Abstract
The modified Ti/SnO2-Sb-Mn/Re electrodes were prepared using rare earth (Re) Gd, Eu, Ce, and Er and various molar ratios of tin and manganese by thermal decomposition. To investigate the electrocatalytic performance of electrodes, phenol was applied as a model pollutant. Phenol removal pursued pseudofirst-order kinetics in the experimental range. The experimental outcomes show that the phenol degradation at ~95% and ~97% was found in Mn (1 mol%)/Re and Mn (2 mol%)/Re electrodes after treatment for 140 min. Accelerated lifetime testing showed that the best-accelerated service life could be measured only in Mn (3 mol%)/Re and Mn (4 mol%)/Re compared with other prepared Mn (0%,1%, and 2%)/Re electrodes under the condition of 500 mAcm-2 current density in this study. The modified electrodes were then characterized, including oxygen evolution potential (OEP), crystal structure, and surface composition of the electrode coatings.
1. Introduction
Industrial wastewater has been causing genuine ecological and medical issues [1], and this problem cannot be solved by conventional procedures [2]. Various methods are applied to treat wastewater relying upon its purpose, including filtration [3], ultrasonic oxidation [4], biological treatments [5, 6], adsorption [7], coagulation [8], electrocoagulation [9], and electroflotation [10]. Regardless, these methodologies have a few drawbacks, for example, high living arrangement time required, utilization of explicit and costly chemicals, solid waste generation, and in some cases low effectiveness.
Electrochemical oxidation procedure stands apart as a clean innovation that creates modest quantities of squanders, being exceptionally reproducible and effectively controllable and permitting mechanization and moderately minimized plants of simple establishment [11, 12]. The stability and electrocatalytic properties of the anodic material was advanced with the development of dimensionally stable anodes (DSA), in which a metallic base of Ti is covered with some metal oxides, for example, RuO2, TiO2, and SnO2, among others [13, 14]. DSA show high electroactive zones because of their mud-split morphology, permitting the direct oxidation of organic pollutants on the electrode surface at low possibilities, and can advance the development of active intermediates to play out the indirect oxidation of pollutants [15]. The exhibition of a DSA is essentially impacted by the electrode material [16]. PbO2 [17], IrO2 [18], RuO2 [19], and SnO2 [20] have high oxygen overvoltage. Of these, the metal oxides SnO2 and PbO2 are believed to be unrivaled for the oxidation of phenol. SnO2 as a promising material and an n-type semiconductor has high oxygen overpotential; a suitable amount of Sb consistently is doped to improve the conductivity just as an electrocatalytic activity [21]. Including an interlayer, Ti/Sb-SnO2/PbO2 [22, 23], Ti/Sb-SnO2/MnOx [24], and Ti/IrO2/Sb-SnO2 [25], significantly draw out the service life of a Ti/Sb-SnO2 electrode [21]. The degradation efficiency is also reduced at different degrees.
Including an interlayer, Mn-doped Sb-SnO2 between the substrate and the Sb-SnO2 covering can enormously build the service life of the electrode [26, 27]. Rare earth has been exhibited to upgrade chemical catalytic processes by either acting as powerful oxidants or helping catalytic processes such as purification of vehicle exhaust gas [28–30]. The addition of some rare earth elements can enhance the electrocatalytic performance of the Ti/Sb-SnO2 electrode [31]. In the present research, the Ti/SnO2-Sb-Mn/Re electrodes were prepared with different tin/manganese molar ratios and four kinds of rare earth (Re) Gd, Eu, Ce, and Er by thermal decomposition. And their electrochemical performance was evaluated based on the degradation of phenol.
The modified electrodes were then characterized including X-ray diffraction (XRD), scanning electron microscopy (SEM), an energy dispersive spectroscopy (EDS), and cyclic voltammetry (CV) techniques.
