Immobilized Photocatalysts

The reaction progress is monitored ofF-Une by HPLC. Flow rates, residence times and initial concentrations of 4-chlorophenol are varied and kinetic parameters are calculated from the data obtained. It can be shown that the photocatalytic reaction is governed by Langmuir-Hinshelwood kinetics. The calculation of Damkohler numbers shows that no mass transfer limitation exists in the microreactor, hence the calculated kinetic data really represent the intrinsic kinetics of the reaction. Photonic efficiencies in the microreactor are still somewhat lower than in batch-type slurry reactors. This finding is indicative of the need to improve the catalytic activity of the deposited photocatalyst in comparison with commercially available catalysts such as Degussa P25 and Sachtleben Hombikat UV 100. The illuminated specific surface area in the microchannel reactor surpasses that of conventional photocatalytic reactors by a factor of 4-400 depending on the particular conventional reactor type. 

The stated reaction pathway is the photodecomposition of phenol via a singlet oxygen mechanism. Kitamura et al. used a 300 W high-pressure mercury lamp (1.0 M 

CUSO4 solution filter, X 330nm) as a light source. They achieve phenol photode-composition yields as high as 93% at solution flow rates of 0.5 L min with a reaction time of 42 s, whereas under similar conditions in a bulk reactor the PyTPP-Si02 suspension provides yields of only 73% with reaction times of 2 h. 

DifTerent types of microreactors, ranging from single-channel to multi-channel designs, and even more complex falling film reactors, have been investigated for carrying out photochemical reactions. Reported channel dimensions of photomicroreactors range from 10 to 1000 pm. 

Due to their inherent superior process control capabilities (precise flow rate, residence time, etc.), photomicroreactors proved to be a powerful tool in reaction engineering if secondary or consecutive reactions must be minimized. 

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The majority of the photocatalytic reactors cuiTently in use for water treatments are of the well-mixed slun-y variety (Augugliaro et al., 1990 Pichat, 1988). Slun-y systems have shown the lai gest photocatalytic activity when compared to photocatalytic reactors with immobilized photocatalyst (Matthews and McEvoy, 1992 Pai ent et al., 1996 Pozzo et al., 1999 Wyness et al., 1994b). 

Certain inorganic materials can be employed as photocatalysts for the synthesis or degradation of compounds in heterogeneous systems. Relevant devices contain, for example, films incorporating immobilized photocatalyst particles. Typically, titania, Ti02, is used for the treatment of water contaminated with chemical pollutants and/or bacteria [9]. The contaminants are oxidized by reactive species, i.e. hydroxyl and superoxide radicals, generated by reaction of electron/hole pairs with O2 and water adsorbed at the particle surface. Electron/hole pairs are formed when UV light (>, <400 nm) is absorbed by titania (see Scheme 14.5). 

In the following sections, the different reaction types are discussed exemplarily and advantages associated with carrying out these reactions in microstructures are explained in detail. Reactions are classified into three categories according to the number and type of phases involved in the reaction single-phase reactions, multiphase reactions and reactions on the surface of an immobilized photocatalyst. 

Most studies related to such photocatalytic oxidation reactions have been carried out using suspensions of powdered Ti02(usually P-25) in polluted aqueous solution. However, the manipulation of powdered Ti02 and its removal from water are difficult. Recent researches have focused on the preparation of active immobilized photocatalysts for water treatment[5,12,13]. 

Purification of waste water containing CICH COOH and PhOH is carried out by ozone treatment and UV-radiation in the presence of immobilized photocatalyst. Thus the rate of decomposition is higher than the rate obtained only in the presence of ozone and UV-radiation [26]. 

These show the largest photocatalytic activity compared with the immobilized photocatalyst and provide a high total surface area of photocatalyst per unit volume which is one of the most important factors configuring a photoreactor. However, these reactors require an additional downstream separation unit for the recovery of photocatalyst particles [116, 137]. Table 7.4 summarizes the advantages and disadvantages of both slurry and immobilized systems. 

Photocatalytic reactors with immobilized photocatalyst are those in which the photocatalyst is fixed to support by physical surface forces or chemical bonds. These reactors extend the benefit of not requiring catalyst recovery and permit the continuous operation [114, 137]. 

Systems that contain the photocatalyst as a suspended powder or colloid are not convenient in continuous flow or circulation arrangements in which the photoactive system is held in place and can readily be removed for regeneration. CdS can be produced or immobilized in a Nafion membrane together with finely dispersed platinum... 

