Titanium dioxide - activated carbon composite for photoelectrochemical degradation of phenol published by Ural Federal University eISSN 2411-1414; chimicatechnoacta.ru ARTICLE 2022, vol. 9(4), No. 20229416 DOI: 10.15826/chimtech.2022.9.4.16 1 of 11 Titanium dioxide - activated carbon composite for photoelectrochemical degradation of phenol L. H. Q. Anh a , Uyen P. N. Tran b , P. V. G. Nghi c, H. T. Le c, N. T. B. Khuyen d*, T. D. Hai e* a: Faculty of General Sciences, Ho Chi Minh city University of Natural Resources and Environment, Ho Chi Minh city 70000, Vietnam b: Faculty of Engineering and Technology, Van Hien University, Ho Chi Minh city 70000, Vietnam c: Faculty of Applied Science, Ton Duc Thang University, Ho Chi Minh city 70000, Vietnam d: Office of R&D and External Relations, Ho Chi Minh city University of Natural Resources and Environment, Ho Chi Minh city 70000, Vietnam e: Faculty of Environment, Ho Chi Minh city University of Natural Resources and Environment, Ho Chi Minh city 70000, Vietnam * Corresponding author: ntbkhuyen@hcmunre.edu.vn, tdhai@hcmunre.edu.vn This paper belongs to a Regular Issue. © 2022, the Authors. This article is published in open access under the terms and conditions of the Creative Com- mons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract In this study, titanium dioxide (TiO2) and titanium dioxide – activated carbon composite (TiO2–AC) were prepared by sol-gel method for pho- toelectrochemical (PEC) applications. Characterization of the materi- als was performed by scanning electron microscope, energy dispersive X-ray analysis, Fourier transform infrared spectroscopy, X-ray dif- fraction, and diffuse reflectance spectroscopy. The results show that TiO2 was successfully loaded on activated carbon (AC), producing TiO2–AC with 2.61 eV of bandgap energy, lower than that of TiO2 (3.15 eV). Photoanodes based on TiO2 and TiO2–AC were fabricated and applied to PEC experiments for phenol degradation. In comparison with the TiO2 photoanode, the TiO2–AC one exhibited superior photo- catalytic activity, which was indicated by a high current density and effective phenol removal. A mechanism of phenol PEC degradation on the TiO2–AC photoanode was proposed, which includes interaction be- tween protonated phenol and active sites bearing oxygen on the pho- toanode surface. A kinetic model according to this mechanism was also established and fitted to experimental findings, resulting in rate con- stants of elementary reactions. Keywords Photoelectrochemical Titanium dioxide Phenol degradation Photocatalyst Kinetic model Received: 17.09.22 Revised: 15.10.22 Accepted: 16.10.22 Available online: 25.10.22 1. Introduction Phenol and phenolic compounds are commonly used in pharmaceuticals, insecticides, cosmetics, and other indus- trial substances [1]. Phenol is known as a hazardous pollu- tant due to its toxicity and high stability for a long period of time in the environment [2, 3]. Phenol exposure may cause acute and/or chronic diseases on the skin, eye, res- piratory and nervous systems [4, 5]. There are some tech- niques for phenol removal from wastewater, as summa- rized in [6]. Among advanced oxidation processes, photoca- talysis is described as an effective choice for phenol degra- dation. Based on TiO2 photocatalyst synthesized by the sol- gel method, phenol degradation was attained at 58.8% af- ter 240 minutes of UV illumination [7]. In the presence of carbon, photogenerated charge recombination of TiO2 was delayed [8], suggesting an improvement in phenol degrada- tion. Synergy effect between TiO2 and carbon-based mate- rials was explored in the previous reports on multi-walled carbon nanotubes – TiO2 [9], activated carbon – TiO2 [10], graphene – TiO2 [11], and carbon fiber – TiO2 [12]. Difficult recovery and fast photoexcited electron-hole pairs recombination of catalysts are two considerable drawbacks of photocatalysis, which can be minimized with the photoelectrochemical (PEC) method [13, 14]. According to the PEC principle, a photocatalyst is coated onto a pho- toelectrode, to which a bias voltage is applied to improve photogenerated charge carrier separation, thus enhancing