Photoelectrocatalytic Process Research Articles

Triethanolamine (TEA) Aqueous Solution as an Electrolyte Promoter in CO<sub>2</sub> Photoelectro-Conversion under Simulated Solar Irradiation Assisted by Cu-Doped Graphene-Titania Catalyst

Copper doped graphene-TiO2 nanocomposite were prepared via sol-gel process and embedded onto ITO coated glass films using electrophoretic deposition technique. The catalyst films were used for photoelectrocatalysis (PEC) of CO2 induced under simulated solar irradiation. Triethanolamine (TEA) aqueous solution was used as an electrolyte in this experiment. CO2 capture in the electrolyte solution plays vital role in the PEC process and in this case, the effect of TEA was investigated. Although the absorption rate of CO2 is lower in TEA solution than primary or secondary amines, CO2 loading was found higher in the solution. Less reactivity of TEA towards CO2 was also observed. The impedance results showed that, increased concentration of TEA in aqueous solution helps to reduce charge transfer resistance and thus facilitates CO2 reduction process.

Photoelectrocatalytic/photoelectro-Fenton coupling system using a nanostructured photoanode for the oxidation of a textile dye: Kinetics study and oxidation pathway

In this study, a coupled photoelectrocatalytic/photoelectro-Fenton reactor was designed to enhance the degradation efficiency of organic pollutants and tested using the azo dye Orange G as a model compound. Pt-decorated TiO2 nanotubes were used as a photoanode with an air-diffusion polytetrafluoroethylene cathode for H2O2 generation. The sum of individual effects of coupling the photoelectrocatalytic and photoelectro-Fenton processes was evaluated as a function of the decolorization and mineralization of Orange G solutions. The dye solutions were only completely decolorized in more acidic conditions (pH 3.0). The mineralization of the Orange G solutions increased in the sequence photoelectrocatalytic<electro-Fenton<coupled photoelectrocatalytic/photoelectro-Fenton due to the gradual increase in the production of OH radicals. Total organic carbon reductions of 80% for photoelectrocatalysis, 87% for electro-Fenton and 97% for the coupled processes were obtained when using an applied electric charge per unit volume of electrolyzed solution of 200mAhL−1. The Orange G decays for all treatments followed pseudo-first-order kinetics, suggesting the attack of a constant concentration of OH radicals. Aromatics such as naphthalenic and benzenic compounds were formed as by-products and were identified using LC–MS/MS analysis. In addition, the generated aliphatic acids were identified using ion-exclusion high-performance liquid chromatography. The final by-products of oxalic and formic acid were identified as ultimate by-products and formed Fe(III) complexes that were rapidly mineralized to CO2 by UV–Vis irradiation. Then, according to the identified oxidation by-products, a plausible pathway was proposed for the degradation of Orange G dye by the coupled process.

Photovoltammetric behavior and photoelectrochemical determination of p-phenylenediamine on CdS quantum dots and graphene hybrid film

A photoelectroactive film composed of CdS quantum dots and graphene sheets (GS) was coated on F-doped SnO2 (FTO) conducting glass for studying the electrochemical response of p-phenylenediamine (PPD) under photoirradiation. The result indicated that the cyclic voltammogram of PPD on CdS–GS hybrid film became sigmoidal in shape after exposed under visible light, due to the photoelectrocatalytic reaction. Such a photovoltammetric response was used to rapidly optimize the photoelectrocatalytic activity of hybrid films composed of different ratios of CdS to GS toward PPD. The influences of scan rate and pH on the photovoltammetric behavior of PPD on CdS–GS film revealed that although the controlled step for electrochemical process was not changed under photoirradiation, more electrons than protons might participate the photoelectrocatalytic process. Furthermore, the photoelectroactive CdS–GS hybrid film was explored for PPD determination based on the photocurrent response of film toward PPD. Under optimal conditions, the photocurrent signal on CdS–GS film was linearly proportional to the concentration of PPD ranging from 1.0×10−7 to 3.0×10−6molL−1, with a detection limit (3S/N) of 4.3×10−8molL−1. Our work based on CdS–GS hybrid film not only demonstrated a new facile photovoltammetric way to study the photoinduced electron transfer process of PPD, but also developed a sensitive photoelectrochemical strategy for PPD determination.

