Zhen Wei,Xuanqi Kang,Shangyuan Xu,Xiaokang Zhou,Bo Jia,Qing Feng

1 Xi’an TaiJin Industrial Electrochemical Technology CO.,Ltd,Xi’an 710016,China

2 Key Laboratory of Porous Metal Materials,Shaanxi Institute for Materials Engineering,Northwest Institute for Non-ferrous Metal Research,Xi’an 710016,China

Keywords:Ti/PbO2 electrodes Electrocatalytic oxidation

ABSTRACT Regulation of the electronic structure and interface property becomes a major strategy in the preparation of electro-catalyst.This paper reports the synthesis of cerium(Ce)and sodium dodecyl benzene sulfonate(SDBS) co-modified Ti/PbO2 electrodes (Ti/PbO2-Ce-SDBS).Ce and SDBS could greatly change the electronic structure and interface property of PbO2.Ti/PbO2-Ce-SDBS exhibited excellent electrocatalytic activity and stability in Rhodamine B(RhB)electrocatalytic oxidation reaction.The improved electrocatalytic activity associates with the synergistic effect of electronic and interface factors.In the electrochemical degradation of RhB,the removal efficiencies of RhB and COD are about 0.880 and 0.694 respectively after the electrolysis of 220 min with Ti/PbO2-Ce-4-SDBS-40,which are higher than the contrast Ti/PbO2 electrodes.In the meantime,the accelerated lifetime of Ti/PbO2-Ce-4-SDBS-40 is more than 6.2 times than that of Ti/PbO2.

1.Introduction

As indispensable industrial products,dyes are widely used in many fields,most of which are toxic and carcinogenic,resulting in a deleterious impact on the environmental ecological equilibrium [1].With the rapid development of dyestuff industry,it has become a major challenge to degrade dye pollutants prior to the discharge of wastewater into the local water body.Up to now,there are several approaches employed to remove dye pollutants,including physical and chemical degradation [2],Fenton’s oxidation [3],adsorption [4],and electrochemical degradation [5–7].However,various chemical compositions and high stability of dyes erect barriers for removing dyes from wastewater with chemical or biological methods[8,9].Therefore,a highly efficient and low-cost wastewater treatment technique is essential.Electrochemical degradation is considered as a kind of feasible,effective,economic,and simple method,attracting much interest.

Recently,a promising prospect awaits the dye wastewater treatment technique of electrochemical degradation for its mild reaction conditions,no secondary pollution,high efficiency and friendly environmental compatibility.There are two mechanisms that could explain for the decomposition of organic dyes into small molecules-H2O,CO2,and other intermediates:direct electrochemical oxidation on the anode surface and an indirect one mediated by electrogenerated oxidants [10].As is known to all,the electrochemical degradation efficiency is dependent on the electrode materials.Right now,there are several electrode materials of graphite:Pt,PbO2,SnO2,RuO2and boron-doped diamond have been developed[11].Among them,PbO2,a good anode material in electrochemical degradation,gains the most of the focus due to its low cost,long lifetime,and good resistance to corrosion[12].However,its low oxygen evolution potential leads to poor electrochemical degradation,limiting the practical engineering application.How to increase the oxygen evolution potential of PbO2is a problem to be solved before the application.

One solution is to introduce catalytic active elements into PbO2to adjust the electronic structure.Ce,which has a peculiar electronic structure,is often chosen a common selection as a doping agent to improve the catalytic activity of electrodes [9,13,14].But Ce and PbO2are loosely bound which requires a binder that has surfactants with hydrophilic heads and hydrophobic tails to satisfy the requirement of compatibility.Sodium dodecylbenzene sulfonate (SDBS) is an excellent choice to be introduced into the reaction system,which could not only improve the uniformity of the reaction solution but also enhance the impact on the interface reaction.The introduction of SDBS into electroplating bath greatly affects the interface property of Ti/PbO2electrode.Our work proposed an electrochemical active Ce and SDBS co-modified Ti/PbO2-Ce-SDBS electrodes,which were characterized by SEM,PXRD,and XPS.Linear sweep voltammetry (LSV),cyclic voltammetry curve(CV)and electrochemical impedance spectroscopy(EIS)were adopted to investigate the influence of the introduction of Ce and SDBS on the electrochemical performances of the modified Ti/PbO2-Ce-SDBS.The accelerated lifetime test was also carried out to determine the electrochemical stability of Ti/PbO2-Ce-SDBS.The effect of the RhB removal and mineralization by electrocatalytic oxidation with Ti/PbO2-Ce-4-SDBS-40 was estimated with UV–Vis,COD,and fluorescence measurements,including the durability of Ti/PbO2-Ce-4-SDBS-40.

