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mediates and carboxylic acids generated in the BA degradation were identified, and a ... pulse and cathodic current pulse, alternating the redox reactions over .... ing diffusion layer (negative concentration gradient) is formed in front ... sample (after grinding and polishing); small cracks appear in PbO⁠2 coatings due to the.
Journal of Electroanalytical Chemistry xxx (2018) xxx-xxx

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Journal of Electroanalytical Chemistry

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PbO2⁠ electrodes prepared by pulse reverse electrodeposition and their application in benzoic acid degradation Zhen Hea⁠ , Muhammad Dilawer Hayata⁠ , Saifang Huanga⁠ , Xingang Wangb⁠ , Peng Caoa⁠ ,⁠ ⁎⁠ a b

Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92010, Auckland 1142, New Zealand School of Materials Science and Engineering, Chang'an University, Xi'an, China

ABSTRACT

Keywords: Lead dioxide Pulse reverse electrodeposition Electrochemical oxidation Benzoic acid

In this work, PbO2⁠ electrodes were prepared by pulse reverse current (PRC) electrodeposition. The effect of different duty cycles was investigated, which confirms that PRC deposition is an effective technique to optimize PbO2⁠ electrodeposition. Surface morphologies and crystal structures of the PbO2⁠ electrodes were characterized, and the voltammetric study was conducted to elucidate their electrochemical behavior. PRC deposition markedly changed the crystal structural and morphologic features of the resulting PbO2⁠ electrodes, giving rise to the increased content of β phase and a rough surface morphology. High-performance liquid chromatography (HPLC) was employed to monitor the galvanostatic electrolysis of benzoic acid (BA) occurring over the PbO2⁠ electrodes. A pseudo first-order reaction was found to well fit these BA degradation reactions, and the highest degradation efficiency was observed for the PbO2⁠ electrode made from 95% duty cycle. Concurrently, main aromatic intermediates and carboxylic acids generated in the BA degradation were identified, and a probable pathway was proposed for the BA degradation course on the PbO2⁠ anode.

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ARTICLE INFO

1. Introduction

Electrochemical oxidation is a novel technique widely accepted in the field of wastewater treatment due to its superior benefits of compatibility, energy efficiency, versatility, and adaptability to automation [1]. This technique largely relies on the anode material where electrochemical oxidation reactions occur. By far, many types of anodes have been investigated and explored, such as boron-doped diamond (BDD), graphite, TiO2⁠ , RuO2⁠ , IrO2⁠ , SnO2⁠ and PbO2⁠ [2–4]. Among these candidates, PbO2⁠ has been extensively studied owing to its distinctive combination of chemical stability, catalytically activity, and affordability [5]. Recently, methanesulfonate electrotype has been employed in PbO2⁠ preparation, due to its great Pb2⁠ + solubility, biodegradability and chemical stability [6,7]. Sirés et al. [8] and Velichenko et al. [9] have studied the PbO2⁠ coatings made from methanesulfonate source, and confirmed the efficient and clean deposition process using this type of electrolyte. The PbO2⁠ preparation conventionally employs the method of galvanostatic anodic deposition. Recently, pulse electrodeposition was studied to improve PbO2⁠ electrodes [10–12]. For instance, Yao et al.



[10,11] developed suitable PbO2⁠ electrodes for organic pollutant treatment through the pulse electrodeposition. Ghasemi et al. [13] explored the pulsed current deposited nanostructured PbO2⁠ electrode. Apart from this, a novel deposition technique – pulse reverse current (PRC) deposition – has been scarcely studied in PbO2⁠ electrodeposition. In a PRC process, an applied periodic waveform consists of anodic current pulse and cathodic current pulse, alternating the redox reactions over electrodes. This influences the crystallization process, and the underlying mechanism primarily corresponds to the dissolution/re-deposition process occurring at the electrode/electrolyte interface. The PRC deposition is capable to tailor the physicochemical properties of the resulting deposits [14]. Adopted in extensive studies to fabricate the desired coatings [14–16], PRC displays several advantages such as the independently controlled parameters and replenishment of the diffusion layer [17]. Moreover, PRC is presumed to have additional influence in PbO2⁠ electrodeposition, and may contribute to the formation of suitable phase constituent. Based on the previous works by Li et al. [18] and Devilliers et al. [19], who investigated PbO2⁠ electrodes cycling in sulphuric acid solutions, the formation of β-PbO2⁠ is favored in the dissolution/re-precipitation process during the Pb(II) ↔ PbO2⁠ transformation.