Photoelectrocatalysis Applied in Water Disinfection Contamined By C. Parapsilosis using Nanoporous Electrodes of W/WO3 Prepared By Electrochemical Anodization

The photoelectrocatalysis is a successful alternative for the inactivation of microorganisms, because it promotes inactivation in a short time and no formation of undesirable by-products and harmful to the people [1]. Tungsten trioxide has been an excellent alternative material in photoelectrocatalytic process, since presents small band gap energy (2.4 - 2.8 eV) and can to be photoexcited in the visible region [2]. This work reports the application of photoelectrocatalysis for disinfection water contaminated by fungus C. parapsilosis by using W/WO3 thin films grown onto tungsten substrate by electrochemical anodization with the aim to treat hemodialysis water. The nanoporous electrode was characterized by scanning electron microscopy and showed high porosity and organization, respectively. The presence of W and O was confirmed by EDS and the monoclinic form by X-ray diffraction. The PEC reactor is based on W/WO3 film acting as photoanode, Pt grid as counter electrode and Ag/AgCl as reference electrode. A solution of NaCl 0.1 mol.L-1 was tested as supporting electrolyte (pH 7) in function the high percentage of this salt in the hemodialysis water. High pressure Hg lamp (125W) was used as a source of electromagnetic radiation and the applied potential was 1.5V. In order to evaluate the method efficiency it was investigated the microbial death, system mineralization, chlorate and perchlorate ions and active chlorine generation during degradation. Therefore, aliquots were removed during 120 min of treatment and analyzed for microbial count, total organic carbon removal, ion chromatography and spectrophotometric measurements, respectively. The best condition for treatment was investigated and after only 1 min of the photoelectrochemical treatment Candida sp was inactivated under UV and visible light irradiation and 59.5 % of the organic matter mineralization, with kinetic constant equal to -0.0056 min-1. According to the chromatograms obtained was not verified the formation of potentially harmful ions. Spectrophotometric measurement confirms the low active chlorine formation at pH 7, but is higher at acidic medium. Ion chromatographic test indicates the no occurrence of chlorate and perchlorate. These result shows that the chloride ion present in the reaction system is only acting in conducting electrical current, not attending possible secondary reactions and death the fungus occurs due breaking the cell wall by the generation of hydroxyl radicals in rational way. The obtained results indicated that photoelectrocatalysis is a promising method for fungi degradation and disinfection of water and hemodialysis water under UV and visible light irradiation. Further studies are in progress to test organochlorides formation.

Study of synergistic effect among photo-, electro-, and sonoprocesses in photocatalyst degradation of phenol on tungsten-loaded titania nanotubes composite electrode

The degradation of 4-nitrophenol (4-NP) in aqueous solution with different processes was investigated. Tungsten-loaded titania nanotubes (WT-NTs) were used as electrode in photocatalytic (PC), sonophotocatalytic (SPC), photoelectrocatalytic (PEC), and sonophotoelectrocatalytic (SPEC) processes. WT-NTs electrode was fabricated by in situ anodization of titanium in a single-step process using sodium tungstate as the tungsten source. The morphology and structure were characterized by FE-SEM, XRD, and EDX. Experimental results showed that the hybrid processes could efficiently enhance the degradation efficiency of 4-NP and followed pseudo-first-order kinetics. At the optimized experimental conditions, the rate constants of degradation of 4-NP were 0.0594 min−1 for SPEC process, 0.0293 min−1 for PEC process, 0.0211 min−1 for SPC process, and 0.0116 min−1 for PC process. The rate constants indicated that there existed synergistic effect in the ultrasonic, electro-assisted, and photocatalytic processes.

