Redox heterogeneous photocatalysis

High-surface-area inorganic materials with ordered mesoporous structures have also been oT major interest Tor numerous applications including photocatalysis [99-102], The ultra-high-surface-area of mesoporous materials is appealing in applications of heterogeneous photocatalysis where it is desirable to minimize the distance between the site of photon absorption and electron-hole redox reactions to improve efficiency [103-105],... 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

Having reviewed photochemical valence adjustment of Np in nitric acid solution, we will briefly consider the effect of heterogeneous Pt particle catalysts on Np speciation reactions in the dark before discussing applications of heterogeneous photocatalysis in Np redox chemistry. 

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 extent of this recombination at shorter time is summarised in Table 5.1. This prompted the realisation that the quantum yield of formation of surface redox species that initiate the redox chemistry in heterogeneous photocatalysis is probably around 10%. This conclusion is in accord with quantum yields of degradation of some substances reported elsewhere and with the quantum yield of -4% for the formation of OH radicals reported by Sun and Bolton (1995) for an Aldrich Ti02 specimen illuminated at appropriate wavelengths. 

Serpone N., Lawless D., Terzian R. and Meisel D. (1992), Redox mechanisms in heterogeneous photocatalysis. The case of holes s. OH radical oxidation and free s. surface-bound OH radical oxidation processes , in Electrochemistry in Colloids and Dispersions, McKay R. and Texter J., eds., VCH Publishers, New York, pp. 399-416. 

According to the expected mechanisms of heterogeneous photocatalysis (Section 5.4.1), this should provide mainly redox chemical reactions. 

Figure 29.1 Electronic scheme of a photochemical process for heterogeneous photocatalysis (a) and redox potential and band gap energy position of several semiconductors (b).

Adsorption influences the reactivity of surfaces. It has been shown that the rates of processes such as precipitation (heterogeneous nucleation and surface precipitation), dissolution of minerals (of importance in the weathering of rocks, in the formation of soils and sediments, and in the corrosion of structures and metals), and in the catalysis and photocatalysis of redox processes, are critically dependent on the properties of the surfaces (surface species and their strucutral identity). 

Due to the low temperatures, the thermal heterogeneous catalytic processes which prevail in the atmosphere are simple hydrolytic reactions (such as the hydrolysis of N2O5 in acidic water droplets to form nitric acid). However, photocatalysis provides more complicated redox reactions (such as ammonia synthesis from N2 and H2O over aerosols containing Ti02). 

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