Pb-0.6%Sb/α-PbO2/β-PbO2-MnO2 Electrode Electrodeposited In Methanesulfonic Acid With Application To The Electrocatalytic Degradation of AYR Wastewater

In the present work, a novel Pb-0.6%Sb/α-PbO 2 /β-PbO 2 -MnO 2 composite electrode with high electrocatalytic activity was obtained by electrodeposition in methanesulfonic acid and further investigated in the electrochemical degradation of alizarin yellow R(AYR) wastewater. The selection of temperature ranges was found to cause a quantitative difference in the formation of α/β-PbO 2 phases. In this way, both phases were simultaneously electrodeposited in the same methanesulfonic acid, and electrodes with corresponding proportions of phases were fabricated. Furthermore, performance tests indicated that composite electrodes with the most appropriate corresponding proportions of phases co-deposited with proper amount of MnO 2 could obviously improve the COD removal eciency and degradation eciency of AYR to 78.1% and 80.3%. They also showed commendable recyclability and ne economic applicability. Ultimately, the paper proposed a proper electrocatalytic degradation pathway of AYR based on the identication of the major intermediate products. The results proved that MnO 2 -co-doped composite electrodes had more promising application potential in the electrocatalytic degradation of AYR wastewater.


Introduction
Followed the development of modern electrochemical technology and environmental protection, more and more researchers are gradually paying close attention to the utilization of electrode coating materials [1][2][3]. Lead dioxide (PbO 2 ) is counted as such a kind of electrode coating material of superior performance and has been used in quite a few applications, especially in the wastewater treatment [4][5][6].
AYR wastewater is a typical representative azo dye wastewater, which has impacted the growth of aquatic creatures and human health for ages [7,8]. For the composition of benzene rings and azo bond of the AYR molecule, AYR wastewater is hard to be degraded. Former scholars' researches proved that electrocatalysis could be considered as an eco-friendly method in AYR wastewater treatment due to its high e ciency and little pollution [9,10]. Based on this, the application of PbO 2 composite electrode in electrocatalysis, where reactive oxygen species are produced to degrade organics in e uent, is naturally recognized as of particular interest. However, previous studies mainly focus on PbO 2 coatings obtained in situ on lead or lead alloys. Even later researches were only limited to single or stochastic phase structures of PbO 2 . All these electrodes were facing the problem of easy corrosion or other ine ciencies problems [11][12][13]. It is urgent to make full use of the phase structures composition of α-and β-PbO 2 that separately show corresponding performance characteristics to nally possess longer service life and better removal e ciency of organic pollutant in AYR wastewater treatment.
Among the current preparation methods of PbO 2 , electrodeposition has always been favored for its advantages in obtaining stable coatings of different phase structures with minimum pollution [14,15]. Up to now, α-and β-PbO 2 are mainly obtained in alkaline and acid solutions separately by electrodeposition.
Orthorhombic α-PbO 2 is relative compact and can promote longer life cycles of electrodes. Whereas tetragonal β-PbO 2 is porous and provides more active surface area [16]. It's rare to simultaneously prepare both phase structures of PbO 2 in the same solution, as well as the crystal proportion be quantitatively controlled. Through a large number of researches and experiments, our previous study has worked out the problem and put forward a method of preparing PbO 2 coating materials composed of two phase structures in the same methanesulfonic acid (MSA) solution by electrodeposition, which not only avoids the di culty of preparing different phase structures in separate solutions, but also makes the PbO 2 coating materials performance more stable since MSA is an environmentally friendly electrolyte for its chemical stability and excellent solubilisation of metal salts [17]. Based on this, the further promotion on chemical stability and oxidation activity of PbO 2 coatings by introducing active particles (MnO 2 , CNTs etc.) appears to be of interest to practice [18][19][20].
In this connection, herein we rst presented a thorough study of the temperature in uence on the nucleation and growth process of α-or β-PbO 2 obtained by electrodeposition in MSA solutions so as to judge the guiding factors for obtaining different crystal forms of PbO 2 and prepare anodes with ideal crystal ratio. Afterwards, the co-deposition of MnO 2 was realized and its content in uence upon Pb- During the whole nucleation process, the growth of PbO 2 particles was closely related to the diffusive transport of metal ions controlled by the current density across the electrodes that directly determined the overpotential of the solution. All these factors were in uenced by the experimental conditions [21]. It should be noted that neither of the two modi cations of PbO 2 were fully stoichiometric and initial crystal forms could be converted to