Redox initiation

Small semiconductor particles also act like microelectrodes upon illumination. Electrons and positive holes are created in the particles which initiate redox reactions. The charge carriers may also recombine and emit fluorescence light. Reaction with a solute leads to quenching of the fluorescence. 

Upon excitation of a semiconductor, the electrons in the conduction band and the hole in the valence band are active species that can initiate redox processes at the semiconductor-electrolyte interface, including photocorrosion of the semiconductor, a change in its surface properties (photoinduced superhydrophilicity [13]), and various spontaneous and non-spontaneous reactions [14-19]. These phenomena are basically surface-mediated redox reactions. The processes are depicted in Fig. 16.1. Owing to the slow spontaneous kinetic of the reactions between the... 

For (a) a reduction reaction has occurred because the oxidation number of the product is lower than that of the initial redox state (Bi is converted to Br ), while (b) and (c) are both oxidation reactions because the oxidation number increases during reaction. 

A compelling conclusion is that the reaction and the product are the same for both TEA and TMBS. We suggest that the function of the added activator is to initiate redox disproportionation similar to that suggested by Schaefer and Zimmermann (82). The reaction is not, however, dependent on the presence of base. 

At higher potentials, a further redox process is observed that can be described as a coupled electrochemical/chemical process involving the disproportionation of the initial redox product this RuIII/IV redox process is best described by the following reaction sequence ... 

The major photoelectrochemical reactions that initiate redox processes in the electrode compartments can be summarized as follows ... 

The electronic structure of semiconductors is characterized by a gap between electronic states populated by valence band (VB) electrons and empty states in the conduction band (CB), as shown in Fig. 2. The former can be promoted to the CB upon excitation with photons carrying energy in excess of Eg, the band-gap energy. This energy is calculated as the difference between the energies at the bottom of the CB and the top of the VB. Such a process yields CB electrons (e ) and VB holes (byB), which initiate redox reactions at the particulate/solution interface. For these reactions to occur the highest... 

The Belousov-Zhabotinsky reaction demonstrated here is set in train by the reduction of potassium bromate to elemental bromine by malonic acid and manganese(II) sulfate this is shown by the orange coloration. The reaction of the bromine with malonic acid to give mono or dibromomalonic acid leads to decolorisation. At the same time more bromine is formed in the initial redox process, and this again replaces one or two hydrogen atoms of the malonic acid. The process is repeated many times the start reaction is inhibited by complexa-tion of the brominated malonic acid by Mn(ll) ions, so that the oscillation slowly comes to an end. ... 

At the surface, these carriers are trapped by defect sites, surface states or oxidising or reducing agents. They are then poised to initiate redox chemistries with other substtates. A simple illusttation of the complex sequence of events is shown in Fig. 5.10, which is a modification of the scheme of Fig. 5.2 to emphasise other steps in the overall process sequence. The trapped electton reduces the pre-adsorbed acceptor A to A, and the trapped hole oxidises the pre-adsorbed electron donor D to D these are followed by other secondary steps. 

Fig. 11. Q-cycle model (A) Location of Cyt in the thylakoid membrane (B) Enlarged model of the Cyt b f complex showing the quinone binding sites Qo and Qr as well as the redox components Cyt be, R-[2Fe-2S], and Cyt f along with symbols for indicating their redox states. (C) Sequence of electron and proton transfers. See text for details. It Is assumed that the Initial redox state of the system has been conditioned by the photochemical oxidation of P700 in the PS-I reaction center. (C) adapted from Cramer and Knaff (1990) Energy Transduction in Bioiogical Membranes, p 343. Springer.

Amines are generally good electron donors. They readily undergo photoinduced electron transfer (PET) processes, in which amine donates an electron to the reaction partner either in its ground or excited electronic state (entry 10). In contrast, electron-deficient, nitrogen-containing molecules, such as aromatic nitriles, may serve as electron acceptors (entry 11). Many organic metal complexes can also be involved in photochemically initiated redox reactions (Section 6.4.4). 

Another expectation that follows from Scheme 45 is that Cr(II) /silica catalysts (those reduced in CO at 350 °C) should be more responsive to cocatalyst than Cr(VI)/silica, because the divalent form should not consume cocatalyst in the initial redox reactions. Again, this expectation fits the observed pattern [27,238,681,682,699,700]. The addition of cocatalyst to Cr(II)/silica usually has less effect (or little effect) on activity, but it does generate a considerably larger concentration of ot-olefins, and the polymer density can drop rather dramatically. 

Attempts were made to correlate the Bo response with the initial capacitance for a given antibody coverage. There was, however, a lower correlation between the Bo response and the initial capacitance than for the hydrogen peroxide assays. This was due, in part, to the lower signal obtained from the immunoassay compared to the hydrogen peroxide assay. Because of the variation in initial redox state and polymer porosity, a relatively large capacitance change compared with the initial capacitance is needed to observe this relationship. 

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