Preparation of effective TiO<Subscript>2</Subscript>/Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> photocatalysts for water treatment

Photocatalytic oxidation using semiconductors is one of the advanced oxidation processes for degradation of organic pollutants in water and air. TiO₂ is an excellent photocatalyst that can mineralize a large range of organic pollutants such as pesticides and dyes. The main challenge is to improve the efficiency of the TiO₂ photocatalyst and to extend TiO₂ light absorption spectra to the visible region. A potential solution is to couple TiO₂ with a narrow band gap semiconductor possessing a higher conduction band such as bismuth oxide. Therefore, here we prepared Bi₂O₃/TiO₂ heterojunctions by the impregnation method with different Bi/Ti ratio. The prepared composites have been characterized by UV–Vis diffused reflectance spectra and X-ray diffraction. The photocatalytic activity of the heterojunction has been determined from the degradation of orange II under visible and UV light. Results show that Bi₂O₃/TiO₂ heterojunctions are more effective than pure TiO₂-anatase under UV-A irradiation, with an optimum for the Bi/Ti ratio of 5 %, for the photocatalytic degradation of Orange II. However, the photocatalytic activity under irradiation at λ higher than 420 nm is not much improved. Under UV–visible radiation, the two semiconductors are activated. We propose a mechanism explaining why our products are more effective under UV–visible irradiation. In this case the charge separation is enhanced because a part of photogenerated electrons from the conduction band of TiO₂ will go to the conduction band of bismuth oxide. In this composite, titanium dioxide is the main photocatalyst, while bismuth oxide acts as adsorbent photosensitizer under visible light.     

Preparation and characterization of a new TiO2/SiO2 composite catalyst for photocatalytic degradation of indigo carmin

The TiO₂/SiO₂ composite was prepared by means of the SiO₂-particle-entrapment method. The FTIR data showed the presence of Si–O–Ti stretching vibration band at 970 cm⁻¹ in the TiO₂/SiO₂ composite, suggesting a reaction between TiO₂ and silica on the TiO₂ particle surface during the silicagel formation around the TiO₂ particles. The photocatalytic efficiency of TiO₂ immobilized in silicagel was compared with that of the conventional TiO₂ Degussa P25 catalyst. For this purpose, the degradation of indigo carmin (IC) dye was used as model molecule in the tests. The effect of operational parameters such as catalyst loading and dye concentration on the photocatalytic degradation of the model dye was investigated. The rate of degradation increased with increasing catalyst loading, and when the concentration of the dye decreases.     

Unexpected release of HNO<Subscript>3</Subscript> and related species from UV-illuminated TiO<Subscript>2</Subscript> surface into air in photocatalytic oxidation of NO<Subscript>2</Subscript>

We have discovered that HNO₃ and related species are released from the TiO₂ surface into air in the TiO₂ photocatalytic oxidation of NO₂ (1 ppm) under continuous UV light illumination (1 mW cm⁻²) by dehumidifying the outlet gas of the reaction and analyzing the recovered condensate liquid by ion chromatography. The origin of the HNO₃ recovered in the dehumidifier could not be explained by a simple desorption of HNO₃ overproduced on the TiO₂ surface. The produced HNO₃ must be activated on the TiO₂ surface and causing the unidentified reaction.     

Photocatalytic degradation of the diuron pesticide

The occurrence of chlorinated pesticides in wellwaters is a major problem of public health in Ivory Coast and other African countries. Here, we studied the photocatalytic degradation of the pesticide diuron in aqueous solution in presence of two commercial TiO₂ catalysts, P25 and PC500. The capacity of diuron adsorption at the TiO₂ surface is lower for both photocatalysts. The efficiency of photocatalytic degradation of diuron, it is higher using P25 Degussa than PC500 Millenium TiO₂ catalyst.     

