Synergistic effect of TiO2 and iron oxide supported on fluorocarbon films: Part 2: Long-term stability and influence of reaction parameters on photoactivated degradation of pollutants

The degradation of hydroquinone (HQ) and nalidixic acid (NA) mediated by TiO₂ and iron oxide immobilized on functionalized polyvinyl fluoride films (PVF^f–TiO₂–Fe oxide) in the presence of H₂O₂ under simulated solar light has been examined. The results show that the contribution of homogeneous photo-Fenton oxidation to the initial mineralization process was low. The degradation rates were not dependant of initial pH. Heterogeneous photocatalytic activity of PVF^f–TiO₂–Fe oxide was enhanced by increasing temperature, NaCl addition and by long-term utilization.The PVF^f–TiO₂–Fe oxide surface was characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) at different states of utilization. Correlations between the catalyst surface composition and degradation kinetics are discussed. Long-term stability evaluated by repetitive pollutant degradations was outstanding. The presence of TiO₂ seems to (i) limit contact between polymer film and highly reactive radicals in the solution and (ii) act as a charge trap. Moreover, during the photocatalysis mediated by PVF^f–TiO₂–Fe oxide, some leaching of supported iron increased the amount of the top TiO₂ layer exposed to the light increasing the synergistic effects between the two oxides leading to enhanced pollutant degradation.     

Photocatalytic conversion of benzene to phenol using modified TiO2 and polyoxometalates

The application of photocatalytic reactions to organic synthesis has attracted interests in view of the development of environmentally benign synthetic processes. This study investigated the effects of various parameters (electron acceptor, surface modification, and the combination of photocatalysts) on the direct synthesis of phenol from benzene using photocatalytic oxidation processes. The OH radicals generated on UV-illuminated TiO₂ photocatalyst directly hydroxylate benzene to produce phenol, hydroquinone, and catechol. The addition of Fe³⁺, H₂O₂, or Fe³⁺ + H₂O₂ highly enhanced the phenol production yield and selectivity in TiO₂ suspension. Surface modifications of TiO₂ had significant influence on the phenol synthetic reaction. Depositing Pt nanoparticles on TiO₂ (Pt/TiO₂) markedly enhanced the yield and selectivity. Surface fluorination of TiO₂ (F-TiO₂) increased the phenol yield two-fold because of the enhanced production of mobile (free) OH radicals on F-TiO₂. Polyoxometalate (POM) in phenol synthesis played the dual role both as a homogeneous photocatalyst and as a reversible electron acceptor in TiO₂ suspension. POM alone was as efficient as TiO₂ alone in the phenol production. In particular, the addition of POM to the TiO₂ suspension increased the phenol yield from 2.6% to 11% (the highest yield obtained in this study). Reaction mechanisms for each photocatalytic system were discussed in relation to the phenol synthesis.     

Photocatalytic reactivity and diffusing OH radicals in the reaction medium containing TiO2 particles

The generation of OH radicals on UV-illuminated TiO₂ surface is mainly responsible for the photocatalytic oxidation of pollutants in various contaminated environmental media. Although the reactivity of OH radicals is largely limited within the surface region, the possibility of OH desorption and diffusion into the reaction medium has been often raised. This study provides several examples for the presence of diffusing OH radicals in aqueous solution and polymer matrix containing TiO₂ particles. The photocatalytic degradation rates of (CH₃)₄N⁺ in TiO₂ suspension were comparable between acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. From the present mechanistic study, it is suggested that the photocatalytic oxidation of (CH₃)₄N⁺ at acidic pH mainly proceeds through free OH radicals in the solution bulk, not on the surface of TiO₂. The diffusing OH radicals also played the role of main oxidants in the solid phase. The photolysis of TiO₂-embedded PVC composite films generated cavities around the imbedded TiO₂ particles and the development of cavity diameter continued even after the direct contact between the PVC and TiO₂ was prohibited. This implied that active oxygen species that were photogenerated on TiO₂ surface desorbed and diffused across a few micrometers to react with the polymer matrix.     