Optimisation of parameters in a solar light-induced photoelectrocatalytic process with a TiO2/Ti composite electrode prepared by paint-thermal decomposition

Abstract This paper presents an outline of the effects of photoelectrocatalytic operating parameters in a batch reactor on the kinetics of photo(electro) catalytic (PEC) oxidation, using phenol as a model compound. Process parameters and electrode configuration were varied to examine which mechanism becomes dominant under which conditions. The parameters studied were: the effect of the TiO2 layer thickness, agitation, different light intensities (UV300–400), and the initial concentration of phenol in an aqueous solution as encountered in practice. The findings show that there is an optimal catalyst-layer thickness for oxidation efficiency, which is six layers. A higher initial concentration of phenol led to a higher overall degradation percentage of phenol as well, highlighting the importance of the surface area of the catalyst. Increasing the solar light intensity increased the kinetics of phenol oxidation due to the increasing concentration of reactive OH. However, the energy efficiency for the lower intensities was in the range of 0.1–2.7 kW h m−3 and was up to 1.3 times more energy efficient than the system using higher intensities. At low light intensity reactions, the electron–hole recombination effect was negligible. A trade-off exists between the energy efficiency of the system and the desired level of degradation of the chosen pollutant to obtain the desired rate of the reaction with an optimal energy consumption. The results can be used to optimize the performance of photoelectrocatalytic reactors to be used for the treatment of water containing micropollutants.

H2O2-assisted TiO2 generation during the photoelectrocatalytic process to decompose the acid green textile dye by Fenton reaction

Abstract TiO 2 electrode was used as photo-driven anode for oxygen reduction at carbon-felt cathode to produce H 2 O 2 used in the Fenton reaction to decompose an acid green 50 textile dye (AG50). The H 2 O 2 generated was catalyzed with ferrous ions in the photoelectroFenton process to decompose AG50 in cathode compartment. TiO 2 nanocomposite electrode was prepared by electrochemical anodization of pure Ti substrates. The results showed that TiO 2 anode exhibited a porous surface and organized nanotubes annealed at 450 °C and 600 °C with anatase phase, while a surface with less porous density with compact bars was obtained at 700 °C with anatase/rutile phase. The influence of applied bias potential, concentration of methanol in the anolyte, annealing temperature of TiO 2 nanocomposite and pH of the anolyte were studied on the photoelectrocatalytic degradation of methanol, whereas the catholyte composition remained unchanged for H 2 O 2 electrogeneration. Higher mineralization of both organic compounds was obtained by increasing the annealing temperature under acidic conditions using a bias potential of 1.02 V.

Charge transfer behavior of graphene-titania photoanode in CO2 photoelectrocatalysis process

In the present study, a graphene-titania composite photoelectrode was synthesized, characterized and examined for the photoelectrocatalytic (PEC) response. The charge transfer process on the semiconductor/electrolyte interface was investigated via electrochemical impedance spectroscopy (EIS) and voltammetry. In addition, the influence of pH toward the photoanode performance was also investigated and it was noticed that a high pH condition was favorable higher photocurrent response from the EIS measurements. The main reason could be attributed to the decrease of recombination process at the photoanode with fast quenching of the photogenerated holes with OH− ions at high pH. The experiment was also run for CO2 photoreduction and increased photocurrent was observed.

Novel wedge structured rotating disk photocatalytic reactor for post-treatment of actual textile wastewater

A wedge structured rotating disk photocatalytic (PC) reactor was used for post-treatment of actual textile wastewater collected after activated sludge treatment. Compared with a conventional planar disk, the wedge structured disk had larger surface area and exhibited better light utilization efficiency. Treatment conditions such as pH and rotating speed were optimized. The COD content in effluent was reduced from 380mgO2L−1 to 109 and 56mgO2L−1 respectively after 3h and 5h treatment at pH 6.0 and rotating speed of 20rpm. The effect of anions (NO3−, SO42− and Cl−) in actual textile wastewater on treatment was investigated. Benefit from high light utilization efficiency, the wedge rotating disk reactor could compensate COD removal inhibition caused by anions. A 1.5V voltage was also applied to perform photoelectrocatalysis process to further improve the performance, and no additional electrolyte was needed. COD in effluent was reduced to as low as 75mgO2L−1 after 3h treatment. In addition, this reactor was used to treat other water samples including the actual textile wastewaters collected before activated sludge treatment (initial COD of 880mgO2L−1) and after activated carbon adsorption post-treatment (initial COD of 108mgO2L−1). Experimental results suggested that wedge structured rotating disk photocatalysis process was stable, efficient and worth further scale-up investigation.