2. Experimental
2.1. Titanium Sheet Surface Treatment
The dimensions of titanium sheets (, >99.6 purity, BaoTi Co. Ltd., China) were polished thoroughly with 320-grit abrasive papers. Then, they were degreased with mixed solution of acetone and 1 mol·L-1 NaOH ( 1 : 1) by the use of ultrasonic bath for 30 min After removing solid impurities and grease, they were subsequently etched in 10% oxalic acid at 98°C for 2 h to result in active rough surface on the Ti sheets. At last, they were thoroughly washed with deionized water. The pretreated Ti substrates lost their metallic sheen and were gray in color.
2.2. Preparation of SnO2-Sb-Mn Intermediate Layer
The interlayer was coated on the pretreated Ti sheets by the thermal decomposition method [20]. In the intermediate layer coating solution, 17.5 g SnCl4·5H2O (98%, Sigma Aldrich), 0.570 g SbCl3 (99.5%, Merck), and Mn (NO3)2 (molar ratios of Sn/Mn of 100 : 0, 100 : 1, 100 : 2, 100 : 3, and 100 : 4, respectively) were dissolved into 50 ml isopropanol with 2.5 ml hydrochloric acid. The pretreated Ti sheets were dipped in the coating solution and dried at 120°C for 5 min to evaporate the solvents. The dipping and drying were repeated 5 times, and the Ti sheets were heated at 550°C in a muffle oven for 20 min. After dipping, drying and heating were repeated 3 times and the electrodes were finally annealed at 550°C for 2 h.
2.3. Preparation of Rare Earth Outer Layer
For the outer layer coating solution, 17.5 g SnCl4·5H2O, 0.570 g SbCl3, 2.5 ml of concentrated (37%) HCl, and proper rare earth nitrates (molar ratio of Re/Sn of 2 : 100) were prepared in 50 ml isopropanol. The electrodes coated with Mn layers were dipped in rare earth coating solution and dried at 120°C for 5 min. After five times repeating of both dipping and drying, the Ti plates were heated in a muffle oven (550°C for 20 min) for coating pyrolysis. The annealing process as discussed earlier (dipping, drying, and pyrolysis) was repeated 3 times, and finally, the electrodes were annealed at 550°C for 2 h.
2.4. Characterization of Electrodes
The crystal structure of the electrodes was characterized by a Bruker D8 ADVANCE X-ray diffractometer (Germany) with Cu-Kα radiation with an operation voltage of 40 kV and current of 40 mA. A Zeiss Supra 55 instrument was employed to analyze the surface morphologies and EDS of the different electrodes. The electrochemical workstation (CHI 660E, Chenhua Instrument Shanghai Co. Ltd., China) with a conventional three-electrode cell was employed to execute cyclic voltammetry (CV) in the range of 0.0–2.5 V (vs. SCE) at a scan rate of 50 mV s-1 in 0.5 M Na2SO4 solutions. A platinum sheet was used as a counter electrode, and a saturated calomel electrode (SCE) served as a reference electrode. To characterize service lifetime and to reduce the experimental time, the accelerated life test was conducted in two-electrode system cell. The as-prepared electrodes were applied as the working electrode, and titanium plate as the counter electrode was placed at a 10 mm distance apart under 500 mAcm-2 controlled current density. The electrolyte solution 3 M H2SO4 was used, and the cell temperature was controlled at 40°C. The deactivated electrodes were assumed when the cell potential reached 10 V.
