnO2-based electrodes

SEM was used to characterize the morphology and surfacetructure of the four different SnO2-based electrode coatings. Ashown in Fig. 1a, the SnO2–Sb2O5 coating prepared using thehermal decomposition method exhibits a “cracked-mud” struc-ure which is typical for oxide electrodes. This coating has widend deep cracks with an average width of 2 �m which surroundislands”. The ternary oxide coatings SnO2–Sb2O5–PtOx (Fig. 1b),nO2–Sb2O5–RuO2 (Fig. 1c) and SnO2–Sb2O5–IrO2 (Fig. 1d) havehe similar “cracked-mud” structure to the SnO2–Sb2O5 coating;

he cracks get progressively smaller and shallower.

The EDS spectra shown in Fig. 2 display clear oxygen, tin, andntimony peaks for each coating. The distinct additive metal peaks present for each of the ternary oxide coatings. Further quantita-ive analysis shows that the additive metal oxide for each ternary

nO2–Sb2O5–PtOx , (c) SnO2–Sb2O5–RuO2, and (d) SnO2–Sb2O5–IrO2.

omposition is present in approximately 10 wt.%. These percent-ges of PtOx, RuO2, and IrO2 are consistent with the composition ofhe three ternary metal oxide coating solutions.

The electrochemical performance of the four SnO2-based elec-rodes was characterized using CV, CP, and EIS. Fig. 3 displays typicalyclic voltammograms of the Ti/SnO2–Sb2O5, Ti/SnO2–Sb2O5–PtOx,i/SnO2–Sb2O5–RuO2 and Ti/SnO2–Sb2O5–IrO2 electrodes

Fig. 2. EDS spectra of the coating surfaces.

1494 B. Adams et al. / Electrochimica Acta 54 (2009) 1491–1498

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ig. 3. CV curves of the prepared electrodes in 0.5 M NaOH at a scan rate of0 mV/s: (a) SnO2–Sb2O5, (b) SnO2–Sb2O5–PtOx , (c) SnO2–Sb2O5–RuO2, and (d)nO2–Sb2O5–IrO2.

imilar shapes with broad oxidation and reduction peaks at around0.3 V. These peaks correspond to the oxidation and reductionf the iridium and ruthenium species. The Ti/SnO2–Sb2O5–RuO2lectrode also has a reversible oxidation and reduction peak atround 0.35 V. It is known that ruthenium has several differentxidation states and can be oxidized from 0 to +6. At the end ofhe forward scan for the Ti/SnO2–Sb2O5–IrO2 electrode there is anncrease in current density which is due to the onset of oxygenvolution. The Ti/SnO2–Sb2O5–PtOx voltammogram is quite dis-inct, with several oxidation peaks and one large reduction peak.he characteristic voltammogram of this electrode is due to theact that Pt oxide is not stable at the temperature at which thelectrodes were fabricated. The peaks appearing in the potentialange between −0.8 V and −0.6 V are attributed to hydrogendsorption and desorption on the doped Pt. The several oxidationeaks presenting in the potential range between −0.5 V and 0.4 Vre due to Pt oxide formation. When the electrode potential iscanned from 0.4 V back to −0.8 V, a large reduction peak appearst around −0.38 V which corresponds to the reduction of the

latinum oxide formed during the positive potential scan.

CP experiments were done where the current was stepped up to00 mA and the potential was monitored with time. Fig. 4 shows theP curves for the four SnO2-based electrodes in 0.5 M NaOH. The

Fig. 4. CP curves of the prepared electrodes in 0.5 M NaOH.

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ig. 5. Nyquist plots of the prepared electrodes in 0.5 M NaOH at 0.6 V; thei/SnO2–Sb2O5 electrode in the inset.

i/SnO2–Sb2O5–RuO2 electrode has the lowest electrode potential,hich is closely followed by the Ti/SnO2–Sb2O5–IrO2 electrode.

he Ti/SnO2–Sb2O5–PtOx electrode has an intermediate potentialesponse, and the CP curve of the Ti/SnO2–Sb2O5 electrode displayshe highest potential. The high potential of the Ti/SnO2–Sb2O5 elec-rode is because of its known high oxygen evolution overpotential.y incorporating other metal oxides (PtOx, RuO2, and IrO2) into thenO2–Sb2O5 coating here, the electrical conductivity is increasednd the oxygen evolution overpotential is also lowered.