Photoelectrocatalytic degradation of p-hydroxybenzoic acid at the surface of a titanium/titanium dioxide nanotube array electrode using electrochemical monitoring

In this study, the titanium/titanium dioxide nanotube array (Ti/TiO2-NTA) electrode was prepared with anodic oxidation of Ti foil electrode. The morphology of Ti/TiO2-NTA electrode was evaluated with scanning electron microscopy images. The results showed that the inner diameter of nanotubes is below of 100nm. The electrochemical behavior of the as-prepared Ti/TiO2-NTA electrode was studied using the cyclic voltammetry. In addition, a significant photoelectrochemical behavior of the p-hydroxybenzoic acid (p-HBA) was observed on the Ti/TiO2 electrode using the hydrodynamic photoamperometry experiments. Then, the photoelectrocatalytic (PEC) degradation of the p-HBA was performed by this electrode, and compared with photocatalytic (PC), electrooxidation (EC), and direct photolysis by ultra-violet ray. It was found from mechanistic studies that the rate constant for the PEC process of Ti/TiO2-NTA electrode was higher than other degradation processes. The p-HBA concentration monitoring was carried out with the differential pulse voltammetry. Finally, the effects of the solution pH, applied potential, and the p-HBA concentration on the degradation efficiencies were studied and the results showed that the optimum pH for the photoelectrocatalytic degradation was equal to 7.00. The optimum potential and the optimum concentration were about 0.5V (vs. Ag|AgCl|KCl (3M) as reference electrode) and 0.129mM in the studied ranges, respectively.

Effect of oxidants on photoelectrocatalytic decolourization using α-Fe2O3/TiO2/activated charcoal plate nanocomposite under visible light

Effect of oxidants on the decolourization efficiency of Lanasol Yellow 4G in photoelectrocatalytic process using α-Fe2O3/TiO2/ACP nanocomposite under visible light.

Growth of TiO2 nanorod bundles on carbon fibers as flexible and weaveable photocatalyst/photoelectrode

TiO2 nanorod bundles were grew on carbon fibers (CFs), and the resulting CFs/TiO2 cloth can degrade 94.0% Rhodamine B in 100 min by photoelectrocatalytic (PEC) process, which is higher than the efficiency from photocatalytic (PC, 60.8%) or electrocatalytic (EC, 5.6%) route.

Photoelectrochemical determination of intrinsic kinetics of photoelectrocatalysis processes at {001} faceted anatase TiO2 photoanodes

Intrinsic degradation kinetic constant of oxalic acid at a double layered anatase TiO2 photoanode with dominantly exposed {001} facets.

Extending the photoelectrocatalytic detection range of KHP by eliminating self-inhibition at TiO2 nanoparticle electrodes

The photoelectrocatalytic oxidation of potassium hydrogen phthalate (KHP) at a titanium dioxide (TiO2) nanoparticle electrode was investigated in the presence of phosphate, which is usually considered an inhibitor of the photocatalytic process. Self-inhibition was observed during the photoelectrocatalytic degradation of KHP in the electrolyte solution with a little and without phosphate. This self-inhibition phenomenon cannot be eliminated by improving the light intensity but can be improved in the presence of a specified amount of phosphate, which extends the detection range of KHP. The adsorption of KHP indicated that the effective separation of KHP molecules adsorbed on the surface of the TiO2 electrode is a key prerequisite for eliminating the self-inhibition phenomenon. It is well-known that phthalic acid is one of the principal intermediates in the photocatalytic degradation of polynuclear aromatic hydrocarbons. Therefore, this study can provide a solution to the self-inhibition that occurs during the photocatalytic/photoelectrocatalytic degradation of polynuclear aromatic hydrocarbons. More importantly, this study can also provide an approach to extending the detection range of chemical oxygen demand using the photoelectrocatalytic process in the presence of polynuclear aromatic hydrocarbons.