Corresponding author. Email address: [email protected] (P. Cao)

https://doi.org/10.1016/j.jelechem.2018.01.044 Received 7 November 2017; Received in revised form 23 December 2017; Accepted 22 January 2018 Available online xxx 1572-6657/ © 2017.

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Inspired by these observations, we presume that the similar dissolution/ re-deposition in PRC may modify the phase composition of PbO2⁠ electrodes, and serve as a novel alternative to generate β-PbO2⁠ which is widely adopted in electrocatalytic fields due to its excellent conductivity and large surface area to volume ratio [20]. Therefore, it is important to elucidate the morphologic and crystal-structural modifications for PbO2⁠ electrodes as a result of PRC deposition. The primary objective of our work is therefore, to study the physiochemical properties for the PbO2⁠ electrodes produced from PRC deposition at different duty cycles. The electrocatalytic activity of the electrodes was examined on benzoic acid degradation. Benzoic acid (BA) is a common organic pollutant in chemical manufacturing, coking and dyeing waste effluents [21]. However, limited information is available in the literature regarding the BA electrochemical oxidation over a PbO2⁠ electrode. This work may shed light on optimizing the deposition process of PbO2⁠ electrodes by PRC, thus promoting their future scale-up applications in environmental engineering.

plied current was identical for both anodic and cathodic periods during each pulse, and can be calculated as: (2)

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where Ja and Jc are the anodic and cathodic current densities in each pulse, and J is the average current density applied in PRC deposition. The average current density in PRC deposition is a core parameter, determining the deposition rate, and equivalent to the applied current density in a direct current deposition process. In our case, both direct current (DC) and PRC depositions were conducted with an identical (average) current density (J) of 40 mA/cm2⁠ for 20 min. As indicated in Eq. (2), the anodic pulse requires an increased current density (Ja) to achieve the same anodic charge at a decreased duty cycle. The different duty cycles of 85%, 90%, and 95% were applied in order to study its effect on the PbO2⁠ deposition. The electrodeposited PbO2⁠ electrodes are categorized as PbO2⁠ -95%, PbO2⁠ -90%, and PbO2⁠ -85% accordingly. On the other hand, the PbO2⁠ electrode fabricated from direct current is labeled as PbO2⁠ -DC. After the deposition, the samples were immediately rinsed with ethanol. In Scheme 1, the schematic of pulse reverse current waveform for PbO2⁠ electrodeposition is illustrated.

2. Experimental 2.1. Materials All chemicals were of analytical grade and used without further purification. Most of the chemicals were purchased from Sigma-Aldrich, namely tin chloride, antimony oxide, isopropanol, trifluoroacetic acid, methanesulfonic acid, lead methanesulfonate and hydrochloric acid. The other chemicals were purchased from J.T. Baker. The high-purity titanium coupons-used as the substrate-were cut into a thickness of 1 mm. Aqueous solutions were freshly prepared using ultrapure water.

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2.3. Characterization of PbO2⁠ electrodes The micromorphology of the Ti/SnO2⁠ -Sb/PbO2⁠ electrodes was characterized by scanning electron microscopy (SEM, FEI, USA). The crystal structure was determined with a Bruker X-ray diffractometer (XRD, D2 Phaser, Germany) machine with the Cu-Kα (λ = 1.54056 Å) incident radiation. XRD patterns of the synthesized PbO2⁠ coatings were recorded in the 2θ range between 20° and 80° at a scanning rate of 0.02°/s. Cyclic voltammetry (CV) measurements were carried out using a CHI604D (Chenhua, China) electrochemical workstation. A typical three-electrode configuration was employed for electrochemical measurements. The PbO2⁠ electrode was used as the working electrode, a platinum sheet was the counter electrode, and a saturated calomel electrode (SCE) was the reference electrode. The CV tests were conducted at room temperature. The accelerated service lifetime of the PbO2⁠ electrodes was investigated in an electrolyte of 1 M H2⁠ SO4⁠ solution at room temperature. A titanium plate was used as the cathode, and the current density was selected as 500 mA/cm2⁠ . During the lifetime tests, the cell potential was