each other to some extent [22,23]. Compared to MSA and lead(II) methanesulfonate concentrations, temperature and current density showed more obvious effect on the overpotential of the solution thus on PbO 2 nucleation process [24]. When maintaining overpotential at a lower range, Pb(II) and OH ads could continuously and stably react over the electrode surface. As particles had similar deposition trails, the differences of particle size and morphology mainly depended on the nucleation moment. A uniform and compact α-PbO 2 lm was easy to obtain then. However, the increase of overpotential would lead to the increase of charge transfer, which directly caused the lack of Pb(II) and OH ads around growing particles. New particles then tended to form on the growing particles, thus the transition and proliferation of lead dioxide were easier to happen in a 3-dimensional growth way. Comprehensively, particles obtained always rst accomplished the full growth of metastable α-PbO 2 and then prefer to be the growth of porous β-PbO 2 ( Fig. 1) [25]. The key step to separately obtain either phase form was to effectively control the overpotential.
Through a large number of experimental studies, our researches found that keep the solution concentration and current density stable while control the temperature at 25 ℃ could continuous and stable supply of Pb(II) and OH ads be ensured to get relative pure α phases. Besides, the temperature of 60 ℃ was discovered to be an appropriate boundary of obtaining relative better pure β phases [26]. Since the increase of current density could cause the change of magnetic eld force in the solution, two different phase forms of PbO 2 prefer to be obtained more stably by controlling the temperature limit.
Having in mind that a proper proportion of the crystalline composition was essential, MnO 2 were further optimized by ultrasonic dispersion and then used in the co-deposition with β-PbO 2 to promote the chemical stability and oxidation activity of PbO 2 coatings.
3.2. Electrochemical behavior of Pb-0.6%Sb/α-PbO2/β-PbO2-MnO2 electrodes 3.2.1 Surface topography analysis As follows from the SEM data, PbO 2 obtained in the rst 30 s was a set of large polycrystalline blocks with a slightly pronounced predominant orientation (Fig. 2a). While the co-deposition of MnO 2 allowed one to obtain PbO 2 (Fig. 2e), of which the surface tended to be more compact and the crystalline size was evidently reduced.
In Fig. 2, within 30 s after the beginning of the co-deposition process, the nucleation and growth of β-PbO 2 were the main reactions that carried out on the surface of the composite anode. Since the conductivity of MnO 2 active particles were not that ideal [27][28][29], they hadn't been adsorbed on the surface of composite anode then. After 2 min, MnO 2 active particles began to co-deposited on the surface of the composite anode, which agglomerated in a small range under the action of electric eld force, providing nucleation sites for the deposition of PbO 2 in the system. As the co-deposition proceeded on a constantly renewable surface, complex particles could be adsorbed on the surface of the growing oxide, which were probably involved in the oxidation of Pb(II) and thus lead to different effects, in particular reduced crystal size. The heterogeneous nucleation rate was thus accelerated and nucleation growth of new PbO 2 squints towards the sides around MnO 2 active particles. When the deposition process reached 5 min, PbO 2 around MnO 2 active particles had grown larger and new formed PbO 2 grains were re ned as well. When they grew to a certain extent, original MnO 2 active particles were covered and new MnO 2 were continuously adsorbed around them. The continuous diffusion co-deposition and growth of the two particles were realized alternately, till most MnO 2 were co-deposited into the β-PbO 2 deposition layer. The surface of the composite anode was compact and smooth then.
As been suggested from the EDS data in Fig. 3a, the agglomeration of MnO 2 active particles at point 1 was not as obvious as that at point 2. Moreover, the proportion of Pb atoms at point 1 was higher than that at point 2, while the proportion of Mn atoms was opposite, which was consistent with the results of SEM analysis in previous studies, indicating that the effective dispersion and adsorption of MnO 2 active particles could provide more nucleation sites for PbO 2 deposition. In Fig. 3b, point 1 and 2 separately represented MnO 2 active particles exposed outside and coated inside the β-PbO 2 grains. The increase of Pb and Mn atoms proportion showed that MnO 2 had been effectively coated in the β-PbO 2 grains.
Besides, there was a slight raise of Pb proportion at point 1 and 2, which proved that the β-PbO 2 deposition layer was also growing and thickening then. In Fig. 3c, the adsorption position of MnO 2 was higher at point 1 and lower at point 2. It could be found from the Pb and Mn atoms proportion that PbO 2 grains seemed to grow preferentially around MnO 2 active particles close to the surface of the matrix. Also the proportion of Pb atoms then was lower than that at 5 min, which suggested the nucleation and growth of β-PbO 2 deposition layer had become slow.

