Editorial [Research on Photocatalytic Properties and Degradation Mechanisms of Organics Guest Editor: Jingfei Luan]

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.