Photoelectrochemical Degradation Research Articles

Photoelectrochemical degradation of humic acid on a (TiO2)0.7(RuO2)0.3 dimensionally stable anode

This study reports the results obtained for the degradation of humic acid (HA) in aqueous solution, by electrochemical and photoelectrochemical oxidation. The experiments were carried out using electrolysis and photo-assisted electrolysis on a thermally prepared oxide electrode mounted in a flow cell reactor. Results showed that both process are effective for the degradation of humic acid. However, by assisting electrolysis with UV radiation, the total organic carbon (TOC) reduction rate was greatly improved. For instance, at 20mAcm−2, electrolysis was responsible for 25% of TOC reduction after 180min of reaction time. On the other hand, the decrease during the photo-electrolysis reached 65% at the same current density and processing time.

Evaluation of color removal and degradation of a reactive textile azo dye on nanoporous TiO 2 thin-film electrodes

The feasibility of the photobleaching of a textile azo dye, reactive orange 16 (C.I. 17757), in aqueous solution using titanium dioxide thin-film electrodes prepared by the sol–gel method was investigated. The best conditions for maximum photoelectrocatalytic degradation were found to be pH>10 for Na 2SO 4 medium and pH<6 for NaCl. In both situations, an applied potential of +1.0 V and low dye concentration are recommended, when 100% of color removal is obtained after 20 min of photoelectrocatalysis. The effects of side reaction pathway on the degradation rate of dye in sulfate and chloride medium were presented and the best performance are optimized to situations closed to that verified in the textile effluent. The influence of variables as applied potential, pH, supporting electrolyte and dye concentration on the kinetics of photoelectrochemical degradation also were investigated. Oxalic acid is identified by HPLC and UV-Vis spectrophotometric methods as the main degradation product generated after 180 min of photoelectrocatalysis of 4×10 −5 mol l −1 dye in sodium sulphate pH 12 and NaCl pH 4.0 and a maximum reduction of 56 and 62% TOC was obtained, respectively.

Photoelectrochemical determination of chemical oxygen demand based on an exhaustive degradation model in a thin-layer cell

This work focuses on the experimental optimisation of a newly developed photoelectrochemical system for the determination of chemical oxygen demand (PECOD). This method is developed based on a photoelectrochemical degradation principle. Chemical oxygen demand (COD) is directly quantified by measuring the amount of electron transferred at a TiO 2 nanoporous thin-film electrode during an exhaustive photoelectrocatalytic degradation process in a thin-layer photoelectrochemical cell. In this fashion, the COD value of a sample can be determined in a simple, rapid and accurate manner. The PECOD is a direct and absolute method that requires no calibration. The effects of important experimental conditions, such as light intensity, applied potential bias, supporting electrolyte concentration and oxygen concentration, on analytical performance have been investigated and optimum experimental conditions were obtained. Analytical linear range of 0−360 ppm COD with a practical detection limit of 0.2 ppm COD were achieved. Real sample analyses were also carried out. The results demonstrated that the measured COD values using the PECOD and the standard methods were in an excellent agreement.

Photoelectrochemical determination of chemical oxygen demand based on an exhaustive degradation model in a thin-layer cell

This work focuses on the experimental optimisation of a newly developed photoelectrochemical system for the determination of chemical oxygen demand (PECOD). This method is developed based on a photoelectrochemical degradation principle. Chemical oxygen demand (COD) is directly quantified by measuring the amount of electron transferred at a TiO 2 nanoporous thin-film electrode during an exhaustive photoelectrocatalytic degradation process in a thin-layer photoelectrochemical cell. In this fashion, the COD value of a sample can be determined in a simple, rapid and accurate manner. The PECOD is a direct and absolute method that requires no calibration. The effects of important experimental conditions, such as light intensity, applied potential bias, supporting electrolyte concentration and oxygen concentration, on analytical performance have been investigated and optimum experimental conditions were obtained. Analytical linear range of 0−360 ppm COD with a practical detection limit of 0.2 ppm COD were achieved. Real sample analyses were also carried out. The results demonstrated that the measured COD values using the PECOD and the standard methods were in an excellent agreement.

