Phenol Electrooxidation Research Articles

Bifunctional Chiral Electrocatalysts Enable Enantioselective α‐Alkylation of Aldehydes

AbstractHerein, we describe an innovative approach to the asymmetric electrochemical α‐alkylation of aldehydes facilitated by a newly designed bifunctional chiral electrocatalyst. The highly efficient bifunctional chiral electrocatalyst combines a chiral aminocatalyst with a redox mediator. It plays a dual role as a redox mediator for electrooxidation, while simultaneously providing remarkable asymmetric induction for the stereoselective α‐alkylation of aldehydes. Additionally, this novel catalyst exhibits enhanced catalytic activity and excellent stereoselective control comparable to conventional catalytic systems. As a result, this strategy provides a new avenue for versatile asymmetric electrochemistry. The electrooxidation of diverse phenols enables the C−H/C−H oxidative α‐alkylation of aldehydes in a highly chemo‐ and stereoselective fashion. Detailed mechanistic studies by control experiments and cyclic voltammetry analysis demonstrate possible reaction pathways and the origin of enantio‐induction.

Heterostructural PbO2/Co3O4 composite for anodic oxidation of phenol: An energy-efficient hybrid process

The PbO2/Co3O4 composite electrodes were synthesized using the composite co-deposition method, which involved the incorporation of Co3O4 nanoparticles into a PbO2 coating. These composite electrodes were employed as anodes for phenol electrooxidation in wastewater treatment. The results demonstrate that the PbO2/Co3O4 composite electrode exhibits a three-dimensional porous structure, leading to a significant increase in the electrochemically active surface area. Electrochemical measurements indicate that the composite electrode achieves a minimum oxygen evolution potential of 526.5 mV. Moreover, the electrode facilitates the indirect degradation of phenol, even at low voltage (0.45 V). The unique structure, improved conductivity, and enhanced surface area of the electrode contribute to its exceptional performance in phenol degradation. Additionally, this approach offers the potential for synergistic integration with other treatment methods, providing a sustainable and energy-efficient solution for removing persistent organic compounds from wastewater.

Studies of Phenol Electrooxidation Performed on Platinum Electrode in Dimethyl Sulphoxide Medium. Determination of Unreacted Phenol by the Effect of 4‐Vinylbenzenesulphonate on the Electrooxidation Process

AbstractThe electrooxidation of phenol was studied in the presence of sodium 4‐vinylbenzenesulfonate in various quantities. When the sulphonate was dissolved in equal or higher quantities than phenol the peak currents increased gradually and suppression of electropolymerization was observed. When the sulphonate quantity was smaller than that of phenol brown films fouled the electrode. The observed phenomenon was utilized in an analytical procedure namely determination of unreacted phenol in electrolysis solution with linear dynamic range between 0 and 50 mM and 2.96 mM detection limit. The shape of curve became saturated‐like above 50 mM.

Reactive species conversion into 1O2 promotes substantial inhibition of chlorinated byproduct formation during electrooxidation of phenols in Cl−-laden wastewater

The generation of chlorinated byproducts during the electrochemical oxidation (EO) of Cl−-laden wastewater is a significant concern. We aim to propose a concept of converting reactive species (e.g., reactive chlorines and HO• resulting from electrolysis) into 1O2 via the addition of H2O2, which substantially alleviates chlorinated organic formation. When phenol was used as a model organic compound, the results showed that the H2O2-involving EO system outperformed the H2O2-absent system in terms of higher rate constants (5.95 × 10−2 min−1vs. 2.97 × 10−2 min−1) and a much lower accumulation of total organic chlorinated products (1.42 mg L−1vs. 8.18 mg L−1) during a 60 min operation. The rate constants of disappearance of a variety of phenolic compounds were positively correlated with the Hammett constants (σ), suggesting that the reactive species preferred oxidizing phenols with electron-rich groups. After the identification of 1O2 that was abundant in the bulk solution with the use of electron paramagnetic resonance and computational kinetic simulation, the routes of 1O2 generation were revealed. Despite the consensus as to the contribution of reaction between H2O2 and ClO− to 1O2 formation, we conclude that the predominant pathway is through H2O2 reaction with electrogenerated HO• or chlorine radicals (Cl• and Cl2•−) to produce O2•−, followed by self-combination. Density functional theory calculations theoretically showed the difficulty in forming chlorinated byproducts for the 1O2-initiated phenol oxidation in the presence of Cl−, which, by contrast, easily occurred for the Cl•-or HO•-initiated phenol reaction. The experiments run with real coking wastewater containing high-concentration phenols further demonstrated the superiority of the H2O2-involving EO system. The findings imply that this unique method for treating Cl−-laden organic wastewater is expected to be widely adopted for generalizing EO technology for environmental applications.

