TiO2 Photocatalytic Degradation Research Articles

Mechanism of photocatalytic degradation of dye MG by TiO2-film electrode with cathodic bias potential

Photoelectrocatalytic degradation of malachite green (MG) under visible light irradiation with TiO2-film electrode has been investigated to reveal the mechanism for TiO2 photocatalytic degradation of dyes. The supported TiO2 electrode was prepared in laboratory and detected by scanning electron micros-copy and X-ray diffractometry. We have examined the degradation kinetics, change in degradation rate of MG and photocurrent change with the bias potential, voltage-induced adsorption of dyes, accumu-lation of electrons in the TiO2 electrode, effect of various additives such as benzoquinone (BQ), and formation of active oxygen species by UV-visible spectroscopy, TOC and spin-trap ESR spectrometry. It was found that the dyes could controllably interact with TiO2 surface by external bias and charge of dyes. The cationic dye MG underwent efficient mineralization at negative bias under visible light irradiation, but N-dealkylation process predominated at positive bias under visible light irradiation. At negative bias of −0.4 V vs SCE, the O2·− and dye·+ were formed simultaneously at the electrode/electrolyte interface during degradation of cationic MG. Experimental results imply both the superoxide radical and the dye cationic radical are essential to the mineralization of the dyes under visible-light-induced photocatalytic conditions.

Photocatalytic oxidation of phenolic compounds using zinc oxide and sulphate radicals under artificial solar light

Photocatalytic degradation of phenolic compounds in aqueous solution was investigated in TiO2/persulphate/UV–vis and ZnO/persulphate/UV–vis light. It is found that ZnO exhibits higher activity than TiO2 in photocatalytic degradation of phenol under simulated solar light. The oxidation of phenol in either ZnO/UV–vis or persulphate/UV–vis system occurs at a similar rate while the degradation will be significantly enhanced with ZnO/persulphate/UV–vis system. The kinetics of phenol oxidation in ZnO/persulphate/UV–vis system follows the first order model. Several parameters influencing the reaction rate were also investigated including power intensity, phenol, ZnO and persulphate concentrations. It is shown that stronger light intensity will increase phenol oxidation. An optimal concentration of ZnO and persulphate will be required for maximum degradation of phenol. For phenol, dichlorophenol and trichlorophenol, photocatalytic activity in ZnO/persulphate/UV–vis shows an order of phenol > dichlorophenol > trichlorophenol.

TiO2-Photocatalyzed degradation of phenol in saline media in an annular reactor: hydrodynamics, lumped kinetics, intermediates, and acute toxicity

The photocatalytic degradation of phenol in aqueous suspensions of TiO2 under different salt concentrations in an annular reactor has been investigated. In all cases, complete removal of phenol and mineralization degrees above 90% were achieved. The reactor operational parameters were optimized and its hydrodynamics characterized in order to couple mass balance equations with kinetic ones. The photodegradation of the organics followed a Langmuir-Hinshelwood-Hougen-Watson lumped kinetics. From GC/MS analyses, several intermediates formed during oxidation have been identified. The main ones were catechol, hydroquinone, and 3-phenyl-2-propenal, in this order. The formation of negligible concentrations of 4-chlorophenol was observed only in high salinity medium. Acute toxicity was determined by using Artemia sp. as the test organism, which indicated that intermediate products were all less toxic than phenol and a significant abatement of the overall toxicity was accomplished, regardless of the salt concentration.

Preparation of Co-doped TiO2 for Photocatalytic Degradation of NOx in Air under Visible Light

AbstractCo-doped visible light active photocatalysts were obtained by chemical modification of TiO

A comparative study of photocatalytic degradation of phenol of TiO 2 and ZnO in the presence of manganese dioxides

The effects of the presence of α-MnO 2, β-MnO 2 and δ-MnO 2 on the photocatalytic degradation of phenol by TiO 2 and ZnO have been investigated. Experiments were conducted in a batch reactor, at initial pH 6.0 and 25 °C. The results showed that the TiO 2-photocatalytic degradation of phenol was inhibited completely in the first 20–60 min in the presence of three crystal types of MnO 2 whereas the ZnO-photocatalytic degradation of phenol only declined slightly. UV-vis diffuse reflectance spectroscopy (DRS) and surface photovoltage spectroscopy (SPS) analysis implied that the interaction of MnO 2 with TiO 2 and ZnO could change the energetic of these two photocatalysts.