2.Materials and Methods

2.1.Materials

All chemicals were used directly as purchased from Sinopharm Chemical Reagent Co.,Ltd.(Shanghai,China) without further purification,including Lead nitrate(Pb(NO3)2,A.R.),Copper nitrate(Cu(NO3)2﹒3H2O,A.R.),Cerous nitrate (Ce(NO3)3﹒6H2O,A.R.),sodium dodecyl sulfate (C12H25OSO2ONa,A.R.),sodium sulfate(Na2SO4,A.R.),hydrochloric acid (HCl,35%),terephthalic acid(HOOCC6H4COOH,A.R.),sodium hydroxide (NaOH,A.R.),oxalic acid (C2H2O4﹒2H2O,A.R.),and sulfuric acid (H2SO4,98%).All solutions were prepared with ultrapure water.

2.2.Preparation

Ti/PbO2electrodes were prepared according to the classic threestep method as is depicted in Fig.1 [15].The first step is the pretreatment of titanium meshes and titanium meshes (20 mm × 10 mm × 1.0 mm).The pretreatment process is polishing,etching in 3%oxalic solution at 80°C for 2 h and ultrasonic cleaning in acetone and ultrapure water successively [16];The second step is the addition of bottom layers:Sn-Sb2O3bottom layers were prepared by the thermal decomposition in 450 °C [17],Sn-Sb2O3bottom layers could not only increase the binding force between titanium substrate and active layers,but also improve the electrical conductivity of titanium substrate,which is conducive to engineering application;The third step is the deposition of active layers:β-PbO2active layers were electroplated in an acidic electrolyte consisting of 150 g﹒L-1Pb2+and 80 g﹒L-1Cu2+,According to the sequence table of metal activity,the introduction of Cu2+can realize the priority deposition of copper at the cathode and reduce the precipitation of lead at the cathode.Both undoped and doped PbO2films were deposited on the electrode in 15 mA﹒cm-2current density at 60 °C within 60 min.Ti/PbO2-Ce electrodes were prepared with Ce in the acidic electrolyte,which were denoted as Ti/PbO2-Ce-n (n=2,4 and 8,representing the Ce concentrations:2,4,and 8 mmol﹒L-1).Comparably,with SDBS in the electrolyte,Ti/PbO2-SDBS electrodes were prepared and denoted as Ti/PbO2-SDBS-m (m=5,10 and 20 mg﹒L-1of SDBS concentrations).As the Ce concentration fixed at 4 mmol﹒L-1,Ti/PbO2-Ce-4-SDBS-m electrodes were prepared with SDBS,which concentrations range from 5 to 50 mg﹒L-1,whose preparations were similar to literature methods [18–21].

2.3.Characterizations and performance tests

The powder X-ray diffraction(PXRD)data of the materials were collected at ambient temperature with a Rigaku MiniFlex 600(Japan) diffractometer (Cu-Kα1,2X-radiation,λ1=0.1540598 nm and λ2=0.1544426 nm),equipped with an X’Celerator detector and a flat plate sample holder in a Bragg–Brentano para-focusing optics configuration (40 kV,15 mA).Intensity data were collected by a step counting method(the step being 0.02°)in the continuous mode in the 2θ range of 20°–80°.The surface morphologies were acquired by scanning electron microscopy (SEM) with an Energydispersive X-ray spectroscopy (EDS) on a Philips–FEI Quanta 200.The chemical states of the elements in the materials were studied by X-ray photoelectron spectroscopy(XPS)on an Axis Ultra(Kratos Analytical Ltd.,UK).UV–Vis absorption spectra were measured on a UV-1800 spectrophotometer (Shimadzu,Japan).Fluorescence spectra were recorded in the range of 380–510 nm on a Hitachi F-7000 with the excitation and emission silts at 5 nm,using an excitation wavelength of 315 nm.Due to the fact that·OH radicals can efficiently degrade pollutants,the·OH radicals generation capacity is one of the important indicators to evaluate the performance of an anode.As known,terephthalic acid can react with·OH radicals to form fluorescent 2-hydroxy terephthalic acid.Terephthalic acid acted as a probe for·OH radicals.The·OH radicals generation levels of electrodes were determined with trapped terephthalic acid by fluorescence spectrophotometer.The electrolysis was performed at a current density of 30 mA﹒cm-2at 30°C in 100 mL solution of 0.5 mmol﹒L-1terephthalic acid,0.5 g﹒L-1NaOH and 0.25 mol﹒L-1Na2SO4.The electrolyzed solutions were drawn from the reactor with a step of 5 min and diluted 10 times with ultrapure water,and then tested their fluorescence.Chemical oxygen demand(COD)is a measure of the amount of the oxygen used in the chemical oxidation of inorganic and organic matter present in wastewater.In this study,COD was measured by a titrimetric method using dichromate as the oxidant.

