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Photochemical energy dissociation rate of methanol on TiO2 (110) surface is affected by different photon energies
Recently, the research team led by Academician Yang Xueming from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has made new progress in the study of surface photochemical reaction kinetics. The research results are strong. Photon Energy Dependence of the Photocatalytic Dissociation Rate of Methanol on TiO2 (110) (Different photon energy The effect of methanol on photocatalytic dissociation rate on TiO2 (110) surface) was published in the latest issue of the American Chemical Society (J. Am. Chem. Soc., 2013, 135 (50), pp 19039. -19 045) (Corresponding author: Guo, YANG Xueming, Timothy K. Minton).
Titanium dioxide has a very wide range of applications in the field of photocatalysis, but the study of its photocatalytic reaction mechanism has been explored. A series of studies have shown that electrons and holes generated by titanium dioxide photoexcitation quickly relax to the bottom of the conduction band or the valence band, and the excess energy is converted into phonon energy, which basically does not contribute to the reaction. The widely accepted photocatalytic model considers that the photocatalytic reaction rate is mainly determined by the number of effective electron-hole pairs generated by illumination, that is, the chemical reaction process is related to the luminous flux, and the excitation light wavelength (or the electrons and holes generated by light excitation). The energy has no obvious relationship.
Yang Xueming's surface photochemical reaction kinetics team used a self-developed surface photochemical device based on high-sensitivity mass spectrometry to systematically study the reaction kinetics of monolayer methanol-coated TiO2 (110) surface after UV irradiation. Early results (J. Am. Chem. Soc., 2012, 134 (32), 13366–13373, J. Am. Chem. Soc., 2013, 135 (28), 10206–10209) show that methanol in the light Formaldehyde is formed by cleavage of the OH bond and the CH bond, and a large number of dissociated hydrogen atoms are transferred to the adjacent bridge oxygen atom. During the surface warming process, a part of the hydrogen atoms will seize the surface of the bridge oxygen atoms and desorb from the surface in the form of water (H2O) to generate surface oxygen vacancies. As the surface oxygen vacancy concentration increases, the remaining hydrogen atoms on the bridge oxygen are more easily combined into hydrogen molecules.
In this study, the photo-induced dissociation of methanol molecules yields about two orders of magnitude higher light yield than the wavelength of 355 nm at 266 nm, while the 266 nm absorption efficiency of TiO2 is only twice that of 355 nm. The experimental results show that photon energy (energy of electrons and holes) has an important influence on photocatalytic efficiency. One possible explanation is that the electrons and holes generated by photoexcitation interact directly with the methanol molecule and induce a chemical reaction, instead of relaxing and reacting with the methanol molecule to induce a reaction. Another possible explanation is that the electrons and holes generated by photoexcitation combine to generate a phonon with a high excitation mode. After coupling with methanol molecules, certain vibrational modes of the excited molecules cause the methanol to dissociate. If we can truly understand the essential mechanism of photocatalytic reaction on the surface of TiO2, it will provide better support for designing and developing high-efficiency catalysts and perfecting the photocatalytic model.
The above research has been funded by the National Natural Science Foundation of China and the Chinese Academy of Sciences.
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