Enhancement of photocatalytic performance in sonochemical synthesized ZnO–rGO nanocomposites owing to effective interfacial interaction

Nanocomposites composed of two or more components with desirable performance have attracted tremendous attention, mainly due to the synergic effect between the components. The effective combination of ZnO and reduced graphene oxide would lead to ameliorate the photocatalytic performance. To enhance applicability of semiconductor photocatalytic, the composites used should be good interfacial contact governed by suitable particle size distribution. Herein we aim to fabricate the different crystallize size of ZnO nanoparticles (NPs) in ZnO–reduced graphene oxide (ZnO–rGO) nanocomposites by sonochemical synthesis and subsequent facile drying treatment method. The Zn precursor, Zn(Ac)₂, with a plenty of functional groups, was used as a starting source for both reduction of graphene oxide and formation of ZnO on rGO sheets through chemical bonds without the addition of hazardous reducing agents. LiOH was chosen as an assistive reagent to enhance the complete reaction between Zn(Ac)₂ and GO in the formation of ZnO–rGO nanocomposites. More remarkably, drying condition has the great influence on the crystallize size of ZnO NPs in as-prepared ZnO–rGO nanocomposites. It is found that ZnO–rGO nanocomposites dried at ?50 °C (freeze drying) show the highest photocatalytic efficiency in the degradation of rhodamine B (RhB) as compared to ZnO–rGO nanocomposites by other drying conditions under visible-light irradiation. Correlating the crystallize size obtained by different drying temperatures with the photocatalytic activity, it is probed that the smaller crystallize size in ZnO–rGO nanocomposites enhances the interfacial contact and a chemical bonding between rGO and ZnO NPs leading to the effective separation of electrons and holes. In addition, the O ₂ ^·? anion was determined to be the main active oxidant by free radicals trapping experiment and a photodegradation mechanism of ZnO–rGO nanocomposites over rhodamine B (RhB) was proposed based on our observations.     

Degradation and reduction of acute toxicity of environmentally persistent perfluorooctanoic acid (PFOA) using VUV photolysis and TiO2 photocatalysis in acidic and basic aqueous solutions

Degradation and toxicity reduction of perfluorooctanoic acid (PFOA) were investigated using TiO₂ adsorption, vacuum ultraviolet (VUV) photolysis, and VUV/TiO₂ photocatalysis in acidic and basic aqueous solutions. Chemical analyses of PFOA and its selected by-products and an acute toxicity assessment using the luminescent bacteria Vibrio fischeri (Microtox®) were conducted during and after the various treatment methods. PFOA was found to be best treated by VUV/TiO₂ at pH 4 with HClO₄, as illustrated by the almost complete degradation of PFOA within 360?min and rapid removal of acute microbial toxicity within 60?min. This difference in the efficiency may be attributed to the strong oxidation effectiveness of the radical species generated in acidic media and the electron scavenger effect of the addition of HClO₄ in VUV/TiO₂ photocatalysis. In addition, the proposed method could effectively decompose other perfluorocarboxylic acid (PFCA) species (C₃–C₇ perfluoroalkyl groups) if the initial intermediates formed were longer-chain species that degraded stepwise into shorter-chain compounds by VUV photolysis and VUV/TiO₂ photocatalysis in acidic and basic aqueous solutions.     

Influence of environmental temperature and relative humidity on photocatalytic oxidation of toluene on activated carbon fibers coated TiO2

TiO₂ supported on active carbon fiber (TiO₂/ACF), an absorbable photocatalyst, is a new kind of material applied in air purification. In this paper, the influence of environmental temperature (T) and relative humidity (RH) on the gas-solid adsorption of toluene and the photocatalytic oxidation (PCO) efficiency of adsorbed toluene on TiO₂/ACF were studied, and then, the purification capability of TiO₂/ACF was estimated. PCO results showed that although the PCO efficiency increased under high RH and T levels, the amount of adsorption of toluene decreased. Moreover, quantitative analysis results of intermediates indicated that more environmental risk emerged when PCO of toluene was carried out at higher environmental T and RH levels because more toxic intermediates would be accumulated on the TiO₂/ACF. So, it is significant to control the environmental T and RH conditions in the application of the PCO technique. T = 25°C and RH530% is the optimal condition for purifying toluene in our experimental system.     

