Photooxidation Mechanism of Methanol on Rutile TiO₂ Nanoparticles

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Abstract

The use of nanoparticulate TiO2 as a photocatalyst for the conversion of organic molecules has grown tremendously in recent years; however, the roles of excited electrons, holes, and surface adsorbates in titania photochemistry remain poorly understood. In this work, detailed infrared measurements, which are sensitive to both vibrational and electronic transitions within the material, are used to uncover the mechanism of methanol oxidation on 4 nm rutile nanoparticles in both anaerobic and aerobic conditions. These experiments are performed in an ultrahigh vacuum cell where the coverage of methanol and exposure to oxygen are precisely controlled. Our measurements reveal that the primary pathway for initial methanol adsorption on TiO2 is dissociative, leading to the production of adsorbed methoxy groups. Upon exposure of the sample to ultraviolet photons, the results show that the electron–hole pairs (e––h+) generated within TiO2 have significant lifetimes because the holes are efficiently trapped by the surface methoxy groups. The subsequent photochemistry induces a two-electron oxidative degradation process of the surface methoxy groups to formate. Formate production proceeds through the formation of a radical anion, the result of hole oxidation, followed by prompt electron injection by the radical anion into the TiO2. Furthermore, these studies show that the role of O2 in promoting methanol photodecomposition is to scavenge free electrons, which opens acceptor sites for the injection of new electrons during methoxy group oxidation. In this way, O2 increases the efficiency of methoxy oxidation by a factor of 5 relative to anaerobic conditions, yet does not affect the hole-mediated oxidation mechanism that leads to final formate production.

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