Synthesis and characterization of Fe doped titanium dioxide nanopowders

Titanium dioxide nanopowders, doped with different amounts of Fe³⁺ ions (0.3-5 mass%), were synthesized by acid-catalyzed sol-gel method in a non-aqueous medium. The obtained powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and determination of izoelectric points as well as particle diameters. Careful investigation of porous structure was provided by application of nitrogen adsorption-desorption method. Structure analysis showed that the obtained nanopowders exhibited the anatase crystal structure, independent of the amount of iron dopants. The presence of Fe³⁺ ions in anatase decreases the value of isoelectric point of undoped TiO₂. Unlike crystal structure, porosity parameters are strongly affected by the amount of iron ions incorporated in TiO₂ lattice. The mesoporosity of TiO₂ can be successfully controlled by changing the amount of iron dopants.     

Photocatalytic mineralisation of organic compounds: a comparison of flame-made TiO2 catalysts

A comparative photocatalytic analysis was carried out on TiO₂ made in a Flame Spray Pyrolysis (FSP) process and flame-made Degussa P25. Both have similar crystallinity, phase composition, phase segregation and a non-porous surface. Hence comparison was made based on their difference in specific surface area, organic adsorption and the amount of OH• generated upon illumination. The photocatalytic activity tests were carried out using the following series of organic compounds: sucrose, glucose, fructose, maleic acid, glyoxylic acid, oxalic acid, isobutyric acid, phenol and methanol. FSP-made TiO₂ outperformed P25 for saccharides mineralisation, while for phenol and methanol mineralisation P25 was better than FSP-made TiO₂. Similar mineralisation rates were observed for both FSP-made and P25 TiO₂ for the mineralisation of carboxylic acids. This shows that the relative performance of the photocatalysts depends on the type of organic compounds to be degraded. The high surface area and possibly a more efficient interfacial charge transfer of FSP-made TiO₂ provided an efficient pathway for saccharides mineralisation. As for phenol and methanol, the mineralisation rates were higher when using P25 due to the greater amount of OH• radicals generated by this photocatalyst. The fast mineralisation rates of carboxylic acids made degradation of these organic compounds to be less affected by the TiO₂ photocatalyst properties and conditions tested in this work.     

A total solution for simultaneous organic degradation and particle separation using photocatalytic oxidation and submerged microfiltration membrane hybrid process

Advanced oxidation process (AOP) with reactor capacity of 150 L, using ultraviolet (UV) radiation and titanium dioxide (TiO₂) photocatalyst, was evaluated for the destruction of toxic organic chemical, bisphenol A (BPA). TiO₂ in the form of powder, was suspended as slurry in the water, as against the commonly adopted practice of immobilizing it onto a carrier material such as glass, concrete or ceramics. Adsorption of BPA by TiO₂ was evaluated and was performed as a pretreatment to AOP. The combined effect of ozone with the AOP process was also studied. Applying ozone along with UV/TiO₂, brought about a synergistic effect on BPA degradation. Within three hours, entire 10 ppm of BPA and the intermediate organic compounds were completely removed. The highlight of this study was the simultaneous degradation of BPA and separation of TiO₂ particles from water after photocatalysis, in order to obtain reusable quality water. Separation of TiO₂ particles was carried out by a unique two stage coagulation and settling process followed by submerged hollow fiber microfiltration membrane technique. With initial turbidity of 4,000 NTU, the turbidity of the final permeate water was well below 0.1 NTU. Almost complete removal of TiO₂ particles was achieved. Some of the main advantages of this hybrid treatment system include, large scale treatment, complete and efficient BPA and its organic intermediates degradation; easy separation of TiO₂ after treatment and reuse as it is free from chemical coagulant contaminants; reusable quality water, and the potential for continuous operation with simple process modifications.     

Immobilization of TiO2 on an ITO substrate to facilitate the photoelectrochemical degradation of an organic dye pollutant

In this study, we developed an immobilized TiO₂ semiconductor on an ITO glass substrate (TiO₂/ITO) and investigated its photocatalytic and electrochemical performance. The TiO₂/ITO samples were prepared via a spin-coating process followed by calcination and were used for the photocatalytic or electrochemical degradation of an organic dye pollutant. The measured photocatalytic performance was comparable to that reported in previous publications; however, a remarkable result was obtained in our electrochemical system. The formation of hydroxyl radicals (OH) strongly dominated the electrochemical system, which resulted in outstanding degradation performance. Therefore, we propose a commercializable photoelectrochemical system that can maximize the degradation of pollutants in wastewater treatment plants.     