Photocatalytic oxidation of aqueous ammonia in model gray waters

This study investigated the TiO2 photocatalytic degradation of aqueous ammonia (NH4+/NH3) in the presence of surfactants and monosaccharides at pH∼10.1. Initial rates of NH4+/NH3 photocatalytic degradation decreased by approximately 50–90% in the presence of anionic, cationic, and nonionic surfactants and monosaccharides. Through correlation analysis, we concluded that scavenging of hydroxyl radical (OH) by the products of surfactant/monosaccharide photocatalytic degradation, including carbonate and formate, could explain approximately 80% of the variance in initial rates of NH4+/NH3 removal in our system. Addition of a supplemental OH source (H2O2) enhanced the rate of NH4+/NH3 degradation in the presence of the surfactant Brij® 23 lauryl ether (Brij® 35), further supporting the idea that OH scavenging is the mechanism by which surfactants and monosaccharides decreased initial rates of NH4+/NH3 photocatalytic degradation. Despite slowed rates of NH4+/NH3 degradation, both surfactants/monosaccharides and NH4+/NH3 were removed by TiO2 photocatalysis, indicating that this process can effectively remove both carbonaceous and nitrogenous biochemical oxygen demand from gray water.

Chemometric study on the TiO 2-photocatalytic degradation of nitrilotriacetic acid

A chemometric study on the TiO 2-photocatalytic degradation of nitrilotriacetic acid (NTA) in aqueous media under UV radiation has been carried out taking into account the multiple variables that take part in the system. To save redundant number of experiments, the system has been managed under chemometric techniques for several variables as NTA and TiO 2 concentrations, pH and irradiation time. Multiple-way analysis of the variance (MANOVA) has been applied to find the statistically significant variables. An artificial neural network (ANN) has been used to build an empirical model of the system. All measurements have been driven under experimental designs: a full-factorial design (FFD) was used to analyze significant factors through MANOVA, and a Doehlert design, which was modified by spatial rotation, was applied in order to have a satisfactory number of levels for the factor time to be able to train the ANN. The study allows the knowledge and prediction of the behavior of the system as well as to work out kinetic parameters and to optimize their variables. The results of kinetic parameters obtained with the neural network agreed with independent experimental results, confirming a Langmuir–Hinshelwood kinetic regime. The difference between NTA and ethylenediaminetetraacetic acid (EDTA), which has been previously studied, is also established.

Photocatalytic degradation of methyl thionine chloride in aqueous solution over nanometer (CdS/TiO2)/MCM-41

(CdS/TiO2)/MCM-41 loaded nanometer photocatalyst was prepared by the sol-gel method and dipping process, the photocatalytic degradation of methyl thionine chloride in water was investigated by using the photocatalyst. The experimental results show that the optimum concentration of CdS over TiO2 was 3% (molar ratio), the photocatalytic activity was enhanced when making TiO2 the anatase phase with a rise of the roasting temperature, and the carrier, mesoporous molecular sieve MCM-41, was beneficial to improving the photocatalytic activity of TiO2 for photocatalytic degradation of methyl thionine chloride. The morphology and the crystalline phase of the photocatalyst were discussed by means of XRD and SEM techniques, and the reaction mechanism of catalytic properties was also discussed.

Photocatalytic Degradation of Pesticide Pyridaben. 3. In Surfactant/TiO2 Aqueous Dispersions

The effective TiO2 photocatalytic degradation of pyridaben in an acetronitrile/water dispersion has been investigated in previous work, but could not be achieved in the case of real waters. In this paper, photocatalytic degradation of pyridaben on TiO2 particles under UV light (lambda> 360 nm) illumination in surfactant CTAB (cetyltrimethyl ammonium bromide) aqueous dispersions was studied. 1H NMR was used to determine quantitative information about the adsorption mode of pyridaben in CTAB micelles. The results showed that the upfield 1H shifts were largest for long chain protons of CTAB, indicating that the hydrophobic aromatic rings were primarily located in this region. Adsorption models on TiO2 surface were thus proposed. The reaction rates decreased with the increase of pH value, which can be attributed to the surface charge variations of pyridaben adsorbed onto TiO2 particles. The adsorption isotherms at different pH values confirmed that preadsorption on the surface of TiO2 particles was prerequisite for efficient degradation. Furthermore, an oxidation reagent such as H2O2 was added to the photocatalytic system, which may act as an alternative electron acceptor and result in a notably enhanced rate of pollutant destruction. On the basis of intermediates identified by GC/MS, a degradation pathway was proposed.

