TiO2-fixed-bed reactor for water decontamination using solar light

A photocatalytic reactor using immobilized TiO₂ (Degussa P25) on a glass plate was studied on a bench scale using solar light as the source of radiation. The influence of parameters such as the slope of the plate, solar light intensity, flow rate and molar flow rate, as well as the geometry of the reactor, was studied using dichloroacetic acid (DCA) as a model compound. A linear dependence of degradation with solar light intensity, measured at 365 nm, was observed. Experiments with recirculation as well as a single pass of solution suggested no mass transfer limitations in this system. The mineralization of DCA resulted in the production of quantitative amounts of chloride ions. An initial concentration of 5 mmol/L of DCA decayed to 2 mmol/L in about 2 min of irradiation. An exponential decay of degradation was observed with an increase of the molar flow rate, achieving saturation around 1.5 mmol DCA/min.     

Photocatalytic destruction of organic dyes in aqueous TiO2 suspensions using concentrated simulated and natural solar energy

The photocatalytic destruction of several classes of organic dyes utilizing highly concentrated solar energy is reported. Several commercial samples of anatase TiO₂ varying in particle size and purity were studied to determine catalytic activity with Degussa P25 grade being the most active. When immobilized on glass beads it remained highly active. The rate and extent of decomposition of the organic compounds were determined by UV/Vis spectroscopy and High Performance Liquid Chromatography (HPLC). Complete mineralization was confirmed by total organic carbon analysis as well as wuantitative measurements of CO₂ and various inorganic ions. In general, sulfur in the organic compounds is converted to sulfate ions and nitrogen is converted to ammonium and/or nitrate ions. The effect of increasing the flux of the radiation on the rate of decomposition of the dye was also studied. There is a dramatic rate increase as the flux is increased from 15 “suns” (solar simulator) to 150 “suns” (solar dish concentrator) with the increase being slightly greater than the square root of the increase in flux. This work suggests that potential exists for the use of highly concentrated sunlight in the removal of textile dyes and biological stains from wastewater.     

Photocatalytic degradation of oxytetracycline using TiO2 under natural and simulated solar radiation

The main objective of the present study was to assess the photocatalytic degradation over TiO₂ of an aqueous solution containing 20 mg L⁻¹ of the antibiotic Oxytetracycline (OTC) using simulated solar radiation, seconded by a solar radiation experiment carried out in a pilot plant equipped with Compound Parabolic Collectors (CPCs) under the optimal conditions found in preliminary lab-scale experiments. These comprehended a set of 1 L aqueous experiments with TiO₂ loads ranging from 0.1 to 0.5 g L⁻¹ starting from different initial pH values. These experiments were carried out in a Solarbox equipped with a 1000 W Xe-OP lamp. OTC degradation was followed by HPLC-DAD, while its mineralization was followed by the removal of Total Organic Carbon.Results suggested that 0.5 g L⁻¹ of TiO₂ with no initial pH adjustment (pH ∼ 4.4) was the best combination for the removal of both OTC (100% after 40 min of irradiation; 7.5 kJ L⁻¹ of UV dose) and TOC (>90% after 180 min of irradiation; 38.3 kJ L⁻¹ of UV dose). Under these conditions, the BOD₅/COD ratio rose from almost 0 to nearly 0.5, showing a remarkable improvement in biodegradability, while inhibition percentage of bioluminescence of Vibrio fischeri after 15 min of exposition measured by Microtox® decreased significantly from 35% down to 7%. A scheme of the OTC degradation pathway is proposed, based on the results obtained from this particular experiment.The solar photocatalytic experiment done under the same conditions was carried out in a solar pilot plant equipped with CPCs. OTC and TOC removal was followed as a function of accumulated UV energy entering the reactor. Results showed a 100% OTC and almost 80% TOC removal with 1.8 kJ L⁻¹ and 11.3 kJ L⁻¹ of photo treatment energy, respectively.     