2.2. Preparation of lead dioxide electrode

The Ti/SnO2⁠ -Sb/PbO2⁠ electrode was made up of three layers: a pre-treated titanium substrate inner layer, thermally deposited SnO2⁠ -Sb interlayer, and an electrodeposited PbO2⁠ outer layer. Firstly, Ti coupons were pre-treated by grinding and ultrasonic cleaning, followed by chemical etching in boiling oxalic acid (10%) for 1 h. After several washes by ethanol, the Ti coupons were used (henceforth, pre-treated substrate). A SnO2⁠ -Sb interlayer was then coated on titanium substrates. The as-prepared Ti substrate was firstly soaked for 10 s in a precursor solution (made up of 14 mL C3⁠ H8⁠ O, 10 mL concentrated HCl, 0.12 g Sb2⁠ O3⁠ and 1.6 mL SnCl4⁠ ), followed by subsequent heating in a muffle furnace at 100 °C for 10 min and at 500 °C for another 10 min. This procedure was repeated six times before the final annealing at 500 °C for 1 h. Many published reports have confirmed the significantly enhanced long-term stability with the presence of the SnO2⁠ -Sb interlayer [22–24]. The anodic deposition of PbO2⁠ was performed in an electrolyte of 1.0 M Pb(CH3⁠ SO3⁠ )2⁠ + 0.5 M CH3⁠ SO3⁠ H. A typical bipolar beaker cell was employed in electrodeposition with two parallel electrodes at a constant inter-electrode gap of 15 mm. The Ti/Sb-SnO2⁠ substrate (30 mm × 10 mm working area) was the anode, and a copper sheet was the cathode. A water bath was used to control the temperature at 65 °C, and the agitation was triggered by a PTFE stirrer at 300 rpm. As a critical parameter during the PRC electrodeposition, the duty cycle in our anodic deposition is defined as follows: (1)

where TA⁠ is the anodic (forward) time, and TC⁠ is the cathodic (reverse) time. The full wavelength (i.e. TA⁠ + TC⁠ ) was set as 20 ms, which defines a constant frequency of 50 Hz. In our PRC deposition, the ap

Scheme 1. Waveform of pulse reverse current electrodeposition for lead dioxide.

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recorded periodically, and the accelerated lifetime was recorded when the cell potential steeply increased and surpassed 10.0 V.

Table 1 Crystallite sizes of PbO2⁠ electrodes deposited from DC and PRC with different duty cycles.

2.4. Electrochemical degradation of BA

Crystal sizes (nm)

PbO2⁠ -DC PbO2⁠ -95% PbO2⁠ -90% PbO2⁠ -85%

10 16 18 22

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The electrochemical oxidation of benzoic acid was performed in a beaker cell. The prepared PbO2⁠ electrode was used as the working anode, and a Ti sheet was used as the cathode. For each electrolysis test, 80 mL of BA solution (50 mg/L) was placed in the beaker cell with 0.1 M Na2⁠ SO4⁠ as the supporting electrolyte. The galvanotactic electrolysis was conducted at a current density of 150 mA/cm2⁠ at 25 °C with a spinning rate of 400 rpm. The concentration change of BA was monitored by a high-performance liquid chromatography (HPLC, Shimadzu, Japan) equipped with an Agilent TC-C18 column. The mobile phase was made up by 90% water (0.1% trifluoroacetic acid) and 10% acetonitrile (0.1% trifluoroacetic acid).