EIS studies were employed to further corroborate thendings of the CP experiments. Fig. 5 presents the Nyquistlots for Ti/SnO2–Sb2O5–PtOx, Ti/SnO2–Sb2O5–RuO2, andi/SnO2–Sb2O5–IrO2, with Ti/SnO2–Sb2O5 as the inset. Forhese experiments, a DC potential of 0.6 V was applied alongith an AC amplitude of modulation potential of 10 mV, and

he frequency was varied from 40 kHz to 40 mHz. An equivalentlectrical circuit, Rs(RctCPE), was used to fit the EIS data, In thisircuit, Rs represents the invariable solution resistance; while thearallel combination of the charge-transfer resistance (Rct) andhe constant phase element (CPE) take into account the oxygenvolution on the electrode coatings. The parallel combinationRctCPE) leads to a depressed semicircle in the correspondingyquist impedance plot. The CPE is defined by the parametersPE-T and CPE-P according to the formula:

= 1

T(iω)P

here T is a frequency independent term with units of (mF/cm2),is the angular velocity and i is the imaginary component

i = (−1)1/2). The value of the P exponent ranges between 0 and 1. A-value of zero corresponds to a pure resistor and a P-value of 1 cor-esponds to a pure capacitor with capacitance T. All the associatedrrors are within 3%, indicating that the proposed model can fit thexperimental data effectively. The values of CPE-P are all close to.9 indicating that the CPE component is close to a pure capacitoror all four electrodes. The CPE-T values decrease in the order

2

f: Ti/SnO2–Sb2O5–IrO2 (44.29 mF/cm ) > Ti/SnO2–Sb2O5–RuO223.01 mF/cm2) > Ti/SnO2–Sb2O5–PtOx (18.29 mF/cm2) > Ti/SnO2–b2O5 (9.35 mF/cm2). The charge-transfer resistanceecreases in the order of: Ti/SnO2–Sb2O5 (492.70 �/cm2) >i/SnO2–Sb2O5–PtOx (45.82 �/cm2) > Ti/SnO2–Sb2O5–IrO2 (8.94

B. Adams et al. / Electrochimica A

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ig. 6. (a) UV spectra of 0.10 mM 4-NPh in 0.5 M Na2SO4 at pH 2 oxidized usinghe Ti/SnO2–Sb2O5 electrode at 100 mA. The inset shows the absorbance at �max

320 nm) vs. time. (b) Plots of ln(C/C0) vs. time for the oxidation of 0.10 mM 4-NPhsing the Ti/SnO2-based electrodes at 100 mA.

/cm2) > Ti/SnO2–Sb2O5–RuO2 (4.44 �/cm2). This correspondsell with the CP experiments as resistance is related proportionally

o potential and inversely proportional to current.

.2. Lifetime tests

Lifetime tests of the prepared electrodes were done to com-are their stabilities under anodic polarization. Here, stability isonsidered as the ability of the electrodes to maintain a low poten-ial for long periods of time as a constant current is applied. At anpplied current of 200 mA, the Ti/SnO2–Sb2O5 electrode only lastedor 12 h. The Ti/SnO2–Sb2O5–PtOx also had a very short lifetimef 75 h. Both the Ti/SnO2–Sb2O5–RuO2 and Ti/SnO2–Sb2O5–IrO2nodes remained active for an exceptionally long time, with therO2 doping (155 days) outlasting the RuO2 doping (124 days). Allhese results demonstrate that the incorporation of IrO2 or RuO2ignificantly increases the service life of the Ti/SnO2–Sb2O5 anode.

.3. Activity of the SnO2-based electrodes towards thelectrochemical oxidation of 4-NPh

The activity of the SnO2-based electrodes was evaluated basedn their ability to oxidize 4-NPh. UV/vis spectroscopy was used to initu monitor the changes in absorbance of the chemical during theourse of the electrochemical oxidation. Shown in Fig. 6a are the

ime dependant UV spectra, taken every 30 min, of 0.10 mM 4-NPhn 0.5 M Na2SO4 at pH 2 oxidized at the Ti/SnO2–Sb2O5 electrode

ith 100 mA applied current; the inset displays the absorbance atmax (320 nm) versus time. The linear regression equation of 4-NPht 320 nm was used to convert these absorbances into concentra-