Hydrotalcite Mineral Stabilization of Titanium Dioxide Photocatalyzed Degradation of Unplasticized Poly(vinylchloride)

Hydrotalcite additions (up to 10 PHR) can stabilize un- plasticized-poly-vinyl-chloride against TiO2 photo-catalyzed degradation. Without stabilization, hydrogen chloride, produced through de-hydrochlorination, catalyses the TiO2 sensitized degradation after a set period of reaction leading to an increase in rate from 13 mu-mol m-2 min-1 to 32 mu-mol m- 2 min-1. Hydrotalcite removes chloride by anion exchange releasing carbonate ions in the PVC removing this autocatalytic pathway. Hydrotalcite performs very well in comparison to commercial Barium/Zinc salt stabilizers.

Photocatalytic Degradation of p-Halophenols in TiO2 Aqueous Suspensions: Halogen Effect on Removal Rate, Aromatic Intermediates and Toxicity Variations

The influence of the halogen upon the TiO2 photocatalytic degradation of p-halophenols in water has been investigated. Phenol was used as the reference compound. Compared with its value for phenol, the apparent first-order rate constant of removal, k, was slightly but significantly higher for p-fluorophenol and p-chlorophenol, and slightly but significantly lower for p-bromophenol. For p-iodophenol, k was about half that of phenol. The relative values confirm that k is roughly correlated to the Hammett constant; this constant reflects the electron density on the aromatic ring and, accordingly, the reactivity towards electrophilic species generated by UV-irradiation of TiO2. All compounds were found to be poorly adsorbed on TiO2. Accordingly, k was not related to the differences observed in the very low adsorbed amounts. The detected aromatic intermediate products included hydroquinone (HQ), benzoquinone (BQ) and various halodihydroxybenzenes. HQ, BQ, 4-chloro-1,2 (and 1,3)-dihydroxybenzenes and 4-bromo-1,3-dihydroxybenzene were quantified. Mechanisms are tentatively suggested to interpret the differences in the degradation pathways of the p-halophenols. The organic intermediate products accounted for only a few percents of the total carbon during the degradation. The toxicity (1/EC50) measured by the Microtox test almost did not vary in the course of the degradation of phenol, p-chlorophenol and p-bromophenol until complete removal of these compounds. By contrast, the value of 1/EC50 was multiplied by ca. 2.5 when ca. 45% of p-iodophenol had been removed; concentrations of BQ higher than with the other p-halophenols are tentatively suggested to be at the origin of this increase. Interpretation of a surprising substantial increase in the 1/EC50 value when the removal of p-fluorophenol increased from 80 to 95% requires further investigation.

Determination of phenol in the presence of its principal degradation products in water during a TiO 2-photocatalytic degradation process by three-dimensional excitation–emission matrix fluorescence and parallel factor analysis

This paper describes a simple and rapid way of monitoring a photocatalytic degradation of phenol in aqueous suspensions of TiO 2. A three-way analytical methodology based on fluorescence excitation–emission matrix (EEM) and parallel factor analysis (PARAFAC) was developed to resolve the species present in the reaction mixture and quantify the concentration of phenol and its principal degradation products throughout the degradation. Parameters such as core consistency, fit% and correlation coefficients between recovered and pure spectra were used to determine the appropriate number of factors for the PARAFAC model. The accuracy of the model was evaluated by the root mean square error of prediction (RMSEP). Using a four-factors PARAFAC model, phenol, hydroquinone, resorcinol and catechol, were satisfactorily determined. The proposed method is an interesting alternative to the traditional techniques normally used for monitoring degradation reactions.

