Abstract
In order to improve the performance of well-established photocatalysts and to develop new potential photocatalyst materials, an understanding of the underlying mechanisms of photocatalytic reactions is of the utmost importance. An often neglected method for studying the mechanism is the investigation of isotope effects. Although just a few studies related to isotope effects exist, it has been shown to be a powerful tool for exploring mechanisms of photocatalytic processes. Most of the reports are focused on TiO2, which is the most studied photocatalyst, while there is a lack of data for other photocatalyst materials. This mini-review represents an overview of research utilizing isotope effects in the area of photocatalysis. The benefits and the importance of these studies will be highlighted, and the potential for these processes to be applied for the study of further photocatalytic reactions and different photocatalyst materials will be shown.
Highlights
Semiconductor photocatalysis is a versatile technology that has been applied to a broad range of applications from treatment of contaminated water and air to energy conversion and storage.1a−e In designing and developing this process for practical commercial applications, it is critical to have a robust understanding of fundamental mechanistic processes that are occurring on the surface of the semiconductor material.1e−j A broad range of physical and chemical methods have been used in developing our understanding of these surface processes over the past four decades.1j−o One area that has not been applied to the same extent is the application of isotope effects to probe photocatalytic processes and mechanisms
Robertson and co-workers investigated the solvent isotope effect on the degradation of microcystin-LR (MC-LR) and another cyanobacterial chemical metabolite, geosmin (GSM), using a Hombikat K01/C TiO2 photocatalyst.23b In this case a solvent isotope effect of 1.5 was observed for microcystin and geosmin,23b which was approximately 50% lower than that found in the previous studies by Robertson et al and Cunningham and Srijaranai (Table 1)
Hydroperoxide was generated through a photocatalytic process under aerobic conditions, which was believed to be produced as a result of the reduction of molecular oxygen adsorbed at the TiO2 surface by the photogenerated conduction band electrons, as opposed to being generated via water oxidation from valence band holes
Summary
Semiconductor photocatalysis is a versatile technology that has been applied to a broad range of applications from treatment of contaminated water and air to energy conversion and storage.1a−e In designing and developing this process for practical commercial applications, it is critical to have a robust understanding of fundamental mechanistic processes that are occurring on the surface of the semiconductor material.1e−j A broad range of physical and chemical methods have been used in developing our understanding of these surface processes over the past four decades.1j−o One area that has not been applied to the same extent is the application of isotope effects to probe photocatalytic processes and mechanisms. Hydroperoxide was generated through a photocatalytic process under aerobic conditions, which was believed to be produced as a result of the reduction of molecular oxygen adsorbed at the TiO2 surface by the photogenerated conduction band electrons, as opposed to being generated via water oxidation from valence band holes It was demonstrated from the spectroscopic studies that under conditions where the percentage of H2O was significantly less than that of D2O there was an exchange of with the OD− ions, having solvent groups on the a stronger adsorption. The technique could be used for kinetic studies, in the case of rapidly decomposing intermediates, which may be more followed in the heavy water solvent
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