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cta 54 (2009) 1491–1498 1495

ions. This procedure was also carried out for the oxidation of 4-NPhith an initial concentration of 0.10 mM using each of the three

ernary SnO2-based mixed metal oxide electrodes.In our previous study, the elimination of 4-NPh at the

i/SnO2–Sb2O5–IrO2 anode was found to be described by pseudorst-order kinetics [42]. Its removal rate can be expressed as:

d[4-NPh]dt

= −kapp[4-NPh]

here the combined effect of direct and indirect process is con-idered without quantifying their relative contributions. Afterntegration, the above equation gives:

n(

[4-NPh]t

[4-NPh]0

)= kappt

here kapp is the apparent rate constant, and [4-NPh]0 (generally,0) and [4-NPh]t (generally, C) correspond to the initial concentra-ion of the nitrophenol and the concentration of the nitrophenolfter a period of time, respectively.

A plot of ln(C/C0) versus time (Fig. 6b) was produced for theime frame of 3 h of electrolysis and had a linear relationship, con-rming that the oxidation of 4-NPh by these electrodes displaysseudo first-order kinetic behaviour. The apparent kinetic rate con-tants were determined from the magnitude of the slopes of thetraight lines for each electrode. The first-order rate constant washe highest for Ti/SnO2–Sb2O5 with a value of 7.31 × 10−3 min−1.he next highest rate constant was for the oxidation using thei/SnO2–Sb2O5–PtOx electrode with a value of 5.84 × 10−3 min−1.his was closely followed by Ti/SnO2–Sb2O5–IrO2 which hadrate constant of 5.48 × 10−3 min−1, and Ti/SnO2–Sb2O5–RuO2ith the lowest activity towards the oxidation of 4-NPh withrate constant of 4.22 × 10−3 min−1. The above results show

hat the Ti/SnO2–Sb2O5–IrO2 anode is the best overall elec-rode among the prepared four SnO2-based oxide coatings forhe electrochemical oxidation of 4-NPh since the Ti/SnO2–Sb2O5nd Ti/SnO2–Sb2O5–PtOx anodes had such short service life-imes and the Ti/SnO2–Sb2O5–IrO2 electrode outperformed thei/SnO2–Sb2O5–RuO2 electrode in both the lifetime tests and activ-ty experiments.

.4. Influence of current density on the oxidation of 4-NPh withhe Ti/SnO2–Sb2O5–IrO2 electrode

For the electrochemical oxidation of organic compounds, it ismportant to achieve high current efficiency in order to provide

cost-effective process. The effect of current density on 4-NPhemoval was tested using the Ti/SnO2–Sb2O5–IrO2 anode withn initial concentration of 4-NPh 0.15 mM. Various currents werepplied, increasing from 20 mA to 150 mA. UV/vis spectroscopy wassed to in situ monitor the time-dependant absorbance of the 4-Ph during the course of the electrochemical oxidation, and thesebsorbance readings were converted to concentration using the cal-bration plots described in Section 2.5. First-order kinetic plots ofn(C/C0) versus time were produced to investigate the effect of thepplied currents on the kapp. As shown in Fig. 7, by increasing theurrent, the rate of degradation of 4-NPh also increased up to aaximum at 100 mA. When increasing the applied current further

o 150 mA, no increase was seen in the rate constant, indicatinghat 100 mA is the most efficient current for this process. When thexidation of 4-NPh is done at anodic currents greater than 100 mAhe extra electricity is being wasted, most likely being used for theompeting OER reaction.

1496 B. Adams et al. / Electrochimica A

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tssat246 nm and 286 nm were formed during the course of the oxidation.

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ig. 7. The dependence of kapp on different current densities applied to oxidize.15 mM 4-nitrophenol on the SnO2–Sb2O5–IrO2 electrode.

.5. Electrochemical oxidation of 2-NPh, 3-NPh, and 4-NPh at thei/SnO2–Sb2O5–IrO2 electrode

A common initial concentration of 2-NPh, 3-NPh, and 4-NPhas chosen in order to compare the relative rate constants for

hese chemicals using the Ti/SnO2–Sb2O5–IrO2 electrode. A con-entration of 0.15 mM was used since this concentration was withinhe dynamic linear range of the UV calibration curves for each ofhe chemicals. Three separate solutions consisting of 0.15 mM of

ach chemical in 0.5 M Na2SO4 adjusted to pH 2 were prepared. Annodic current of 100 mA was applied. Fig. 8 presents the UV spec-ra for each chemical taken every 30 min during the course of theirlectrochemical oxidation.