Nanostructured semiconducting materials for efficient hydrogen generation

Massive production of hydrogen by water decomposition triggered by a solar light active photocatalyst is a major objective in chemistry and a promising avenue to overcome the global energy crisis. The development of efficient, stable, economically viable and eco-friendly photocatalysts for hydrogen production is a challenging task. This article reviews the use of nanocomposite in three combinations: metal oxide–metal oxide semiconductor, metal–metal oxide semiconductor and metal chalcogenide–metal oxide core–shell nanostructures. These core–shell structures occur in two forms: a simple form where the photocatalyst is either in the core or the shell or in a more complex system where the core–shell structure comprises a co-catalyst deposited on a semiconducting material. We discuss the design, synthesis and development of semiconductor-based nanocomposite photocatalysts for hydrogen production. The major points are the role of catalytic active sites, the chemical nature of sacrificial agents, the effect of light sources, the variable light intensity and the energy efficiency calculation. For TiO₂-based nanocomposites, the metal oxide or metal co-catalyst loading of 1.0–3.0 wt% was optimal. TiO₂ nanotube–CuO hybrid nanocomposites produce 1,14,000 µmol h^?1 \({\text{g}}^{ - 1}_{\text{cat}}\), whereas TiO₂/Au nanocomposites display 1,60,000 µmol h^?1 \({\text{g}}^{ - 1}_{\text{cat}}\). For core–shell catalysts, a shell thickness of 2–20 nm was found for the best activity, and its performance is as follows: (a) CdS–NiO system produces around 19,949 µmol h^?1 \({\text{g}}^{ - 1}_{\text{cat}}\) and (b) CuO–Cr₂O₃ as co-catalyst immobilized on TiO₂ system produces around 82,390 µmol h^?1 \({\text{g}}^{ - 1}_{\text{cat}}\).     

Pd nanoparticles entrapped in TiO<Subscript>2</Subscript> nanotubes for complete butane catalytic combustion at 130 °C

Air pollution by volatile organic compounds is a major health issue due to increasing industrialization and urbanization, notably in the developing countries. Cleaning organic pollutants by catalytic combustion is a potential solution, but actual methods require relatively high temperatures, thus increasing remediation costs. There is therefore a need for methods that operate at mild temperatures. Here we prepared a novel catalyst made of Pd nanoparticles entrapped in TiO₂ nanotubes by vacuum-assisted impregnation. Then, we tested this catalyst for butane combustion. The catalyst was characterized by N₂ adsorption–desorption isotherms, transmission electronic microscopy, energy-dispersive X-ray analysis coupled with a scanning transmission electron microscope, X-ray photoelectron spectroscopy and temperature programmed oxidation. Results show a complete combustion of butane at 130 °C, which is about 20 °C lower than temperatures required by actual catalysts made of Pd nanoparticles deposited on the exterior surface of TiO₂ nanotubes. Structure characterization suggests that this higher performance at lower temperature is explained by the confinement of TiO₂ nanotubes. Such a confinement could hinder the metal sintering and, in turn, facilitate the formation of PdO during oxidation on the entrapped Pd nanoparticles.     

Exposure and Health Effects of Cadmium

The results of a study of photocatalytic degradation of phenol using aqueous oxygenated TiO₂ (anatase) suspensions in a batch Pyrex photoreactor are reported. The influence on the photodegradation rate of various parameters as pH, phenol and TiO₂ content, oxygen partial pressure, anions present in the dispersions was investigated. A complete oxidation of phenol was observed. Intermediate compounds, catechol and quinone, were detected. It was observed that the photodegradation also proceeded with sunlight radiation. A mechanistic and kinetic model, which accounts for the results obtained, is given. Likely reasons for inactivity of the rutile modification for this reaction are also given.     

