Photocatalysis indirect

In the majority of cases, the oxidizing species involved in the photooxidation reactions is not well known because the nature of the species is inferred from indirect experiments. The 02 ion has been invoked as the precursor of the oxidizing species in a number of reactions because it is often the only species observed at room temperature using EPR. However, a variety of other species such as O and OJ have been identified on surfaces when the low-temperature photoreactions are observed by EPR. In addition, O" formed on the surface may have a very short lifetime (as discussed in Ref. 1, p. 93) and can only be detected by its reactivity. With these points in mind, we conclude that O-, and particularly OJ in the presence of excess oxygen, may play a much more important role in photocatalysis than has yet been generally realized. In this connection, the paper of Kubokawa et al. (410) is of particular importance but the use of 170-labeled oxygen is necessary to confirm the nature of the species involved. In order to explore... 

This AOT will be discussed later in this book therefore, only a brief introduction is included here. Heterogeneous photocatalysis is a process based on the direct or indirect absorption of visible or UV radiant energy by a solid, normally a wide-band semiconductor. In the interfacial region between the excited solid and the solution, destruction or removal of contaminants takes place, with no chemical change in the catalyst. 

Chapter 2 considers the removal of inorganic water contaminants using photocatalysis. Metal cations react via one-electron steps first leading to unstable chemical intermediates, and later to stable species. Three possible mechanisms are identified (a) direct reduction via photo-generated conduction band electrons, (b) indirect reduction by intermediates generated from electron donors, and (c) oxidative removal by electron holes or hydroxyl radicals. The provided examples show the significance of these mechanisms for the removal of water contaminants such as chromium, mercury, lead, uranium, and arsenic. 

Light with supraband gap energy absorbed by a semiconducting particle (e.g., Ti02) can excite an electron from its valence band to the conduction band. This process also creates a positive hole in the valence band, which is an oxidizer and may react with adjacent oxygenated species to produce OH radicals. At the same time, the excited electron is capable of directly or indirectly reducing species in contact with the semiconductor. Such phenomena are electrochemical in nature—they are under intense study for application in the environmental remediation arena where they are globally called photocatalysis. 

Here, is a constant and m depends on the nature of the optical transition m = Vi for a direct bandgap, and ni = 2 for an indirect gap. From (2.6), extrapolation of a plot of (ahv) vs. hv plot gives the indirect bandgap, while a plot of (ahv) vs. hv yields the direct bandgap of the material. Such a plot is called a Tauc plot [7] and is often encoimtered in the photoelectrochemistry and photocatalysis literature. 

---