Photocatalysis light sources

A similar combination of ultrasound and photocatalysis has also been reported to destroy 2,4,6-trichlorophenol in aqueous solution [39]. An ultrasonic probe (22 kHz) with a uv light source (15 W) was used to examine the effect of changing such operating conditions as ultrasonic intensity, reaction temperature and uv transmission. The experiments involved using 2,4,6-trichlorophenol (100 ppm) and TiOj (0.1 g L ) and showed that the degradation rates increased with the temperature of the solution. The cumulative effect was more pronounced at lower ultrasonic intensities with little additional benefit derived at increased ultrasonic powers. 

Light sources are among the most important parts of photocatalytic devices, based on the fact that photons are often regarded as the most expensive component of photocatalytic reactors (Nicolella and Rovatti, 1998). Hence, it is obvious that criteria for effective use of photons should be very important in the design and operation of photocatalytic devices. Unfortimately (or not), the odds that lamp manufacturers will produce UV lamps especially designed for photocatalysis for a competitive price are very slim. As a consequence, the design and even the size of a feasible reactor is very much constrained by the commercial availability of the radiation source (Imoberdorf et al., 2007). 

With photocatalysis the main problems encountered in the previously examined applications of PVRs are avoided. It is also worth noting that (i) the coupling of photocatalysis with the pervaporation process allows for a complementary exploitation of the solar spectrum. In fact, photocatalysis is normally only able to use its ultraviolet component, while the remaining thermal part of the spectrum can be utilized to heat the fluid and to evaporate the permeate (ii) photocatalysis and pervaporation have common operative conditions liquid solutions (often aqueous solutions), low concentrations of the reactants and consequently of the products, low temperature and atmospheric pressure and (iii) once the type of light source is chosen, photocatalysis is a modular process like pervaporation. Therefore the integration of the two processes is straightforward and advantageous. 

In Fig. 21.7 a laboratory scale PMR coupling photocatalysis with MF is shown. The PMR was applied for the removal of trichloroethylene (TCE) from water (Choo et al., 2008). The system was composed of a photocatalytic reactor (volume of 700 cm ) and a hollow fiber MF module (effective membrane surface area of 20.7 cm ). A UV-A light source was placed in the inner chamber of the photoreactor, whereas in the outer chamber the solution undergoing the photocatalytic reaction was flowing. Feed from the feed tank was pumped through the photoreactor to the membrane module. The PMR was operated either in batch or in continuous mode. In batch operations, the permeate and retentate were recycled to the photoreactor. In continuous mode, the permeate was discharged and the same volume of the solution was fed into the reactor. Thus the working volume of the photoreactor was maintained at a constant level. 

The reaction is catalytic in Pt2(pop)4 but it is not photocatalytic. The definition used for photocatalysis is that more than a single mole of product is formed for each einstein of light quanta that is absorbed by the reaction. For the photolysis of a mixture of 2-propanol and Pt2(pop)4", removal of the light source from the reaction results in termination of the formation of hydrogen. The first step of the reaction involves abstraction of the methine hydrogen of 2-propanol by the A2u state of... 

Finally, photoelectrocatalytic (PEC) technologies based on the use of a photoactive anode illuminated with UVA or solar light have also been recently developed for the destmction of pollutants [1], In classical Ti02 photocatalysis, which has been the most explored variant, these energy sources promote one electron from the valence to the conduction band (e cs) producing a hole (h ) by reaction (14). The adsorbed organic matter can then be directly oxidized under the action of h", by reaction (15), or OH produced by reaction (16) ... 

Titanium dioxide-based materials have proved to be the most suitable for the majority of environmental applications. TiOa is abundant, chemically inert, stable to photo- and chemical corrosion, inexpensive, relatively non-toxic, with good electronic and optical properties. However, it should be noted that the toxicity of Ti02 in the form of nanoparticles is currently under study [33, 34]. Ti02 is of special interest since it can use natural solar-UV radiation for excitation, which makes it a promising candidate in photocatalysis using solar light as energy source. 

Heterogeneous photocatalysis is not selective and can be used to treat different types of contaminants, including mixtures of contaminants. But it is a complex process and it depends on a number of variables, such as the nature (composition and structure) of the contaminant, its type and concentration, its adsorption processes, the medium of reaction (aqueous or air), the presence of O2 in the air, the configuration of the photoreactor, the source of light used... 

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