Reactant transport electron-transfer reactions

The semiconductor model is analogous to processes created in a system of redox reactants namely, after a redox reaction occurs the products must be moved apart as quickly as possible. The processes of diffusional mass transport or convection are the only means of separating the products in solution, and these processes are slow compared to the mechanism involved in semiconductors. To enhance mass transport, it is possible to introduce intermediates so that the oxidant and reductant are separated by fast electron-transfer reactions. In this case, recombination is prevented (to some extent) by the physical separation of oxidant and reductant using intermediary donors and acceptors (63). 

One of the simplest of all chemical reaction is the exchange of a proton. Unlike electron-transfer reactions, which involve mainly the exchange of charge between reactants, the transfer of a proton also results in the transport of mass. Reactions involving protons are ubiquitous because of their simplicity. They have been studied in a large number of systems... 

All the reactions in the electron transport chain are electron-transfer reactions, but some of the reactants and products inherently transfer either one or two electrons, as the case may be. 

The three steps associated with electrochemical reactions, i.e., transport of reactant(s) to the interface, the electron transfer (surface) reaction and transport of product(s) from the interface, are sequential. Therefore, the overall rate of reaction is controlled by the slowest of the three steps. When the transport processes are capable of operating at high rates relative to the electron transfer reaction, the rate of the overall reaction can be described by equations of electrodekinetics. These types of electrode reactions are said to be "under activation control". 

Mass transport control of electrode reactions can be caused by one of two physical processes. In the first case, the interfacial concentration of the reactant drops to zero, i.e., the electron transfer reaction consumes the species as quickly as it arrives at the interface. In the second case, the interfacial concentration of the product reaches saturation. When these conditions prevail, the rate of mass transport is at its limiting (maximum) value. Limiting current densities, ij and in, for anodic and cathodic partial reactions, respectively, are... 

ELECTRODE KINETICS OF ELECTRON-TRANSFER REACTION AND REACTANT TRANSPORT IN ELECTROLYTE SOLUTION... 

Effect of Reactant Transport on the Electrode Kinetics of Electron-Transfer Reaction 57... 

Besides the kinetics of electron-transfer reaction discussed above, the process of reactant transport near an electrode... 

In the above sections, we have presented the electrode kinetics of electron-transfer reaction and reactant transport on planar electrode. However, for practical application, the electrode is normally the porous electrode matrix layer rather thtin a planner electrode siuface because of the inherent advantage of large interfacial area per unit volume. For example, the fuel cell catalyst layers are composed of conductive carbon particles on which the catalyst particles with several nanometers of diameter are attached. On the catalyst particles, some proton or hydroxide ion-conductive ionomer are attached to form a solid electrolyte, which is uniformly distributed within the whole matrix layer. Due to the electrode layer being immersed into the electrolyte solution, this kind of electrode layer is called the flooded electrode layer . 

This first chapter to Volume 2 Interfadal Kinetics and Mass Transport introduces the following sections, with particular focus on the distinctive feature of electrode reactions, namely, the exponential current-potential relationship, which reflects the strong effect of the interfacial electric field on the kinetics of chemical reactions at electrode surfaces. We then analyze the consequence of this accelerating effect on the reaction kinetics upon the surface concentration of reactants and products and the role played by mass transport on the current-potential curves. The theory of electron-transfer reactions, migration, and diffusion processes and digital simulation of convective-diffusion are analyzed in the first four chapters. New experimental evidence of mechanistic aspects in electrode kinetics from different in-situ spectroscopies and structural studies are discussed in the second section. The last... 

As the study of ET reactions is difficult at large interfaces, these have been studied using different experimental approaches such as SECM or liquid-fihn modified electrodes as presented in Section 1.6. Of course, cyclic voltammetry is a ubiquitous electrochemical method that can be applied to study electron-transfer reactions. However, the mass transport equations differ from those for classical cyclic voltammetry at a solid electrode, as we have to consider the mass transport equations for the two incoming reactants and the two outgoing products. As a result, the classical... 

At higher current densities, the primary electron transfer rate is usually no longer limiting instead, limitations arise tluough the slow transport of reactants from the solution to the electrode surface or, conversely, the slow transport of the product away from the electrode (diffusion overpotential) or tluough the inability of chemical reactions coupled to the electron transfer step to keep pace (reaction overpotential). 

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. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

Experiment shows that when the transport of reactants cannot keep pace with the charge-transfer reaction, the potential d observed at the current density i is not equal to the zero-current, or equilibrium potential difference J< > =0 = J< >e. If an electronation reaction is considered,... 

This relation emphasizes that only part of the double-layer correction upon AG arises from the formation of the precursor state [eqn. (4a)]. Since the charges of the reactant and product generally differ, normally wp = ws and so, from eqn. (9) the work-corrected activation energy, AG orr, will differ from AG. [This arises because, according to transition-state theory, the influence of the double layer upon AG equals the work required to transport the transition state, rather than the reactant, from the bulk solution to the reaction site (see Sect. 3.5.2).] Equation (9) therefore expresses the effect of the double layer upon the elementary electron-transfer step, whereas eqn. (4a) accounts for the work of forming the precursor state from the bulk reactant. These two components of the double-layer correction are given together in eqn. (7a). 

---