N-type Cu2O Film for Photocatalytic and Photoelectrocatalytic Processes: Its stability and Inactivation of E. coli

Photoelectrocatalytic (PEC) inactivation of Escherichia coli K-12 by cuprous oxide (Cu2O) film irradiated by visible light is firstly reported. A complete inactivation of about 7 log of E. coli was obtained for Cu2O film within 6 h. The bacterial inactivation efficiency was significantly improved in a photoelectrochemical cell, in which 7 log of E. coli could be completely inactivated within 2 h by Cu2O film with a 0.1 V bias. Electric charge transfer between electrodes and E. coli, and electric charge inactivation towards E. coli were investigated using membrane-separated reactor combined with short circuit photocurrent technique. H2O2, hole, and toxicity of Cu2O film were found responsible for the inactivation of E. coli. Toxicity of copper ions (including Cu2+ and Cu+) leakage from Cu2O films was determined and the results showed that the amount of leakage copper ions was not toxic to E. coli. Finally, the Cu2O film was proved to be effective and reusable for PC and PEC inactivation of E. coli.

Combined photoelectrocatalytic/electro-Fenton process using a Pt/TiO2NTs photoanode for enhanced degradation of an azo dye: A mechanistic study

The combined photoelectrocatalytic/electro-Fenton (PEC/EF) process containing a Pt/TiO2NTs photoanode and an air-diffusion PTFE cathode is investigated in the degradation of the Acid Red 29 (AR29) dye. The photoelectrocatalytic (PEC), electro-Fenton (EF) and photoelectrocatalytic/electro-Fenton (PEC/EF) processes are compared in the performance of dye treatment. The mineralisation ability of the AR29 dye increased in the sequence PEC<EF<PEC/EF with total organic carbon (TOC) decays of 81%, 90% and 98%, respectively. The AR29 decays followed a pseudo first-order kinetic. The higher mineralisation capacity obtained for the PEC/EF process in comparison with the PEC and EF processes was ascribed to the additional production of OH by the photolytic reactions in bulk and on the Pt/TiO2NTs surface. The synergetic effect from the photocatalytic reduction of H2O2 onto the electrode surface (which minimises the electron/hole pair recombination) and the photolysis of Fe(III)–carboxylate complexes also enhanced the degradation efficiency of the coupled process. Finally, a plausible pathway for AR29 degradation via the PEC/EF process was proposed based on the by-products detected by HPLC and LC–MS/MS.

Preparation and application of α-Fe2O3/TiO2/activated charcoal plate nanocomposite as an electrode for electrosorption-assisted visible light photoelectrocatalytic process

A novel α-Fe2O3/TiO2/activated charcoal plate (α-Fe2O3/TiO2/ACP) nanocomposite was prepared by electrophoretic deposition method and characterized by X-ray diffraction, transmission electron microscope, scanning electron microscope, and energy dispersive X-ray. The prepared nanocomposite was used as an electrode for decolorization of reactive yellow 39 solution using a new electrosorption-assisted electrophotocatalytic process under visible light irradiation. Response surface methodology was used to model and optimize the decolorization process. The dye concentration of 10mgL−1, Na2SO4 concentration of 7.9gL−1, voltage of 700mV, and time of 150min were achieved as optimum conditions. Results showed that the decolorization efficiency in electrosorption-assisted visible light photoelectrocatalytic process using α-Fe2O3/TiO2/ACP nanocomposite was higher (94%) compared with those of TiO2/ACP (45%) and α-Fe2O3/ACP (60.26%) at optimum conditions. Moreover, the decolorization efficiency of 28.55% in adsorption, 43% in electrosorption, and 38.1% in photocatalytic processes were achieved using α-Fe2O3/TiO2/ACP nanocomposite at optimum conditions. The obtained results indicated that electrosorption-assisted photoelectrocatalytic process using α-Fe2O3/TiO2/ACP nanocomposite was more effective than other processes in the decolorization of the contaminated water.