the activity of the photocatalyst [15, 16]. Therefore, the PEC method has attracted considerable interest in water split- ting [17] as well as organic pollutants degradation [13]. Us- http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2022.9.4.16 mailto:ntbkhuyen@hcmunre.edu.vn http://creativecommons.org/licenses/by/4.0/ https://doi.org/10.1039/D0FO02324H http://dx.doi.org/10.1088/1674-0068/22/04/423-428 https://doi.org/10.1016/j.apsusc.2009.12.113 https://doi.org/10.1021/ie501673v https://doi.org/10.1016/j.partic.2010.03.013 https://doi.org/10.1021/acsaem.1c02548 https://orcid.org/0000-0001-7855-1337 https://orcid.org/0000-0001-6038-3873 https://orcid.org/0000-0002-0103-8866 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2022.9.4.16&domain=pdf&date_stamp=2022-10-25 Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 2 of 11 ing a TiO2-based photoanode for the PEC experiment, phe- nol degradation achieved 73.76% after 120 minutes of UV illumination under 0.8 V of applied voltage [18]. By adding peroxymonosulfate into the PEC system, a photoelectrode based on Co3O4-loaded carbon fiber demonstrated 100% phenol degradation within 90 minutes under UV radiation at 1.5 V of applied voltage [19]. The contribution of carbon to PEC behavior of the C/TiO2 composite was explored by Haro et al. [20]. It was found out that carbon promoted charge transfer reactions on C/TiO2 photoelectrode surface through enhancement of charge carrier generation and sep- aration. However, the application of C/TiO2 in PEC degra- dation of organic compounds has not been reported in the literature. In this study, TiO2 and TiO2-activated carbon (TiO2–AC) composites were synthesized by a sol-gel method for PEC degradation of phenol. Effects of AC on PEC properties of the TiO2/AC photoanode for phenol were determined. More- over, a mechanism of phenol PEC degradation was pro- posed, revealing a kinetic model describing the surface re- sponses of the TiO2/AC photocatalyst. 2. Experiment 2.1. Materials, chemicals and apparatus Pure titanium tetrachloride (TiCl4) (99.99%) was pur- chased from Shanghai Aladdin Bio-Chem Technology Co., Ltd (China). Commercial activated carbon (AC), ethanol (99%), phenol (>99%), and hexane (>96%) were obtained from Xilong Scientific Co., Ltd (China). Hydrochloric acid (HCl, 36.5% w/w), sodium hydroxide (NaOH, >99%) were bought from Merck (Germany). Liquid polyester resin was collected from En chuan Chemical Industries Co., Ltd (Tai- wan). 1-Butyl-1-Methylpyrrolidinium hexafluorophosphate (BMIM FP6, 97.5%) was supplied by IndianMart. Tap water was used to prepare all solutions. Characterization of materials was conducted with ana- lytic methods such as scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) using Prisma E SEM, X- ray diffraction (XRD) recording by D2 PHASER, diffuse re- flectance spectra carried out by FL-1039 (HORIBA), Fou- rier-transform infrared spectroscopy (FT-IR) with Nicolet iS5, current − voltage (J–V) curve and electrochemical im- pedance spectroscopy (EIS) using MPG2 Biologic system. 2.2. Preparation of TiO2, TiO2–AC composite, and photoanodes 10 ml of TiCl4 and 10 g of AC were added into an Erlenmeyer flask containing 200 ml of hexane under N2 atmosphere in a glovebox. The Erlenmeyer flask was then covered and moved to an ultrasonic tank. The mixture was dispersed un- der 40 kHz of sonication for 15 minutes before adding 500 ml of distilled water. After 30 minutes of sonication, the solid phase was separated and washed with distilled water in a vacuum filtration system until the filtrate reached neu- tral pH. The obtained solid was thermally treated at 550 °C for 30 minutes to produce the TiO2–AC photocatalytic com- posite. To prepare the TiO2 photocatalyst, the above procedure was applied without AC. A coating mixture (consisting of 80% w/w of polyester, 19% w/w of ethanol, and 1% w/w of BMIM PF6) was de- posited onto a SUS 304 stainless sheet (0.8 mm of thick- ness, 100100 mm of dimension) as a photoanode substrate by the dip-coating method. Then, an abundant amount of photocatalyst was spread on the photoanode surface and pressed under 3 N/cm2 of pressure to create contact be- tween the photocatalyst and the photoanode substrate. The obtained photoanodes based on TiO2 and TiO2–AC photo- catalysts were stored at room temperature for one week be- fore being utilized in the PEC system. 