Photocatalytic Degradation of Phenol with Fe-Titania Catalysts

Iron titanate (FeTiO₃) obtained from rich titanium and iron mineral presented some absorption of UV light and photocatalytic activity for the degradation of phenol in aqueous solution. However, most of the reactant is converted to carboxylic acids that remain in the reaction mixture because natural ilmenite do not produce enough HO^? radicals to completely mineralize the original reactant at the same rate as pure titania (Degussa P25). The XRD analysis of Fe-titania catalysts prepared by simple mixing of FeTiO₃ and TiO₂ showed that the structure of TiO₂ is not modified when small amounts of FeTiO₃, in the order of 10% weight, are well mixed with pure titania. Therefore, the photocatalytic activity of these catalysts is not affected by the presence of FeTiO₃. But, heavy ilmenite particles induce the separation of the catalysts particles after the photocatalytic degradation processes.     

Influence of hydrogen bonding upon the TiO2 photooxidation of isopropanol and acetone in aqueous solution

This study investigates the aqueous photocatalytic degradation of small polar organic compounds (SPOCs) that bear hydrogen-bonding capabilities but do not readily adsorb to the TiO₂ catalyst. The effect of pH on the TiO₂ surface hydroxyl speciation and surface acid/base equilibria was used to elucidate the possible role of hydrogen-bonding interactions in the degradation of acetone and isopropanol in aqueous TiO₂ photocatalytic systems. The kinetic parameters describing the decomposition of these two model compounds were obtained by gas chromatographic analysis of their photoreaction systems and interpreted on the grounds of the Brönsted acid/base properties of the TiO₂ surface speciation and solute hydrogen-bonding numerical scales. The results showed that the fastest initial degradation rates of acetone and isopropanol occurred in a pH range where the optimal conditions for adsorption through hydrogen bonding to the TiO₂ surface and optimum concentration of hydroxyl radicals (OH) coincide. The fastest degradation constants were observed at pH 6.04 and 8.61 for acetone and isopropanol, respectively. The hypothesis of hydrogen bonding to surface hydroxyl groups presented in this study challenges the common assumption that these model compounds do not adsorb to surface sites, and that their oxidative pathways of degradation only occur via homogeneous-phase reaction with free OH radicals.     

Kinetic Analysis of Heterogeneous Photocatalysis: Role of Hydroxyl Radicals

This article provides the current research activities that concentrate on the role of hydroxyl radicals in heterogeneous photocatalysis by transition metal oxides for different nanostructures. We devote most attention to Al-based Fe₂O₃ nanostructures that have been synthesized using chemical methods. The visible light photocatalytic activity of nanocrystalline pure and Al-based Fe₂O₃ photocatalysts for degradation of Salicylic acid, 4-Cholorphenol, and Acid orange 7 (Azo dye) is reported. The catalytic activity and selectivity for organic species are remarkably influenced by the size of the different photocatalyst. Utilization of various structures, advanced oxidation processes, heterojunction between Al-based Fe₂O₃ and TiO₂, and application of solar energy for heterogeneous photocatalysis of water impurities were discussed. Extent of complete mineralization of such compounds by measuring COD and TOC was discussed. We conclude this review with personal perspectives on the directions towards which future research on this new class of nanostructured materials for photocatalysis might be directed.     

Synergistic effect of TiO2 and iron oxide supported on fluorocarbon films. Part 1: Effect of preparation parameters on photocatalytic degradation of organic pollutant at neutral pH

Iron oxide and TiO₂ were immobilized on modified polyvinyl fluoride films in a sequential process. Synergic effects of iron oxide and TiO₂ on the polymer film were observed during the heterogeneous degradation of hydroquinone (HQ) in the presence of H₂O₂ at pH close to neutrality and under simulated solar irradiation. Within the degradation period, little iron leaching (<0.5 mg/L) was observed.The surface of commercial polyvinyl fluoride (PVF) film was modified by TiO₂ under light inducing oxygen group (C–OH, CO, COOH) formation on the film surface. During this treatment, TiO₂ nanoparticles simultaneously bind to the film, leading to PVF^f–TiO₂. The possible mechanistic pathway for the TiO₂ deposition and the nature of the polymer–TiO₂ interaction are discussed. Furthermore PVF and PVF^f–TiO₂ were immersed in an aqueous solution for the deposition of iron oxide layer by hydrolysis of FeCl₃, leading to PVF–Fe oxide and to PVF^f–TiO₂–Fe oxide respectively.HQ degradation and mineralization mediated by PVF^f–TiO₂, PVF–Fe oxide and PVF^f–TiO₂–Fe oxide were investigated under different conditions. Remarkable synergistic effects were observed for PVF^f–TiO₂–Fe oxide possibly due to Fe(II) regeneration, accelerated by electron transfer from TiO₂ to the iron oxide under light.     

