Current Organic Chemistry - Volume 14, Issue 7, 2010

Photocatalytic properties and degradation mechanisms of organics play an important role in removing the poisonous organic compounds from wastewater. In order to reveal the degradation mechanisms and photocatalytic properties of all kinds of organic compounds, some corresponding articles should be reviewed, and different preparation methods for some new photocatalysts should be summarized, at the same time, the different intermediate products and the variation of possible degradation pathway of all kinds of organic compounds should be analyzed in this thematic issue of Current Organic Chemistry. Moreover, we should find a better way to produce new photocatalysts, which should have high degradation efficiency for decomposing organics. From the related articles, we may try to find some new photocatalysts which have larger surface area and perfect crystallinity, consequently, these photocatalysts have potential for decomposing organics effectively. We may also try to seek new ways for utilizing solar energy to remove organic compounds from wastewater. More specifically, the preparation, characterization and photoactivity measurements of new photocatalysts should be reviewed and we should find some new photocatalysts which can suppress the recombination of holes and electrons not only under UV light irradiation but also under visible light irradiation. Within reviewing the related articles, the framework of studying those new photocatalysts is intended to find or summarize several approaches for increasing the photoefficiency. These approaches should reveal how to improve crystallinity such as optimization of production procedures and how to obtain larger surface area such as inducing porosity of new photocatalysts. At last, by reviewing the related articles, we may also find or summarize how to obtain high quality photocatalyst film electrodes, which should decompose organics effectively. Photocatalysis offers a promising way for the degradation of organic pollutants. Zou et al. have discussed the photocatalytic degradation properties of organic pollutants with different new photocatalysts from three aspects, namely, wide band gap and narrow band gap as well as band gap modified photocatalysts. Wide band gap semiconductor photocatalysts have strong oxidative ability and preferably photostability, which can be used for efficient treatment of persistent organic pollutants such as polycyclic aromatic hydrocarbons in the environment, however, they have an awful drawback that they can only absorb UV light, therefore they can only utilize less than 5% of solar energy. On the other hand, narrow band gap semiconductor photocatalysts which can be used for the photodegradation of unstable pollutants have been investigated to improve the photocatalytic efficiency and utilization efficiency of solar energy, and they can utilize visible light, while their photocatalytic efficiency is still low and their photostability is poor. Investigation on the simultaneous enhancement of photoactivity and visible light absorption is still a challenge. In addition, although numerous organic pollutants can be readily photocatalyzed to CO2 and H2O, the mass treatment of organic compounds still meets with limited success due to the accumulation of stable reaction intermediates on the surface of the catalyst that loses photocatalytic functions (Zou et al.). Photocatalytic degradation technology has developed very rapidly in wastewater treatment for its efficiency in the degradation of organics. The degradation mechanisms, degradation pathways and intermediate products of photocatalytic degradation of organic compounds are considerably important for investigators who work in the environmental protection field. Three degradation mechanisms are expatiated in detail by Luan et al.. One degradation mechanism is that holes in valence bond and resulting OH˙ radicals generated from holes oxidize the organics, and another degradation mechanism is that resulting OH˙ radicals and the O2 ˙ generated from conduction band electrons oxidize the organics, and the third degradation mechanism is dye sensitization. These mechanisms can provide references for researchers who work on photocatalysis technology. Actually, by the analysis on degradation pathway and intermediate products of organic compounds, some toxic products during the degradation process can be obtained. Therefore, some novel photocatalysts can be prepared and new degradation methods can be found to remove these hazardous compounds efficiently. Moreover, Luan et al. present the detailed degradation pathways and intermediate products of 48 organic compounds. The degradation pathways and intermediate products are indispensable segments for the research of photocatalytic degradation of organic compounds. By the detailed illumination for the degradation pathways and intermediate products of these organic compounds, novel and ideal photocatalysts with perfect crystallinity can be synthesized successfully. In addition, it can also provide theoretic evidences for seeking new and high efficient degradation methods for organic compounds. TiO2 is used as a promising photocatalyst because of its capability for degradation of organic compounds in air and water. But the inefficient utilization of solar energy, the difficulty for separation after the reaction and improvement of adsorption property still exist. Thus, many strategies are utilized to improve the photocatalytic properties of TiO2. For exploitation of the strong oxidation potential of TiO2, it is important to select support materials having adequate structures and morphologies, which will affect the crystallinity of TiO2 particle, adsorption property and accessibility of organic molecules to the active TiO2 sites according to the nature of the target organic molecules (molecular size and hydrophobicity) and the operating conditions where catalysts are used. Yamashita et al. summarize these methods that include F and N doping, transition metal doping, TiO2 immobilized to various support materials, and TiO2 supported by micro/mesoporous silica materials. The support materials with hydrophobic surface are most suitable for photocatalytic degradation. Especially zeolites or mesoporous silica adsorbents modified using fluorine-containing silane coupling agent show excellent hydrophobic properties as well as structural retention and thermal stability, which are very favorable to TiO2 photocatalysis. The combination of these zeolites, mesoporous silica materials and TiO2 photocatalysts is a good candidate for efficient photodegradation system. The continuous breakthroughs in the synthesis and modifications of TiO2 materials will bring new approaches in solving environmental pollutions occurring on a global scale (Yamashita et al.). Photocatalyst is the core part of photocatalytic system and some new photocatalysts have been studied in detail. A great number of new photocatalysts with high photocatalytic activity have been prepared by various preparation methods. All preparation methods for photocatalysts are based upon the common characteristics of good photocatalysts such as photoactive and photostable property, chemically and biologically inert character, available and inexpensive property and non-toxic characteristic. Thus we can see that preparation methods of photocatalysts play an important role in the photocatalytic process. Recent advances for photocatalytic process have been reviewed in this issue. Luan et al. summarize the published methods to provide the specific process as well as advantages and disadvantages of each method. The preparation methods can be separated into physical and chemical methods. Physical methods include ball milling, magnetron sputtering, pulsed laser deposition, electron spinning and solid state reaction methods. Chemical methods include chemical vapor deposition, sol-gel, liquid phase deposition, chemical solution decomposition, precipitation/coprecipitation, hydrolysis, hydrothermal and solvothermal, electrophoretic deposition, layer-by-layer, photoreduction, microemulsion, molecular adsorption-deposition and spray pyrolysis methods. In addition, some comparisons offered by researchers among different preparation methods are provided. It is expected that these powerful methods can owe their existence to the discovery of new highly effective photocatalysts. It is anticipated that these informations will help researchers choose the right methods for the preparation and applications of new photocatalysts.