Photocatalytic degradation of TCE in water using TiO2 catalyst

Wastewater is generally released untreated into the rivers and streams in developing countries. Industrial wastewater usually contains highly toxic pollutants, cyanides, chlorinated compounds such as trichloroethylene (TCE). Ultraviolet (UV) radiation from sunlight also decomposes organic compounds by oxidation process. However, the process is less effective due to large amount of toxic effluent entering the main stream water. The solar radiation can effectively be applied to accelerate the process by using suitable catalyst for economically cleaning the major fresh water sources. This paper describes photocatalytic degradation of trichloroethylene in aqueous solution using TiO₂. Variable parameters such as initial concentration of TCE, type and concentration of TiO₂ and reaction time are investigated. The powder TiO₂ is found more effective than the sand TiO₂ for decomposing TCE. The effect of sand TiO₂ as photocatalyst is investigated at various water depths. It is observed that up to 45 mm water depth, sand TiO₂ shows photo-degradation of TCE. The degradation rate increases as the concentration of TCE is increased up to 45 μl of TCE per litre of water. Similarly the photocatalytic degradation increases with TiO₂ concentration up to 0.7 g L⁻¹ of solution but then starts decreasing. The optimum values of TiO₂ and TCE concentration obtained are 0.7 g and 35 μl L⁻¹ of the solution, respectively.     

TiO2-assisted degradation of acid orange 7 textile dye under solar light

Photocatalytic degradation of acid orange 7 (AO7) in aqueous systems was successfully achieved by the combination of TiO₂ with potassium persulphate under solar light using a photochemical reactor with recirculation. Degradation of AO7 involves color removal and mineralization. The employment of TiO₂ removed 85% of color from the 0.2 mM AO7 aqueous solution under solar light; while, 66% of color was abated using the persulphate ion as oxidant in the absence of TiO₂ under similar conditions in 2 h. However, over 90% of color removal was achieved by combining TiO₂ and the persulphate ion for the same solution under similar conditions. Color removal was faster at pH 3. Mineralization of AO7 was followed by measuring chemical oxygen demand (COD). Negligible COD abatement of the textile dye was observed in the absence of persulphate ions (S₂O₈²⁻) while over 70% of COD abatement was observed for the initial dye concentrations of 0.2–0.7 mM employing a mix of TiO₂–S₂O₈²⁻ under solar light.     

Al2O3-coated nanoporous TiO2 electrode for solid-state dye-sensitized solar cell

This paper reports the preparation of a core-shell nanoporous electrode consisting of an inner TiO₂ porous matrix and a thin overlayer of Al₂O₃, and its application for solid-state dye-sensitized solar cell using p-CuI as hole conductor. Al₂O₃ overlayer was coated onto TiO₂ porous film by the surface sol–gel process. The role of Al₂O₃ layer thickness on the cell performance was investigated. The solar cells fabricated from Al₂O₃-coated electrodes showed superior performance to the bare TiO₂ electrode. Under illumination of AM 1.5 simulated sunlight (89 mW/cm²), a ca. 0.19 nm Al₂O₃ overlayer increased the photo-to-electric conversion efficiency from 1.94% to 2.59%.     

Slow interfacial charge recombination in solid-state dye-sensitized solar cell using Al2O3-coated nanoporous TiO2 films

Al₂O₃-coated TiO₂ porous films were used to fabricate solid-state dye-sensitized solar cells using CuI as hole conductor. Investigation with transient photovoltage measurements showed that the Al₂O₃ interlayer slowed down the interfacial recombination of electrons in TiO₂ with holes in CuI by forming a potential barrier at the TiO₂/CuI interface. As a consequence, the cell made from Al₂O₃-coated TiO₂ film showed superior cell performance than the cell made from TiO₂ film only, especially under relative high intensity of simulated sunlight.     

Enhancement of the photoelectric performance of dye-sensitized solar cells by using a CaCO3-coated TiO2 nanoparticle film as an electrode

Core-shell-type nanoparticles with TiO₂ cores and CaCO₃ shells were applied as the electrode of dye-sensitized solar cells. The performance of the cell was significantly improved (as high as 26.7%) compared to the case when un-coated TiO₂ particle film was used as electrode. The improved energy conversion efficiency has been ascribed to (i) enhanced dye adsorption due to the high isoelectric point of the overlayer, and (ii) the prevention of the back electron transfer by the insulating nature of the overlayer.     