2.3. Photoelectrochemical measurements A three-electrode cell was used for PEC measurements with a Pt grid as a counter electrode, an Ag/AgCl as a reference electrode, the photoanode as a working electrode, and a UV- C mercury lamp (9 W) vertically soaked in a solution. All PEC measurements were conducted under fluorescent light (400−600 lux of illumination) in a 10 mg/L of phenol solution, pH = 5 with UV and non-UV radiation. The J–V curves were recorded according to the linear sweep voltam- metry technique in a potential range of 0 V to +1.5 V at a scan rate of 50 mV/s. The EIS characteristics were obtained over a frequency range of 10 mHz to 10 kHz with 10 mV amplitude. 2.4. Photoelectrochemical degradation of phenol Bath experiments for PEC degradation of phenol under UV and non-UV illumination were performed in a stirred PEC reactor containing 1 liter of phenol solution. The Pt grid and photoanode were vertically dipped into phenol solution and connected to a controllable DC voltage source. Between the Pt grid and the photoanode, the UV-C lamp was also im- mersed in the solution. Phenol concentration was deter- mined by the colorimetric method using HI 3864 phenol test kit with the instrumental error up to 0.1 mg·L−1. The efficiency of phenol removal was calculated by Equation (1). ( )   −    t 0 C Phenol removal % = 1 ×100 C (1) where Ct, C0 are the concentrations of phenol after contact time t and at the start (initial concentration) in mg·L−1, re- spectively. 3. Results and Discussion 3.1. Characterization of materials The morphology of TiO2, AC and TiO2–AC can be observed in SEM images. As shown in Figure 1a, TiO2 particles are irregular polygonal shapes that are less than 25 m in size. Porous structure with cavities and pits of AC is disclosed in Figure 1b. https://doi.org/10.1016/j.dib.2019.103949 Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 3 of 11 Figure 1 SEM images of TiO2 (a), AC (b), TiO2–AC (c), and EDX spec- trum of TiO2–AC (d). For TiO2–AC, a large number of TiO2 particles was loaded on AC structure by filling into the pits instead of the surface of cavities, as shown in Figure 1c. Similar observa- tion was also reported in a previous publication [21]. Lo- cated in pits, TiO2 particles could avoid being washed out from AC in photoanode preparation and utilization. As ex- pected, C, O, and Ti elements in TiO2–AC were indicated in the EDX results, as shown in Figure 1d. Crystal phase characteristics of TiO2, AC, and TiO2–AC were explored with X-ray diffraction analysis as shown in Figure 2. The reflections from the (002) and (100) planes of aromatic rings of amorphous carbon structure caused the diffraction peaks at 26.2° and 43.5° [22]. For the XRD pat- tern of TiO2, peaks appearing at 27.7°, 36.1°, 41.2°, 54.2°, and 56.7° were indexed as (110), (101), (111), (211), and (220) planes of rutile phase, whereas the presence of ana- tase phase was identified by the peaks at 25.3° and 48.3°, according to (101) and (200) planes [23]. However, only characteristic peaks of rutile TiO2 exist in TiO2–AC material, as shown in the XRD pattern of TiO2–AC. This may be due to the effect of carbon on anatase to rutile transformation of TiO2 through the formation of oxygen vacancies, as men- tioned in [24–26]. This is a disadvantage of our TiO2–AC be- cause rutile TiO2 exhibits lower photocatalytic activity than anatase TiO2 [27]. Chemical bonds in TiO2, AC, and TiO2–AC were identified by FTIR analysis (Figure 3). As shown in the FTIR spectrum of TiO2, a strong absorption band at about 500 cm−1 reflects the vibration of Ti−O bond, and a band at 1653 cm−1 corre- sponds to O−H bending in absorbed water molecules [28]. Stretching vibrations of C−O and C=C bonds in AC structure were identified by bands at 1097 cm−1 and 1554 cm−1, re- spectively [29]. A band appearing at 3469 cm−1 is associated with the stretching vibration of O−H bond and free water [28]. It can be observed in the FTIR