Fig.1.Synthetic process of Ti/PbO2.

Electrochemical measurements were carried out on CHI 660E electrochemical workstation equipped with a three-electrode system in 0.25 mol﹒L-1Na2SO4electrolyte.Saturated calomel electrode (SCE) was used as a reference electrode,platinum sheet as a counter electrode,and as-prepared Ti/PbO2electrodes as working electrode.CV potential of 0.5–2.0 V vs.SCE (0.25 mol﹒L-1Na2SO4electrolyte) at a rate of 100 mV﹒s-1.EIS is an AC impedance test with an output potential of 1.6 V,an amplitude of 5 mV and a frequency range of 1.0–105Hz.

The accelerated lifetime test was carried out in 1 mol﹒L-1H2SO4solution at an applied current density of 1 A﹒cm-2to evaluate the stability of the electrodes with the temperature of the electrolyte at about 60 ℃,the tested electrode as working electrode and Ti plate as counter,respectively.When the anode potential increased to 10 V,the electrolysis time was regarded as the accelerated life.

2.4.Electrochemical degradation of RhB

The electrochemical degradation experiments were carried out in a batch reactor which connected with a DC power supply,Ti/PbO2electrodes and Ti electrodes(both of 20 mm×10 mm)were respectively used as the anode and the cathode with a 1.0 cm interelectrode gap.In each experiment,the electrolysis was performed in 100 mL RhB solution of 25 mg﹒L-1.The electrolyzed solutions were drawn from the reactor every 5 min.

COD were used to determine the removal rate of RhB based on the combustion-infrared method (Eqs.(1) and (2)),

where Aoand Atare the initial and final absorbance of RhB.

where CODoand CODtare the initial and final COD (mg﹒L-1) of the solution.

A well-fitted pseudo-first-order kinetic model of RhB degradation was determined by Eq.(3).

where Coand Ctare the initial and final concentrations of RhB at time t,and k is the reaction rate constant.

3.Results and Discussion

3.1.Surface topography analysis

3.1.1.PXRD

On the PXRD pattern of the Ti/PbO2electrode,it is easy to find the PXRD pattern of Ti/PbO2comprises of the mixed-phase of α-PbO2and β-PbO2.By adding Ce,the diffraction peak intensities of β-PbO2increase while those of α-PbO2decrease and finally disappear (Fig.2(a)),which indicates that Ce contributes to the structural transformation from α-PbO2to β-PbO2.As is known to all,β-PbO2has better electrocatalytic activity than α-PbO2.Therefore,it can be expected that the synthesized Ti/PbO2electrode will have better electrochemical performance[13,22].With the increased Ce content in Ti/PbO2-Ce electrode,the full width at half maximum(FWHM)of the PXRD pattern becomes wider,suggesting the crystal size is smaller and smaller.The relationship between the Ce content and crystal size is consistent with SEM pictures (Fig.S1).Further,it is noteworthy that the preferred crystal plane (1 1 0)in Ti/PbO2changed to (101) in Ti/PbO2-Ce under the intervention of Ce.Nevertheless,the increased Ce content leads to lower crystallinity of PbO2,which might have a causal relationship with the precipitation of Ce from the PbO2phase and acting as the oxidant for the reaction.