Synergetic degradation of 2,4-D by integrated photo- and electrochemical catalysis on a Pt doped TiO2/Ti electrode

Abstract The photoelectrochemical degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) at a Pt doped TiO 2 /Ti electrode was investigated. The electrode was prepared by first oxidizing the surface of a titanium sheet to form TiO 2 and then doping platinum in H 2 PtCl 6 solution. The degradation efficiency of 2,4-D in the solution with 51 mg/l concentration was, respectively, 16% for electrochemical process and 27% for photochemical process after 5 h reaction. It became 53% when photo- and electrochemical processes were integrated. The 10% higher 2,4-D degradation efficiency by the integrated method than the sum of the two individual processes demonstrates an obvious synergetic effect between the photo- and electrochemical catalysis. Besides, a decrease in current density was observed with reaction time. The degradation efficiency increased with the increase of potential bias. The degradation of 2,4-D was also dependent on pH and concentration of sodium sulfate acting as a supporting electrolyte.

Fotocatálise eletroquímica de atrazina sobre dióxido de titânio: efeito de diferentes parâmetros experimentais

The atrazine photoelectrochemical degradation has been examined in solutions containing TiO2 on immobilized films under a variety of experimental conditions. It was possible to observe that the supporting electrolyte nature affects the intensity of the photocurrent, being an indicative of the adsorption process. The disappearance of the organic molecule follows approximately a pseudo-first order kinetic. As mineralization product, NH4+ and NO3- ion have been identified. These results indicated that the formation of NH4+ ion can be associated to the effect of atrazine adsorption, due to need of potential adaptation together with a variation in the supporting electrolyte concentration.

Mechanism and kinetics of 2-chlorophenol degradation in drinking water by photo-electrochemical synergic effect

The synergic effect mechanism of photo-electrochemical oxidation is investigated in detail through reaction products and kinetics analysis in a photo-electric integral reactor with 2-chlorophenol (2-CP) as the model pollutant. A kinetics model is constructed for the combinatorial photo-electrochemical (CPE) degradation. A remarkable synergetic effect, which can significantly enhance the mineralization rate of the CPE process, is verified by the comparison of apparent kinetic constants. In the CPE process, complemental effects with multi-level and multi-pathway for pollutants degradation under our experimental conditions are speculated. It is proved that the degradation pathways are not only the simple summation ofthat of photolysis and electrolysis, but the formation of synergic effect through combination of several new acting approaches. The degradation efficiency is enhanced considerably by three factors, control of electrode poisoning by the UV irradiation, control of excitation and reaction trend of pollutants molecules by the UV irradiation, and control of activation effect and transfer trend by the oriented direct current (DC) electric field. An advanced oxidation system is set up through manifold of free radicals chain reactions in the CPE reactions, so that the aqueous organics can be mineralized fast and completely. It is proven by the kinetics analysis that the mineralization of organic pollutants is mainly attributed to the generation of very active hydroxyl radicals (OH) in bulk solution from the CPE synergetic effect.

Photoelectrocatalytic degradation of 4-chlorophenol and oxalic acid on titanium dioxide electrodes

Photocatalytically active thin TiO 2 films were produced by spin-coating or dip-coating an alkoxy precursor onto a transparent conducting electrode substrate and by thermal oxidation of titanium metal. The thin films were used to study the photoelectrocatalytic or photoelectrochemical degradation of oxalic acid and 4-chlorophenol (4-CP) under near UV (monochromatic, 365 nm) light irradiation. Degradation was monitored by a variety of methods. In the course of oxalic acid degradation, CO 2 formation accounted for up to 100% of the total organic carbon degradation for medium starting concentrations; for the degradation of 4-CP, less CO 2 was detected due to the higher number of oxidation steps, i.e. intermediates. Incident-photon-to-current conversion efficiency, educt degradation and product formation as well as Faradaic efficiencies were calculated for the degradation experiments. Quantum yields and Faradaic efficiencies were found to be strongly dependent on concentration, with maximum values (quantum yield) around 1 for the highest concentrations of oxalic acid.

Feasibility study of photoelectrochemical degradation of methylene blue with three-dimensional electrode-photocatalytic reactor

The photoelectrochemical degradation of methylene blue in aqueous solution was investigated with three-dimensional electrode-photocatalytic reactor. It was found that the methylene blue could be degraded more efficiently by photoelectrochemical process than by photocatalytic oxidation or electrochemical oxidation alone. The decolorization efficiency and COD reduction were 95% and 87% for a photoelectrochemical process, respectively, while they were only 78% and 68% for a single electrochemical process and 89% and 71% for a single photochemical process. The TOC reduction of the former also reached as high as about 81% within a reaction time of 30.0 min. And these degradation reactions conformed to pseudo-first-order kinetics.