Systematic approach to in-depth understanding of photoelectrocatalytic bacterial inactivation mechanisms by tracking the decomposed building blocks.

A systematic approach was developed to understand, in-depth, the mechanisms involved during the inactivation of bacterial cells using photoelectrocatalytic (PEC) processes with Escherichia coli K-12 as the model microorganism. The bacterial cells were found to be inactivated and decomposed primarily due to attack from photogenerated H2O2. Extracellular reactive oxygen species (ROSs), such as H2O2, may penetrate into the bacterial cell and cause dramatically elevated intracellular ROSs levels, which would overwhelm the antioxidative capacity of bacterial protective enzymes such as superoxide dismutase and catalase. The activities of these two enzymes were found to decrease due to the ROSs attacks during PEC inactivation. Bacterial cell wall damage was then observed, including loss of cell membrane integrity and increased permeability, followed by the decomposition of cell envelope (demonstrated by scanning electronic microscope images). One of the bacterial building blocks, protein, was found to be oxidatively damaged due to the ROSs attacks, as well. Leakage of cytoplasm and biomolecules (bacterial building blocks such as proteins and nucleic acids) were evident during prolonged PEC inactivation process. The leaked cytoplasmic substances and cell debris could be further degraded and, ultimately, mineralized with prolonged PEC treatment.

Photocatalytic and photoelectrocatalytic degradation and mineralization of small biological compounds amino acids at TiO2 photoanodes

Quantitative evaluation of photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of small biological compounds like amino acids (AAs) were systematically investigated at illuminated nanoparticulate TiO2 photoanode using a thin-layer photoelectrochemical cell. Three model AAs like phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp), were found to be photocatalytically and photoelectrocatalytically degradable. PEC degradation efficiency was much higher than that of PC method for all compounds investigated, and the superiority becomes more obvious at higher concentrations. Organic nitrogen atoms in AAs can be oxidized to either NH3/NH4+ or NO3–, or both, depending the chemical structures of AAs and degradation methods used. HPLC analysis found that, for a given AA except Phe, the hydrophilicity characteristics of intermediates are differed slightly between PC and PEC process. For a given method, similar hydrophilicity characteristics were obtained from all AAs investigated except Phe. Theoretically calculated results showed that initial reaction sites for all single-ringed AAs are likely to occur on the atoms within 6-membered ring structure, while for double-ringed AA, the initial reaction sites are likely to occur at 6-membered ring structure (PC) or at 5-membered ring structure (PEC). Both experimental and theoretical results demonstrated PEC reaction mechanisms differed remarkably from that of PC process.

WO₃/W nanopores sensor for chemical oxygen demand (COD) determination under visible light.

A sensor of a WO3 nanopores electrode combined with a thin layer reactor was proposed to develop a Chemical Oxygen Demand (COD) determination method and solve the problem that the COD values are inaccurately determined by the standard method. The visible spectrum, e.g., 420 nm, could be used as light source in the sensor we developed, which represents a breakthrough by limiting of UV light source in the photoelectrocatalysis process. The operation conditions were optimized in this work, and the results showed that taking NaNO3 solution at the concentration of 2.5 mol·L−1 as electrolyte under the light intensity of 214 μW·cm−2 and applied bias of 2.5 V, the proposed method is accurate and well reproducible, even in a wide range of pH values. Furthermore, the COD values obtained by the WO3 sensor were fitted well with the theoretical COD value in the range of 3–60 mg·L−1 with a limit value of 1 mg·L−1, which reveals that the proposed sensor may be a practical device for monitoring and controlling surface water quality as well as slightly polluted water.