Titanium dioxide (TiO2) is ideally most promising photocatalyst for degradation of organic compounds under environmentally benign conditions. For further exploitation of its catalytic properties, some strategies to improve adsorption of organic substrates by hybridization with various support materials have become important. This article reviews recent developments in designing TiO2 photocatalyst with the objective of fabricating efficient photodegradation systems toward organic compounds diluted in water and air. We highlight that the hydrophobic support materials can offer significant enhancement in photodegradation, where adsorption properties of organics onto TiO2 surface play much more dominant role than other structural and compositional factors in photocatalysis of TiO2.

The codoping of TiO2 provides a promising approach to extend the photo-response of TiO2 to the more abundant visible region and emerges as a very active research field in the photocatalytic degradation of organic pollutants very recently. The present review first gives an overview on the general performance of the co-doped TiO2 visible-light photocatalysts and then the current progresses in the synthetic methods and structure characteristics of the codoped TiO2 are summarized and discussed. The synergistic effects of co-doping on the optical absorption, the redox ability, the mobility and surface trapping of the photogenerated carriers are highlighted in the terms of the electronic, crystalline, and surface properties in the codoped TiO2, relative to the monodoped ones.

In this review, three degradation mechanisms, pathways and intermediate products of various organic compounds for photocatalytic degradation by using different photocatalysts are discussed in detail. Three degradation mechanisms are as following: (1) Holes in valence bond and resulting OH˙ radicals generated from holes oxidize the organics, (2) resulting OH˙ radicals and the O2 ˙¯ generated from conduction band electrons oxidize the organics, (3) dye sensitization. The organic matters can be oxidized by several oxidizing species. Under the condition of various light irradiation, photocatalyst can produce electron and hole to form oxidizing species, which have enough oxidization capability to decompose those organic compounds. Most researchers agreed with the mechanisms of electron /O2 or hole oxidizing, which happened in the photodegradation process. However, dye sensitization is also an important mechanism for photodegradation. In addition, photodegradation intermediate products and pathways of 48 organic compounds are illuminated in detail. They are 1,2-diethyl phthalate, 2,3-dichlorophenol, 2-naphthol, 3,3'-dimethylbiphenyl-4,4'-diamine (o-Tolidine), 3-chloropyridine, 3- nitroacetophenone, methylene blue, trimelitic acid, 4-chlorophenol, pirimiphos-methyl, propanil, molinate, imzazpyr, 4-nitrophenol, methomyl, iodosulfuron methylester, imipramine, chlorfenvinphos, rhodamine B, atrazine, chlorfenapyr, methylparaben, malachite green, C.I. acid red, aniline, 4-chlorophenol, chlorothalonil, dichlofluanid, cyromazine, decabromodiphenyl ether, dipyrone, ethanol, fluroxypyr, hexachlorocyclohexane, maleic acid, fumaric acid, malic acid, metanil yellow, methyl orange, methyl red, methyl-tert-butyl ether, naphthalene, tetrachloroethylene, trichloroethylene, triane, Ph-triole, Ph-triane, dicyclanil. The degradation routes and the intermediate products of these 48 organic compounds will provide scientific researchers with the proof to prepare novel and ideal photocatalysts with perfect crystallinity and photodegradation methods in this field.

