无机物对光催化降解苯酚的影响

采用TiO_2悬浮体系,考察了无机物对光催化降解苯酚的影响。实验结果表明,当H_2O_2质量浓度为0.45mg/L或K_2S_2O_8质量浓度为0.05g/L时,苯酚的降解率较高。     

Preparation of polyaniline-modified TiO2 nanoparticles and their photocatalytic activity under visible light illumination

Titanium dioxide nanoparticles were modified by polyaniline (PANI) using ‘in situ’ chemical oxidative polymerization method in hydrochloric acid solutions. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy spectrum (XPS) and UV–vis spectra were carried out to characterize the composites with different PANI contents. The photocatalytic degradation of phenol was chosen as a model reaction to evaluate the photocatalytic activities of the modified catalysts. Results show that TiO₂ nanoparticles are deposited by PANI to mitigate TiO₂ particles agglomeration. The modification does not alter the crystalline structure of the TiO₂ nanoparticles according to the X-ray diffraction patterns. UV–vis spectra reveal that PANI-modified TiO₂ composites show stronger absorption than neat TiO₂ under the whole range of visible light. The resulting PANI-modified TiO₂ composites exhibit significantly higher photocatalytic activity than that of neat TiO₂ on degradation of phenol aqueous solution under visible light irradiation (λ ≥ 400 nm). An optimum of the synergetic effect is found for an initial molar ratio of aniline to TiO₂ equal to 1/100.     

Photocatalytic degradation kinetics of di- and tri-substituted phenolic compounds in aqueous solution by TiO2/UV

In this study, photocatalytic degradation of 2,4,6-trimethylphenol (TMP), 2,4,6-trichlorophenol (TCP), 2,4,6-tribromophenol (TBP), 2,4-dimethylphenol (DMP), 2,4-dichlorophenol (DCP) and 2,4-dibromophenol (DBP) has been studied by TiO₂/UV. Although degraded phenolic compound concentration increased by increasing initial concentration photocatalytic decomposition rates of di- and tri-substituted phenols at 0.1–0.5 mM initial concentrations decreased when the initial concentration increased. The fastest degradation observed for TCP and the slowest for TMP. Photodegradation kinetics of the compounds has been explained in terms of Langmuir–Hinshelwood kinetics model. Degradation rate constants have been observed to be extremely depended on electronegativity of the substituents on phenolic ring. Degradation rate constant and adsorption equilibrium constant of TCP were calculated as k 0.0083 mM min⁻¹ and K 9.03 mM⁻¹. For TBP and TMP the values of k and K were obtained as 0.0040 mM min⁻¹, 19.20 mM⁻¹, and 0.0017 mM min⁻¹, 51.68 mM⁻¹, respectively. Degradation rate constant of DBP was similar as DCP (0.0029 mM min⁻¹ for DBP and 0.0031 mM min⁻¹ for DCP) whereas adsorption equilibrium constants differed (48.40 mM⁻¹ for DBP and 30.52 mM⁻¹ for DCP). K and k of DMP found as 83.68 mM⁻¹ and 0.0019 mM min⁻¹, respectively. The adsorption equilibrium constants in the dark were ranged between 1.11 and 3.28 mM⁻¹ which are lower than those obtained in kinetics. Adsorption constants have inversely proportion with degradation rate constants for all phenolic compounds studied.     

Phenol photodegradation with oxygen and hydrogen peroxide over TiO2 and Fe-doped TiO2

Photodegradation of phenol was investigated with two types of oxidant agents in water, oxygen and hydrogen peroxide, at two different reaction pH with a series of nanosized iron-doped anatase TiO₂ catalysts with different iron contents. The catalysts have been prepared by a sol–gel/microemulsion method. Firstly, iron-doped titania catalysts were studied with respect to their activity behavior when oxygen was used as oxidant agent in the photocatalytic degradation of aqueous phenol in comparison with un-doped reference catalysts. Secondly, two catalysts (TiO₂ and 0.7 wt.% Fe-doped TiO₂) were selected to extend the study for the employment of hydrogen peroxide as oxidant at different concentrations and two initial reaction pHs. An enhancement of the photocatalytic activity is observed only for relatively low doping level (ca. 0.7 wt.%) in catalyst calcined at 450 °C preferably using hydrogen peroxide as oxidant agent which is attributable to the partial introduction of Fe³⁺ cations into the anatase structure. Nevertheless, it has been demonstrated that catalyst surface properties can play an important role during phenol photodegradation process on the basis of the analysis of differences found in the photoactivity as a function of reaction pH.     