SYNERGIC DEGRADATION OF REACTIVE BRILLIANT RED X-3B USING THREE DIMENSION ELECTRODE-PHOTOCATALYTIC REACTOR

A new reactor, three dimension electrode-photocatalytic reactor, was designed and used to investigate the photoelectrochemical degradation of reactive brilliant red X-3B (RBRX) in simulated wastewater. The reactor was characterized by a series of parameters, the current change, decolorization ratio, COD removal and degradation ratio. It was found that the three dimensional electrode-photocatalytic reactor could effectively destroy the RBRX within a reaction time of 30 min. The results also showed that the photoelectrochemical process is more efficient than the single application of electrochemical oxidation or photocatalytic degradation. The degradation reactions of RBRX conformed to pseudo first order kinetics in the three processes, and an apparent synergic effect in the increase of the photocurrent and the disappearance of RBRX was observed by combining the electrochemistry with photocatalytic process in the three dimension electrode-photocatalytic reactor.

Photoelectrochemical mineralization of textile diazo dye pollutants using nanocrystalline WO 3 electrodes

Photoelectrochemical degradation processes of Naphthol Blue Black (NBB) dye in aqueous solutions were investigated seeking an effective method of degrading textile diazo dyes. Nanocrystalline WO 3 film on a Pt mesh electrode was used as the photoanode for photoelectrochemical degradation of NBB dye. Higher rates of NBB degradation were observed in chloride-containing environments than in sulfate solutions. A strong pH dependence of the NBB dye degradation rate has been found in chloride media. The pH dependence follows the trend of decreasing oxidizing power of electron holes, h VB,s, trapped at the surface of WO 3 particles, with increasing solution pH. In the low pH range (1.0–4.5), photooxidation of the medium favors the formation of Cl and Cl 2 − radicals which are powerful oxidants for the NBB dye. In alkaline solutions, the formation of OH radicals is predominant over Cl and Cl 2 − radicals but the kinetics of NBB degradation is much slower than in acidic media. In sulfate solutions, the rate of NBB photodegradation is slower than in chloride media. Evidently, the lower oxidizing power of the dipersulfate ion S 2O 8 −2 being the oxidant in this environment, is the limiting factor. The intermediates of the NBB dye degradation were detected using HPLC analysis. Possible dye degradation mechanism is proposed.

Photoelectrochemical degradation of naphthol blue black diazo dye on WO 3 film electrode

Photoelectrochemical degradation of a textile diazo dye, naphthol blue black (NBB), has been investigated using different semiconductor electrodes. A higher photoelectrocatalytic activity has been observed for WO 3 film electrodes, prepared by electrodeposition, than for TiO 2 nanoparticulate film electrodes. By annealing of the WO 3 film deposited on a Pt substrate, we were able to increase the photodegradation rate of NBB, owing to the reduced losses due to the electron–hole recombination. This treatment also increased the long-term photostability of the electrode. The effect of the supporting electrolyte was examined using NaCl, NaClO 4, KNO 3, and Na 2SO 4 solutions. The highest NBB degradation rate was found in the acidic Cl − medium. With increasing solution pH, the degradation rate decreased to a lower level indicating a change in the NBB photodecomposition mechanism. The reaction is of the first order with respect to NBB in the concentration range up to ca. 6×10 −5 M NBB. The apparent rate constants for the photoelectrocatalytic degradation of NBB in 0.5 M NaCl+8.8×10 −5 M NBB solution, on a WO 3 film electrode at E=1.08 V versus SCE, are 1.302×10 −4 and 3.41×10 −5 cm −2 s −1, under illumination with ultraviolet and visible light, respectively. It has been found that the mechanism of the sub-band excitation with the dye self-sensitization is a minor degradation path for NBB, while the main path involves the electron–hole generation and heterogeneous oxidation with participation of valence-band electron holes. The latter reactions involve oxidation of solution components with generation of powerful oxidants, such as the OH , Cl 2 −, and Cl radicals, being directly responsible for the fast dye decomposition.