PbO2⁠ electrodes

cording to the Scherrer equation through full width at half maximum (FWHM) of the strongest diffraction peak [25]. It is clear that the application of PRC leads to an apparent crystallite growth. The PbO2⁠ -DC sample has the smallest calculated crystallite size of 10 nm, while the PbO2⁠ -95% sample shows a crystallite size of 16 nm. Further reducing the duty cycle increases the crystallite size and the largest crystallite size of 22 nm is observed in the PbO2⁠ -85% sample. In PRC deposition, an increased anodic current density is applied with the reduced duty cycle to offset the prolonged reverse current. In a DC deposition process, an increased current density usually helps to accelerate the nucleation process and results in smaller crystallite size [5,8]. This seems contradictory to our results. Such discrepancy may be attributed to the dissolution/re-deposition process occurring in PRC deposition. During the short cathodic period, the dissolution of deposited PbO2⁠ occurs at the electrode surface and gives rise to the aggregation of the dissolved Pb2⁠ + near the PbO2⁠ electrode surface. Therefore, a pulsating diffusion layer (negative concentration gradient) is formed in front of the PbO2⁠ electrode, as illustrated in Scheme 2. At the cathodic current, if an inert cathode were used, the Pb2⁠ + ions could be reduced to Pb cathodically. However, when the PbO2⁠ is the cathode, the reduction of PbO2⁠ to Pb2⁠ + dominates and the accumulation of Pb2⁠ + leads to the pulsating diffusion layer [26–28]. When the current is reversed to anodic, the sufficient Pb2⁠ + ions within this pulsating diffusion layer are attached and crystalize into PbO2⁠ at the electrode surface, significantly alternating the electrodeposition kinetics.

3. Results and discussion 3.1. Phase compositions and surface morphology

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Lead dioxide exhibits two polymorphic forms, namely orthorhombic α-PbO2⁠ and tetragonal β-PbO2⁠ . As shown in Fig. 1, the application of PRC during the deposition process has a substantial influence on the formation and stability of different phases of PbO2⁠ electrodes. The PbO2⁠ -DC sample is composed of a mixture of α- and β-PbO2⁠ , demonstrating two distinctly favored crystal planes of α (021) at 29.9° and β (301) at 62.5°. On the other hand, in the samples produced by the PRC electrodeposition, nearly pure β-PbO2⁠ is present. The PbO2⁠ -95% sample exhibits preferred orientations of β (101) and β (301) along with a weak α peak. Further reducing the duty cycle continues to alter the phase composition and results in different orientated crystal planes belonging to β phase (Fig. 1). For the PbO2⁠ -90% and PbO2⁠ -85% electrodes, a sharp peak belonging to β (110) can be seen at 25.5°, while the β (301) peak reduces at the same time. This modification pattern in phase constituent from PRC deposition can be conducive to the electrocatalytic fields where β-PbO2⁠ is more attractive owing to its better conductivity, good corrosion resistance, and a high overpotential for oxygen generation [20]. Table 1 presents average values of estimated crystallite size for different PbO2⁠ electrodes. The average crystallite size was estimated ac

Fig. 1. X-ray diffractograms of PbO2⁠ electrodes deposited from DC and PRC with different duty cycles.

Scheme 2. Concentration profile of Pb2⁠ + in the electrolyte during the cathodic current pulse; the dash line (—·—) indicates the original concentration gradient in the pulsating diffusion layer. 3

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ture of the pyramid- and rectangular-shaped morphology for the sample PbO2⁠ -85%. The presence of pulse current substantially transforms the surface morphologies of these PbO2⁠ electrodes. The morphologic modification depends on varied electrodeposition conditions, and is correlated to the crystal structural transition [7,8]. In our experiments, the open and ordered microstructure corresponded to those nearly pure β-PbO2⁠ electrodes prepared from PRC deposition; on the other hand, the flat and rice-shaped surface was observed for PbO2⁠ -DC comprising a mixture of α and β-PbO2⁠ . The cross sections of the PbO2⁠ coatings were examined. As depicted in Fig. 3a, the average thickness of the SnO2⁠ -Sb interlayer is ~2 μm. Also, it is clear that the PRC deposition can uniformly deposit the PbO2⁠ layer of ~60 μm thickness on the Ti/SbO2⁠ -Sb substrate, Fig. 3b. This is further confirmed in Fig. 4a, which shows a compact array of vertically grown PbO2⁠ crystals for PbO2⁠ -95%. On the other hand, PbO2⁠ -DC exhibits a rather fine microstructure with a similar coating thickness, Fig.