spectrum of TiO2–AC that characteristic bands of the bonds in TiO2 and AC ap- peared again without a new band, indicating TiO2 did not conjugate to AC in TiO2–AC by a chemical bond. A similar result could be found in [21, 30]. Figure 2 XRD patterns of materials. http://dx.doi.org/10.1155/2016/8393648 http://dx.doi.org/10.1155/2012/702503 http://dx.doi.org/10.1039/C8RA06681G http://dx.doi.org/10.1155/2012/702503 Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 4 of 11 As presented in Figure 4a, UV-V is diffuse reflectance spectra of TiO2 and TiO2–AC shows the absorption edge of the TiO2 and TiO2–AC at about 370 nm and 420 nm, respec- tively. It indicates the red-shift of the TiO2–AC towards the visible region in comparison with TiO2. Tauc plots, relation- ships of h vs (h)1/2 of materials, are illustrated in Figure 4b, revealing band gap energies of 3.15 eV and 2.61 eV for TiO2 and TiO2–AC, respectively. Carbon not only promotes the 4p → 4s electronic transition at defect points in the ti- tanium atoms [31], but also improves the electron transfer due to the high electronic conductivity of carbon, thereby resulting in the lower band gap energy of TiO2–AC com- pared to TiO2. This result indicates that TiO2–AC can effec- tively separate electron-hole pairs in the visible region, which promotes photocatalytic activity under the solar illu- mination. 3.2. Characterization of photoanodes A TiO2–AC photoanode was selected for morphological anal- ysis by the SEM technique. It can be observed in Figure 5a that TiO2–AC particles in random shapes were widely spread on the photoanode surface generating a rough structure. The original morphology of TiO2–AC (Figure 1c) exposed on the photoanode surface (as shown in Figure 5b) indicated that TiO2–AC particles were not immersed completely in the coat- ing mixture. It proves that photoactive sites in TiO2–AC were disclosed on the photoanode surface, enabling photoexcita- tion of the TiO2–AC photoanode in the photoelectrochemical system. The mean thickness of a coated layer on the pho- toanode was determined to be 136 m. Photoelectrochemical properties of the TiO2 and TiO2– AC photoanodes were examined using a three-electrode cell in phenol solution at pH = 5. Figure 6 demonstrates the cur- rent density of the TiO2 and TiO2–AC photoanodes under non-UV and UV illumination applying linear sweep voltam- metry. In the absence of UV (non-UV), photoanodes present low current density. Under UV radiation, photocurrent den- sities of photoanodes significantly increase. The TiO2–AC photoanodes generated a current density of 283 A/cm2 (at 1.45 V vs. Ag/AgCl), which is approximately 2.2 times higher than that of the TiO2 photoanode. This result may be due to the decrease of TiO2 band gap in the presence of AC. Photocatalytic activity of a photoanode driving a reaction can be evaluated through an onset potential, which is a po- tential at the intersection point between J-V curve in non- UV radiation and the tangent line with a maximum slope of J-V curve in UV radiation [32, 33]. The onset potential of the TiO2 photoanode driving phenol oxidation (~1.0 V vs Ag/AgCl) is about 350 mV higher than that of the TiO2–AC photoanode (~0.66 V vs Ag/AgCl), indicating heterojunc- tion formation of TiO2/AC in the TiO2–AC material [34]. The low onset potential of the TiO2–AC photoanode demon- strated an effective charge separation and transfer, mani- festing a favorable application of the TiO2–AC photoanode for PEC degradation of phenol. Figure 3 FTIR spectra of materials. 350 400 450 500 0 1 2 A b s o r b a n c e Wavelength, nm (a) TiO2 TiO2-AC 2,5 3,0 3,5 0 1 2 3 2.6 3.15 ( h  )1 / 2 , e V 1 / 2 h, eV (b) TiO2TiO2-AC Figure 4 Curves of UV-Vis diffuse reflectance spectra (a) and Tauc plots of TiO2 and TiO2–AC materials (b). https://doi.org/10.1021/acsaem.9b01101 Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 5 of 11 Figure 5 SEM images of TiO2-AC photoanode surface in 80x (a) and 1200x (b). Figure 6 Current – voltage (J–V) curves of photoanodes in 10 mg/L of phenol solution (pH = 5). Figure 7 presents Nyquist plots of TiO2 and TiO2–AC pho- toanodes under UV and non-UV illumination. There