A batch LED reactor for the photocatalytic degradation of phenol

Photocatalytic degradation of phenol by titanium dioxide illuminated by one light emitting diode (LED) in a batch photocatalytic reactor is reported in this paper. The effect of catalyst loading, catalyst type, phenol–hydrogen peroxide ratio, pH, initial phenol concentration and irradiance by applying pulse width modulation (PWM) was studied. The effect of the beam width on photocatalytic degradation of phenol is also included in this paper as is the use of different type of reflectors outside the reactor. With both an LED beam width of 120° and optimal chemical conditions of 10 ppm phenol concentration with a hydrogen peroxide–phenol molar ratio of 100 and pH of 4.8, a degradation rate of 42% was achieved after 4 h. Decreasing the beam width to 40° raised degradation to 87%.In order to study the irradiance distribution and its effect on the reactor performance, experiments were conducted incorporating various catalysts loading, reactor heights and beam widths. The irradiance was measured for different amount of catalyst loading ranging from 0.17 to 1.8 g L⁻¹at different reactor heights. The results are compared with optimal catalyst loading measurement to assess the correlation between phenol degradation and irradiance distribution. The UV LED in combination with titanium dioxide is appropriate for water treatment to degrade organic pollutants at low concentration.     

Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. I: Effect of charge-trapping species on the degradation kinetics

The effects of charge-trapping species on the kinetics of phenol decomposition were studied in near-UV-irradiated aqueous TiO₂ (anatase) suspensions in a batch photoreactor. The influence of catalyst loading, initial phenol concentration, dissolved O₂ concentration, Ag⁺ content and H₂O₂ concentration on the rate of phenol degradation is reported. The observed heterogeneous degradation of phenol followed apparently zero-order kinetics up to ca. 70% conversion. The Langmuir–Hinshelwood kinetic model successfully described the influence of the initial phenol concentration and dissolved O₂ concentration on the rate of heterogeneous photooxidation of phenol. The data obtained by applying the Langmuir–Hinshelwood treatment are consistent with the available kinetic parameters. The results of the experiments in the presence of Ag⁺ indicated that the phototransformation of phenol can proceed via direct electron transfer, neither dissolved O₂ nor its reduction forms playing a significant role in the degradation mechanism.     

Gas-phase photocatalytic degradation of trichloroethylene on pretreated TiO2

This paper reports the effects of preillumination, prechlorination and prehydroxylation of TiO₂ glass fiber cloth (TiO₂-GFC) on the photocatalytic degradation (PCD) reaction of trichloroethylene (TCE) in gas–solid regime. The reaction was monitored in situ by FT-IR spectroscopy at room temperature (298 K). Product analysis by gas chromatography–mass spectrometry (GC–MS) revealed the formation of a new by-product, 1,1-dichloroethane (1,1-DCE) in significant amount along with other known by-products. The photocatalytic activities of TiO₂-GFC and the mineralization of TCE were dependent on preillumination, prehydroxylation and prechlorination while the product yield was significantly influenced by prehydroxylation and prechlorination of TiO₂-GFC. Prechlorination increased the yields of phosgene (COCl₂) and pentachloroethane (C₂HCl₅) while prehydroxylation decreased the yield of COCl₂ with corresponding increases in the yield of oxalyl chloride (COClCOCl) and the mineralization of TCE, suggesting a possible surface-mediated hydrolysis of phosgene to COClCOCl in the latter case. Reaction schemes have been proposed to account for the formation of 1,1-DCE and COClCOCl. The photocatalytic activity of TiO₂-GFC and the mineralization of TCE have been found to correlate with the concentration of HCl employed for the prechlorination of TiO₂-GFC.     

Application of TiO2-mounted activated carbon to the removal of phenol from water

TiO₂-mounted activated carbon was prepared through hydrolytic precipitation of TiO₂ from teraisopropyl orthotitanate and following heat treatment at 650–900 °C for 1 h under a flow of nitrogen. The removal of phenol from its aqueous solution under UV irradiation was measured on TiO₂-mounted activated carbons thus prepared. Although BET surface area of TiO₂-mounted activated carbons decreased drastically in comparison with the original activated carbon, the efficiency of phenol removal under UV irradiation was high. The sample heated at 900 °C, which consisted mainly of rutile phase, showed the highest total removal of phenol. Efficiency of phenol degradation is reduced because of phenol adsorption on the catalyst.     