Synthesis and characterization of interpenetrating polymer network based on sodium alginate and methacrylic acid and potential application for immobilization of TiO2 nanoparticles

An interpenetrating polymer network (IPN) based on the sodium alginate (A) and partially neutralized poly(methacrylic acid) (MAA) was prepared by free radical polymerization followed by additional cross‐linking of sodium alginate with calcium ions. Obtained material (A/MAA IPN) was characterized by FTIR spectroscopy, thermogravimetric analysis, scanning electron microscopy, and rheological measurements. Swelling behavior of synthetized IPN has been also investigated. TiO₂ nanoparticles (TiO₂ NPs) were immobilized onto A/MAA IPN by dip‐coating method and obtained TiO₂/IPN nanocomposite was used for removal of the methylene blue (MB) from aqueous solution. The photodegradation (under illumination) and sorption (in the dark) processes for dye removal were monitored through decrease of dye concentration in the solution by UV/VIS spectrometer. The TiO₂/IPN nanocomposite sorbed approximately 93% of the MB from a 10 mg L^?1 MB solution in the dark, but no degradation occurred. Likewise, more than 93% of dye was removed after 8 h of illumination. However, after 24 h of illumination, the samples were discolored indicating that dye molecules were successfully degraded. Thus, the TiO₂/IPN nanocomposite could be utilized in the photodegradation–sorption process for the abatement of pollutants in water. POLYM. ENG. SCI., 55:2511–2518, 2015. © 2015 Society of Plastics Engineers     

The dependence of the photocatalytic activity of TiO2/carbon nanotubes nanocomposites on the modification of the carbon nanotubes

Nanostructural TiO₂/modified multi-wall carbon nanotubes photocatalysts were prepared by hydrolysis of Ti(iso-OC₃H₇)₄ providing chemical bonding of anatase TiO₂ nanoparticles onto oxidized- or amino-functionalized multi-wall carbon nanotubes (MWCNT). The processes of functionalization of the MWCNT and the deposition of TiO₂ influence the photocatalytic activity of the synthesized nanocomposites. The phase composition, crystallite size, and the structural and surface properties of the obtained TiO₂/modified-MWCNT nanocomposite were analyzed from XRD, FEG-SEM, TEM/HRTEM and FTIR data, as well low temperature N₂ adsorption. In the photocatalytic study, the TiO₂/oxidized-MWCNT catalyst showed the highest and the TiO₂/amino functionalized-MWCNT catalysts somewhat lower degradation rates, indicating that the enhancement of photocatalysis was supported by the more effective electron transfer properties of the oxygen- than amino-containing functional groups, which support the efficient charge transportation and separation of the photogenerated electron-hole pairs.     

Photocatalytic Oxidation Processes In The Presence Of Polymers

The objective of the study was to investigate the removal of synthetic organic compounds and THM precursors by two photocatalytic oxidation processes in the presence of synthetic polymers. H₂0₂/UV and O₃/UV processes were tested on solutions of nitrobenzene and aquatic fulvic substances in the absence and presence of Polyethylene Oxide (PEO) and a Betz Polymer. The presence of high concentrations of PEO reduced the removal rate of nitrobenzene by competing with it for hydroxyl radicals. The presence of low concentrations of PEO did not significantly alter the removal rate of nitrobenzene and THM precursors. Experimental results indicated that even in the presence of the polymers, decomposition rather than coupling was the favored reaction pathway.     

Phorate degradation by TiO2 photocatalysis: Parameter and reaction pathway investigations

The photocatalytic degradation of phorate in aqueous suspensions was examined with the use of titanium dioxide (TiO₂) as a photocatalyst. About 99% of phorate was degraded after UV irradiation for 60 min. The photodegradation of phorate followed pseudo-first-order kinetics and parameters such as pH of the system, TiO₂ dosage, and presence of anions were found to influence the reaction rate. To obtain a better understanding of the mechanistic details of this TiO₂-assisted photodegradation of phorate with UV irradiation, the intermediates of the processes were separated, identified, and characterized by the solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) techniques. The probable photodegradation pathways were proposed and discussed. To the best of our knowledge, this is the first study that reports the degradation pathways of phorate. The electrical energy consumption per order of magnitude for photocatalytic degradation of phorate was also calculated and showed that a moderated efficiency (E_EO = 96 kWh/(m³ order)) was obtained in TiO₂/UV process.     

