Reaction Mechanisms Energy conduction band

Figures 8-16 and 8-17 show the state density ZXe) and the exchange reaction current io( ) as functions of electron energy level in two different cases of the transfer reaction of redox electrons in equilibrium. In one case in which the Fermi level of redox electrons cnxEDax) is close to the conduction band edge (Fig. 8-16), the conduction band mechanism predominates over the valence band mechanism in reaction equilibrium because the Fermi level of electrode ensa (= nREDOK)) at the interface, which is also dose to the conduction band edge, generates a higher concentration of interfadal electrons in the conduction band than interfadal holes in the valence band. In the other case in which the Fermi level of redox electrons is dose to the valence band edge (Fig. 8-17), the valence band mechanism predominates over the conduction band mechanism because the valence band holes cue much more concentrated than the conduction band electrons at the electrode interface.

Table XIII lists these values for small-size aggregates. Clearly, Ag2 and Ag4 centers in CNDO calculation cannot accept a photoelectron from the conduction band. As size increases, however, the larger even-sized neutral aggregates can accept electrons, as the trend in LUMO shows in Table XI. The path of growth for the CNDO calculation that releases the most energy for each step is shown in Fig. 23. Kinetic factors are not included in this mechanism it must be remembered that these may exert a controlling influence on the reaction path.

In order to account for such a mechanism, photochemical excitation of a semiconductor surface might induce the promotion of an electron from the valence band to the conduction band. Since relaxation of the high-energy electron is inhibited by the absence of intra-states, if the lifetime of this photo generated electron-hole pair is sufficiently long to allow the interfacial electron transfer from an accumulation site to an electron acceptor, as well as the interfacial electron transfer from an adsorbed organic donor to the valence-band hole, the irradiated semiconductor can simultaneously catalyze both oxidation and reduction reactions in a fashion similar to multifunctional enzymes reactions [232]. 

In any case, the energy released through this mechanism of transfer must have an average value in the order of the energy gap between the valency and conduction bands, but may not exceed the maximum extraction energy"of an electron from the valency band. If the AH of an endothermic reaction or of the endothermic step of a reaction is greater than this... 

The ultrashort time constants of <25 fs for electron transfer, found by Hannappel et al., indicate a different reaction mechanism for the electron transfer process [53]. The electron transfer occurs more quickly than the vibrational relaxation within the dye molecule, i.e. the electron is transferred from any excitation level directly into a corresponding energy level in the conduction band. Because of this strong coupling, this electron transfer cannot be described by the simple Marcus-Gerischer model which is only valid for comparably weak interactions. 

For Na, K, and Ba azides no such simple correlation can be made. The activation energy of the alkali-metal azides is not high enough for promotion of an electron even to the first exciton level. On the other hand, the mechanism proposed by Mott specifically to explain the thermal decomposition of barium azide is energetically more favorable. In this mechanism, an electron is first promoted to the conduction band of the metal to form metal, which catalyzes the reaction. Young [16], in fact, observed that the decomposition of potassium azide is promoted in the presence of potassium vapor, which prevents the evaporation of potassium nuclei as they are formed by decomposition. 

When titanium oxides are irradiated with UV light that is greater than the band-gap energy of the catalyst (about X < 380 nm), electrons (e ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which, together, cause catalytic reactions on the surfaces namely photocatalytic reactions are induced. Because of its similarity with the mechanism observed with photosynthesis in green plants, photocatalysis may also be referred to as artificial photosynthesis [1-4]. As will be introduced in a later section, there are no limits to the possibilities and applications of titanium oxide photocatalysts as environmentally harmonious catalysts and/or sustainable green chemical systems. ... 

---