纳米TiO2-UV光催化降解乙酰甲胺磷影响因素的研究

考察了UV-纳米TiO2光催化降解乙酰甲胺磷的可行性,就高压汞灯照射时间及照射方式、催化剂种类及用量、pH值、反应温度、乙酰甲胺磷初始浓度等对光催化降解效率的影响进行了研究,并探索了催化剂的重复利用。结果表明,当添加0.1 g/L的纳米TiO2(德国P25),乙酰甲胺磷的起始浓度为20 mg/L,温度控制在25℃,反应体系的pH为11时,以高压汞灯持续照射80 min,即可实现对乙酰甲胺磷99.9%的光催化降解。     

HIPed TiO2 dense pellets with improved photocatalytic performance

The effects of heating method and temperature on physical, structural and photocatalytic behaviors of TiO₂ pellets prepared by conventional heating and hot isostatic pressing have been evaluated. The pellets of submicron TiO₂ powders were heated to 600, 650, 700, 750 and 1000 °C using both processing methods in order to compare anatase to rutile phase transformation and densification behaviors. Bulk densities and porosities were calculated using the Archimedes method. XRD analysis were performed to calculate anatase/rutile ratios. Microstructures were characterized using SEM. Photocatalytic experiments have been performed under full spectrum irradiation. Degraded methylene blue samples were periodically monitored through UV–vis spectrophotometer to determine degradation kinetics. Anatase to rutile transformation is slightly faster and densification is better for lower temperatures for conventional heating, however HIPing gives better densification above 750 °C as it also retards rutile transformation. Mixed phase structures and HIPed samples showed the best photocatalytic performance which makes this method advantageous.     

Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. II. Effect of charge-trapping species on product distribution

The photodegradation of phenol was investigated in the presence of TiO₂ (anatase) as photocatalyst in near-UV-irradiated aqueous unbuffered suspensions. The distribution of degradation products and a reduction in total organic carbon are reported, focusing on the influence of charge-trapping species (O₂, Ag⁺ and H₂O₂). In the presence of dissolved O₂, the degradation proceeds predominantly via OH, where hydroxylated aromatics were detected: catechol, hydroquinone and 1,2,4-trihydroxybenzene; in the presence of Ag⁺, the direct hole oxidation dominates, with p-benzoquinone as the only identified transient product; in the presence of H₂O₂, both OH and direct oxidation by positive holes contribute to the degradation of phenol. Besides the identified compounds, the formation of presumed ring-opening products occurs simultaneously. In contrast with the general view, it is pointed out that the appearance of aliphatic products in the early stage of the oxidation process is not unambiguous proof of the participation of direct hole oxidation in the degradation mechanism.     

Comparison of photocatalytic efficiencies of TiO2 in suspended and immobilised form for the photocatalytic degradation of nitrobenzenesulfonic acids

The photocatalytic efficiency of TiO₂ immobilised on various supports (glass, cement, red brick and inorganic fibres), using different techniques (sputtering, sol–gel dip-coating, patented method for inorganic fibres), are compared with the photocatalytic efficiency of TiO₂ Degussa P25 in suspension 2 g l⁻¹, for the degradation of 3-nitrobenzenesulfonic acid (3-NBSA) and 4-nitrotoluenesulfonic acid (4-NTSA). In all cases, the fixation of TiO₂ on solid supports appreciably reduces the photocatalytic efficiency. The best results were obtained with TiO₂ on inorganic fibres.     

Treatability studies on domestic wastewater using UV/H2O2 process

Advanced oxidation processes (AOPs) are emerging and promising technology both as an alternative treatment to conventional wastewater treatment methods and enhancement of current biological treatment methods especially dealing with highly toxic and low biodegradable wastes. In this paper, the results of domestic wastewater treatment using H₂O₂/UV process in both batch and continuous mode are presented. Over 95% reduction in COD was achieved in less than 60 min of reaction time. Optimum conditions for pH and H₂O₂ dosage for this process was found to be 3 and 50 mg L⁻¹, respectively. A pretreatment in the form of removal of turbidity is recommended for the success of the process in the long run. Electric energy required is estimated to be 10 kWh kg⁻¹ COD on the average.     

