Sonophotocatalytic reaction

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

Utilization of Sonophotocatalytic Reaction 1 12.2.1 Sonophotocatalysis of Water A. General Considerations... 

I) Sonophotocatalytic reaction, which means simultaneous light and ultrasound irradiations with titanium oxide (Ti02) photocatalyst... 

In order to confirm the dependence of the product ratio on surface area, other experiment was performed. The amount of photocatalyst was changed and the sonophotocatalytic reaction was performed. The product ratio in the early stage of the reaction decreased to the desirable value when the amount of photocatalyst was reduced, i.e., the surface area was reduced, as shown in Table 12.3. Although improvement of the product ratio was confirmed, the ratio of H2 to 02 indicated a high value, 3.6, even in the case of a small amount (lOmg) of photocatalyst. In order to explain the phenomenon, another factor must be considered, i.e., the difference in product ratio and the induction period discussed in the following section. 

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. 

Heterogeneous photocatalytic reaction products and their production rates depend on the kind of photocatalysts. As noted above, each photocatalyst powder has different properties. In the case of the sonophotocatalytic reaction, products or their yields also depend on the kind of photocatalysts. The effect of surface area on product ratio was discussed in section (12.2.1C). The influence of surface area on product ratio was noted, but factors other than surface area must be introduced to explain the difference in product ratios. It is known that there are several crystal structures of Ti02. The major structures are anatase and rutile. 2... 

Fig. 12.11 Time dependencies of sonophotocatalytic reaction products from pure water using visible light. Pre-irradiation was carried out for 3 hours. Photocatalyst BiV04 200 mg Light 500 W-Xe equipped with shorter wavelength cut filter (>430 nm) Ultrasound 200kHz, 200W Temperature 25°C Atmosphere Ar.

Fig. 12.12 Isolation of H2 and 02 by sonophotocatalytic reaction using alternating irradiation method.

As seawater is the most available water on earth, the possibility of application of sonophotocatalytic reaction system to seawater cleavage is also examined. It is known that sodium chloride (NaCl) is the principal mineral constituent of seawater and chloride ion (Cl ) concentration is nearly 2%.29) When the desired amount of NaCl is added into the system, effects of NaCl on the reaction of each process are assumed to occur. 

Table 12.4 lists the sonophotocatalytic reaction products from pure water and those from 4% NaCl solution.18) The results from 4% NaCl solution were obtained under the most desirable conditions. The sonophotocatalytic reaction was impeded by the addition of NaCl to water. However, only a slight influence was observed at low concentration such as seawater. 

In conclusion, the decomposition of seawater into H2 and 02 is expected to be accomplished using the sonophotocatalytic reaction. Furthermore, H2 and 02 were obtained separately by the alternating irradiation technique, as described in section 12.2.1G. 

Table 12.4 Sonophotocatalytic reaction products from water and 4% NaCl solution under Ar atmosphere at 35°C...

Figure 12.14 shows the effect of ultiasound on the amount of the main products of the Photo-Kolbe reaction. The reaction appears to have been accelerated by ultrasonic irradiation. The product ratio of the sonophotocatalytic reaction, however, was not satisfactory. A reasonable value of methane (CH4) to carbon dioxide (C02) must be 1.0 for Photo-Kolbe reaction, as shown in Eq. (12.14). 

On other contribution of sonophotocatalytic reaction, it may be thought to construct the system like Photosynthesis, since oxygen evolution and carbon dioxide fixation proceed simultaneously by sonophotocataiysis. 

Fig. 12.2 Time dependencies of sonophotocatalytic reaction products from pure water. As powdered photocatalyst, Ti02-A (200mg, Soekawa, Commercial Reagent, rutile-rich type and specific surface area 1.9 m2/g) was used without further treatment. Liquid water (150 cm3, Wake, Distilled water for HPLC was used as reactant and was purged with argon, a Pyrex glass bulb (250-300 cm3) was used as a reactor and was placed m a temperature-controlled bath (EYELA NTT-1200 and ECS-0) all time. After the glass bulb was sealed, the irradiation was carried out under argon atmosphere at 35°C. Photo and ultrasonic irradiations were performed from one side with a 500 W xenon lamp (Ushio, UXL500D-O) and from the bottom with an ultrasonic generator (Kaijo. TA-4021-4611, 20C kHz 200 W), respectively.

As deocubed above, each reaction consisting of a two-step process is influenced by the addition of NaCl. Thus, it is assumed that the presence of NaCl in solution aftects sonophotocatalytic reaction products or/and their yields. In this section, the influence of NaCl dissolved in water on the sonophotocatalytic reaction is considered after reconfirmation of the influence of NaCl on the reaction of each process. 

In this section, the sonophotocatalytic reaction of an organic compound is demonstrated. As a reactant, acetic acid is introduced. The reaction of acetic acid is a typical and a very famous photocatalytic reaction, callec the Photo-Kolbe reaction.345... 

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