Promotion of Phenol Electro-oxidation by Oxygen Evolution Reaction on an Active Electrode for Efficient Pollution Control and Hydrogen Evolution.

We report an electrolysis system using NiFe layered double hydroxide/CoMoO4/nickel foam (NFLDH/CMO/NF) as the anode and CMO/NF as the cathode for simultaneous phenol electro-oxidation and water electrolysis. This system shows high performance for both phenol degradation and hydrogen evolution. We demonstrate that the degradation rate of phenol on the active anode is governed by the mass transfer rate at a low phenol concentration (0.5-2 mM) and by the electro-oxidation rate at a high phenol concentration (5 mM). The anodic oxygen evolution reaction (OER) can promote the phenol degradation through enhanced mass transfer efficiency. More importantly, the common deactivation issue of phenol electro-oxidation on the inert anode can be eliminated by the high OER activity of the active anode. The constructed full electrolytic cell only needs a low potential of 1.498 V to achieve 10 mA/cm2 for water electrolysis. The reported promotion effect of phenol degradation by OER as well as the improved anode resistance to deactivation offer new insights into efficient and robust waste-to-resource electrolysis system for water treatment.

Effect of Anodic Pretreatment on the Performance of Glassy Carbon Electrode in Acetonitrile and Electrooxidation of Para-substituted Phenols in Acetonitrile on Platinum and Glassy Carbon Electrode

In the first part of the work electropolymerisation of phenol was studied at glassy carbon electrode. Rapid fouling of its surface indicated the formation of coherent poly(phenyleneoxide) layer which was demonstrated by the repeated cyclic voltammetric scans. Effect of anodic pretreatment potential in acetonitrile solvent was also investigated and the results showed that at potentials higher than 2 V glassy carbon electrode becomes deactivated. Preanodisation of glassy carbon electrode at 3 V in acetonitrile resulted in diminished anodic peak currents by phenols. It was due to the partial deactivation of electrode surface and its extent increased with the pretreatment time. The electrooxidation of para-substituted phenols (p-Cl-phenol, p-NO2-phenol, p-tertbutylphenol, p-methoxyphenol) in acetonitrile resulted in no fouling layer on platinum electrode and the peak currents were significantly higher than in the first scan of unsubstituted phenol in the same concentration. Glassy carbon deactivated continuously by repeating the scans due to the solvent and bonding of products on the surface.

Investigation of phenol electrooxidation in aprotic non-aqueous solvents by using cyclic and normal pulse voltammetry

Electrooxidation of phenol was studied on platinum electrode in five aprotic non-aqueous solvents (acetonitrile, dimethyl sulfoxide, dimethyl formamide, acetone, and tetrahydrofuran) with cyclic and normal pulse voltammetry. The cyclic voltammetric results showed that fouling of the electrode surface by the polyphenol took place continuously in dimethyl formamide and a weak passivation of the electrode could be observed in dimethyl sulfoxide. From this solvent, a coherent and mechanically removable, weakly adsorbed layer could be obtained. In the other three solvents, the electrode passivated completely after five scans. Diffusion barrier properties of the polymer formed in acetone were the most pronounced of all solvents towards a redox probe. Normal pulse voltammetric investigations showed that the extent of electrode passivation is insignificant in three of the solvents used. It was due to the application of the short anodic potential pulses where the oxidation of phenol occurred and the formed oligomers diffused into the bulk of solution.

Regeneration of expanded graphite electrodes by joined electrochemical and ozone treatment in liquid phase

The researches presented in this work were devoted to electrochemico-chemical regeneration of exhausted electrode made of expanded graphite (EG). The aimed process was conducted by electrochemical treatment and ozone flow performed together in wet environment. EG was covered with insoluble products of incomplete oxidation of phenol formed during cyclic voltammetry measurement. The same electrochemical technique was applied for evaluation of regeneration efficiency. To understand the process of EG regeneration, the electrode was characterized by calculating of BET surface, FTIR and XPS analysis. Moreover, SEM images of the investigated samples were also done. Obtained results have showed the success of regeneration treatment, which led to significant enhancement of electrode activity compared to original EG. The present work also revealed that the mechanism of phenol electrooxidation is changed after the regeneration treatment of electrode material. This effect is probably caused by the modification of chemical composition of EG surface due to its interactions with OH radicals intensively generated during the process of regeneration.