Photocatalytic Degradation of Organic Pollutants Under Visible Light Irradiation

Several TiO2-based photocatalytic systems that have considerable visible light response have been developed, such as the photodegradation of organic pollutants on sensitized TiO2 by visible light, construction of visible-light-active novel TiO2 photocatalysts by matrix or surface modification. In this paper, we review briefly our recent progress in the TiO2 photocatalytic degradation of organic pollutants by visible light, some related work by other groups is also involved.

On the mechanism of TiO 2-photocatalyzed degradation of aniline derivatives

Interaction between aromatic amines and TiO 2 takes place preferentially through the amino group when pH > p K a(BH +) and, as a minor but mechanistically relevant mode, via a π-interaction if pH < p K a(BH +). No significant direct photodegradation of aniline or N, N-dimethyl-aniline is detected in acidic medium using λ > 290 nm, but it is enhanced in alkaline medium. 2-Aminophenol and benzoquinone are the main photoproducts of direct irradiation of aniline. The main photoproducts of photocatalytic degradation of aniline at the pH of maximum adsorption are 2-aminophenol and phenol. Scavenging HO • with t-BuOH shows that adsorbed aniline is oxidized by positive holes (h +), with participation of the anilinium radical cation. In the case of N, N-dimethyl-aniline at the pH of maximum adsorption, N-methyl-aniline is the main photoproduct, formed also via the dimethyl anilinium radical cation. Photocatalytic degradation in acid medium is inhibited due to electrostatic repulsion between the positively charged surface and the protonated amines. Aniline is mainly transformed into phenol and 2-aminophenol, and N, N-dimethyl-aniline into aniline, that undergoes hydroxylation to phenol. In alkaline medium the main photoproduct of degradation of aniline is nitrobenzene, formed with involvement of the anilinyl radical.

TiO2-photocatalyzed degradation of phenol in saline media: lumped kinetics, intermediates, and acute toxicity

The photocatalytic degradation of phenol in aqueous suspensions of TiO2 under different salt concentrations has been investigated. In all cases, complete removal of phenol and mineralization degrees above 90% were achieved. However, the time needed to effectively remove phenol and degrade organics was considerably increased in high salinity medium. The photodegradation of the organics followed a Langmuir–Hinshelwood–Hougen–Watson lumped kinetics, showing rate and adsorption/desorption constants that decrease as salinity increases. From GC/MS analyses, various hydroxylated intermediates formed during oxidation have been identified. The main ones were catechol, hydroquinone, and 4,4′-dihydroxybiphenyl, in this order. It was shown that salinity inhibited catechol formation and promoted 4,4′-dihydroxybiphenyl production. The formation of negligible concentrations of 4-chlorophenol was observed only in high salinity medium. Acute toxicity during degradation process was determined by using Artemia sp. as the test organism, which is suited for saline environments. Intermediate products were all less toxic than phenol and a significant removal of the overall toxicity was accomplished, regardless of the salt concentration.Photocatalytic phenol degradation in the absence of salt and in low salinity media (2gL−1) was quite the same (100% for a 150min irradiation time). However, high salinity media (50gL−1) showed a significant difference (81%). Organic matter mineralization showed a similar behavior regarding the media (94 and 44%, respectively).

Early stages in the TiO 2-photocatalyzed degradation of simple phenolic and non-phenolic lignin model compounds

The TiO 2-photocatalyzed degradation of phenolic and non-phenolic benzyl alcohols, which can be considered simple lignin model compounds has been studied in aqueous solution under a variety of conditions, with the aim of acquiring fundamental information on the early stages of the degradation process. The role of the electron scavengers O 2, K 2S 2O 8 and N 2O on the degradation process as well as the influence of pH have been investigated. A major role in the degradation process is played by OH and O 2 −, while SO 4 − plays a minor role. OH plays a fundamental role in the conversion of non-phenolic substrates into phenolic ones, while O 2 − exerts its role mainly by reacting with phenoxyl radicals to give aromatic ring-opening products. Addition of K 2S 2O 8 as electron scavenger allows substantial substrate degradation also in the absence of O 2, while N 2O which is an excellent scavenger for the hydrated electron seems to be a very poor electron scavenger under the heterogeneous conditions employed. A general mechanistic scheme which proceeds through the formation of aromatic radical cations via one-electron oxidation of the neutral substrates by TiO 2(h +), accounting for all the experimental observations has been proposed.