Titanium dioxide photocatalysis for pharmaceutical wastewater treatment

Heterogeneous photocatalysis using the semiconductor titanium dioxide (TiO₂) has proven to be a promising treatment technology for water purification. The effectiveness of this oxidation technology for the destruction of pharmaceuticals has also been demonstrated in numerous studies. This review highlights recent research on TiO₂ photocatalytic treatment applied to the removal of selected pharmaceuticals. The discussions are tailored based on the therapeutic drug classes as the kinetics and mechanistic aspects are compound dependent. These classes of pharmaceuticals were chosen because of their environmental prevalence and potential adverse effects. Optimal operational conditions and degradation pathways vary with different pharmaceutical compounds. The main conclusion is that the use of TiO₂ photocatalysis can be considered a state-of-the-art pharmaceutical wastewater treatment methodology. Further studies are, however, required to optimize the operating conditions for maximum degradation of multiple pharmaceuticals in wastewater under realistic conditions and on an industrial scale.     

Photocatalytic oxidation of humic substances with TiO<Subscript>2</Subscript>-coated glass micro-spheres

The photocatalytic oxidation of humic substances in aqueous solutions and natural waters with TiO₂ attached to buoyant, hollow glass micro-spheres was studied. A maximum oxidation efficiency of 3.6 mg W^–1 h^–1 was achieved in neutral or alkaline media at a plane surface concentration of the catalyst attached to the micro-spheres of 25 g m^–2. Proceeding by different mechanisms in acidic and alkaline media, the photocatalytic oxidation efficiency did not benefit from an excessive presence of hydroxyl radical promoters, hydrogen peroxide and alkali.     

Recent advances in the photocatalytic reduction of carbon dioxide

Fossil fuels are currently the major energy source and are rapidly consumed to supply the increasing energy demands of mankind. CO₂, a product of fossil fuel combustion, leads to climate change and will have a serious impact on our environment. There is an increasing need to mitigate CO₂ emissions using carbon–neutral energy sources. Therefore, research activities are devoted to CO₂ capture, storage and utilization. For instance, photocatalytic reduction of CO₂ into hydrocarbon fuels is a promising avenue to recycle carbon dioxide. Here we review the present status of the emission and utilization of CO₂. Then we review the photocatalytic conversion of CO₂ by TiO₂, modified TiO₂ and non-titanium metal oxides. Finally, the challenges and prospects for further development of CO₂ photocatalytic reduction are presented.     

Evaluation of environmental stress by comet assay on freshwater snail Lymnea luteola L. exposed to titanium dioxide nanoparticles

The use of aquatic organisms to monitor for contamination is well-established. Therefore, this study was designed to assess the adverse effects of titanium dioxide nanoparticles (TiO₂NP) in freshwater snail Lymnea luteola L. (L. luteola). For TiO₂NPs ecotoxicity tests, snails were exposed for seven days. A dose and time-response relationship was observed for TiO₂NP-induced genotoxicity. Induction of oxidative stress in digestive gland was observed by a decrease in glutathione and gluthathions-S-transferase levels accompanied by elevated malondialdehyde levels at TiO₂NP (9 and 28 µg/mL). Superoxide dismutase activities were markedly reduced at TiO₂NP (9 and 28 µg/mL) at days 1 and 3, but not at day 7. Catalase activities were decreased at days 1 and 3 but increased at higher concentration of TiO₂NP at day 7. DNA fragmentation occurring in L. luteola due to ecotoxic impact TiO₂NP was further substantiated by alkaline single-cell gel electrophoresis assay and expressed in terms of percent tail DNA and olive tail moment. The results indicate that the interaction of these TiO₂NP with snail influences the toxicity, which is mediated by oxidative stress in a dose- and time-dependent manner. The measurement of DNA integrity in L. luteola thus provides an early warning signal of contamination of the aquatic ecosystem by TiO₂NP. Data suggest the freshwater snail L. luteola is a potential biomonitor organism.     

