Transfer Kinetics

In most experiments and applications with titanium dioxide photocatalysts, molecular oxygen is present to act as the primary electron acceptor. Usually the electrons trapped as Ti(III) are transferred to dioxygen adsorbed at the semiconductor surface yielding peroxyl radical anions (reaction (7.16)) [16]. 

Depending on the pH of the suspension these superoxide radical anions can also exist in the protonated form (reaction (7.17)) [17]. 

Beside the electron transfer from the semiconductor to adsorbed molecular oxygen also the direct transfer to an organic molecule is possible. This type of photocatalytic reaction, yielding an organic radical anion, has been found to occur with 1,4-benzoquinone [18], tetrachloromethane [19], and several nitroaromatic compounds [20]. But electrons can also be transferred very efficiently to (adsorbed) metal cations [21]. 

In the investigations of Bahnemann et al. different decay kinetics and evolution of the transient absorption spectra of titanium dioxide colloidal solutions upon bandgap irradiation have been observed depending upon the presence of molecular oxygen, air, or molecular nitrogen, respectively [7]. In every case, a biphasic decay of the transient absorption signal was 

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Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

Lindemann <8> has made an interesting application of the new theory in the determination of the frequency of atomic vibration, r, from the melting-point. He assumes that at the melting-point, T the atoms perform vibrations of such amplitude that they mutually collide, and then transfer kinetic energy like the molecules of a gas. The mean kinetic energy of the atom will then increase by RT when the liquid is unpolymerised and the fusion occurs at constant volume this is the molecular heat of fusion. 

This chapter attempts to give an overview of electrode processes, together with discussion of electron transfer kinetics, mass transport, and the electrode-solution interface. 

For reversible systems (with fast electron-transfer kinetics), the shape of the polarographic wave can be described by the Heyrovsky—Ilkovic equation ... 

Substantial loss in sensitivity is expected for analytes with slow electron-transfer kinetics. This may be advantageous for measurements of species with fast electron-transfer kinetics in the presence of a species (e.g., dissolved oxygen) that is irreversible. (For the same reason, the technique is very useful for the study of electron processes.) Theoretical discussions on AC voltammetry are available in the literature (16-18). 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

R.L. McCreery, Carbon Electrodes Structural Effects on Electron Transfer Kinetics, in A.J. Bard, Ed., Electroanalytical Chemistry, Vol 18, Marcel Dekker, New York, 1991. 

Charge transfer kinetics for electronically conducting polymer formation, 583 Charge transport in polymers, 567 Chemical breakdown model for passivity, 236... 

Butler-Volmer kinetics (and mechanism of electron transfer), 587 charge transfer kinetics (for... 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

EPR studies on electron transfer systems where neighboring centers are coupled by spin-spin interactions can yield useful data for analyzing the electron transfer kinetics. In the framework of the Condon approximation, the electron transfer rate constant predicted by electron transfer theories can be expressed as the product of an electronic factor Tab by a nuclear factor that depends explicitly on temperature (258). On the one hand, since iron-sulfur clusters are spatially extended redox centers, the electronic factor strongly depends on how the various sites of the cluster are affected by the variation in the electronic structure between the oxidized and reduced forms. Theoret-... 

Energy can be transferred from one object to another. Anyone who has played a game of pool has firsthand experience with energy transfers. When the cue ball strikes the pack, the balls carom off in all directions. The cue ball loses most of its speed, and it may even come to a complete stop. The collision transfers kinetic energy from the cue ball to the other balls. Whereas transfers of energy among pool balls occur when initiated by a pool cue. 

Main axis experimental conversions ( ) intrinsic kinetic model (solid line) kinetic model + mass transfer kinetics (dashed line). Secondary axis variation of computed k,o with flow rate. 

The interpretation of phenomenological electron-transfer kinetics in terms of fundamental models based on transition state theory [1,3-6,10] has been hindered by our primitive understanding of the interfacial structure and potential distribution across ITIES. The structure of ITIES was initially studied by electrochemical and thermodynamic analyses, and more recently by computer simulations and interfacial spectroscopy. Classical electrochemical analysis based on differential capacitance and surface tension measurements has been extensively discussed in the literature [11-18]. The picture that emerged from... 

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