Comparative assessment of the efficiency of TiO 2/OTE thin film electrodes fabricated by three deposition methods : Photoelectrochemical degradation of the DBS anionic surfactant

This study reports on a comparative assessment of the efficiencies of TiO 2/SnO 2-doped OTE thin film electrodes fabricated by three different deposition methods. Process efficiencies, as measured by the kinetics of degradation, were examined by monitoring the photo-oxidative fate of two test substrates, namely the anionic surfactant sodium dodecylbenzenesulfonate (DBS) and the model compound sodium benzenesulfonate (BS). Methods chosen to fabricate the TiO 2/OTE electrodes comprised (i) pulsed laser deposition, (ii) deposition of a TiO 2 paste on the OTE plate, and (iii) sol–gel deposition. Both DBS and BS are readily photo-oxidized and, under our conditions, were partially mineralized to CO 2 with the process being made particularly more efficient when the TiO 2/OTE electrodes were anodically biased at +0.3 V, which generated a photocurrent during the photoelectrolysis. The sol–gel {C} and anatase {B1} electrodes proved especially useful under the +0.3 V bias. In the absence of any bias, however, DBS photodegraded faster on the TiO 2-pasted electrode. Under otherwise similar conditions, the pulsed laser rutile TiO 2/OTE electrode showed very little photoactivity. Some of the characteristics of the TiO 2 film, such as the TiO 2 crystalline form, film thickness, surface roughness, and film transparency appear to have some effect on the overall photodegradative process efficiency.

Photoelectrochemical decomposition of amino acids on a TiO 2/OTE particulate film electrode

The photoelectrochemical degradation of amino acids and derivatives such as glutamic acid, glutamine, glutaric acid, lysine, β-alanine, 8-aminooctanoic acid and phenylalanine has been examined on an irradiated TiO 2/OTE particulate film electrode. The photooxidative disappearance of the substrates ultimately transforms the nitrogen into NO 3 − and NH 3 (as ammonium ions under our conditions), whereas the carbonaceous residues are converted into CO 2. Variations in photocurrent were observed during the temporal course of the photodegradative process. The rates of conversion and the quantity of degraded products depend on the external applied bias and appear to be closely related to the molecular structure of the substrates. A photoelectrochemical degradative pathway is discussed briefly on the basis of the experimental observations.

Photoelectrochemical Degradation of 4-Chlorocatechol at TiO2 Electrodes: Comparison between Sorption and Photoreactivity

The TiO2-catalyzed photodegradation of a strongly adsorbing substrate, 4-chlorocatechol, has been investigated as a function of solution concentration and pH at illuminated polycrystalline TiO2 electrodes operated at a constant current density. The results are compared to the previously determined sorption behavior of 4-chlorocatechol. The initial rates of 4-chlorocatechol photodegradation measured at pH 3, 5, and 8 and solution concentrations of 20, 50, and 200 μM show a linear correlation with the concentration of the sorbed substrate.

Applied studies on immobilized titanium dioxide films ascatalysts for the photoelectrochemical detoxification of water

Preliminary results are presented on the photochemical and photoelectrochemical degradation of aqueous organic solutions by anodic, thermal and sol–gel TiO2 films. The films were tested in a photochemical falling film reactor, and a photochemical/photoelectrochemical vortex reactor, and preliminary results are presented on the degradation of a range of model organics using these reactors. The former showed the best mass transport characteristics and most efficient light usage, whilst the latter reactor clearly showed the efficacy of the electric field enhancement effect. The results on the vortex reactor effectively represent a proof-of-concept of the electric field enhancement approach in large scale photoelectrochemical reactors. From time to time it is necessary to recoat the substrates, and the importance of the procedure adopted to remove ’old‘ TiO2 films prior to the fabrication of ‘new’ films is highlighted, as well as the mode of operation of the sol–gel films, and problems encountered in reactor design.

Photoelectrochemical properties of ‘Q-state’ CdS particles in arachidic acid Langmuir–Blodgett films

The photoelectrochemical (PEC) characteristics of thin films of ‘Q-state’ CdS semiconductor particles in arachidic acid Langmuir–Blodgett (LB) layers have been examined. Initially, ‘Q-state’ size CdS particles of ca. 2 nm were formed in the film and then progressively grown to ca. 10 nm by a cyclic sequence, which involves exposing the film to H2S(g) following its immersion in a CdCl2(aq) solution. A PEC cell was constructed using a layered glass/ITO–CdS/LB plate as the photoanode and a standard calomel electrode (SCE) as the reference electrode. The photoelectrical response of electrodes coated with LB films containing CdS particles of various sizes was investigated using white light. The reproducibility of current–voltage polarisation curves indicated the relatively good stability of the film to PEC degradation processes over several scanning cycles in the range 0 to –1000 mV vs. SCE. Values for the open-circuit (Voc) and the short-circuit current for the PEC cells were obtained and fill factors in the range 20 to 70–75% were calculated. Voc decreased from ca. –200 to ca. –800 mV vs. SCE with increasing particle size. This effect was attributed to the dielectric properties of the arachidic acid LB film matrix.