A quantitative evalution of the photocatalytic performance of TiO2 slurries

This paper reports the results of a comparison between two TiO₂ photocatalysts that differ for particle size and absorption/scattering optical properties. The catalyst with larger particles and lower surface area performed better in the degradation of phenol than the specimen with smaller particles and larger surface area. Following carefully designed experiments, it is possible to assess the relative role of light absorption/scattering properties and catalyst-related efficiency by means of a basic kinetic model for the rate of photocatalytic reactions. Explicit relationships are derived in the framework of the steady-state approximation for the quantum yield as a function of one a-dimensional number collecting surface kinetic constants for charge carrier reactions at the interface, absorbed light and surface substrate concentrations. The dimensionality change to volume-defined quantities allows derivation of the explicit dependence of the quantum yield on substrate concentration and partition constants, catalyst concentration, and the rate of volumetric light absorption. Following this approach, the rate expression for slurry systems, valid in the absence of back reactions, is directly derived. Some further simplification of the rate equation for the case of low quantum yield regime leads to analytical relationships able to account for the dependence of the rate on catalyst concentration and absorbed light in the case of stirred and unstirred conditions. The reported properly designed experiments allow the estimation of catalyst-specific micro-kinetic constants.     

The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead

In this investigation, a packed bed filled with coated titanium dioxide glass beads to study the kinetics of photocatalytic degradation of trichloroethylene under irradiation of 365 nm UV light. In the range of 100–500 ml/min of flowrate, the reaction rate for 6 μM TCE increased with an increasing flowrate upto 300 ml/min, while was not affected by the flowrate at the values higher than 300 ml/min. For moisture in the range of 9.4–1222.2 μM, the reaction rate of TCE was decreased with an increasing humidity. The adsorbed water on the catalyst surface could compete with the adsorption of TCE on the sites. The reaction rate of 6 μM TCE increased as the light intensity increased, and was proportional to the 0.61 order of the light intensity. Among the three L–H bimolecular form models, the experimental data had the best fit for one of models:

     

Photocatalytic degradation of Chromotrope 2R using nanocrystalline TiO2/activated-carbon composite catalysts

Composite catalysts made of nanocrystalline TiO₂ and carbon were prepared by a modified sol–gel method over activated carbon (AC). The composite catalysts were characterized by N₂ adsorption–desorption isotherm, TG, diffuse reflectance UV–vis spectroscopy, XRD and SEM. The photocatalytic activity was tested on the degradation of Chromotrope 2R (C2R) in aqueous medium under UV radiation. The composite catalysts exhibited higher activities than commercial Degussa P25 alone and the photocatalytic process was more efficient than the pure photolytic degradation. A modified Langmuir–Hinshelwood approach was used to study the kinetics and to determine the adsorption equilibrium constant and the reaction rate constant. Two different mechanisms are proposed and discussed in order to explain the observed synergy.     

Immobilization of hydrothermally produced TiO2 with different phase composition for photocatalytic degradation of phenol

Hydrothermally produced TiO₂ powders with different phase composition (anatase, rutile and mixed phase) were immobilized on glass fibers and tested in the phenol mineralization process. Both H₂O₂ and O₂ were used as oxygen donors, and their performances were compared with those of the same TiO₂ samples as slurries.The catalytic properties of the immobilized different crystalline phases, rutile and anatase, show the same trend as the slurry samples: pure rutile displays the highest catalytic efficiency in the presence of H₂O₂, while samples containing anatase improve the photodegradation efficacy with O₂. It was suggested that the stability of the photogenerated electron–hole couple allows high activity of rutile in the presence of H₂O₂, while the relevant oxygen chemisorption on anatase causes high catalytic activity in the presence of O₂. A four parameters kinetics model shows that both reaction steps, the phenol degradation and the mineralization of the intermediates, are photoactivated by TiO₂.Photoactivity of the coated glass fibers is generally lower than that of slurries, even if their efficiencies are almost comparable when the oxidation is performed by H₂O₂, while much lower when the oxygen donor is O₂. As a matter of fact, the morphology of immobilized catalysts shows the presence of chestnut burr aggregates of large rutile crystalline rods on the glass fiber, which are much less compact than the aggregates of small anatase particles. This preserves rutile surface area from the coarsening effects; thus, when rutile is the more active species, as in the presence of H₂O₂, the photocatalytic activity is less affected by immobilization.     