PXRD patterns of Ti/PbO2-SDBS were shown in Fig.2(b)depending on SDBS modifying in mixed-phase of α-PbO2and β-PbO2.The same PXRD patterns of Ti/PbO2-SDBS as Ti/PbO2demonstrates that SDBS does not affect the crystal structure.The experimental PXRD patterns of the dual doped Ti/PbO2-Ce-SDBS match well with the standard diffraction peaks (JCPDS Card No.41-1492).The diffraction peaks at 25.4°,31.9°,36.2°,49.0° and 63° are well indexed to β-PbO2.Therefore,it demonstrates not only Ti/PbO2-Ce-SDBS isolated as a pure phase,but PbO2is the β-PbO2phase (Fig.2(c)).Similarly,the widened the full width at half maximum (FWHM)of the PXRD pattern suggests that the smaller particles were formed in Ti/PbO2-Ce-SDBS,which is consistent with SEM pictures.As shown in Fig.S5,the cross-section of the coating shows that the thickness of the PbO2coating(500 μm)exceeds the X-ray penetration capacity,so the diffraction peak of the titanium substrate is not detected.

3.1.2.SEM

SEM was carried out to investigated the influence of doping on the surface morphology of the Ti/PbO2electrodes with the comparison of the undoped electrode.The situations of the doping agents of Ce and SDBS coating on Ti/PbO2were studied by EDS(the result is shown in Fig.3(e)and Table.S1).Compared with EDS of Ti/PbO2,the result shows that there are elements S,O,and Na elements in Ti/PbO2-SDBS-10,as well as more content of C elements,indicating that the modifying of SDBS is fruitful.Ce can be detected in Ti/PbO2-Ce-4,revealing that Ce is successfully introduced into the coating.Ce,S,and Na elements can be detected in Ti/PbO2-Ce-4-SDBS-40 simultaneously,confirming the successful preparation of co-modified electrode (Fig.3(e)),which is consistent with the following XPS analysis results.

Based on the pattern of SEM,the typical pyramid structure of PbO2can be clearly seen on the surface of the undoped Ti/PbO2with large PbO2particles unevenly scattering (Fig.3(a)).Nonetheless,the surface morphology of the Ti/PbO2was greatly modified by Ce.When 4 mmol﹒L-1Ce was added in the electroplating bath,it is clearly presented in Fig.3(b)that the Ti/PbO2-Ce-4 coating was shaped.The magnification diagram Fig.3(f) shows that the size of the crystal particles is small,whose edges and corners are not prominent in the meantime.Furthermore,with the increased Ce content,the crystal particles became smaller and coated tightly on PbO2[23].Notwithstanding,when 8 mmol﹒L-1Ce was introduced,though the crystal particles were still smaller,the edges of PbO2pyramids became obscure and the coating severely deteriorated to make partial coatings peel off with the adhesion decreasing,even the pyramid structure of PbO2particles completely disappeared (Fig.S1(d)).Comparison between the morphology of Ti/PbO2before and after modifying SDBS by their SEM images,the coatings of Ti/PbO2-SDBS-10 accumulated a lot of PbO2pyramid grains (see Fig.3(c)),which grains were smaller and denser(see Fig.S2).With Ce content fixed as 4 mmol﹒L-1,the PbO2particles tended to be smaller and more coherent depending on SDBS concentration (Fig.S3).It is in line with the previous analysis[22].With co-modified Ce and SDBS,Ti/PbO2-Ce-4-SDBS-40 displayed the smallest PbO2particles (Fig.3(d)),The magnification diagram Fig.3(g) reveals that the crystal particles have the smallest size and porous structure,which not only increases the specific surface area of the reaction but is conducive to the subsequent electrocatalytic reaction.

Fig.2.PXRD spectra of surface coating:(a) Ti/PbO2-Ce,(b) Ti/PbO2-SDBS,(c) Ti/PbO2-Ce-SDBS.

Fig.3.SEM of surface coating:(a) Ti/PbO2,(b) Ti/PbO2-Ce-4,(c) Ti/PbO2-SDBS-10,(d) Ti/PbO2-Ce-4-SDBS-40,(e) EDS of the Ce doped electrode,(f) Local magnification diagram of (b),(g) Local magnification diagram of (d).