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The above crystal structural data validates our earlier assumption that the dissolution/re-deposition process leads to the formation of β-PbO2⁠ , and concurs with previous reports of the diminished content of α phase in the outer layer of PbO2⁠ electrodes cycling in acid solutions [18]. It is speculated that both α and β phases at the electrode surface are reduced into Pb2⁠ + and dissolve in the methanesulfonate electrolyte during the cathodic current. In the following anodic current period, dissolved Pb2⁠ + is preferably re-precipitated into β-PbO2⁠ on the electrode surface. Fig. 2 presents surface morphologies of PbO2⁠ deposits made from different duty cycles. It can be observed larger crystals form when the duty cycle decreases. This coincides with our analysis above. The flat and smooth surface morphology with rice-like crystallites is clearly visible in the case of PbO2⁠ -DC, Fig. 2a. However, when a reverse current is applied, the flat surface is replaced by pyramid-shaped morphologies for the samples PbO2⁠ -95% and PbO2⁠ -90% (Fig. 2b and c), and a mix

Fig. 2. Surface morphology of the PbO2⁠ electrodes deposited with different duty cycles: (a) DC and PRC with different duty cycles: (b) 95%, (c) 90%, and (d) 85%.⁠

Fig. 3. The cross-sectional microstructures of (a) the SnO2⁠ -Sb interlayer and (b) the PbO2⁠ –95% sample (after grinding and polishing); small cracks appear in PbO2⁠ coatings due to the sample grinding and polishing. 4

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Fig. 4. The cross-sectional microstructures of (a) the PbO2⁠ –95% sample and (b) the PbO2⁠ –DC; the debris were the broken pieces caused when the PbO2⁠ coatings were peeled off.

4b. It is noted that the measured coating thickness in both cases (DC- or PRC-deposited) is approaching the theoretical thickness of 64 μm [29], indicating a high current efficiency in the electrodeposition process.

These nearly pure β-PbO2⁠ electrodes are produced with the pyramidal surface, and this morphology type is proposed to possesses the large active area owing to its rough structure [8]. Compared with the flat surface morphology of PbO2⁠ –DC, the three-dimensional pyramid-shaped electrode illustrates an assorted topography comprising of numerous peaks and valleys [8]. In other words, the roughness of the PRC-processed PbO2⁠ is much greater than that of the DC-processed coating and therefore better electrocatalytic performance. Considering the similar threshold potential for OER coupled with the highest surface activity, it is speculated that the PbO2⁠ –95% electrode exhibits a higher resistance to oxygen generation, as compared to other examined electrodes, implying a more efficient organic electro-oxidation by using this PbO2⁠ –95% electrode. Table 2 summarizes the results from the accelerated life tests. It can be seen the PbO2⁠ – 95% electrode exhibits the highest lifetime in the examined electrodes. As suggested in the previous literature [32,33], a uniform and compact surface should guarantee a better stability performance. One can see that the PbO2⁠ –95% sample demonstrates a uniform and compact surface morphology free from cracks, pits or pores (Fig. 2b). Compared to other electrodes in Fig. 2, the surface microstructure of the PbO2⁠ – 85% sample exhibits the inferior uniformity with the existence of some cracks, and this could be correlated to its shortest tested life among the studied electrodes.

3.2. Electrochemical behaviors

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Fig. 5 presents the CV curves recorded over different types of PbO2⁠ electrodes. During forward scans, an upsurge of anodic current can be observed at ~1.8 V for all electrodes. This can be ascribed to oxygen evolution reaction (OER). It should be noted that all the studied electrodes have a similar high OER overpotential, meaning oxygen generation is inhibited during electrochemical oxidation reactions occurring on the electrode surface. As for cathodic branches, a broad reduction peak can be seen when the potential is lower than 1.3 V, corresponding to the transformation of PbO2⁠ to Pb(II). It is observed that the PbO2⁠ –95% sample demonstrates the highest anodic current density. Both the detected anodic charge and the cathodic peak areas decrease in the order of PbO2⁠ –95%, PbO2⁠ –90%, PbO2⁠ –85%, and PbO2⁠ – DC. This indicates the PbO2⁠ –95% electrode possesses highest catalytic activity owing to its large active surface area which is available for the generation of highly oxidizing hydroxyl (OH) radicals in electrolysis [30,31]. The superior surface activity of these PRC deposited PbO2⁠ electrodes is believed to originate from their different surface morphologies (Fig. 2).

