Conduction heterogeneous photocatalysis

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukhetjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. 

There are very few reports in the literature concerning heterogeneous photocatalysis for uranium treatment in water. In our previous review, only one case of photocatalytic reaction on uranium salts was reported (Amadelli et al., 1991). Taking into account the standard reduction potentials, U(VI) can be photocatalytically reduced by Ti02 conduction band electrons to U(V) and then to U(IV) (E° = +0.16 V and +0.58 V, respectively, Bard et al., 1985). However, more reduced U(III) and U(0) forms cannot be generated because of very negative redox potentials (Bard et al., 1985). In addition, U(V) rapidly disproportionates to U(VI) and U(IV), and its chemistry is very complex (Selbin and Ortego, 1969). For example, uranyl... 

The effect of temperature is not so significant within the studied interval, probably due to the counter effect of a decrease in the adsorption constant of solute on the catalyst with increasing thermal levels. It is also to be noted that reactions carried out in heterogeneous photocatalysis display low activation energies. Moreover, most of the photocatalytic experiments are conducted at isothermal conditions (usually at room temperature), and therefore, temperature changes do not play a major role (Chen and Ray, 1999). 

Different from the two above-mentioned catalysis, heterogeneous catalysis does not involve extra substance (such as photos, electrons), and all the behaviors performed in the reaction are done by the catalyst itself. A review of hterature shows that the amount of works reporting the application of perovskite catalyst in heterogeneous catalysis is far more than that in photocatalysis or in electrocatalysis. Possible reasons could be the various reactions in heterogeneous catalysis and no restriction for the catalyst is required at this time, which is different from that for Photocatalysis or Electrocatalysis, where special properties, such as negative energy level, high-electronic and ionic conductivities are usually required. 

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