ZnO photocatalyst can also be coupled with other materials in order to improve its chemical and physical properties [183] and photocatalytic activity [184]. Nanosized ZnO was immobilized on aluminum foil for the degradation of phenol [185]. Lanthanum and ZnO were combined to degrade 2,4,6-trichlorophenol [186]. Compared with Ti02 nanomaterial, ZnO nanomaterial generally absorbs a significant amount of the solar spectrum in the visible range therefore, ZnO nanomaterials were combined with Ti02 nanomaterials used as a photocatalyst [187]. 

Other composite photocatalysts were prepared by mounting immobilized anatase particles on mesoporous silica and silica beads [189-191], The behavior of anatase-mounted activated carbons was also studied in detail [192-194], It was even suggested that carbon-coated anatase exhibits better performance in photocatalysis than anatase itself, demonstrating high adsorptivity, inhibition of interaction with organic binders, etc. [195,196],... 

The abihty to modify the surface of the CdS nanoparticle within the cavity of the w/o microemulsions has been extended for their immobihzation. CdS nanoparticles (e.g., prepared in AOT/isooctane w/o microemulsions) were immobilized onto thiol-modified aluminosihcate particles [218] and thiol-modified alumina [219] by a simple addition of thiol-modified aluminosih-cates and alumina, respectively, in the micellar solution. The resulting CdS nanoparticles-aluminosihcate composites were used as photocatalysts for H2 generation from 2-propanol aqueous solution. 

The idea of using fluidized bed as both uniform light distribution and an immobilizing support for photocatalysts has been originally proposed and theoretically evaluated by Yue and Khan [3]. Experimental application of this idea has been demonstrated by Dibble and Raupp [4] who designed a bench scale flat plate fluidized bed photoreactor for photocatalytic oxidation of trichloroethylene (TCE). Recently, Lim et al. [5,6] have developed a modified two-dimensional fluidized bed photocatalytic reactor system and determined the effects of various operating variables on decomposition of NO. Fluidized bed photocatalytic reactor systems have several advantages over conventional immobilized or slurry-type photocatalytic reactors [7,8]. The unique reac-... 

Heterogeneous catalysis is a surface phenomenon, therefore the overall kinetic parameters are dependent on the real exposed catalyst surface area. In the supported systems only a part of the photocatalyst is accessible to light and to substrate. Besides, the immobilized catalyst suffers from the surface deactivation since the support could enhance the recombination of photogenerated electron-hole pairs and a limitation of oxygen diffusion in the deeper layers is observed. 

The recovery of the photocatalyst from the reaction environment represents one of the main problems of the photocatalytic process that limits its industrial application. Although this process step can be obviated by the use of immobilized catalyst, the suspended system has more attractive features [76]. Therefore, the separation of the photocatalyst from the treated solution and its recycle is one of the challenges in further development of this technology. 

Photocatalytic methods involve illumination of a large band gap semiconductor particle such as Ti02 either dispersed as a slurry in the contaminated aqueous solutions or as immobilized films. While a substantial body of literature exists on photocatalysis [29-40], barriers to successful commercialization still prevail. The major problems with this technique include deactivation of the photocatalyst surface, and recovery of the photocatalyst in slurry systems. 

The study of the possible cooperation of the photo-Fenton reagent with a semiconductor photocatalyst showed that the Fenton catalyst immobilized in Ti02 reveals enhanced photocatalytic activity and is more effective in pollutant degradation this was revealed in the case of such electron donors as citric acid [20], maleic acid [41], chlorinated solvents [42], resorcinol [43], quinoline [44], herbicides [45, 46], and pesticides [47], The optimization of the combined photocatalytic systems... 

Dhananjeyan, M.R., Mielczarski, E., Thampi, R.K., Buffat, P., Bensimon, M., Kulik, J., Mielczarski, J. and Kiwi, J. (2001) Photodynamics and surface characterization of Ti02 and Fe203 photocatalysts immobilized on modified polyethylene films. J. Phys. Chem. B 105, 12046-12055. 

Lee, S., Nishida, K., Otaki, M. and Ohgaki, S. (1997) Photocatalytic inactivation of phage Q-beta by immobilized titanium-dioxide mediated photocatalyst. Water Sci. Technol. 35, 101-106. 

To increase catalyst recovery, efforts have been directed toward immobilization of the photocatalyst on a wide variety of materials. However, there is an inherent decrease of catalytic activity due to a reduced surface area. As such, Fe could be used to minimize or compensate for the catalyst deactivation due to the support preparation. New methods must be developed to ensure Fe ions stay adsorbed onto the photocatalyst surface to avoid further reactivation posttreatments. 

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