3.3. Electrochemical oxidation of benzoic acid HPLC measurements were utilized to monitor the BA concentration during electrochemical oxidation courses over PbO2⁠ anodes. Fig. 6 demonstrates the decay of BA concentration after different periods of electrolysis according to the UV absorbance in the HPLC system. It can be seen that PRC deposition provides greater anodic oxidation ability against the organic pollutant than that of PbO2⁠ – DC. After the electrolysis for one hour, 75% and 68% of BA was degraded on the PbO2⁠ –95% and PbO2⁠ –90% electrodes respectively, while only 51% of BA was electrochemically oxidized on the surface of the PbO2⁠ – DC electrode. Moreover, the reduction in BA concentration with time on PbO2⁠ anodes is demonstrated from semi-logarithmic plots in Fig. 7. The presented Table 2 The accelerated service life of Ti/SnO2⁠ -Sb/PbO2⁠ electrodes prepared from DC and PRC with different duty cycles.

Fig. 5. Cyclic voltammograms of PbO2⁠ anodes deposited with different duty cycles; scan rate was 20 mV/s, supporting electrolyte was 0.1 M Na2⁠ SO4⁠ .

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PbO2⁠ electrodes

Accelerated service life

PbO2⁠ – DC PbO2⁠ – 95% PbO2⁠ – 90% PbO2⁠ – 85%

31 h 34.5 h 32 h 24.5 h

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In electrochemical oxidation reactions, hydroxyl radicals generate during water electrolysis at active sites on the electrode surface, and these hydroxyl radicals are considered as the pivotal oxidizing agent in organic oxidations and OER – the side reaction. The most accepted oxidation reaction model for organics is described below in equations:

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(1) (2)

Upon our analysis in above sections, as for the PbO2⁠ – 95% electrode, the larger number of surface active sites and greater resistance to oxygen evolution warrant the effective oxidation reactions for organic pollutants. Therefore, the PbO2⁠ – 95% electrode exhibits the best electrocatalytic performance, which is supported by our BA degradation tests displaying that PbO2⁠ – 95% has the greatest degradation ability. Moreover, other PbO2⁠ electrodes deposited from PRC also demonstrate better electrocatalytic ability than PbO2⁠ – DC electrode. Fig. 6. Benzoic acid concentration as a function of time during degradation over the PbO2⁠ anodes deposited from DC and PRC with different duty cycles.

3.4. Degradation mechanism

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The HPLC analysis of electrolyzed solutions was carried out to investigate degradation pathways during the BA degradation course. The electrochemical oxidation course over the PbO2⁠ −95% electrode was studied to detect the generated intermediates including aromatic products and carboxylic acids. All the recognized chemicals were precisely identified by matching the retention time of standard chemicals. Table 4 illustrates the main degradation products and their retention times based on the HPLC tests. The result shows that the aromatic intermediates produced during degradation process are 4-hydroxybenzoic acid (4-HBA), hydroquinone,1,4-benzoquinone, and catechol. On the other hand, the carboxylic by-products formed during the degradation process are maleic, fumaric, and oxalic acids. Fig. 8 demonstrates the concentration change of some primary intermediates during the electrolysis, including 4-HBA, hydroquinone, 1,4-benzoquinone and carboxylic acids. During initial electrolysis, concentrations of all intermediates surge, accompanied by a significant concentration drop of BA. The aromatic products of 4-HBA, hydroquinone, and benzoquinone are formed in the early degradation stage, and as the electrolysis process proceeds, their concentrations decrease. It is worth noting that this concentration abatement starts after electrolysis for 30 min, concurrently the concentrations of carboxylic acids increase (Fig. 8). Hence, the main reaction in the early electrochemical oxidation is the degradation of BA into aromatic products. This is followed by cleavage of the aromatic intermediates (the primary oxidation reaction), as indicated by the rapid increase in the concentration of carboxylic acids. Moreover, it should be noted that the carboxylic acids existed in the electrolyzed solution even after the complete removal of BA (Fig. 8). A similar phenomenon has been observed previ

Fig. 7. The corresponding kinetic analysis associated with the first-order reaction for BA on the PbO2⁠ anodes deposited from DC and PRC with different duty cycles; C: the BA concentration C0⁠ : the initial concentration.

plots reveal a linear relationship between ln(C/Co) and the degradation time, which implies pseudo-first-order kinetics for all electrochemical oxidation courses. Table 3 summarizes kinetics coefficients (k) and correction coefficients (r2⁠ ) for the BA degradation processes for the prepared electrodes. The first-order kinetics constant for PbO2⁠ – 95% electrode is appropriately twice as high as that of PbO2⁠ – DC, and the kinetics constant decreases in the following order: PbO2⁠ – 95% > PbO2⁠ – 90% > PbO2⁠ – 85% > PbO2⁠ – DC.