3.1.3.XPS

XPS was adopted to test the surface chemical states of Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10,and Ti/PbO2-Ce-4-SDBS-40 electrodes.The characteristic peaks of Pb,O,C,Ce and S elements for the above electrodes were investigated with the binding energies calibrated with respect to the C 1s peak at 284.8 eV (Fig.4(a)).The detectable Ce3d peak can be observed in Ti/PbO2-Ce-4 and Ti/PbO2-Ce-4-SDBS-40,which means Ce was successfully doped into the PbO2coatings.The detectable S2p peak can be observed in Ti/PbO2-SDBS-10 and Ti/PbO2-Ce-4-SDBS-40,indicating SDBS was successfully doped into the PbO2coatings (Fig.4(b)).Furthermore,two well-defined Pb 4f5/2and Pb 4f7/2peaks were observed in both samples by the detailed scan spectra(Fig.4(c)).The binding energies of Pb 4f5/2and Pb 4f7/2peaks located at 142.0–142.3 eV and 137.1–137.6 eV,respectively.The binding energy difference of Δ(Pb 4f5/2)– (Pb 4f7/2) was almost 5.0 eV in accordance with the literature [14,24,25].Therefore,it could be confirmed that PbO2in Ti/PbO2-Ce-4 was the β-PbO2phase.Interestingly,comparing to Ti/PbO2,the binding energy of Pb 4f peaks in Ti/PbO2-Ce-4 shifted to lower binding energy,which may stem from the powerful oxidation ability of the doped Ce.The outer electron of Ce can tune the valence electrons of Pb,leading to different binding energies of Pb 4f5/2and Pb 4f7/2states in β-PbO2.The electronic structure character of β-PbO2can also exert an impact on its catalytic property.Therefore,the doped Ce can effectively improve the catalytic property of PbO2.Fig.4(d) shows the high-resolution XPS spectra of O 1 s of the four electrodes.All collected spectra showed two peaks:the one at the lower binding (529.1–529.4 eV) was assigned to lattice oxygen species (O1at);the one at the higher binding energy (531.2–531.6 eV) was attributed to the adsorbed oxygen (Oads),Adsorbed oxygen includes adsorptive bulk oxygen and/or the weakly physisorption of bonded oxygen species (e.g.hydroxyl group and water) on the surface of PbO2electrodes,which was likely to participate in redox reaction at the electrode/water interface and play an important role in the oxidation process.What also should be noted is that positive shift for O 1 s peak was observed in the spectra of Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10 and Ti/PbO2-Ce-4-SDBS-40 electrodes,and the chemical shift was strengthened with the increase of SDBS and Ce concentration,which may be caused by the decrease of electron density around Pb4+and O2-due to the introduction of SDBS and Ce.

Fig.4.XPS spectra of surface coating:(a) Survey spectra,(b) Ce 3d and S 2p spectra,(c) Pb 4f spectra,(c) O 1s spectra.

3.2.Electrochemical performance

3.2.1.LSV curves

Table 1 is the anode potentials (APs) at different current densities and the electrochemical performances of Ti/PbO2,single doped Ti/PbO2with Ce or SDBS,and dual doped Ti/PbO2-Ce-4-SDBS were studied(Fig.5).APs of Ti/PbO2-Ce grew with the increased Ce contents or the increased current density.Apart from Ti/PbO2-Ce-2,both Ti/PbO2-Ce-4,and Ti/PbO2-Ce-8 possessed higher APs than Ti/PbO2.The higher APs imply the introduction of Ce improves the oxygen evolution potential,which is helpful for preventing the oxygen evolution reaction.A presumed explanation is that the introduction of Ce weakens the interactions between theTi/PbO2-Ce coatings and the adsorbed species,thus increasing the oxygen evolution potential.Ti/PbO2-Ce could form more hydroxyl radicals,which is propitious to the removal of organics.

Table.1APs of Ti/PbO2,Ti/PbO2-Ce,Ti/PbO2-SDBS and Ti/PbO2-Ce-4-SDBS with different Ce and/or SBDS contents at different current densities

Fig.5.LSV curves of different content for Ti/PbO2 electrode:(a)PbO2-Ce,(b)PbO2-SDBS,(c) PbO2-Ce-SDBS.