is only one semicircle for each Nyquist plot, indicating that PEC process on the photoanodes for phenol was controlled by trap electron transfer [35]. Smaller diameter semicircle demonstrates lower charge transfer resistance [36]. It can be observed clearly from Figure 7 that the semicircle in the Nyquist plot of the TiO2 and TiO2–AC photoanodes under UV radiation is smaller than that under the non-UV excitation. This finding proves that the charge transfer of photoanodes was improved under UV illumination. The TiO2–AC pho- toanode exhibited a better charge transfer in comparison to TiO2 one because Nyquist plots of the TiO2–AC photoanode show smaller semicircles in both UV and non-UV excitation. This result indicates that the presence of AC not only did not reduce the photocatalytic properties of TiO2, but also improved the electrical conductivity of TiO2–AC, advancing the applicability of the TiO2–AC photoanode in PEC process. 3.3. Photoelectrochemical degradation of phenol Applied external voltage (Vapp) can improve PEC perfor- mance of TiO2 by enhancing the generation and separation of charge carriers [37]. After 30 minutes of PEC treatment under different Vapp in the range of 0−0.7 V, phenol and COD removal at the TiO2–AC photoanode were recorded and shown in Figure 8a. Phenol and COD removal exhibit simi- lar trends under Vapp variation. Phenol can be converted to organic intermediates during the photocatalysis process be- fore it is completely oxidized to CO2 [38]. If COD removal equates to phenol removal, the total removed phenol is ox- idized to CO2 without the formation of intermediates. As il- lustrated in Figure 8a, the ratio of phenol and COD removal decreases to approximately 1 with the increase of Vapp, cor- responding to 15.50, 13.67, 7.45, 2.58, 1.04, and 1.04 for 0, 0.2, 0.4, 0.5, 0.6, and 0.7 V of Vapp, respectively. Depend- ences of phenol and COD removal as a function of Vapp pre- sent a break-like point at 0.4 V and an exhaustion-like point at 0.6 V (Figure 8a), which is close to the onset potential of the TiO2–AC photoanode, suggesting a decisive contribution of the applied external voltage to the PEC degradation of phenol. Significant improvement of phenol degradation with PEC process as compared to photocatalysis is shown in Figure 8b. Under UV illumination, electrons and holes were photogenerated on TiO2, enhancing charge transfer between phenyl ring and photoanodes [39] and promoting phenol degradation. However, fast recombination of these electron-hole pairs causes a low limit of phenol removal, as seen in the TiO2/UV and TiO2–AC/UV curves. The re- combination time of electron-hole pairs can be longer than charge transfer time in the redox reaction under appropri- ate Vapp. As expected, at Vapp = 0.7 V, TiO2/PEC and TiO2– AC/PEC curves in Figure 8b exhibit an increase in phenol removal. Moreover, the TiO2–AC photoanode presents a higher efficiency of PEC degradation of phenol than the TiO2 photoanode owing to the contribution of electric con- ductivity of AC. After 60 min of contact time, phenol PEC degradation achieved 75.9% on TiO2–AC, which is lower than that on TiO2 nanotubes [40], but significantly higher than on TiO2 [18]. http://dx.doi.org/10.1134/S1023193515110130 Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 6 of 11 Figure 7 Nyquist plots from electrochemical impedance measure- ments on photoanodes in 10 mg/L of phenol solution (pH = 5). 18,6 20,5 24,6 63,4 71,4 73,2 1,2 1,5 3,3 24,6 68,2 70,4 0,0 0,2 0,4 0,6 0,8 0 25 50 75 P h e n o l r e m o v a l, % Applied voltage, V (a) C0 = 10 mg/L pH0 = 5 0 50 100 C O D r e m o v a l, % 0 40 80 120 0 20 40 60 80 TiO2-AC/PEC TiO2/PEC TiO2-AC/UVP h e n o l r e m o v a l, % Contact time, min (b) TiO2/UV C0 = 10 mg/L pH0 = 5 Vapp = 2 V Figure 8 Effects of a) applied external voltage and b) PEC degra- dation of phenol on TiO2–AC photoanode. As discussed above, the following experiments for the ki- netic study of phenol PEC degradation were conducted at the TiO2–AC photoanode under Vapp = 0.7 V and UV illumination. 