Electrochemical performance test
The data in Fig. 4 was combined data from studies, which comprehensively analyzed the effects of MnO 2 co-deposition on the performance of composite anodes by separately presenting the anodic polarization curves, EIS curves, Tafel curves and accelerated service life curves.
As could be seen from Fig. 4a that the oxygen evolution potential of the co-deposited composite electrode (1.79 V vs. SCE) was higher than that of Pb-0.6%Sb/α-PbO 2 /β-PbO 2 electrode (1.77 V vs. SCE), which demonstrated that the co-deposition of MnO 2 could increase the electrochemical oxidation e ciency of organic pollutants by inhibiting the oxygen evolution reaction. The Nyquist diagrams in Fig. 4b illustrated the same conclusion, of which the radius decreased as MnO 2 co-deposited. Theoretically, the radius semicircle objectively re ected the charge-transfer resistance. A smaller radius semicircle represents the lower oxygen evolution overpotential and electron transfer resistance of composite anode. The codeposition of MnO 2 had effectively achieved the decrease and promoted the charge transfer. In Fig. 4c, the corrosion potential of the composite anodes were compared, between which the one was higher when MnO 2 was co-deposited. It should be noted that as the ductility of MnO 2 was poor [29], an excessive codeposition of MnO 2 may result in the self-corrosion area of the composite anode increase correspondingly and should be avoided in the optimization of degradation AYR wastewater conditions. The observed effect in Fig. 5 indicated that MnO 2 content in composite anodes has a great effect on the degradation e ciency of AYR. With the increase of MnO 2 content from 10 to 40 g·L − 1 , the COD removal e ciency signi cantly increased from 53.6-78.1% after 150 min (Fig. 5a), while the degradation e ciency of AYR increased from 54.4-80.3% (Fig. 5b). By further observing the growth rate could we summarized that both two kinds of e ciencies changed slightly in the presence of a higher MnO 2 content of more than 30 g·L − 1 , indicating that excessive MnO 2 was not entirely conducive to the degradation of pollutants. Moreover, the COD removal e ciency and degradation e ciency of AYR could still be reached up to 72.3% and 73.4% after ten successive cycles of electrocatalytic degradation, which indicated that Pb-0.6%Sb/α-PbO 2 /β-PbO 2 -MnO 2 electrodes had commendable recyclability and ne economic applicability.
3.3.2. Degradation mechanism of AYR on Pb-0.6%Sb/α-PbO 2 /β-PbO 2 -MnO 2 electrode During the electrochemical degradation process, series of precursor-product relationships of the intermediates were emerged. Theoretically, persulfate ions were generated via direct electron transfer of and then activated to create sulfate ,which could rapidly attack oxidisable compounds including organics and inorganics [30]. Besides, vast amounts of ·OH were generated from the reaction between and water to accelerate the electrochemical degradation of AYR [31].
Compared with , the degradation effect of ·OH tended to be more signi cant. Figure 6 showed the snail shaped UV-Vis spectra of composite anodes at different degradation time. The rapid peak value decrease of the maximum absorption band (360 nm) indicated that azo bond (-N = N-) had been gradually broken. Four possible degradation pathways were proposed according to different initial attacking positions, including C-N cracking, N-N cracking, denitration and decarboxylation.
In order to verify this, LC-MS was employed to identify the oxidization intermediates. The observed organic compounds indicated us three steps of organic degradation.
Step 1, CAN bond and NAN bond were cleaved into organic intermediates by ·OH [32].
Step 2, single-chain small molecules were formed from the organic intermediates by denitration and decarboxylation.
Step 3, these single-chain small molecules were rearranged by further oxidation, reduction, and partial mineralization until completely mineralized into CO 2 , H 2 O and inorganic ions [33]. The whole electrochemical degradation process could be simulated in Fig. 7.

Conclusions
In summary, Pb-0.6%Sb/α-PbO 2 /β-PbO 2 -MnO 2 composite electrode with high electrocatalytic activity was successfully prepared by electrodeposition in methanesulfonic acid and further applied in the electrochemical degradation of alizarin yellow R(AYR) wastewater. Keep the solution concentration and current density stable while control the temperature at 25 ℃ could continuous and stable supply of Pb(II) and OH ads be ensured to get relative pure α phases. Besides, the temperature of 60 ℃ was discovered to be an appropriate boundary of obtaining relative better pure β phases. Meanwhile, the co-deposition of MnO 2 allowed one to obtain PbO 2 layers, of which the surface tended to be more compact and the crystalline size was evidently reduced. With the increase of MnO 2 content from 10 to 40 g·L − 1 , the COD removal e ciency signi cantly increased from 53.6-78.1% after 150 min, while the degradation e ciency of AYR increased from 54.4-80.3%. Moreover, the COD removal e ciency and degradation e ciency of AYR still exhibited superior stability after ten successive cycles of electrocatalytic degradation. Based on the identi cation of the major intermediate products, the paper proposed a proper electrocatalytic degradation pathway of AYR. The results proved that MnO 2 -co-doped composite electrodes had more promising application potential in the electrocatalytic degradation of AYR wastewater.

Declarations Ethical Approval
All analyses were based on previous published studies,thus no ethical approval and patient consent are required.

Consent to Participate and Publish
All participants provided informed verbal consent. They all agree to publish.  Figure 1 Simulation diagram of PbO2 phase growth by electrodeposition in methanesulfonic acid   Effects of MnO2 co-deposition on the performance of composite anodes  Electrochemical degradation pathway simulation of AYR.