The effect of doped SDBS on APs was also explored (Fig.5(b)).APs of Ti/PbO2-SDBS and Ti/PbO2-Ce-4-SDBS were close to 1.86 V of Ti/PbO2at 100 A﹒m-2,but higher than those of Ti/PbO2at 300 and 500 A﹒m-2when SDBS content was less than or equal to 10 mg﹒L-1.APs of Ti/PbO2-SDBS reached the summit of 1.89 V at 10 mg﹒L-1SDBS.However,in the cases of co-modified Ce and SDBS,APs of all Ti/PbO2-Ce-4-SDBS at all fixed current densities were higher than those of Ti/PbO2except for those with SDBS content was less than or equal to 10 mg﹒L-1at 100 A﹒m-2.Moreover,their APs increased with SDBS contents until reaching the maximum at 40 mg﹒L-1(Fig.5(c)).Though AP of Ti/PbO2-SDBS peaked at 10 mg﹒L-1SDBS,Ti/PbO2-Ce-4-SDBS-40 exhibited the highest APs at 40 mg﹒L-1SDBS.The phenomenon suggests a possible synergistic role between Ce and SDBS.Ti/PbO2-Ce-4-SDBS-40 with the highest AP was selected for the following tests.

3.2.2.CV curves

Fig.6.CV curves of different content for Ti/PbO2 electrode:(a) PbO2-Ce,(b) PbO2-SDBS,(c) PbO2-Ce-SDBS.

What is depicted in the Fig.6(a) is the test result of cyclic voltammetry (CV) curves of Ti/PbO2,Ti/PbO2-Ce,Ti/PbO2-SDBS,and dual doped Ti/PbO2-Ce-2-SDBS.All Ti/PbO2-Ce displayed a larger closed area of the CV curve than that of Ti/PbO2,indicating Ce can increase the active surface area and improve the electrocatalytic performance of Ti/PbO2.Among Ti/PbO2-Ce electrodes,Ti/PbO2-Ce-4 has the best reversibility because of its minimum potential difference between the oxidation and reduction peaks.As for Ti/PbO2-SDBS,Ti/PbO2-SDBS-10 had a larger closed area of CV curve than Ti/PbO2(Fig.6(b)),showing the similar role of Ce:increasing the active surface area and improving the electrocatalytic performances of Ti/PbO2.As for dual doped Ti/PbO2-Ce-SDBS,with Ce content fixed as 4 mmol﹒L-1and SDBS contents ranging from 5 to 50 mg﹒L-1,a reduction peak appears on all Ti/PbO2-Ce-SDBS when the potential is 0.9 V (vs.SCE),corresponding to PbO2→ PbO (Fig.6(c)).This phenomenon demonstrates a remarkable consistency with the results of a previous study [6].Especially,Ti/PbO2-Ce-4-SDBS-40 showed the largest reduction peak area and the best electrocatalytic properties.

3.2.3.EIS curves

The electrochemical impedance Nyquist plots of Ti/PbO2,Ti/PbO2-Ce,Ti/PbO2-SDBS,and Ti/PbO2-Ce-SDBS were depicted to evaluated their electron transfer resistances between the solution and the electrode (Fig.7(a)-(c)).As can be seen from Fig.7(a),all Nyquist plots appeared as semicircles,revealing a good conductivity.Among them,Ti/PbO2-Ce-4 exhibited the smallest radius of curvature,meaning the best conductivity and the role of Ce improving the activity of the coating).In the case of Ti/PbO2-SDBS,Ti/PbO2-SDBS-10 displayed a smaller radius though the radii of curvature of Ti/PbO2-SDBS-20 and Ti/PbO2-SDBS-5 were larger than Ti/PbO2(Fig.7(b)).It also demonstrates that 10 mg﹒L-1SDBS added to the plating solution can enhance the activity of the coating.All dual doped Ti/PbO2-Ce-4-SDBS showed larger radii of curvature than Ti/PbO2with SDBS addition ranging 5–50 mg﹒L-1(Fig.7(c)).And Ti/PbO2-Ce-4-SDBS-40 has the smallest radius of curvature.Generally speaking,the best Ce and SDBS concentration added to the electrode was optimized as 4 mmol﹒L-1and 40 mg﹒L-1,respectively.