Enhanced electrochemical oxidation of phenol over manganese oxides under mild wet air oxidation conditions

Low-cost manganese oxide, MnOx-based electrocatalysts, containing α-MnO2 and mixed α-Mn2O3/α-MnO2 phases, were synthesized by scalable anodic and cathodic electrodeposition methods, respectively. Their morphological and chemical composition were characterized by means of Field Emission Scanning Electronic Microscopy (FESEM), X-Ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). These electrodes were tested for the electro-oxidation of a recalcitrant molecule (i.e. phenol) in a lab-scale high temperature and high pressure (HTHP) batch electrocatalytic reactor. Their electrocatalytic activity was compared with that of state-of-the-art anodes for phenol electro-oxidation: antimony-doped tin oxide (SnO2–Sb5+) and ruthenium oxide (RuO2): first, under standard ambient conditions, and then, under the conditions of a Polymeric Electrolyte Membrane (PEM) electrolyzer (i.e. 85 °C and 30 bar) and of mild Catalytic Wet Air Oxidation (CWAO, i.e. 150 °C and 30 bar). Both reaction time and current density were varied to investigate their effect in the performances of the system as well as on the reaction mechanism. Both MnOx electrodes reported enhanced conversion efficiencies, up to ∼75%, at the highest pressure and temperature, and at the lowest applied current density, which influenced the process by improving dissolution of the O2 evolved, the reaction kinetics and thermodynamics, and by minimizing irreversibilities, respectively. The here reported MnOx films achieved conversion and mineralization efficiencies comparable to Sb-SnO2 (that is the more toxic) and RuO2 (that is more expensive) materials, operating under mild CWAO operation conditions, which demonstrate the potential of the electrocatalytic HTHP process as a sustainable advanced oxidation technology for wastewater treatment or electrosynthesis applications.

Electrooxidation of phenol on carbon fibre-based anodes through continuous electrolysis of synthetic wastewater

AbstractThis work reports on the process of phenol electrooxidation, which is carried-out through continuous electrolysis of synthetic, sodium sulphate-based wastewater. Phenol electrodegradation is examined by means of a laboratory size (ca.700 cm3of working volume), poly (methyl methacrylate)-made electrolyser unit for various, carbon fibre and graphite-based anode configurations, and stainless steel cathodes, two different current-densities and concentrations of phenol in synthetically prepared wastewater solution. Proper monitoring of phenol degradation (including quantitative identification of reaction products and calculation of specific energy consumption) in wastewater is performed by means of instrumental, combined HPLC and MS technique in function of electrolysis time.

Influence of expanded graphite coming from the electrochemical oxidation of phenol on cement-polymer matrix

Abstract In presented article the polymer-modified cement mortars with expanded graphite covered by the oligomer film, being a by-product of phenol electro-oxidation, were investigated. The changes in the morphology of EG and EG/oligomer as well as in the microstructure of cement-polymer composites modified with EG/oligomer were verified using the scanning electron microscopy (SEM). The evaluation of adhesion between EG/oligomer and cement-polymer mortar based on the mechanical tests, especially their flexural behavior. It was shown that the oligomer film formed on EG surface made graphite flakes more durable and resistant to bending. Moreover, the oligomer due to the interaction with polymer network in cement-polymer mortar led to the improvement of flexural toughness of composite.

Process of phenol electrooxidation on the expanded graphite electrode accompanied by the in-situ anodic regeneration

The investigations reported in this paper, were devoted to electrochemical regeneration of spent expanded graphite (EG), carried out in-situ during the process of electrochemical oxidation of phenol. The electrochemical process of phenol oxidation followed by the anodic regeneration of EG electrode was investigated by cyclic voltammetry technique in electrolytes composed of 0.1M phenol dissolved in 0.5M and 6M KOH. The obtained results undoubtedly revealed that, owing to the considerable potential lowering of electrochemical decomposition of water, the anodic oxidation of phenol in 6M KOH partially proceeded within the potentials of violent oxygen evolution. The active oxygen and/or hydroxyl radicals, generated during oxygen evolution, impacted both as phenol oxidation supporting oxidants and as in-situ synthesized regenerating agents of spent electrode material. Owing to this feature, the established process may be continued during the consecutive cycles. The enhanced level of electrochemical activity of EG remains almost unchanged during the initial 3cycles of voltammetric process of phenol oxidation. On farther scanning, the EG activity decreases reaching in tenth cycle activity on significantly higher level compared to the process performed in 0.1M phenol in 0.5M KOH. The higher activity of EG demonstrated in 0.1M phenol in 6M KOH is proved by the higher decline in phenol concentration and lower value of total organic carbon. It cannot be excluded that partial damage to the EG structure is responsible for the noted decrease.

