There are two types of photocatalytic reactors: (a) reactors utilizing TiO2 as a suspension of ultrafine particles and (b) reactors incorporating TiO2 nanoparticles immobilized on a support material, as shown in Fig. 4. Suspension-type reactors are often used to study degradation kinetics since they are characterized by large catalytic surface area (i.e., high TiO2 loading) and low mass transfer limitations. However, the systems impose the requirement for filtering the effluent to remove TiO2 particles with nanoscale size (typically below 100 nm) before discharging of the polished water containing TiO2 . Recent studies show that possible toxicity of TiO2 nanoparticles damages brain microglia and human lung epithelial cells [66,67]. In addition, public perception and practical (i.e., catalyst reuse) and esthetic reasons require the complete separation of TiO2 particles from the effluent. The postfiltration process is tedious and costly and unfortunately it does not guarantee complete removal of the ultrafine particles. In most cases, TiO2 photocatalysis is considered as a refining process at the end of the treatment train and the use of suspension-type reactors is not a choice. Searching for answers and solutions to the concerns and questions on the
Figure 4 Photocatalytic reactors employing (a) TiO2 particles in suspension and (b) immobilized TiO2 coatings on a support. UV light can be installed outside or inside of the reactors, depending on the application of the reactors.
potential impact of nanosize TiO2 to human health and the environment mandates immobilization of TiO2 onto various substrates for use in a variety of applications [68-70]. This will be more sustainable and a key feature for practical applications. However, as expected, immobilized-type TiO2 reactors exhibit a low catalytic activity due to limitations in catalyst loading (surface area) on a support and catalyst activation at the near surface, partial loss of catalyst by attrition, and possible mass transfer limitations. The first two problems can be solved by novel preparation routes that aim at the precise fabrication of immobilized nanoporous TiO2 catalyst with enhanced surface area and finely tuned nanoscale dimensions for better adhesion to the support, as discussed in Section 6.1 and will be mentioned in detail in Section 7.1. The third problem can be addressed by the development of innovative photocatalytic reactors that significantly reduce or eliminate the influence of mass transfer.
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