The kinetics of phenol decomposition under UV irradiation with and without H2O2 on TiO2, Fe–TiO2 and Fe–C–TiO2 photocatalysts

H₂O₂ used in the photo-Fenton reaction with iron catalyst can accelerate the oxidation of Fe²⁺ to Fe³⁺ under UV irradiation and in the dark (in the so called dark Fenton process). It was proved that conversion of phenol under UV irradiation in the presence of H₂O₂ predominantly produces highly hydrophilic products and catechol, which can accelerate the rate of phenol decomposition. However, while H₂O₂ under UV irradiation could decompose phenol to highly hydrophilic products and dihydroxybenzenes in a very short time, complete mineralization proceeded rather slowly. When H₂O₂ is used for phenol decomposition in the presence of TiO₂ and Fe–TiO₂, decrease of OH radicals formed on the surface of TiO₂ and Fe–TiO₂ has been observed and photodecomposition of phenol is slowed down. In case of phenol decomposition under UV irradiation on Fe–C–TiO₂ photocatalyst in the presence of H₂O₂, marked acceleration of the decomposition rate is observed due to the photo-Fenton reactions: Fe²⁺ is likely oxidized to Fe³⁺, which is then efficiently recycled to Fe²⁺ by the intermediate products formed during phenol decomposition, such as hydroquinone (HQ) and catechol.     

Conversion of benzoic acid during TiO2-mediated photocatalytic degradation in water

The photocatalytic degradation of benzoic acid in water over Degussa P-25 TiO₂ suspensions was studied. UVA irradiation at 365 nm was supplied by a medium pressure mercury lamp providing 25 mW cm⁻² light intensity. Experiments were conducted at benzoic acid initial concentrations between 25 and 150 mg L⁻¹, catalyst loadings between 0.2 and 1 g L⁻¹ and initial solution pH values between 2 and 10.6. Conversion increased with increasing catalyst loading up to about 0.6 g L⁻¹ and it was favored at alkaline or neutral conditions but impeded at extremely acidic conditions. Increasing initial substrate concentration led to decreased benzoic acid conversion, which was found to follow a Langmuir–Hinshelwood kinetic expression. High performance liquid chromatography (HPLC) was employed to follow benzoic acid concentration profiles as well as to identify reaction by-products, while chemical oxygen demand (COD) and total organic carbon (TOC) analyses were carried out to assess the extent of mineralization. Benzoic acid hydroxylation by-products, namely 2-, 3- and 4-hydroxybenzoic acids as well as phenol were identified as reaction intermediates, although these contributed only a small fraction of the residual organic content. Although benzoic acid at 50 mg L⁻¹ was not ecotoxic to marine photobacteria Vibrio fischeri, its photodegraded solution exhibited substantial toxicity, which, however, proved not to be due to the identified intermediates.     

Heterogeneous photocatalytic removal of the herbicide clopyralid and its comparison with UV/H2O2 and ozone oxidation techniques

Clopyralid is a herbicide that has recently been reported to occur in drinking water at concentrations above the Permitted Concentration Value (PCV) of 0.1 μg/L for an individual pesticide (EU directive 98/83/EC). An extensive laboratory study on clopyralid removal with UV/TiO₂, was carried out and was compared to UV/H₂O₂ and O₃ removal efficiencies. The effectiveness of three TiO₂ photocatalysts (Degussa P25, VP Aeroperl, Hombifine N) was studied and Degussa P25 was selected since it outperformed the other two. Complete removal of clopyralid was achieved with UV/TiO₂ in about 90 min at an optimum catalyst concentration of 1 g/L. Pseudo-zero-order kinetics were suitable to describe the first stage of the photocatalytic reaction in the concentration range 0.078–0.521 mM. pH was found to significantly affect the removal rates of clopyralid due to changes in TiO₂ surface charges and clopyralid ionisation degree. The rate constant was maximum at pH 5 and its value was 2.1 × 10⁻⁶ ± 4.3 × 10⁻⁷ M min⁻¹. Pure oxygen bubbled in solution was found to slightly affect unfavourably the photocatalytic removal of clopyralid as compared to air. With UV/H₂O₂ and O₃ systems, the initial removal rates were high but these systems were not effective in achieving high removal percentages overall.