Photocatalytic process

In the first part of this chapter systems in which solar energy is used for producing electrical energy or a storable fuel have been described. Both processes lead to an increase of free enei, that is, AG 0 (uphill reaction). On the other hand, a photocatalytic reaction is a downhill reaction (AG 0), where light excitation is only used to speed up a reaction which is thermodynamically possible in the dark but is kinetically inhibited. Examples are reactions involved in pollution control or in the synthesis of some organic compounds these are discussed below. 

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Besides improvements in catalyst characteristics [28], the low productivity of a photocatalytic process can also be improved by reactor design. In photocatalytic research on a laboratory scale, the most widely applied reactors are the top illumination or annular reactors containing a suspended catalyst [29]. This type of... 

The primary target of studies on photocatalytic semiconductor suspensions has been water cleavage by visible light. Suspension-based photocatalytic processes are also useful for the removal of inorganic (metal ions) and organic pollutants, the reduction of CO2, the photodestruction of bacteria and viruses, and various organic reactions an example is the use of Pt-loaded CdS for the photocatalytic racemization of L-lysine [210]. 

Photocatalysis is a fundamental feature of life processes on our planet [1] (it provides photosynthesis in plants and bacteria) and of the chemistry of its atmosphere [2]. Work is under way to develop photocatalytic technologies for abatement of environmental problems [3,4]. Photocatalysis is anticipated to become in the coming years important also for selective organic synthesis [4]. In a more distant future thermal catalytic processes induced by heating with solcir radiation, together with photocatalytic processes may become important for environmentally friendly technologies of solar energy utilization [5-9]. 

The use of a UV filter or alternate visible light source is the next step to ensure that no UV induced reactions are occurring in parallel with the photocatalytic process. 

Chemical models of photosynthesis have been used to investigate two types of reactions photosynthesis and photocatalysis. In photosynthetic processes the standard Gibbs free energy of the reaction is positive, and solar energy is utilized to perform work. In photocatalytic processes the free energy is negative and solar energy is used to overcome the activation barrier. 

The formation of singlet oxygen through the Kautsky mechanism [78] was observed not only when sensibilization was induced by adsorbed dye molecules but also in case when sensibilization was provided by deposited oxides of transition metals [92, 93]. This is very important for understanding numerous heterogeneous and photocatalytic processes. 

Shchukin, D.G. and Sviridov, D.V. (2006) Photocatalytic processes in spatially confined micro- and nanoreactors. Journal of Photochemistry and Photobiology C Photochemistry Reviews, 7 (1), 23-39. 

There are new ideas and experiments on the rTCA cycle. A group from Harvard University studied some reaction steps in the rTCA cycle which were kept going by mineral photochemistry. The authors assumed that solar UV radiation can excite electrons in minerals, and that this energy is sufficient to initiate the corresponding reaction steps. In this photocatalytic process, semiconductor particles were suspended in water in the presence of a zinc sulphide colloid (sphalerite) the experiments were carried out in a 500 mL reaction vessel at 288 K. Irradiation involved a UV immersion lamp (200-410 nm) in the photoreactor. Five reactions out of a total of 11 in the rTCA cycle were chosen to check the hypothesis ... 

Fig. 7 SEM images of nanopillars of titanium oxide produced by controlled anodic process and model of virus and bacteria transformation by photocatalytic processes using nanopillars of titanium oxide.242...

Natural nuclear reactors, 17 589 25 397 Natural organic polymers, manufactured fibers produced from, 24 616 Natural photocatalytic processes, in the environment, 19 100-101 Natural plant growth regulators, 13 22-28 Natural polymer hollow fibers, 16 23... 

Solar-grade silicon, production of, 22 507-508 Solar heat control, use of gold in, 12 703 Solarization effect, 19 203 Solar photocatalysis, 23 23-24 Solar photocatalytic detoxification, 19 76 Solar photocatalytic processes, 19 100-101 Solar photocatalytic reactor, using deposited titania, 19 99 Solar photoreactors, 19 95-99 Solar salt harvesting, 22 802, 806-808 Solar spectrum, 23 2 Solar still, 26 89-92 Solar thermal converters, 23 10-13 Solar transmittance, for thin films, 23 19 Solatene, 24 558 Solder, 3 53... 

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

Several articles have reviewed the ongoing work in the photocatalytic degradation of pollutants that involve oxidation or reduction processes (depending on the experimental conditions) [16,18,187,265-273], The addition of external oxidants such as ozone or hydrogen peroxide during the photocatalytic process can improve the degradation of the organic material when they are added in suitable doses [274-275],... 

The photocatalytic process can be divided into elementary processes that take place in different space-time positions (here exemplified for Ti02) ... 

Figure 16.2 depicts the processes considered, including back reactions, but not current doubling. Even if the photocatalytic process is located at the semiconductor-liquid interface [29], it was demonstrated that the dark adsorption constant of the donor does not match the adsorption constant obtained from the fitting of the photocatalytic degradation data against the L-H model [30], The L-H... 

Fig. 16.2 Simplified kinetic model of the photocatalytic process. ps represents the light absorbed per unit surface area of the photocatalyst, e b and h+b are the photogenerated electrons and holes, respectively, in the semiconductor bulk, kR is the bulk recombination rate constant and /R the related flux, whatever recombination mechanism is operating A is the heat resulting from the recombination kDe and kDh are the net first-order diffusion constants for fluxes Je and Jh to the surface of e b and h+b in the semiconductor lattice, respectively e s and h+s are the species resulting from...

Metal oxides are an important elass of heterogeneous catalysts. They find direct application in a variety of reactions, from acid-base to redox reactions, in photocatalytic processes, and as catalysts for environmental protection. In addition, they are widely used as supports for other active components (metal particles or other metal oxides), although often they act not only as a support, but actively participate in the reaction mechanism." ... 

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