Photocatalytic degradation of organic dyes with Er:YAlO3/ZnO composite under solar light

The Er³⁺:YAlO₃/ZnO composite, a new photocatalyst that could effectively utilize visible light, was prepared by ultrasonic dispersion and liquids boiling method in this work. In succession, the Er³⁺:YAlO₃/ZnO composite, Er³⁺:YAlO₃ particle and pure ZnO powder were characterized by X-ray diffraction (XRD). The Acid Red B dye as a model compound was degraded under solar light irradiation to evaluate the photocatalytic activity of the Er³⁺:YAlO₃/ZnO composite. In addition, the effects of Er³⁺:YAlO₃ content, heat-treated temperature and heat-treated time on photocatalytic activity of Er³⁺:YAlO₃/ZnO composite were reviewed through the degradation of Acid Red B dye under solar light. Otherwise, the effects of initial concentration, Er³⁺:YAlO₃/ZnO amount, solution acidity and solar light irradiation time on the photocatalytic degradation of Acid Red B dye were investigated in detail. It was found that the photocatalytic activity of Er³⁺:YAlO₃/ZnO composite is much higher than that of pure ZnO powder for the similar system. Perhaps, the use of this Er³⁺:YAlO₃/ZnO composite may provide a new way to take advantage of ZnO in sewage treatment aspects using solar energy.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

UV light protection through TiO2 blocking layers for inverted organic solar cells

Fully organic solar cells (OSCs) based on polymers and fullerenes have attracted remarkable interest during the last decade and high power conversion efficiencies (PCEs) beyond 8% have been realized. However, air stability of these cells remains poor. The conventional geometry of OSCs utilizes strongly oxidizing metal top contacts like Al or Ca. These metals are easily oxidized in air resulting in rapid decrease of PCE if cells are not perfectly encapsulated. Using a thin electron-selective hole-blocking bottom layer like TiO₂ enables fabrication of solar cells in a so-called inverted geometry. In this geometry, noble metals like Ag or Au can be used as top contacts, which are less sensitive to ambient oxygen. Thus, air-stability of these inverted solar cells is significantly improved. In this study we investigate inverted polythiophene-methanofullerene solar cells. We find significant influence of the TiO₂ layer thickness on light absorption and illumination stability of the solar cells, as well as the trap filling by photoinduced carriers. Even though TiO₂ layers as thick as 500 nm seem not to be detrimental for charge transport, light intensity losses limit the device performance. In turn, illumination stability is better for thicker TiO₂ layers, which can serve as UV filters and protect the photoactive materials from degradation, when compared to thin TiO₂ layers. Considering these different effects we state that a thickness of 100 nm is the optimization of the TiO₂ layer.     

Continuous flow photochemical reactor for solar decontamination of water using immobilized TiO2

A photochemical reactor is designed for solar decontamination of organic pollutants in water, where the nanocrystalline photocatalyst TiO₂ is immobilized on glass. The reactor modules could be connected in series and/or parallel to achieve desired flow rates under different conditions of illumination and degree of contamination. Methyl violet and phenol was found to completely degrade under solor irradiation and flow rates of 102–138 ml/h.     

Photocatalytic properties of redox-treated Pt/TiO2 photocatalysts for H2 production from an aqueous methanol solution

The influence of redox-treated Pt/TiO₂ photocatalysts on H₂ production is investigated. Catalyst characterizations are performed by TEM, XPS, XRD, BET, and UV–vis/DR spectroscopy techniques. In terms of production rate, the oxidation treatment shows higher reactivity than the reduction treatment. The reduction treatment allows the formation of metallic Pt(0), which more easily catalyzes the transition of TiO₂ from the anatase to the rutile phases. Reduction-treated Pt/TiO₂ photocatalysts have lower S_BET values than oxidation-treated Pt/TiO₂ photocatalysts due to the higher percentage of TiO₂ in the rutile phase. Combining the results of XPS and optical analyses, PtO/TiO₂ shows a higher energy band gap than metallic Pt(0)/TiO₂, indicating that oxidation-treated Pt/TiO₂ is more capable of achieving water splitting for H₂ production. According to the results of this study, the oxidation treatment of Pt/TiO₂ photocatalysts can significantly enhance the reactivity of photocatalytic H₂ production because of their homogenous distribution, lower phase transition, higher S_BET, and higher energy band gap.     

Photocatalytic degradation of organophosphorus pesticides using floating photocatalyst TiO2 · SiO2/beads by sunlight