3.3.Hydroxyl radical generation of different electrodes

·OH radicals can degrade pollutants effectively.Hence,·OH radicals generation capacity could serve as a prominent indicator for evaluating the performance of an anode.The capacity partly relies on the surface hydrophilicity of anode which could be evaluated by water contact angle[7,26].The hydrophobic surface of the anode is conducive to release·OH radicals,making it effective to degrade pollutants.Water contact angles of Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10,and Ti/PbO2-Ce-4-SDBS-40 were measured with Ti/PbO2as contrast sample.Their contact angles were 93.4°,87.2° and 111.2° respectively,and were much higher than the contact angle of Ti/PbO2:82.9° (Fig.8).The result indicates that SDBS and Ce in the electroplating bath can largely improve the surface hydrophobic property of PbO2coatings.Thus,it could be inferred that the high hydrophobicity of Ti/PbO2-Ce-4-SDBS-40 will result in significant growth in·OH radicals generation in the electrochemical oxidation process.

The fluorescence spectra of Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10,and Ti/PbO2-Ce-4-SDBS-40 were measured.The fluorescence intensities at 425 nm of them were compared with the influence of the electrolytic time (Fig.9).Ti/PbO2-Ce-4-SDBS-40 gave off the highest fluorescence intensity,which means it has an excellent·OH radicals generation capacity.The fluorescence intensity is also related to the electrolysis time when the degradation time follows the zero-order reaction kinetics.The rate constants of·OH generation were 0.4713,2.9930,0.9460,and 5.1215 min-1for Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10,and Ti/PbO2-Ce-4-SDBS-40,respectively.

3.4.Electro-catalytic degradation of RhB

Fig.7.EIS curves of different content for Ti/PbO2 electrode:(a) PbO2-Ce,(b) PbO2-SDBS,(c) PbO2-Ce-SDBS,(d) the equivalent circuit of the AC impedance spectroscopy.

Fig.8.Contact angle measurements of the Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10 and Ti/PbO2-Ce-4-SDBS-40 electrodes.

Fig.9.Fluorescence spectra as a function of treatment time (a) Ti/PbO2,(b) Ti/PbO2-Ce-4,(c) Ti/PbO2-SDBS-10 and (d) Ti/PbO2-Ce-4-SDBS-40 in presence of 0.5 mmol﹒L-1 terephthalic acid,0.25 mol﹒L-1 Na2SO4 and NaOH 0.5 g﹒L-1.

Fig.10.Uv–vis spectra as a function of treatment time (a) Ti/PbO2,(b) Ti/PbO2-Ce-4,(c) Ti/PbO2-SDBS-10 and (d) Ti/PbO2-Ce-4-SDBS-40 in presence of 0.125 mmol﹒L-1 Na2SO4 and 25 mg﹒L-1 RhB.

The removal of RhB is used to evaluate the electrocatalytic activities of the electrodes with the time-dependent UV–Vis spectra (Fig.10).As time went by,the absorbances of UV–Vis spectra gradually decreased.After 220 min electrolysis,as is shown in Fig.11(a),the RhB removal rates of Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10 and Ti/PbO2-Ce-4-SDBS-40 were 44.0%,87.4%,68.3%and 88.0%respectively.Fig.11(d)indicates that the degradation effect of Ti/PbO2-Ce-4-SDBS-40 is the best.The kinetic rate constants (k) of RhB degradation were 0.00316,0.01153,0.00629,and 0.01216 min-1for Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10 and Ti/PbO2-Ce-4-SDBS-40 respectively,showing the degradation of RhB following pseudo-first-order reaction kinetics model(Fig.11(b)).Furthermore,Ti/PbO2-Ce-4-SDBS-40 performs better electrocatalytic activity than others.Time-dependent COD was used to check the RhB removal(Fig.11(c)).After 220 min electrolysis,the COD removal efficiencies are 42.56%,53.81%,44.90% and 69.39% for Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10 and Ti/PbO2-Ce-4-SDBS-40 respectively.Similarly,Ti/PbO2-Ce-4-SDBS-40 showed the best removal efficiency.The higher COD removal efficiency comes from the high production and utilizing of·OH radicals,fast electron transfer,and large active area for Ti/PbO2-Ce-4-SDBS-40.

