Photocatalyst particle size

Reducing the photocatalyst particle size increases the surface area and the number of surface active sites and reduces the distances excited electrons and holes travel from the bulk to the surface. However, these beneficial effects of reducing the particle size are offset by the reduced number of photons that strike the particle per unit time, which reduces the probability of photons being absorbed by the particle (see also the discussion of absorption depth in Section 11.2.3). In addition, it is important to... 

FIGURE 11.6 (continued) (b) Different depletion layer thicknesses are obtained for different photocatalyst particle sizes. Ohmic contact is assumed at the Pt-TiO interface. Reprinted with permission from Ref. [19]. John WUey Sons. 

Very line photocatalyst particle size can be preserved even after carbon coating. 

The influence of photocatalyst particle size on gas solid photocatalyzed rate has recently been examined [107,108], using titanium dioxide powder entrained in a flowing gas transport reactor. Consideration of the complex number form of the particle extinction coefficient leads to calculated extinction and absorption efficiencies versus particles size that have maxima at 0.09 fim and become "constant above 10 /xm. Since particle extinction rises for particles smaller than 10 /xm, the calculated reactant conversion shows a maximum at about 10 /xm, falling to the left due to scattering and the right due to diminished photocatalyst surface area. 

Tioxide process. This process is similar to that used to produce fumed silicas. Ultra-low particle size titanium dioxide (15-35 nm) is obtained for use as photocatalyst or UV absorber (for instance in sun protective creams). 

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

The sonophotocatalytic system is effective for overall water splitting as shown in Fig. 12.2 and Table 12.1. This system requires, properly, a photocatalyst such as particulate Ti02. As ultrasonic waves pass through the solution, the properties of the solution influence a sonochemical reaction. In particular, negative effects are considered in the presence of powdered photocatalysts. The effects of fine particles in the solution on the sonochemical reaction have been noted so far. For example, Yasuda et al.19) reported the effects of insoluble particles, such as silicon oxide (Si02) or aluminum oxide (Al203), in the reactant solution on the sonochemical reaction and demonstrated that the reaction rate constant depended on particle properties, particle size and number of particles. It is assumed that a powdered photocatalyst suspended in the solution obstructs the transmission of ultrasonic waves. In this section, the influence of the photocatalyst powder suspended in solution on the sonochemical reaction is examined. 

Concluding this section, when very fine particles were dispersed in the reactant solution, i.e., the number of particles and the surface area increased, the reactivity of the sonophotocatalytic reaction decreased and the product ratio became lower. In general, for photocatalytic reactions, the finer the photocatalyst, the better for the reaction. However, for sonophotocatalytic reactions it was found that the finer the particles such as Ti02-B in the reactant solution, the worse the product ratio. Since it is impractical to obtain and use a photocatalyst of very large particle size to increase the activity limitlessly, a suitable particle size must be selected to obtain high performance in the sonophotocatalytic reaction. 

Fig. 3.1 Schematic representation of effect of surface area on photocatalytic activity. If constant density and complete absorption of incident photons are assumed, the number of e and h+ is independent of particle size, i.e., surface area. The amount of the substrates adsorbed on the photocatalyst increases with the increase in the surface area, which, therefore, enhances the reaction of e and h+ with the substrates.

The problem noted above must be pointed out once again. Fig. 12.6 shows the time dependencies of products from water by sonophotocatalysis using Ti02 of large particle size (tentatively named Ti02-C, specific surface area is 8.1 m2/g, further details listed in Table 12.2). The production rate of 02 was very low although surface area was small. It was stated above that the larger particle of the photocatalyst, the smaller difference in product ratio. However, Fig. 12.6 contradicts that explanation. 

Zhang, Z., Wang, C.-C., Zakaria, R., and Ying, J. Y., Role of particle size in nanocrystalline Ti02-based photocatalysts, J. Phys. Chem. B 102(52), 10871 (1998). 

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