4. Kinetic of photoelectrochemical degra- dation of phenol in acidic solution Photocatalytic degradation of phenol can be described by the following Langmuir – Hinshelwood (L-H) kinetic mechanism [41–42], in which phenol is first adsorbed onto the photocatalyst surface and then decomposed. Therefore, this L-H model (2) was used to fit our experi- mental data, revealing negative values of KB (as shown in Table 1). However, this is unreasonable because the adsorption equilibrium constant KB must be positive. • L-H model: − A B dC K C r = = dt 1+K C (2) • First-order model: − 1 dC r = = k C dt (3) In the case of chemical reaction control, a first-order ki- netic model (3) well described the photodegradation of phe- nol on TiO2/AC [43] as well as ZnO, TiO2 and ZnO–TiO2 pho- tocatalysts [44]. Photoelectrochemical degradation rate fol- lowing the first-order model was identified for acidic red 17 dye on ammonium persulphate [45], and phenol on PbO2 anode [46]. However, the determination coefficients (R2) obtained from fitting our experimental data with this model were not close to 1 (as shown in Table 1). It signifies that the PEC degradation of phenol occurred in a complex mech- anism and was not controlled exclusively by a chemical re- action process. We proposed a mechanism of PEC degradation of phenol through reactions (4)–(6) based on the previous reports. Spallart et al. [47] proved that water competed with aro- matic compounds in PEC oxidation. Moreover, oxygen at- oms from water molecules can form O2 •− radicals at photo- excited points [39]. Hence, the active site on the pho- toanode surface (∗𝑛+) was suggested to interact with a wa- ter molecule under the applied external potential and form an active site bearing oxygen (∗𝑛+ O2−) following reaction (4). In other consideration, phenol (Ph) can be protonated into H+Ph form [48] in acid solution, promoting the transfer of electron pair of oxygen in −OH group into the aromatic ring [49] and resulting in polar structure H+Ph− according to reaction (5). We suppose that H+Ph− contacted with ∗𝑛+ O2− and then oxidized according to reaction (6), yield- ing the decomposition products and regenerating the active site. − − + + w w kn+ n+ 2 + 2 k H O + * * O 2H (4)  + − + + − + p p k k Ph H H Ph (5)  − − + − ⎯⎯→ +d k+ n+ 2 n+H Ph * O ze Products * (6) https://doi.org/10.1016/j.mssp.2014.05.031 http://dx.doi.org/10.1016/j.arabjc.2011.03.001 https://doi.org/10.1016/j.electacta.2013.08.080 https://doi.org/10.1021/acs.jpca.8b04446 Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 7 of 11 Table 1 Calculated parameters of kinetic models at different initial concentrations and pH values. Kinetic model Parameters Initial concentrations, mg/L (pH = 5) pH values (C0 = 20 mg/L) 5 10 15 20 3 4 6 L-H KA 0.011 0.012 0.014 0.016 0.033 0.026 0.003 KB −18.62 −9.22 −5.05 −2.78 −3.073 −3.143 −4.595 R2 0.944 0.978 0.995 0.994 0.984 0.998 0.986 First-order k1 0.021 0.021 0.022 0.022 0.034 0.031 0.009 R2 0.054 0.610 0.884 0.959 0.832 0.901 0.704 This study 𝑘𝑤 + 0.131 0.070 0.204 0.122 0.013 0.241 0.231 𝑘𝑤 − ∙ 103 5.608 5.321 6.463 6.339 5.792 6.279 6.325 𝑘𝑝 + 0.099 0.071 0.045 0.034 0.070 0.061 0.023 𝑘𝑝 − 0.032 0.024 0.022 0.013 0.009 0.027 0.028 𝑘𝑑 ∙ 10 3 0.164 0.158 0.156 0.142 0.168 0.192 0.152 [∗𝑛+]𝑒𝑥 452 435 438 392 464 589 471 R2 0.999 0.998 0.994 0.994 0.994 0.998 0.990 k1 [min–1]; KA [min–1]; KB [L·mmol–1]; 𝑘𝑤 + , 𝑘𝑤 − , 𝑘𝑝 +, 𝑘𝑝 − [min–1]; kd [L·mmol–1·min–1], [∗𝑛+]𝑒𝑥 in mmol·L –1 We assume that reactions (4)–(6) occurred in the pres- ence of large amounts of H+ ions and H2O. Therefore, the rates of these reactions depend on the concentration of phe- nol (x1),  −+H Ph (x2), and −n+ 2* O (x3), as shown in equations (7), (8), (9). d𝑥1 d𝑡 = −𝑘p +𝑥1 + 𝑘p −𝑥2, (7) d𝑥2 d𝑡 = 𝑘p +𝑥1 − 𝑘p −𝑥2 − 𝑘d 𝑥2𝑥3, (8) d𝑥3 d𝑡 = 𝑘w + [∗𝑛+] − 𝑘w − 𝑥3 − 𝑘d 𝑥2𝑥3, (9) where x1, x2, x3 are in mmol·L−1, and [∗𝑛+] is an apparent concentration of active sites in mmol·L−1. Under UV illumination, TiO2–AC particles on the pho- toanode surface were excited to create photoexcited sites (∗). Then, under the applied external potential, the photoexcited site lost n electrons and became an active site (∗𝑛+). Supposing that the total number of ∗𝑛+ does not change with an apparent concentration [∗𝑛+]𝑒𝑥, equation (10) is obtained: [∗𝑛+]𝑒𝑥 = 𝑥3 + [∗ 𝑛+]. (10) The value of [∗𝑛+] can be determined from (10) and sub- stituted in to (9) to result in (11): d𝑥3 d𝑡 = 𝑘w + [∗𝑛+]ex − (𝑘w + + 𝑘w − + 𝑘d 𝑥2)𝑥3. (11) The parameters in the proposed model (𝑘𝑤 + , 𝑘w − , 𝑘p +, 𝑘p −, kd, and [∗𝑛+]ex) were determined by fitting experimental data with the model using the least-square method for x1 objective: ∑(𝑥1,𝑖 − 𝑥1,�̂�) 2 → min 𝑛 𝑖=1 (12) where, 𝑥1̂ is the phenol concentration predicted by the model, i = 1, 2, …, n denotes ith value, and n = 12 is the num- ber of experimental data points. The determination coefficient (R2) is used to evaluate the goodness of fit of the model as presented in formula (13). 𝑅2 = 1 − SSR SST = 1 − (𝑥1−𝑥1̂) 2 (𝑥1−𝑥1̅̅ ̅) 2 (13) where SSS = ∑(𝑥1 − 𝑥1̂) 2 is the residual sum of squares, TSS = ∑(𝑥1 − 𝑥1̅̅ ̅) 2 is the total sum of squares, 𝑥1̅̅ ̅ = 1 𝑛 ∑ 𝑥1 𝑛 𝑖=1 is the mean value of x1. Numerical solutions of ordinary differential equations (7), (8), and (11) were carried out using Runge – Kutta 4th order method with initial conditions: 𝑥1(0) = [Ph]0 = 𝐶0 94.11 , 𝑥2(0) = 𝑥3(0) = 0 (94.11 is molecule weight of phenol). The minimization problem (12) was solved with the help of the Excel Solver tool (ver. 2016) with a GRG non-linear option. Figure 9 shows the effects of initial phenol concentra- tion (C0) and initial pH (pH0) on phenol removal under both observations of the experiment and simulation. The pro- posed model exhibited a good description of the experi- mental data due to the closeness of R2 to 1, obtaining kinetic parameters as summarized in Table 1. Experiments of PEC degradation of phenol were performed at different C0 from 5 to 20 mg·L−1 at pH0 = 5, revealing kinetic behavior as presented in Figure 9a. Phenol exhibits a property of UV light interception [39, 50], causing a decrease in the ac- tive site quantity (*n+) and oxidation rate constant (kd) while increasing C0 as shown in Table 1. Consequently, the rate of phenol removal is lower with higher C0. Moreover, contact time for 99.95% phenol removal was predicted to be 1110, 725, 490, and 450 minutes at C0 = 5, 10, 15, and 20 mg·L−1, respec- tively, according to the proposed model. It proves that the PEC degradation of phenol on TiO2–AC photoanode did not reach equilibrium and tended to complete phenol removal. pH0 of the solution is an important parameter in the pho- todegradation of phenol because of the variation of charge properties of phenol at different pH values [51]. In this study, PEC degradation of phenol was studied at various pH0 values of solution (3, 4, 5, and 6) with C0 = 20 mg·L−1. Phenol is pro- tonated to H+Ph− in the presence of ion H+ according to re- action (4). Therefore, the lower pH0 value was, the more H+Ph− was produced, resulting in an improvement in phenol degradation (as shown in Figure 9b). Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 8 of 11 0 40 80 120 0 20 40 60 80 100 5 mg*L−1 10 mg*L−1 15 mg*L−1 20 mg*L−1 P h e n o l r e m o v a l, % Contact time, min (a) Conditions: pH0 = 5 Vapp = 2V 0 40 80 120 0 20 40 60 80 100 pH0 = 3 pH0 = 4 pH0 = 5 pH0 = 6 P h e n o l r e m o v a l, % Contact time, min (b) Conditions: C0 = 20 mg*L -1 Vapp = 2V Figure 9 Simulation (continuous line) and experimental results (discrete points) for PEC degradation kinetic of phenol on TiO2–AC photocathode at different C0 (a) and pH0 (b). Variation of [H+Ph−]/[Ph]0 and [∗𝑛+ O2−] as a function of contact time at different C0 was simulated and presented in Figure 10. As shown in Figure 10a, [H+Ph−]/[Ph]0 in- creased in the first stage, then decreased, reaching the max- ima around 25 minutes of contact time. A similar trend is also observed in the inset in Figure 10a, which shows the dependence of [H+Ph−] on t. The higher C0, the higher max- imum value of [H+Ph−] would be, contrary to the [H+Ph−]/[Ph]0 relation. Moreover, the rate constant 𝑘p + was found to be 0.099, 0.071, 0.045, and 0.034 min−1 (Table 1) at 5, 10, 15, and 20 mg·L−1 of C0, respectively, which proves that high C0 is a disadvantage to protonation of phenol. The curves in Figure 10b present the relationships be- tween the amount of *n+O2− and the contact time. In all stud- ied