Heterogeneous photodegradation of aqueous hydroxy butanedioic acid by microporous polyoxometalates

A new type of heterogeneous photocatalytic materials, microporous polyoxotungstates such as H3PW12O40/SiO2, H4SiW12O40/SiO2 and Na4W10O32/SiO2, were synthesized by incorporating the polyoxotungstates into the silica matrix via a sol–gel technique, and some physico-chemical properties of as-synthesized composites were measured. The photocatalytic activity of the composites was tested via degradation of the model molecule, hydroxy-butanedioic acid (malic acid, (MA)). After 90 or 180min irradiation in the presence of Na4W10O32/SiO2 or H3PW12O40/SiO2 (H4SiW12O40/SiO2), an aqueous MA (100mg/l) was totally degraded into the intermediates such as oxalic acid, glyceric acid, tartaric acid, butenedioic acid, acetic acid, formic acid and so on. After subsequent 90 or 60min irradiation under the same conditions, these intermediates were totally mineralized into CO2 and H2O. Disappearance of MA (in the range of 0–200mg/l) followed Langmuir–Hinshelwood first-order kinetics, and the reaction rate constant and adsorption equilibrium constant were measured. Most of the intermediates, identified during the process of H4SiW12O40/SiO2-photocatalyzed MA degradation, are the same with those detected during the process of TiO2-photocatalyzed MA degradation. The mechanism studies showed that the desolubilized polyoxotungstates appears to photo-oxidative degradation of MA by both OH radical attack and excited state polyoxotungstate direct oxidation.

Multivariate approach to photocatalytic degradation of a cellulose bleaching effluent

TiO2-photocatalytic degradation of a cellulose ECF effluent was evaluated using multivariate experimental design. The effluent was characterised by general parameters such as AOX, TOC, COD, colour, total phenols, acute toxicity and by the analysis of chlorinated low molecular weight compounds using GC/MS. The pH, catalyst amount and hydrogen peroxide were simultaneously varied using multivariate analysis to establish the weight of each variable in the COD and AOX removal after 180min of reaction. The pH has a large effect in the AOX degradation at values below 5 and over 10, while COD removal is improved at low pH. The presence of hydrogen peroxide in the photocatalytic reaction has a small effect in degrading the AOX and COD. The optimal concentration of TiO2 was found to be around 1gl−1. In addition, the reaction was monitored for longer periods of irradiation by carrying out TOC, COD, AOX, acute toxicity and spectrophotometric analysis, concluding that the organic matter contained in the effluent was significantly degraded. After 30min of reaction more than 60% of the toxicity was removed and after 420min of reaction none of the initial chlorinated low molecular weight compounds were detected, suggesting an extensive mineralisation which was corroborated by 95 and 50% AOX and TOC removals, respectively. Colour and total phenols were also removed.

TIO 2-photocatalyzed degradation of phenol and ortho-substituted phenolic compounds

The photocatalytic degradation of phenol, guaiacol, 2-chlorophenol and catechol in aqueous suspensions of TiO 2 under different experimental conditions has been investigated. The photodegradation of the different organics follows a Langmuir–Hinshelwood kinetics, showing rate constants that decrease in the order: guaiacol>2-chlorophenol≅phenol>catechol. A similar trend is also observed, except for catechol, for the stability of the σ-complexes that may be formed between the aromatic ring and the OH radical. From different analytical techniques (HPLC, GC/MS and HPLC/MS), various hydroxylated intermediate compounds have been reported and different mechanisms of degradation of the starting compounds have been proposed. From experiments performed using aqueous solutions containing the four organics, it is observed that the competitive Langmuir–Hinshelwood kinetics for the degradation of each one of the phenolic compounds is obeyed.

Surface state studies of TiO2 nanoparticles and photocatalytic degradation of methyl orange in aqueous TiO2 dispersions

Abstract Nanocrystalline TiO2 is prepared using a stearic acid gel method. Their surface state is analyzed by means of X-ray photoelectron spectroscopy (XPS). The photocatalytic degradation of methyl orange in TiO2 suspension is investigated. The relationships between surface state and photocatalytic activity are discussed.