Table 4 Identification of main degradation intermediates resulting from the degradation reaction identified by HPLC over the PbO2⁠ – 95% electrode. The separation was performed at 25 °C with a flow rate of 1.0 mL/min, and the absorbance wavelength was 254 nm. Chemical name

Table 3 The electrocatalytic activity of Ti/SnO2⁠ -Sb/PbO2⁠ electrodes prepared by DC and PRC with different duty cycles; k is the kinetics coefficients, r2⁠ is the correction coefficient. PbO2⁠ electrodes

⁠ 1) k (h−

r2⁠

PbO2⁠ – DC PbO2⁠ – 95% PbO2⁠ – 90% PbO2⁠ – 85%

−0.0131 −0.0259 −0.0208 −0.0155

0.99705 0.99668 0.99691 0.99707

Benzoic acid 4-Hydroxybenzoic acid Catechol 1,4-Benzoquinone Hydroquinone Fumaric acid Maleic acid Oxalic acid

6

Molecular formula

m/z

Retention time/ min

C7⁠ H6⁠ O2⁠ C7⁠ H6⁠ O3⁠

122 138

30.61 7.56

C6⁠ H6⁠ O2⁠ C6⁠ H4⁠ O2⁠ C6⁠ H6⁠ O2⁠ C4⁠ H4⁠ O4⁠ C4⁠ H4⁠ O4⁠ C2⁠ H2⁠ O4⁠

110 108 110 116 116 90

6.42 5.05 3.24 2.33 2.09 1.81

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Fig. 8. Concentration change of primary intermediates during the BA degradation. The electrolysed solution was analyzed periodically every half hour.

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ously by Yang et al. [34] and Cong et al. [35]. Based on above analysis of the degradation intermediates, it can be concluded the hydroxylation of the aromatics is an important pathway during the early stage of degradation process; and later the cleavage of the aromatic ring becomes the main reaction generating carboxyl acids. A schematic of BA degradation over the PbO2⁠ – 95% anode is proposed in detail in Scheme 3. It is interesting to note that 4-HBA, instead of its ortho- and meta-isomers, is the prevalent aromatic intermediate in the electrolyzed solutions. The formation of 4-HBA in BA degradation was also reported previously in the literature [21,36]. In addition, we noted that phenol – a common intermediate during the oxidation of aromatic acids – was not detected as a degradation product in our case. Similar oxidation mechanism has been previously reported for the BA degradation pathways over a boron-doped diamond (BDD) electrode [37,38]. It is not surprising that PbO2⁠ and BDD electrodes have a similar degradation route, as both the two electrodes are considered as a typical non-active electrode with a relatively high OER overpotential. In these electrodes, a considerable number of hydroxyl radicals are generated on the electrode surface and deeply involved in the organic oxidation process.

Scheme 3. The schematic of BA degradation pathway on the PbO2⁠ electrode.

4. Conclusion

the cleavage of the aromatic ring being the primary reaction to generate carboxyl acids.

PbO2⁠ electrodes were manufactured by PRC electrodeposition with different duty cycles. Based on the XRD and SEM results, the application of PRC deposition significantly modifies crystal structure and surface morphologies of the PbO2⁠ electrodes. PRC deposition sharply increases the content of β phase in PbO2⁠ electrode, and creates a rough pyramid-shaped surface. The CV tests reveal a greater surface activity for the electrodes made from PRC deposition, and the PbO2⁠ – 95% electrode demonstrates the highest electrocatalytic ability among the tested electrodes. The degradation of benzoic acid over the PbO2⁠ electrodes follows the pseudo-first order kinetics. The PbO2⁠ – 95% electrode shows the best performance for the destruction of BA, and the BA removal after 3 h using the PbO2⁠ – 95% electrode was nearly 100%. The excellent BA degradation ability of the PbO2⁠ –95% electrode results from its more surface active sites and its greater resistance to oxygen generation. The HPLC analysis confirms the primary intermediates including the aromatic products and carboxylic acids. The hydroxylation of the aromatics is the main oxidation reaction for initial degradation, followed by

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