C0, [*n+O2−] quickly increases to reach the equilibrium concentration ([*n+O2−]eq). The ratios of [*n+O2−]eq and [*n+]ex are larger than 0.9, as shown in the inset in Figure 10b. It means that most photoexcited sites changed to form active sites bearing oxygen, which acts as a reactant in PEC degra- dation of phenol. Although [*n+O2−]eq decreased with the in- crease of C0, the high value (>350 mmol·L−1) was enough to interact entirely with protonated phenol molecules. The relationship of [H+Ph−]/[Ph]0 vs contact time at dif- ferent pH0 (3, 4, 5, and 6) exhibited maxima as shown in Figure 11a. The maximum value of [H+Ph−]/[Ph]0 at pH0 = 3 was the highest. At pH0 = 4, 5, and 6, the maximum values of [H+Ph−]/[Ph]0 were similar; however, the maximum peaks shifted to longer contact time with the increase of pH0. In other consideration from Table 1, the ratio of 𝑘p +/ 𝑘𝑝 − generally decreased with the rise in pH0 (7.8, 2.3, 2.6, and 0.82 at pH0 = 3, 4, 5, and 6, respectively). These results prove that phenol protonation was promoted in low pH0. Moreover, pH0 also affected the amount of *n+O2− on TiO2–AC photocathode, as shown in Figure 11b. For pH0 = 4, 5, and 6, [∗𝑛+ O2−] tended to reach [*n+O2−]eq after around 30 minutes of contact time; and [*n+O2−]eq at pH0 = 4 was the highest. Because the point of zero charge of the pre- pared TiO2–AC was determined at pH = 5.4, TiO2–AC was positively charged under acidic conditions, advancing to generate *n+O2−. However, the amount of H+ ion was too high at pH0 = 3, causing a strong electrostatic force between H+ and oxygen in ∗𝑛+ O2−, which dissociated oxygen and ac- tive site. Consequently, [*n+O2−] cannot reach equilibrium concentration after 120 minutes of contact time at pH0 = 3, as shown in Figure 11b. 0 40 80 120 0,0 0,2 0,4 0,6 0,8 1,0 20 mg*L−1 C0 = 5 mg*L −1 20 mg*L−1 [H + P h  − ]/ [P h ] 0 Contact time, min C0 = 5 mg*L −1 (a) 0 50 100 0,00 0,04 0,08 [H + P h  − ], m m o l* L − 1 Contact time, min 0 40 80 120 0 100 200 300 400 500 C0 = 20 mg.L −1 [* n + O 2 − ], m m o l* L − 1 Contact time, min (b) 5 0 5 10 15 20 0,00 0,25 0,50 0,75 1,00 [* n + O 2 − ] e q / [* n + ] e x C0, mg*L −1 Figure 10 Simulation of H+Ph− (a) and *n+O2− (b) variations in PEC degradation of phenol on TiO2–AC photoanode at different C0 (pH0 = 5, Vapp = 0.7 V). Chimica Techno Acta 2022, vol. 9(4), No. 20229416 ARTICLE 9 of 11 0 40 80 120 0,0 0,2 0,4 0,6 0,8 1,0 pH0 = 6 pH0 = 5 pH0 = 4 [H + P h  − ] / [ P h ] 0 Contact time, min pH0 = 3 (a) 0 40 80 120 0 100 200 300 400 500 pH0 = 6 pH0 = 5 pH0 = 4 [* n + O 2 − ], m m o l* L − 1 Contact time, min pH0 = 3 (b) 3 4 5 6 0,00 0,25 0,50 0,75 1,00 [* n + O 2 − ] e q / [* n + ] pH0 Figure 11 Simulation of a) H+Ph− and b) [∗𝑛+ O2−] variations in PEC degradation of phenol on TiO2–AC photoanode at different pH0 (C0 = 20 mg·L−1, Vapp = 0.7 V). The inset in Figure 11b presents the effect of pH0 on the ratio of [∗𝑛+ O2−]/[∗𝑛+] ex. It can be seen that this ratio at pH0 = 5 is the highest (close to 1). It means that more sep- aration between the point of zero charge and pH0 caused lower [∗𝑛+ O2−]/[∗𝑛+] ex. 5. Conclusions In this work, TiO2 and TiO2–AC photocatalysts were success- fully synthesized by the sol-gel method to apply to the PEC degradation of phenol. The TiO2–AC photoanode exhibits higher photoactivity for degrading phenol under UV-C illu- mination than TiO2. Effects of applied external voltage, pH, and initial concentration of phenol on the kinetics of phenol PEC degradation were also experimentally investigated in this study. The interaction mechanism between protonated phenol and the active site bearing oxygen was well-demon- strated for the phenol PEC degradation. The kinetic constants and the concentration variations of protonated phenol and the active site bearing oxygen were determined by fitting the established kinetic model to the experimental data. Supplementary materials No supplementary materials are available. Funding This research had no external funding. Acknowledgments This work was supported by the Vietnam’s Ministry of Nat- ural Resources and Environment through project coded CS.2022.12. 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