Photocatalytic degradation of organic chemicals enjoys fast development now and great achievements have been made over the past few decades. Photocatalysts, as the core of photocatalytic system, have been tremendously studied. Various methods to prepare photocatalysts have been developed for changing the photoelectrochemical characteristics and improving the photocatalytic activity. In this way, a great number of new photocatalysts have been prepared. The present review paper seeks to offer an overview of different preparation methods of new photocatalysts. The impacts of preparation methods on the physicochemical properties of photocatalysts are mentioned as well. Moreover, we discuss the advantages and disadvantages of these methods. The most commonly used photocatalysts are introduced firstly. Secondly, we divide all of the methods into physical and chemical preparation methods with each subdivided not strictly. The principle and main process of each method are described. Finally, we provide some comparisons offered by researchers among different preparation methods.

Titanium dioxide photocatalysts having anatase and rutile phases are a promising substrate for photodegradation of pollutants in water and air. However, their photocatalytic activities show only under ultraviolet (UV) light. In order to utilize a wide range of incident light such as solar light, development of the photocatalysts whose activities show under visible light is one of the most important strategies. We and other researchers have reported chemically modified titanium dioxide photocatalysts in which S (S4+ or S2-) substitutes for some of the lattice titanium atoms. They show rather strong absorption for visible light and high activities for degradation of organic compounds under visible light irradiation. The oxidation state of the S atoms incorporated into the TiO2 particles is determined to be mainly S4+ from X-ray photoelectron spectra (XPS). In addition, a new method to adsorb Fe3+ ions only onto the surface of S cationdoped TiO2 is proposed. The photocatalytic activities of Fe3+ ions adsorbed on S cation-doped TiO2 photocatalysts for oxidation of organic compounds are markedly improved compared to those of S cation-doped TiO2 without treatment of Fe3+ ions under a wide range of incident light wavelengths, including UV light and visible light. Furthermore, further improvement in their photocatalytic activities of S cation-doped TiO2 photocatalysts with Fe3+ treated with NaBH4 was observed. After the treatment, the crystal structure of Fe3+ compounds on S cation-doped TiO2 was changed. We also discussed the photocatalytic activity of S cation-doped titania nanotube (TNT) site-selectively loaded with Fe3+ compounds under visible light.

This review describes recent development of photocatalysts and their performance in degrading organics. As the core of photocatalytic technique, photocatalysts with improved performance, high energy efficiency, environmental benignity, low cost and long lifetime are desirable. Favorable strategies to produce photocatalysts with high degradation efficiency are selectively compared. Some new film electrodes with advanced nano-architectures (nanowire, nanotube arrays, nanoporous, hierarchical structures, etc.), which can work under visible light and utilize more photo-induced charges assisted by electric bias, are discussed for their promising application in environmental purification. The effects of common factors, such as light intensity, pH value of solution, original concentration of organics, dissolved oxygen concentration, etc. on the photocatalytic degradation of some organic species are preferentially remarked.

Environmental problems related to hazardous organics attract much attention currently. Photocatalysis, an attractive technology using solar energy, has been employed for pollutants degradation for several decades. In recent years, it develops extremely fast. In this review, the photocatalytic degradation properties of organic pollutants with different new photocatalysts are discussed from three aspects, namely, wide and narrow band gap as well as band gap modified photocatalysts. Some new progresses in the area of traditional photocatalysts are also presented. Semiconductor photocatalysts with wide band gap have strong oxidative ability and preferable photostability, however, they have a serious drawback that they only absorb UV light. On the other hand, narrow band gap semiconductor photocatalysts can utilize visible light, while their photocataytic efficiencies are still low and their photostabilities are rather poor. Further investigation concerning the simultaneous enhancement of photoactivity and visible light absorption is proposed.