The Chinese Academy of Sciences has made a series of progress in photocatalytic degradation of organic pollutants and its mechanism

With the continuous support of the National Natural Science Foundation of China, the research team of Zhao Jincai, Institute of Chemistry, Chinese Academy of Sciences, has conducted systematic and in-depth research on photocatalytic degradation of organic pollutants and its mechanism for a series of important research progress.

Low concentration, high toxicity and refractory organic pollutants are a kind of ubiquitous and long-term harmful environmental pollutants, which are difficult to deal with by traditional methods. TiO2 photocatalysis can use clean sunlight to drive the reaction, and use environmentally friendly molecular oxygen as the oxidant, which is one of the most promising methods for eliminating such pollutants. TiO2 is corrosion-resistant, has good light, heat and chemical stability, and is currently the best photocatalytic system. However, TiO2 can only use ultraviolet light (about 5% of sunlight). Due to the low excitation energy of visible light, which accounts for the main part of sunlight, it is difficult to meet the conduction band electrons at the same time from the band-band excitation principle of traditional semiconductor photocatalysis Activated oxygen and valence band holes oxidize water or pollutants in two necessary conditions of visible light reaction. Therefore, how to realize the visible light reaction is a great challenge to the principle and application of TiO2 photocatalysis.

Zhao Jincai's research group has been working on visible light photocatalytic degradation of dye pollutants and its mechanism since 1995. It was found that dye molecules can absorb electrons after being excited by visible light, and can inject electrons into the TiO2 conduction band to achieve charge separation. The mediator function of the semiconductor conduction band can achieve simultaneous activation of dye molecules and oxygen molecules in the air under visible light irradiation, which successfully oxidizes and degrades organic dye pollutants. . Reveals a visible light degradation mechanism that is fundamentally different from traditional photocatalysis. This mechanism does not involve the band-to-band absorption of semiconductors and the generation and reaction of holes, but uses dye pollutant molecules to absorb visible light-induced active free radicals and molecules. The combined action of oxygen leads to degradation of pollutants.

Through the study of the degradation of dozens of dye pollutants, it is found that as long as the electronic excited state potential of the dye is more negative than the conduction band potential of TiO2, effective electron injection and degradation can be achieved, proving the validity and universality of this principle. This principle also has wide application prospects in the oxidative degradation of coexisting colorless small molecule pollutants, the reduction and dehalogenation of halogenated pollutants, and the synthesis of chemicals by visible light photocatalysis. Related research results have been published in J. Am. Chem. Soc., Angew. Chem. Int. Ed., Environ. Sci. Technol. And other publications.

Recently, at the invitation of the Chemical Society Reviews, a review journal of the Royal Society of Chemistry, he wrote a review paper entitled "Semiconductor-mediated photodegradation of pollutants under visible-light irradiation" (Chem. Soc. Rev. 2010, 39, 4206-4219), The related research achievements made by the research group were systematically introduced.

Recently, they have made new progress in the study of the mechanism of photocatalytic activation of molecular oxygen. How to activate molecular oxygen during the process of photocatalysis has always been a key scientific issue in this research field. When they used isotopic labeling and other experiments to study the photocatalytic oxidation of alcohol molecules with TiO2, they found that the oxygen atom in the alcohol molecule was completely replaced by an oxygen atom in the oxygen molecule during the reaction (replacement rate> 99%) to generate the corresponding carbonyl compound. . Based on the experimental results of paramagnetic resonance, oxygen isotope-labeled Raman spectroscopy, kinetic isotope effect, etc., it revealed the photocatalytic oxygen atom transfer mechanism of TiO2 which is completely different from the catalytic oxidation mechanism of previous precious metals (Angew. , 48, 6081-6084, was selected as Highly Important Paper (HIP) and published as a cover paper).

Under the guidance of this mechanism, they further realized the photocatalytic conversion of alcohol molecules through the adsorption of Bronsted acid on the surface of TiO2, and found that the doping of SiO2 can increase the adsorption site of acid. When Bronsted acid is used for TiO2 / SiO2 After the surface modification of the composite photocatalyst, the acceleration effect is further strengthened. Surface spectrometric titration experiments confirmed that protons can effectively promote the decomposition of Ti-peroxide intermediate species formed on the surface of TiO2, thereby regenerating the photocatalytic active sites on the surface, thus accelerating the photocatalytic cycle and reaction. This research is helpful to understand the microscopic mechanism of TiO2 photocatalytic activation of molecular oxygen, and provides an important scientific basis for the preparation of new photocatalysts and the regulation of photocatalytic reactions in the future. Related research results were published in Angew. Chem. Int. Ed. (2010, 49, 7976-7979), selected as a VIP paper and made a special introduction as an inside cover, Nature China also made this research result Comment (Highlight).

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