Synthesis, physicochemical characterizations and catalytic performance of Pd/carbon-zeolite and Pd/carbon-CeO2 nanocatalysts used for total oxidation of xylene at low temperatures

In this work, xylene removal from waste gas streams was investigated via catalytic oxidation over Pd/carbon-zeolite and Pd/carbon-CeO₂ nanocatalysts. Activated carbon was obtained from pine cone chemically activated using ZnCl₂ and modified by H₃PO₄. Natural zeolite of clinoptilolite was modified by acid treatment with HCl, while nano-ceria was synthesized via redox method. Mixed supports of carbon-zeolite and carbonceria were prepared and palladium was dispersed over them via impregnation method. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller surface area (BET), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) techniques. Characterization of nanocatalysts revealed a good morphology with an average particle size in a nano range, and confirmed the formation of nano-ceria with an average crystallite size below 60 nm. BET analysis indicated a considerable surface area for catalysts (～1000 m²·g^?1). FTIR patterns demonstrated that the surface groups of synthesized catalysts are in good agreement with the patterns of materials applied in catalyst synthesis. The performance of catalysts was assessed in a low-pressure catalytic oxidation pilot in the temperature range of 100° C-250°C. According to the reaction data, the synthesized catalysts have been shown to be so advantageous in the removal of volatile organic compounds (VOCs), representing high catalytic performance of 98% for the abatement of xylene at 250°C. Furthermore, a reaction network is proposed for catalytic oxidation of xylene over nanocatalysts.     

Removal of arsenic from aqueous solution by two types of nano TiO2 crystals

Titanium dioxide (TiO₂) is a promising sorbent for As removal. There are two main and physico-chemically distinct polymorphs of TiO₂ in nature, namely anatase and rutile. Since the difference of arsenic removal by the two polymorphs of TiO₂ is now well known, study on the arsenic removal efficiency and the underlying mechanism is of great significance in developing new remediation strategies for As-polluted waters. Here batch experiments were carried out in combination with instrumental analysis of X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) to investigate the effects, influential factors and mechanisms of As removal from aqueous solution by two types of nano TiO₂ crystals. The adsorption behavior of anatase and rutile for As(V) and As(III) are well described by Freundlich equations. Anatase had higher As removal efficiency and adsorption capacity than rutile. Solution pH had no influence on the As adsorption of anatase TiO₂, whereas the As removal by rutile TiO₂ was increased by 7?C18% with pH from 4 to 10. Presence of accompanying anions such as phosphate, silicate, nitrate and sulfate, decreased the As(V) and As(III) removal by both crystals, with phosphate being the most effective. However, removal of As by rutile TiO₂ was greatly enhanced in the presence of divalent cations i.e. Ca²⁺ and Mg²⁺. Shading of light decreased the removal of As(V) and As(III) of anatase by 15.5% and 17.5%, respectively, while a slight increase of As removal was observed in the case of Rutile TiO₂. FT-IR characterization of As(V) or As(III)-treated nano TiO₂ crystals indicated that both Ti-O and As-O groups participated in As adsorption. Both FT-IR and XPS analysis demonstrated that As(III) was photooxidated into As(V) when adsorbed by anatase under the light condition. Thus, the effect of crystal types and light condition on As removal should be taken into consideration when nano TiO₂ is applied for As removal from water.     

Effect of iron speciation on the photodegradation of Monuron in combined photocatalytic systems with immobilized or suspended TiO2

Photodegradation kinetics of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) in photoreactor with immobilized and suspended TiO₂ photocatalyst were studied. The effect of addition of ferric or ferrous perchlorate was investigated. Whatever the concentration of Fe(III/II) added there is no significant negative effect on the photodegradation rate of pollutants. On the contrary, depending on speciation and concentration of iron salts, slight or marked acceleration of the photodegradation kinetics was observed. This positive influence was more pronounced in the case of TiO₂ suspensions than for TiO₂ layers. Fe(III) was generally more effective than Fe(II).     