Electrochemical properties of exfoliated graphite affected by its two-step modification

Exfoliated graphite (EG) was modified by a two-stage process consisting of electrochemical oxidation followed by the thermal treatment. Within the former one, the process of re-intercalation of H2SO4 into the EG by linear sweep voltammetry was carried out. Thus, obtained re-intercalated EG underwent heat treatment at 800 °C in order to synthesize re-exfoliated EG (re-EG). The electrochemical features of the re-EG were examined in the model process of phenol electrooxidation carried out by cyclic voltammetry technique in alkaline solution with and without phenol addition. Taking into account the anodic charges as a main criterion of electrochemical activity, it was found that the modification of EG caused over twofold improvement of its electrochemical activity. This behavior is related with the changes within the chemical composition of modified EG surface and on much smaller scale with the modification of its structure. The degree of electrochemical activity improvement depends on the conditions under which the processes of re-intercalation and re-exfoliation were performed. The results of Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis supported by the data of the Brunauer-Emmett-Teller (BET) surface area and scanning electron microscope (SEM) observations allow an understanding of the physicochemical properties of re-exfoliated EG and enhancement of its electrochemical activity.

Kinetics of electrooxidation of phenol on polycrystalline platinum

Abstract This work reports on kinetics of phenol electrooxidation reaction (PhER), examined at polycrystalline Pt electrode in 0.5 M H2SO4 and 0.1 M NaOH supporting solutions. Important aspects of PhER kinetics were analysed based on potential-dependent, a.c. impedance-derived values of charge-transfer resistance and capacitance parameters. Special attention was also given to the influence of supporting electrolyte ions on the process of phenol oxidation (pH dependence of the PhER), in relation to an important role of anion adsorption on the Pt catalyst surface.

Effect of calcination temperature and molar ratio of tin and manganese on capacitance of Ti/SnO2–Sb–Mn/β-PbO2 electrode during phenol electro-oxidation

The variations in the capacitance of Ti/SnO2–Sb–Mn/β-PbO2 electrodes prepared using sol–gel and electrodeposition techniques with various calcination temperatures and molar ratios of tin and manganese are discussed in this work. The microscale morphology and physical composition of the fabricated electrodes were characterized by scanning electron microscopy and X-ray diffraction. Cyclic voltammetry and galvanostatic charge–discharge tests were also performed to evaluate the electrochemical performance. The experimental data showed that the capacitance of prepared Ti/SnO2–Sb–Mn/β-PbO2 electrodes was inversely related to the calcination temperature and the molar ratio of tin and manganese in the molding process of the transition layer. The optimal sintering temperature and Sn/Mn molar ratio were 550°C and 4:1, respectively, while the specific capacitance was 36.35mFcm−2 at a charge–discharge current density of 1.0mAcm−2 in a phenolic electrolyte. In addition, the mechanism for improving the capacitance during phenol electro-oxidation in the presence of Mn was investigated by comparing the electrochemical impedance spectra of the Ti/SnO2–Sb–Mn/β-PbO2 electrode with those of the Ti/SnO2–Sb/β-PbO2 electrode. A larger electrolyte resistance, accompanied by a smaller charge-transfer resistance, was obtained for the electrode with Mn doping, which could be ascribed to the decrease in electrical conductivity and increase in electron-capture capacity of MnOx.

Study of Phenol Oxidation Products over Glassy Carbon Electrode in Alkaline Solutions by Photoluminescence Spectroscopy

In this research, a new approach is used to identify the products of electrochemical oxidation of UV visible substances over solid electrodes. In order to determine the product of phenol electrooxidation over glassy carbon electrode, the results obtained using voltammetric method and photoluminescence spectroscopy have been compared. It was proved that in an alkaline solution phenol is present in form of phenolate-ion, adsorbing on the surface of glassy carbon electrode, and oxidized to hydroquinone with transfer of two electrons.