3.5.Electrochemical stability

For an anode in wastewater treatment,the re-utilizing and stability are vital indicators.The stabilities of Ti/PbO2,Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10,and Ti/PbO2-Ce-4-SDBS-40 were tested by repeating the RhB electrocatalytic degradation five-time,and the correlations are shown in Fig.12(a).The RhB removal efficiencies are highly re-utilizing after five cycles without an obvious decrease in the activities of all electrodes.As is shown in Fig.12(b),the accelerated life tests were carried out in 1 mol﹒L-1H2SO4solutions at an applied current density of 1 A﹒cm-2.The different anodic cells had varied potentials.In view of the instability of the outermost layer of β-PbO2,the cell potential initially decreased slightly and then gradually increased.It was found that the accelerated life of Ti/PbO2-Ce-4,Ti/PbO2-SDBS-10 and Ti/PbO2-Ce-4-SDBS-40 electrodes were 27 min,37 min,and 155 min respectively,showing longer accelerated life than that of Ti/PbO2(25 min).Especially,Ti/PbO2-Ce-4-SDBS-40 showed the longest accelerated life,about 6.2 times longer than Ti/PbO2.As is described above,with the addition of Ce and SDBS,smaller crystals and denser film on lead could produce the compact morphology of Ti/PbO2-Ce-4-SDBS-40.Hence,the permeability of the electrolyte into the PbO2coating is prevented by the compact morphology and the hydrophobic nature of Ti/PbO2-Ce-4-SDBS-40,which contributes to the longer accelerated life.The SEM image of the electrode surface after failure is shown in Fig.13(a)-(d),which is rough as is compared with Fig.3.Compared with Fig.3,the electrode surface after failure became rough.The anodic film became loose and porous,and the particles became smaller.There are still unconsumed PbO2particles on the surface of the electrode after failure,indicating that the electrode is gradually consumed,not directly detached.According to the powder diffraction pattern,the lead sulfate is the main component of the electrode surface after failure (Fig.13(e)).The results of EDS showed that the proportion of S and O elements in the failed electrode increased obviously,and lead sulfate was formed on the surface of the electrode (Fig.13(f)),which was in good agreement with PXRD.The main reason for failure is the gradual thinning of the plating as the electrolysis time increased until all the β-PbO2was gone on the electrode surface.It could be concluded that the thinning of the plating was the main cause of the electrode failure.

Fig.11.(a)Variation of RhB removal efficiency with time,(b)first order kinetics fitting curves during electrochemical oxidation,(c)variation of COD removal efficiency with time,(d) The photos of degradation in 0–220 min with a step of 40 min.

Fig.12.Anode potential variation with electrolysis time in accelerated life test.

4.Conclusions

A Ce and SDBS co-modified Ti/PbO2electrode (Ti/PbO2-Ce-SDBS) was synthesized and optimized for electrochemical oxidation treatment of wastewater.After the tests,it is proved that the introduction of Ce and SDBS can tune the electronic structure and interface property of PbO2,thus to refine PbO2crystals and make the PbO2coating more compact.With a comparison of Ti/PbO2,Ti/PbO2-Ce,and Ti/PbO2-SDBS,the optimized Ti/PbO2-Ce-4-SDBS-40 demonstrated the highest·OH radicals generation rate and the longest accelerated life.In the electrocatalytic oxidation of RhB,the electrodes oxidized and degraded RhB follows the order of Ti/PbO2-Ce-4-SDBS-40 >Ti/PbO2-Ce-4 >Ti/PbO2-SDBS-10>Ti/PbO2.Ti/PbO2-Ce-4-SDBS-40 reveals higher oxidation capacity than Ti/PbO2,Ti/PbO2-Ce,and Ti/PbO2-SDBS.After treated with electrolysis of 220 min at the Ti/PbO2-Ce-4-SDBS-40,the RhB,and COD removal efficiencies are about 0.880 and 0.694 respectively,which are manifestly higher than those of the other electrodes.The RhB removal efficiency of Ti/PbO2-Ce-4-SDBS-40 is highly reproducible after five cycles.In brief,Ti/PbO2-Ce-4-SDBS-40 is an effective electrocatalyst for RhB removal.

Fig.13.SEM of surface coating after failure:(a) Ti/PbO2,(b) Ti/PbO2-Ce-4,(c) Ti/PbO2-SDBS-10,(d) Ti/PbO2-Ce-4-SDBS-40,(e) PXRD of the spent electrodes,(f) EDS of the spent electrodes.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Thanks to the financial support from the Science and technology project of Shaanxi Province (2017ZDXM-GY-041).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.09.044.