Preparation of TiO2/MCM-41 by plasma enhanced chemical vapor deposition method and its photocatalytic activity

Titanium dioxide is coated on the surface of MCM-41 wafer through the plasma enhanced chemical vapor deposition (PECVD) method using titanium isopropoxide (TTIP) as a precursor. Annealing temperature is a key factor affecting crystal phase of titanium dioxide. It will transform an amorphous structure to a polycrystalline structure by increasing temperature. The optimum anatase phase of TiO₂ which can acquire the best methanol conversion under UV-light irradiation is obtained under an annealing temperature of 700°C for 2 h, substrate temperature of 500°C, 70 mL·min^?1 of oxygen flow rate, and 100W of plasma power. In addition, the films are composed of an anatase-rutile mixed phase, and the ratio of anatase to rutile varies with substrate temperature and oxygen flow rate. The particle sizes of titanium dioxide are between 30.3 nm and 59.9 nm by the calculation of Scherrer equation. Under the reaction conditions of 116.8 mg·L^-1 methanol, 2.9 mg·L^?1 moisture, and 75°C of reaction temperature, the best conversion of methanol with UV-light is 48.2% by using the anatase-rutile (91.3/8.7) mixed phase TiO₂ in a batch reactor for 60 min. While under fluorescent light irradiation, the best photoactivity appears by using the anatase-rutile (55.4/44.6) mixed phase TiO₂ with a conversion of 40.0%.     

Photocatalytic degradation of phenol with mesoporous TiO2?xBx

We report a facile approach for preparing mesoporous boron-doped TiO₂ materials by combining the sol?Cgel process with the dehydration of glucose. Specifically a high surface carbon material was formed by dehydration of glucose, then used as template. This material and the TiO₂ dry gel were calcinated to produce porous TiO₂. The as-synthesized boron-doped TiO₂ was in pure anatase crystallite phase with high surface area. X-ray photoelectron spectroscopy (XPS) results showed that boron was incorporated into the anatase TiO₂ lattice to form TiO_2?xB_x. The absorption spectra of TiO_2?xB_x extended into the visible region to 460?nm. The TiO_2?xB_x exhibited much higher photocatalytic activity on phenol degradation than pure TiO₂. It showed that the phenol degradation by-products of TiO_2?xB_x were different from that of pure TiO₂. Mechanism of the photocatalytic degradation of phenol at TiO_2?xB_x was also proposed.     

Photocatalytic oxidation of the textile dye basic red 18 with irradiated titanium dioxide

One of the major problems of textile wastewater is the presence of dye materials, because colour is visible to the public even if the dye concentration is lower than other pollutants, and needs therefore to be removed from the wastewater before it is discharged. Techniques based on “advanced oxidative processes” such as photocatalysed oxidation seem to be very promising for industrial wastewater treatment, especially for decolourization of textile effluents. In this work, we describe the photocatalytic degradation of the textile dye Basic Red 18 (BR 18) in aqueous solution using two different types of TiO₂ as photocatalyst: Degussa P25 (80% anatase) and Framitalia (100% anatase). Photooxidation of BR 18 was followed by HPLC analysis, and kinetic parameters were evaluated in order to optimise the treatment procedure. The results obtained in this work showed that the colour became virtually zero and the chemical oxygen demand (COD) is strongly reduced at the end of the treatment. The obtained results are compared with the efficiency of decolourization using the H₂O₂/UV System. Finally, marine mussel test was used to evaluate the efficiency of photocatalytic oxidation with TiO₂ in terms of ecotoxicity. A significant reduction of cumulative mortality was observed for the treated effluent.