avT

ig. 8. UV spectra taken in situ every 30 min during the electrochemical oxidation of: (a)n(C/C0) vs. time plots.

cta 54 (2009) 1491–1498

It can be seen that in scanning the UV spectra of 2-NPh (Fig. 8a),oth absorbance bands decrease with time. The absorbance bandt 279 nm displays a slight blue-shift with time, possibly due toverlapping peaks of intermediates in this region. The absorbanceand at 350 nm was used in the kinetic analysis for this reason since

t showed no shift in either direction. A third characteristic of thecanning kinetic UV spectra is the formation of a new absorbanceand at ca. 404 nm, indicating that intermediates are formed whichlso have characteristic UV spectra within the range of 210–550 nm.his wide band at 404 nm also overlaps with the 2-NPh band at50 nm after a long period of time. Since the absorbance of the 2-Ph band became very small after 3 h of electrolysis, ln(C/C0) versus

ime first-order kinetic rate curves were only plotted for the firsth of oxidation.

For the scanning kinetic UV spectra of 3-NPh (Fig. 8b), bothbsorbance bands (273 nm and 331 nm) decreased during the elec-rochemical oxidation, indicating a decrease in concentration of-NPh. No intermediate peaks appeared during the course of thexidation within the range of 210–550 nm. This is consistent withrevious results indicating that the electrochemical oxidation of 3-Ph proceeds without the quinone intermediates seen for 2-NPhnd 4-NPh [17,41]. The larger peak at �max = 273 nm was used tobtain the rate constant using the linear regression equation devel-ped in Section 2.5.

Fig. 8c presents the spectral absorbances taken every 30 min forhe oxidation of 4-NPh. The peak centered at 320 nm decreasedignificantly with time while the peak centered at 224 nm blue-hifted and decreased slowly with time. The most intense bandt 320 nm was chosen to use as the base in order to converthe absorbances into concentrations. Two new bands centered at

The linear lines displayed in the plot of ln(C/C0) versus time over3-h time span for the oxidation of the three chemicals (Fig. 8d)

erifies that the electrochemical oxidation of all chemicals on thei/SnO2–Sb2O5–IrO2 electrode follow pseudo first-order kinetics.

2-NPh, (b) 3-NPh, and (c) 4-NPh at an initial concentration of 0.15 mM, and (d) the

B. Adams et al. / Electrochimica A

Table 1Kinetic rate constants and half lives for different nitrophenols on theSnO2–Sb2O5–IrO2 electrode with the applied current of 100 mA.

Chemicals Initial concentrations(mM)

Rate constants,×103 (min−1)

Half lives(min)

2-Nitrophenol 0.050 7.74 89.60.150 7.66 90.50.175 7.94 87.30.275 8.17 84.8

3-Nitrophenol 0.050 3.64 190.40.150 3.61 192.00.175 3.86 179.6

4-Nitrophenol 0.050 6.25 110.9

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0.100 6.39 108.50.150 6.53 106.10.175 6.50 106.6

-NPh was found to be the easiest to be oxidized with a kineticate constant of 7.66 × 10−3 min−1. The rate constant for 3-NPh wasound to be the lowest, with a value of 3.61 × 10−3 min−1. It is sug-ested that ring opening to carboxylic acids is the first step of thexidation of 3-NPh which is quite energetically demanding. 4-NPhad an intermediate degradation rate with a corresponding rateonstant of 6.53 × 10−3 min−1. It can be seen from the scanninginetic plots that virtually all of the nitrophenols were oxidizedfter 6 h.

The effect of initial concentration on the rate constant was fur-her examined for the electrochemical oxidation of 2-NPh, 3-NPh,nd 4-NPh using the Ti/SnO2–Sb2O5–IrO2 anode with an appliedurrent of 100 mA. Various initial concentrations were chosen forach chemical that were within the dynamic linear range of theV calibration curves described in Section 2.5. The same tech-ique as described above for comparing the rate constants at aommon concentration were used here to obtain the rate con-tants for different concentrations of the pollutants. The absorbanceeadings taken with time were converted to concentrations usinghe linear regression equations set forth in Section 2.5 and first-rder plots of ln(C/C0) versus time were created for each oxidationxperiment. The kinetic rate coefficients were determined from theagnitude of the slopes of the straight lines for each chemical. In

