Titanium dioxide photocatalysis technology extended to the visible light range

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A key laboratory at the School of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, has recently successfully utilized the principle of dye/TiO2 visible-light photocatalytic reactions to effectively prevent the formation of holes and hydroxyl radicals. By using molecular oxygen under normal temperature and pressure, it is possible to oxidize alcohol compounds into their corresponding aldehydes with high selectivity, thus successfully applying visible-light catalysis to the green and selective oxidation of organic compounds. With the funding from the National Natural Science Foundation of China, the Ministry of Science and Technology, and the Chinese Academy of Sciences, this research group has successfully extended photocatalytic reactions from the ultraviolet region to the visible light region, thereby enabling the study of photocatalytic removal of pollutants under sunlight. Through ongoing in-depth research on the photocatalytic mechanism, researchers have discovered that during a photocatalytic reaction, if the reaction between dye radicals and oxygen as well as reactive oxygen species can be suppressed, the dye molecules can undergo a catalytic cycle without being degraded. They selected the anthraquinone dye alizarin red, which has an appropriate redox potential and is a cheap commercially available dye, to construct a visible-light photocatalytic system. Under visible light irradiation, alizarin red can selectively oxidize alcohol compounds in organic solvents to produce the corresponding aldehyde compounds. This system is suitable for the selective oxidation of primary alcohols such as aromatic alcohols, fatty alcohols, alcohols containing α-bond double bonds, and alcohols containing nitrogen heteroatoms, with a selectivity of over 99% and a turnover number (TON) that can exceed 600. Compared with existing methods for the catalytic oxidation of alcohols, such as catalytic reactions using precious metals under high temperature and pressure, or oxidation methods employing strong oxidants and peracids, this system can operate at room temperature, driven by visible light and using oxygen as the oxidant. It does not involve precious metals or transition metal ions, nor strong oxidants or acids and bases, making it a green method for selective oxidation. It is understood that titanium dioxide photocatalysis, as a green oxidation method, is receiving increasing attention for its application in the degradation of organic pollutants. The holes and hydroxyl radicals generated in titanium dioxide by the excitation of non-visible light—ultraviolet light—used previously possess strong oxidizing properties. Although they can oxidize and decompose most organic pollutants into inorganic products such as carbon dioxide and water, the selective oxidation capabilities resulting from this catalysis are not selective, which has led to slow progress in research on selective photocatalytic oxidation.