In this paper, the floating TiO₂ · SiO₂ photocatalyst beads are prepared by the dip-coating method, which use hollow glass microbeads as the carrier and titanium tetraisopropoxide [Ti(iso-OC₃H₇)₄] and ethyl silicate as the raw materials. The feasibility of photocatalytic degradation of organophosphorus pesticides using TiO₂ · SiO₂ beads as a floating photocatalyst by sunlight is studied. The results show that the best heat treatment condition for TiO₂ · SiO₂ beads is at 650 °C for 5 h. Apart from heat treatment temperature and time, the amount of SiO₂ also influences the photocatalytic activity of TiO₂ · SiO₂ beads. The optimum amount of SiO₂ is 0.20 (molecular fraction). 0.65 × 10⁻⁴ mol/dm³ of four organophosphorus pesticides of three structures can be completely photocatalytically degraded into after 420 min illumination by sunlight. The effects of parameters such as the amount of TiO₂ · SiO₂ beads, initial pH and metal ions on the photocatalytic degradation of the organophosphorus pesticides are also studied. The possible mechanisms of photocatalytic degradation of phosphate ester pesticides are proposed. After 120 h illumination by sunlight, there is no significant loss of the photocatalytic activity of TiO₂ · SiO₂ beads.     

Enhanced TiO2 photocatalysis by Cu in hydrogen production from aqueous methanol solution

Cu particles have been deposited on TiO₂ by incipient-wetness impregnation followed by low-temperature (400°C) calcination/reduction, and the metallization process leads to significant enhancement in photocatalytic activity of TiO₂ for H₂ production from aqueous methanol solution. The activity exhibits up to 10-fold enhancement at the optimum loading of 1.2 wt% Cu. Spectroscopic studies indicated that the Cu particles were oxidized during reaction to have a valence lower those of thermally oxidized particles, which showed inferior activities. Dissolution of Cu ion in TiO₂ lattice, in contrast, resulted in reduction in photocatalytic activity.     

High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2

Photocatalysts CuS/TiO₂ for hydrogen production were synthesized by hydrothermal method at high temperature and characterized by XRD, UV–visible DRS, XPS, EDX, SEM and TEM. When TiO₂ was loaded with CuS, it showed photocatalytic activities for water decomposition to hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the one with 1 wt% CuS-loaded TiO₂ showed the maximum photocatalytic activity for water splitting, which indicated CuS could improve the separation ratio of photoexcited electrons and holes. What's more, the amounts of the produced hydrogen was about 570 μmol h⁻¹, which had exceeded pure titania (P25) 32 times. In the present paper, it is proven that CuS can act as an effective co-catalyst to enhance the photocatalytic H₂ production activity of TiO₂.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

Photocatalytic H2 production from water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled Pt/TiO2 nanocrystal photocatalyst

Sensitized photocatalytic production of hydrogen from water splitting is investigated under visible light irradiation over mesoporous-assembled titanium dioxide (TiO₂) nanocrystal photocatalysts, without and with Pt loading. The photocatalysts are synthesized by a sol–gel process with the aid of a structure-directing surfactant and are characterized by N₂ adsorption–desorption analysis, X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray analysis. The dependence of hydrogen production on the type of TiO₂ photocatalyst (synthesized mesoporous-assembled and commercial non-mesoporous-assembled TiO₂ without and with Pt loading), the calcination temperature of the synthesized photocatalyst, the sensitizer (Eosin Y) concentration, the electron donor (diethanolamine) concentration, the photocatalyst dosage and the initial solution pH is systematically studied. The results show that in the presence of the Eosin Y sensitizer, the Pt-loaded mesoporous-assembled TiO₂ synthesized by a single-step sol–gel process and calcined at 500 °C exhibits the highest photocatalytic activity for hydrogen production from a 30 vol.% diethanolamine aqueous solution with dissolved 2 mM Eosin Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic activity for hydrogen production are 3.33 g dm⁻³ and 11.5, respectively.     

Hydrogen production from organic fatty acids using carbon-doped TiO2 nanoparticles under visible light irradiation

The photocatalytic production of H₂ using carbon-doped TiO₂ (CTiO₂) nanoparticles has been investigated in single or mixed systems of organic fatty acids (OFAs) under visible light irradiation, including acetic acid, propionate acid, butyric acid and lactic acid. When OFAs were applied at the same electron density (10 e-eq L^?1), the H₂ evolution rates followed the order of propionic acid > butyric acid ≈ acetic acid > lactic acid, whereas at the same molar concentration (0.5 mol L^?1), that order changed to lactic acid > acetic acid > butyric acid ≈ propionic acid. This result implied that the electron transfer efficiency differed from four OFAs, probably due to their different affinity with CTiO₂. O₂^?? and CH₃? partially contributed to OFAs degradation and H₂ production. The quantum dynamics simulations of electron transfer revealed that the dominant mechanism of H₂ production was direct electron transfer from adsorbed OFAs to CTiO₂. This work aims to pursue the synergy of solar energy utilization and conversion of OFAs into H₂.