2.5. Phenol Degradation Test
Electrochemical degradation of phenol was performed in 100 ml glass beakers. For each cell, the as-prepared electrodes (4 cm2) () were used as the anode and the titanium sheet cathodes having the same area were placed at a 10 mm spacing distance. For a degradation test, 80 ml of a phenol solution with a predetermined concentration of 100 mg l−1 was placed in the cell with 0.1 M Na2SO4 as the electrolyte and the current density was controlled at 30 mAcm-2. The phenol removal rate was found to fit well the pseudofirst-order kinetics, and the degradation rate could be expressed by equation (2). All the electrochemical oxidation processes were carried out at 25°C for 140 min with a magnetic stirrer. The removal of phenol concentrations was determined by the 4-amino antipyrine spectrophotometric method. A UV2200 UV-VIS spectrophotometer (Ocean Optics, Dunedin, FL) was employed to record the absorbance of the liquid samples (1 ml) at the wavelength of 510 nm, which were withdrawn from the electrolytic cell at fixed time intervals to determine the variation of phenol concentration. The removal efficiency of phenol () was calculated as follows: where and are the initial and remaining concentrations of phenol, respectively. The concentrations were obtained from the standard curve.
3. Results and Discussion
3.1. Electrochemical Degradation Tests
The electrocatalytic activity of the different rare earth electrodes prepared with various Mn doping interlayer was investigated with degradation experiments (2.3 h electrolysis) by using the phenol solution as a target pollutant. The removal of phenol concentration (95%) can be reduced at a Mn (1%)/Gd electrode (Figure 1(a)), and the rate constant was higher than the other electrodes (Table 1). The removal efficiency (94%) was reached with a Mn- (1%) based Eu-doped electrode (Figure 2(a)). The interlayer doping concentration of Mn (2%) of a Ce electrode removed the phenol up to 97% (Figure 3(a)), and that of an Er electrode was 95% removal efficiency (Figure 4(a)), respectively. These conditions and removal rates were better than the Mn (0%) electrode and the other Mn various concentrations. The phenol removal rates were also fitted with pseudofirst-order kinetics (Tables 1 and 2) and the rate equation for the as shown in eqn. where is the initial concentration of phenol, is the concentration of phenol at given time , and is the kinetic rate constant.
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Among all of the rare earth electrodes as mentioned above, the kinetic rates were achieved at the Mn (1%) with Gd (Figure 1(b)), and that with Eu electrodes (Figure 2(b)) were 1.08 times and 1.27 times the rest of Mn (2%)/Ce (Figure 3(b)), and Er electrodes (Figure 4(b)) were 1.45 times and 1.2 times compared with no addition of Mn electrodes. According to the degradation tests, the Mn concentration of 1 mol% and 2 mol% was the proper amount for the inner layer. Therefore, the modified Mn layer-based rare earth electrodes significantly enhance the performance of electrochemical degradation of phenol.
3.2. Surface Morphology and Structure Analysis
SEM was utilized to characterize the morphology of the rare earth- (Gd, Eu, Ce, and Er) (Re-) doped SnO2-Sb-Mn anodes as shown in Figures 5–8. All of the images (Figures 5–8) which present the cracked-mud morphology were perceived as the aftereffect of thermal oxidation [20]. Manganese inserts into the Sb-SnO2 can improve resistance to the thermal stress of coating and construction of a more compact morphological structure which consists of microcracks and agglomerated particles [32]. Figures 9(a)–9(d) observed that the modified electrodes presented a rough structure with good attachments and smaller coating loss after 120 min phenol electrolysis. Formation of a nonconductive TiO2 layer due to the entering oxygen atoms resulting from the electrolysis process can facilitate change that can favor stripping off the coating layers [21]. The prepared oxide films showed demonstrated a progression of diffraction peaks identified with rutile-type SnO2 (PDF#41-1445) and Ti (PDF#44-1294) for the majority of the prepared Re-doped Ti/SnO2-Sb-Mn electrodes including various rare earth and different Mn contents, and the fundamental diffraction peaks were named (Figure 10). The Mn content increased from 1% to 4%; the intensities of the SnO2 diffraction peaks increased. In any case, the degradation abilities were decreased with including Mn amount of 1-4% as the result of these intensities increased. Thus, Mn addition of 1% for Gd and Eu and 2% for Ce and Er is ideal for improving the electrocatalytic capacity of all rare earth electrodes without well SnO2 crystal formation. There are no distinct diffraction peaks of Sb, Mn, and rare earth metal oxides in the XRD spectrums due to either the low doping level or the consolidation of the doping ions into the SnO2 unit cell. EDS confirmed the contents of Sn, Sb, rare earth (Re), and Mn in the Re-doped SnO2-Sb-Mn electrodes (Table 3). The expanded Sb concentration in the SnO2 cross sections is believed to be useful for adsorption of polar molecules and in this manner can upgrade the electroreactant limit of the electrodes [33].
