Interfacial electron transfer reactions

It is also assumed (Hoffmann 1990) that the adsorbed sulfite is oxidized by the valence band holes, h+b, that are formed through absorption of light with photon energies exceeding the band-gap energy (ca. 2.2 eV) of an iron(III)(hydr)oxide, e.g., hematite (a-Fe203). This interfacial electron transfer reaction results in formation of the SO radical anion which reacts with another radical to form S20 , one of the end product, if the reaction is carried out under nitrogen. 

Electrodic reactions are interfacial electron-transfer reactions, with the special circumstance that the electron transfer occurs between the solution and an electronic conductor, or vice versa. In Section 4.2.17 it was pointed out that the jump frequency of an ion is given by... 

From what has been described so far, there can be a flow of cathodic current, or of anodic current at an electrode/solution interface, according to the value (and particularly the sign) of the overpotential, i.e., of the displacement from equilibrium of the electric potential of the electrode. The equilibrium referred to is that of some specific interfacial electron transfer reaction (e.g., the cathodic reduction of 02 (02 + 4H+ + e —> 2H20)) or the anodic oxidation of ethylene, C2H4 + 4H20 — 2C02 + 12H+ + 12e. 

This work showed that the driving forces for the decisive interfacial electron-transfer reactions in DSSCs are not necessarily fixed by the chemical identity of the participants, but can vary depending on the spatial location of the dye in the electrochemical double layer. 

Fig. 14.32. Cyclic voltammogram of coenzyme Q within the bilayer electrode. Phosphate buffer (pH 7.4, ionic strength 0.15), scan rate =100 mV/s. (Reprinted from Y. Xiaoling, J. Cullson, S. Sun and F. M. Hawkridge, Interfacial Electron Transfer Reactions of heme Proteins, Charge and Field Effects in Biosystems, M. J. Allen, S. F. Cleary, and F. M. Hawkridge, eds., vol. 2, p. 87, Plenum, 1989.)...

Graetzel, M. Dynamics of interfacial electron transfer reactions in colloidal semiconductor systems and water cleavage by visible light, Nato Asi Ser., Ser. C 1986, 174, 91. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

Figure 5. Excited state interfacial electron transfer reactions that can lead to a) anodic, and b) cathodic photocurrent at a metal surface in the presence of electron donor D and acceptor A respectively.

Earlier studies on dye-sensitized Ti02 reported nanosecond time constants for the injection kinetics [16, 40-42]. These results were obtained indirectly from the measurement of the injection quantum yield and implicitly assumed that the interfacial electron transfer reaction was competing only with the decay of the dye excited state. Other studies were based on the same assumption but used measurements of the dye fluorescence lifetime, which provided picosecond-femtosecond time resolution [43-45]. Direct time-resolved observation of the buildup of the optical absorption due to the oxidized dye species S+ has been employed in more recent studies [46-51]. This appears to be a more reliable way of monitoring the charge injection process as it does not require any initial assumption on the sensitizing mechanism. 

Gratzel M. and Frank A. J. (1982), Interfacial electron-transfer reactions in colloidal semiconductor dispersions—kinetic analysis , J. Phys. Chem. 86, 2964-2967. 

In this equation, AE represents the measured cathodic-to-anodic peak potential separation, and (AEp) , denotes the value determined as the ordinate at the origin in the AE versus plot for different concentrations of electroactive species. That (AEp)yn value can be directly related with kinetic parameters for the interfacial electron transfer reaction (Nicholson, 1965b). The slope of the above representation allows for calculation of the uncompensated ohmic resistance in the cell. Figure 1.6 shows AEp versus plots for the Fe(CN)g 7Fe(CN)6 couple at zeolite Y- and hydrotalcite-modihed glassy carbon electrodes immersed in K4Fe(CN)6 solutions in concentrations between 0.1 and 10.0 mM. 

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

The electron motion in the conduction band of the mesoscopic oxide film is coupled with interfacial electron-transfer reaction and with ion diffusion in the electrolyte. Bisquert [51] has introduced a transmission line description to model these processes. The mesoscopic film is thought to be composed of a string of oxide nanoparticles (Fig. 12). Apart from recapture by the oxidized dye, the electrons can... 

FIG. 2 Geometric factors for an interfacial electron-transfer reaction. 

The primary species involved in charge transport are thus electrons in the oxide film and the reduced and oxidized forms of the hole conductor. The extremely high surface area means that interfacial electron transfer reactions and electrostatic interactions between species in the two conducting media are particularly important. In addition, the following secondary species may influence charge transport ... 

This hypothesis was discussed by Bockris in 1969, in a comprehensive article in Nature entitled, Are Interfacial Electron Transfer Reactions an Important Step in Biological Reactions These ideas were treated in the textbook. Modern Electrochemistry (1977). ... 

Armstrong, F.A. and Lannon, A.M. (1987) "Fast interfacial Electron Transfer Reactions of Cytochrome c Peroxidase and Graphite Electrodes Promoted by Aminoglycosides Novel Electroenzymatic Catalysis of H 0 Reduction", J. Am. Chem. Soc., 109,7211-2. 

The above reaction terms include homogeneous reactions, interfacial reactions (e.g., phase change), and interfacial electron-transfer reactions. As discussed above, the irreversible heat generation is represented by the activation... 

Hotta, H., S. Ichikawa, T. Sugihara, and T. Osakai, Clarification of the mechanism of interfacial electron-transfer reaction between ferrocene and hexacyanoferrate(III) by digital simulation of cyclic voltammograms, J Phys Chem B, Vol. 107, (2003) p. 9717. 

Another example of a process in which a charge is moved across an interface is interfacial electron transfer reactions. As in the case of ion transfer, experimental data on electron transfer across liquid-liquid interfaces are very limited. For this process, however, there exists a theoretical framework developed within a dielectric continuum model,which built on the fundamental theory of electron transfer in bulk media. Computer simulations, which complement experiments and theory, have not yet dealt with chemically realistic systems but, instead, considered idealized molecules to test the basic assumptions of the continuum model. 

Harriman, A., G. R. MUlward, P. Neta, M. C. Richoux, and J. M. Thomas. 1988. Interfacial electron-transfer reactions between platinum coUoids and reducing radicals in aqueous-solution. J. Phys. Chem. 92 (5) 1286-1290. 

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