ML-WO3/TiO2异质结的制备及其对罗丹明B的光催化降解

采用空间限域法制备了单层三氧化钨纳米片(ML-WO₃),然后将其与TiO₂复合得到ML-WO₃/TiO₂纳米材料,被用来在模拟太阳光下对罗丹明B进行光催化降解。ML-WO₃/TiO₂的组成和光学特性通过扫描电镜、透射电镜、高分辨透射电镜、X射线衍射、紫外-可见吸收光谱和光致发光光谱手段进行表征。结果证实,纳米ML-WO₃/TiO₂克服了纯TiO₂带隙较大的缺陷,在全波段太阳光表现出比ML-WO₃和TiO₂更强的吸收性能,ML-WO₃与TiO₂之间具有明显的协同效应。活性物种捕获实验表明.OH和.O_2^-自由基是RhB降解的主要活性物种。ML-WO₃和TiO₂之间构建的Z型异质结电荷转移路径能够保证光生载流子的高效分离和重组。在5次循环实验后ML-WO₃/TiO₂的光催化活性仍能接近80%,具有良好的光化学稳定性。通过高效液相色谱-质谱检测RhB的中间产物,推测了RhB可能的降解路径。     

Photoassisted mineralization of remazole red F3B over NiO/TiO2 and CdO/TiO2 nanoparticles under simulated sunlight

In this work, a series of titania-supported NiO and CdO materials were synthesized by a modified sol-gel process. The prepared photocatalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), and transmission electron microscopy (TEM). The activities of titania-supported NiO and CdO photocatalysts for photocatalytic degradation of Remazole Red F3B (RR) dye, under simulated sunlight, were investigated. The photocatalytic mineralization of an RR dye solution over various NiO-x/TiO₂ and CdO-x/TiO₂ photocatalysts under simulated sunlight was investigated. It was worthy noticing that the photocatalytic activity of titania improved using the prepared catalysts. The prepared TiO₂, NiO-5/TiO₂, and CdO-2/TiO₂ photocatalysts exhibited higher photocatalytic activity under simulated sunlight than did commercial TiO₂. The prepared photocatalysts were stable after photocatalytic degradation of the dye. The observed photocatalytic mineralization of the dye was 51 and 71% over NiO-10/TiO₂ and CdO-2/TiO₂ after 180 min of irradiation, respectively. Juxtaposing a p-NiO-5/TiO₂ semiconductor provided a potential approach for decreasing charge recombination. The prepared photocatalystsNiO-5/TiO₂ and CdO-2/TiO₂ are promising composites for the solar detoxification of textile wastewater.     

A simple and convenient method for the surface coating of TiO2 nanoparticles with bioactive chiral diacids containing different amino acids as the coupling agent

For practical applications, especially in an organic-solution situation, the surface of TiO₂ nanoparticles (NP)s requires modification with an organic coupling agent. Therefore, in this study, the surface of TiO₂ NPs with average diameter of 30 nm was modified with bioactive dicarboxylic acids (DA)s containing different natural amino acids such as l-valine, l-methionine, l-leucine, and l-isoleucine. The prepared DAs-modified TiO₂ NPs were studied with Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis techniques. The results showed that the modifiers were grafted on the surface of TiO₂ NPs. Modified TiO₂ NPs could be environmentally friendly due to the presence of bioactive DAs.     

Highly visible-light active C- and V-doped TiO2 for degradation of acetaldehyde

C-doped and C- and V-doped TiO₂ photocatalysts were prepared by a sol–gel process. Both catalysts showed high activity for the degradation of acetaldehyde under visible irradiation (>420 nm). The co-doped TiO₂ catalysts also were highly active in the dark; 2.0% V-containing co-doped TiO₂ had the highest activity, comparable with the activity under visible light irradiation. The catalysts were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), and N₂ adsorption–desorption. The results suggest that vanadium ions were introduced both on the surface and into the bulk of TiO₂. A free electron, induced by the formation of V⁵⁺ in the sublayers of TiO₂ during calcination at 500 °C in air, was delocalized and promoted into the conduction band by thermal energy and further transferred to O₂, generating a superoxide radical anion (O⁻₂) that is responsible for degradation of acetaldehyde in the dark. In addition to functioning as a photosensitizer that shifts the optical response of TiO₂ from the ultraviolet (UV) to the visible light region, the doped elemental carbon increased the surface area and improved the dispersion of vanadium.