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3.3. Cyclic Voltammetry Tests
The high oxygen evolution potential (OEP) of anodes benefits the degradation of phenol, because the OEP can control the formation of OH⋅ and O in the electrolyte during the electrolysis processes from forming oxygen and produced more and broaden the lifetime of the hydroxyl radicals on the anode [24], which is good to expand the rate of removal and the current efficiency [34]. Figure 11 represents the CV curves of various electrodes in 0.5 M Na2SO4 supporting electrolyte. As per the CV curves, the edge potential for oxygen evolution lies in Mn (1%) of Gd (2.0329 V) and that of Eu (2.2430 V), for Mn (2%) Ce (2.4915 V) and Mn (2%) Er (2.3788 V), respectively, higher than without the Mn/Re-doped electrode.
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3.4. Accelerated Life Tests
Not only the electrocatalytic activity but also sufficient electrode stability is necessary for the performance of the electrodes. The current density, temperature, and pH of the electrolyte are mainly related to the actual service lifetime. In the actual condition, the applied current density is low and difficult to evaluate the actual service life time. To ensure the best durable performance and reduce the test time, the accelerated lifetime test has been applied to evaluate the whole service life of the electrodes. An empirical relationship between the accelerated service life () and the actual service life () was proposed, which could assess the actual service life of [35]. where is a constant often equal to 2, is the actual current density, and is the accelerated current density. Figure 12 demonstrates that the accelerated life test results and the cell voltage varied with time for the prepared electrodes. The increased service life was observed in the electrode content with Mn (3% and 4%) compared with other electrodes. The rare earth Gd and Eu with interlayer Mn (2%) electrodes show that the service life is higher than those electrodes without the Mn layer but at Mn (1%) are lower as shown in Figures 12(a) and 12(b). Although the Mn (2%)/Ce electrode has great service life, the Mn (1%)/Ce was not having that condition compared with the Mn (0%)/Ce electrode (Figure 12(d)). As mentioned in Figure 12(c), the high life of the electrode was watched at the Mn (1%)/Er by comparing with the Mn (0%)/Er and Mn (2%)/Er electrodes. The four mechanisms can cause the electrode deactivation: metal base passivation, coating detachment, coating consumption, and mechanical damage [36]. The more significant explanation is that electrolyte responds with the Ti substrate and structures a layer of inactive film in the wake of penetrating the middle layer [37].
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4. Conclusions
The distinctive molar proportions of Sn/Mn and the doping influence of rare earth Gd, Eu, Ce, and Er were researched to uncover their impacts on the catalytic performance of the electrodes. The accelerated life test outcomes demonstrated that the service life was significantly higher in the Mn (3% and 4%) of all prepared rare earth electrodes than that of others. The best degradation rate of phenol was found in the Mn (1%) and (2%) doped Re electrodes compared with other Mn adding and no adding Mn-doped rare earth electrodes. XRD analysis investigation demonstrated the crystalline structure of the SnO2 coating on the surface of all the synthesized electrodes. However, due to the consolidation of the doping ions (Sb, Mn, and Re) into the SnO2 structure, it was unrealistic to recognize them through XRD. But EDS affirmed the inclusion of those doping elements. Based on these experimental results, the proper contents of Mn added between the substrate and the rare earth outer layer can enhance the performance of the electrodes.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
Zenghe Li, corresponding author, declares that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by Zenghe Li as a funder, corresponding author at Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, College of Chemistry, Beijing University of Chemical Technology, Beijing, 100029, China.