able 1, the values of the kinetic rate constants obtained at dif-erent nitrophenol initial concentrations are presented along withhe calculated half lives (ln 2/k). For initial concentrations of 2-NPhithin the range of 0.05–0.275 mM, the rate constants all lie within

he range of 7.66 × 10−3 min−1 to 8.17 × 10−3 min−1. For 3-NPh, ini-ial concentrations were chosen within the range of 0.05–0.175 mMnd the rate constants were found to fall between the range of.61 × 10−3 min−1 to 3.86 × 10−3 min−1. The rate constants for 4-Ph with initial concentrations between 0.05 mM and 0.175 mM arelso very similar, all falling within the range of 6.25 × 10−3 min−1

nd 6.53 × 10−3 min−1. The minimal differences in rate constantshen varying the initial concentration of each chemical shows that

nitial concentration has little effect on the process for the range ofoncentrations examined here.

It has been reported that the electrochemical degradation ofrganics is primarily dependent on the hydroxyl radical gener-ted on the anode [41]. As the reaction between the nitrophenololecules and the hydroxyl radical is an electrophilic reaction, the

reater rate constants for the degradation of 2-NPh and 4-NPh canhus be explained. The attack of the electrophilic hydroxyl radical

ccurs at the ring positions activated by the presence of the twoubstituents. The NO2 group is electron-withdrawing while thehenolic OH group is electron-donating for electrophilic aromaticubstitution. Electron-donating substituents increase the electronensity at the ortho and para positions while electron-withdrawing

[

cta 54 (2009) 1491–1498 1497

ubstituents are strongly deactivating and meta directing. Whenoth the substituents ( OH and NO2) are present, the electrophilicttack will occur preferentially in the ortho and para positions withespect to the OH group [45]. No positions on the ring are ener-etically favourable for the attack of 3-NPh by the hydroxyl radical.t is also known that the NO2 group in nitrophenols is a good leav-ng group, and can be easily eliminated by direct attack of hydroxyladicals for 4-NPh since the para directing effect of the OH groups stronger than the ortho directing effect [46]. The rate constant forhe degradation of 2-NPh is larger than that of 4-NPh because theres no interference with the NO2 group for the initial attack of theydroxyl radical at the para position relative to the OH group.

. Conclusions

In this study, four different types of SnO2-based electrodes wereabricated using the thermal decomposition method. Their surface

orphology and composition were analyzed using SEM and EDSnd their electrochemical performance was characterized using CV,P, and EIS. A lifetime test was performed to compare the stabilitiesf the Ti/SnO2–Sb2O5, Ti/SnO2–Sb2O5–PtOx, Ti/SnO2–Sb2O5–RuO2,nd Ti/SnO2–Sb2O5–IrO2 electrodes under anodic current and itas found that by incorporating different metal oxides into the

b-doped SnO2 coating, the lifetime was significantly increased.he greatest improvement in service life was observed for thei/SnO2–Sb2O5–IrO2 electrode which had a lifetime of 155 daysnder 160 mA/cm2 anodic current density in 0.5 M NaOH; while thei/SnO2–Sb2O5 electrode only lasted 12 h under the same experi-ental conditions.The electrochemical activities of the prepared SnO2-based elec-

rodes were characterized using the degradation of 2-NPh, 3-NPhnd 4-NPh. In situ UV/vis spectroscopy was used to monitor theoncentration change of nitrophenols with time. The influence ofurrent density was studied for the electrochemical oxidation of-NPh at the Ti/SnO2–Sb2O5–IrO2 anode and it was concluded thatn applied current of 100 mA was the most efficient for this pro-ess; using a higher current is simply wasting energy. The rateonstants for the electrochemical oxidation of the nitrophenolsxamined in this study were found to decrease in the order of:-NPh > 4-NPh > 3-NPh; little effect was seen on the rate constantf oxidation when the initial concentration of each chemical wasaried in a broad range. The fabricated Ti/SnO2–Sb2O5–IrO2 elec-rode possesses a very long lifetime and high activity towards theegradation of nitrophenols, very promising in the application oflectrochemical treatment of wastewater.

cknowledgments

This work was supported by a Discovery Grant from the Natu-al Sciences and Engineering Research Council of Canada (NSERC).. Chen acknowledges NSERC and the Canada Foundation of Inno-ation (CFI) for the Canada Research Chair Award in Material andnvironmental Chemistry.

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