Anode Polarization Character of Metallic Oxides Loaded Granular Actived Carbon during Phenol Electro-oxidation in a Three Dimensional Electrode Reactor

By the method of sol-gel metallic oxides (SnO2 and TiO2) could be loaded on the surface of granular active carbon. X-ray power diffraction (XRD) and scanning electron microscopy (SEM) are used for phase composition and surface properties analysis, respectively. Results showed that SnO2 and TiO2 existence in different particle shapes based on their binding characteristic with carbonaceous materials. Cyclic voltammetry (CV) conducted in solution of 0.5mol/L potassium ferricyanide showed that with the coverage of metallic oxides on GAC surface, the response current and anodic area would expanded greatly during phenol electrooxidation process. Anode extension coefficient of TiO2-GAC and SnO2-GAC are 2.73 and 5.78, respectively. Further sdudy of circuit characters of three dimensional electrode reactor for phenol oxidation was conducted by electrochemical impedance spectroscopy (EIS). The outcomes showed SnO2-GAC would have an efficient decrease in eletrolyte resistance concurrent with a increased property for electricity storage, while TiO2-GAC are mostly focus on the improving for electron transferring. Metallic oxide SnO2 and TiO2 would positively influence on anode polarization of carbon-supported three dimensional electrode reactor for phenol oxidation.

Electrochemical capacitance behavior of a packed bed electrochemical reactor toward phenol electro-oxidation

The electrochemical capacitance behavior of a packed bed electrochemical reactor (PBER) toward phenol electro-oxidation in aqueous solution was studied. In the absence of phenol, a decrease in the response current (i) coupled with an increase of the specific capacitance (C s ) was obtained for PBER compared with a flat-bed electrochemical reactor. However, following intercalation of phenol, simultaneous improvement of i, and reduction of C s were observed. The reasons for the former were ascribed to an increased resistance, an abundant pore structure for charge, and electrolyte transportation owing to the packing of the fixed granular active carbon (GAC) filler, whereas anode extension and formation of the polyoxyphenylene film on the anode and the GAC surface were responsible for the latter. Although significant attenuation (42.06 % after 300 cycles at current density 1.90 mA cm−2) of C s was observed for PBER in the long cycle test, the non-negligible C s value (30.86 mF cm−2) holds great promise for pulsating power supplies. An equivalent circuit associated with electrochemical capacitance and resistance distribution was proposed, which gives an exhaustive explanation of the electrical characteristics during the phenol electro-oxidation process.

Development, validation and application of an HPLC method for phenol electrooxidation products in the presence of chloride

A high-performance liquid chromatography (HPLC) method was developed and validated to determine phenol and potential intermediates from hydroxylation (hydroquinone, benzoquinone, catechol) and hypochlorination (2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, 2,3,6-trichlorophenol, 2,4,5-trichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol) pathways during electrooxidation in the presence of chloride. A Hypersil ODS column (150 mm L × 4.6 mm I.D.) was used for the separation. The best separation was achieved when using a time variant gradient between a water mobile phase (with 0.1% formic acid adjusted to pH 3.0 with 0.1 mM sulfuric acid) and an organic phase (90:5:5 by volume mixture of acetonitrile:methanol:acetic acid). The flow rate was 0.8 mL min−1 and UV absorbance was monitored at 270, 280, 290 and 300 nm, choosing the wavelength of strongest response for each compound. The intra- and inter-day accuracy and precision were tested using five replicates each day on three consecutive days.

Multiple anodic regeneration of exfoliated graphite electrodes spent in the process of phenol electrooxidation

The present paper deals with the studies concerning anodic regeneration of exfoliated graphite (EG) electrodes coated with oligomer products of incomplete phenol electrooxidation. The electrochemical activity of regenerated samples depends on the concentration of regenerating electrolyte and potential applied during the regeneration. The effective method of a single-step regeneration of spent EG electrodes was found to be anodic treatment proceeded in 6 M KOH at the potential of 1.3 V vs. Hg/HgO/0.5 M KOH. During the oxidative regeneration of EG, the physical and chemical interactions between intensively generated active oxygen and/or OH· radical and oligomer covering the EG surface take place. As has been proved by FTIR and XPS analyses, active oxidative agents cause significant modification of chemical composition of oligomer that resulted in enhanced concentration of surface functionalities mainly involving C=O bonds. The results of multiple regeneration of EG electrode, considered as promising method for practical application, showed that the highest efficiency of regeneration is attained due to potentiostatic treatment carried out in 6 M KOH at the potential of 1.2 V. The electrochemical activity of EG regenerated at this potential increases gradually with the number of regenerating loops and after the third regeneration is almost fourfold higher compared to that of the original EG.