Factors affecting heterogeneous TiO2 photocatalysis, including TiO2 loading, initial concentration of reactant, UV wavelength, radiant flux, quantum yield, oxygen, and solution pH, have been well-established . The initial decomposition rates of organic compounds are proportional to the surface area of TiO2 since the reaction is in a true heterogeneous regime (Fig. 3a). However, at too high loading of TiO2 mass, its screening effect prevents part of TiO2 surface from being irradiated with UV and the TiO2 particles scatter the UV light. The critical or optimum amount of TiO2 (typically ranging from 0.1 to 3.0 g/L particles in slurry systems) is experimentally determined, considering the geometry of TiO2 (e.g., particle size, degree of agglomeration), reactor configuration, UV wavelength and irradiance, and many other process parameters. The kinetics of heterogeneous TiO2 photocatalysis generally follow a Langmuir-Hinshelwood equation although recent studies by Serpone and his coworkers have pointed out some dogmas and misconceptions in heterogeneous photocatalysis [15-17]. For diluted solution (contaminant concentration, Co < 1 mM) in most cases, the reaction follows the apparent first order whereas for solutions with Co higher than 5 mM, the reaction rate is at maximum (zero order) (Fig. 3b). An UV source emitting light at a
wavelength l with a certain photon energy equal to or above the BG of TiO2 should be used for the activation of TiO2. Anatase TiO2 (EG — 3.2 eV) requires !<387nm, while rutile TiO2 (EG — 3.0 eV) needs A <400 nm (Fig. 3c). The reaction rate r is proportional to the radiant flux F (energy per unit time that is radiated from a source) at low values. However, above a certain value (~25mW/cm2), r becomes proportional to F1/2 (Fig. 3d).
Quantum yield is defined as the number of molecules of the contaminants undergoing transformation divided by the number ofphotons absorbed by the catalyst. Although the quantum yield depends on various conditions such as the nature of a catalyst, contaminant, and water-matrix characteristics (solution properties), it is fundamentally important since the activities of different catalysts for the same reaction can be compared in terms of their quantum yields. Molecular oxygen plays a crucial role, either inhibiting or facilitating the reaction depending on the degradation pathway and mechanism of contaminants. Its primary role is to act as an electron acceptor to produce superoxide radical anions, as discussed in Section 3.2. TiO2 is an amphoteric material with point of zero charge around 6.0-6.4. Consequently, solution pH dramatically affects the adsorption of ionic species on the catalyst surface as well as their concentration in the electrical double layer around the catalyst. In addition, the oxidation/ reduction potential of organic compounds, inorganic ions, and oxidizing species is a function ofsolution pH. As a result, changes in surface chemistry of TiO2 and chemicals in water under different pH conditions affect the band edge of TiO2 and adsorption of the contaminants, resulting in different reaction rates and reaction intermediates formation for many compounds. Other parameters affecting the reaction include the presence of coexisting chemicals and ions, and temperature.
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