Reaction layer

Figure C2.18.6. The coverages of fluorosilyl groups in tire reaction layer shown as a function of exposure. The coverages refer to monolayers of SiF groups. The smootli curves are drawn tlirough tire data points. Reproduced from 1411.

Quasi-equilibrium exposure regime. After tire 7 x 7 stmcture has been removed, quasi-equilibrium between etching and growtli of tire reaction layer is established. The reaction layer is about one monolayer tliick, and contains primarily SiF. Defects fonn near tire surface, partly from tire large reaction exotliennicity. 

Transition to steady-state etching. The surface becomes sufficiently disordered to dismpt tlie quasi-equilibrium, and tlie reaction layer becomes a tree stmcture of fluorosilyl chain stmctures tenninated by SiF., groups. 

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... 

Though using a much lower total amount of catalyst than in the homogeneous case, a considerably higher catalyst concentration in the reaction layer can be supplied. 

A general theory based on the quantitative treatment of the reaction layer profile exists for pure redox catalysis where the crucial function of the redox mediator is solely electron transfer and where the catalytic activity largely depends only on the redox potential and not on the structure of the catalyst This theory is consistent... 

Enantioselective electron transfer reactions are not possible in principle because the electron cannot possess chirality. Whenever the choice of enantiodifferentiation becomes apparent, it will occur in chemical steps subsequent (or prior) to electron transfer. Thus, enantioselectivities require a chiral environment in the reaction layer of electrochemical intermediates although asymmetric induction was report-... 

Non-Kolbe electrolysis of carboxylic acids in acetonitrile/water leads to acetamides as main products [294] (Table 10). The mechanism has been investigated by using " C-labeled carboxylic acids. The results are rationalized by assuming a reaction layer rich of carboxylate resulting in the formation of a diacylamide that is hydrolyzed... 

Figure 5.15 More detail than seen in Fig. 5.14 is obtained in a scanning electron image. The reacted glass particles are covered by a distinct reaction layer of silica gel (Barry, Clinton Wilson, 1979).

Each of the particles of Red produced in the chemical reaction will, after some (mean) time t, have been reconverted to A. Hence, when the current is anodic, only those particles of Red will be involved in the electrochemical reaction which within their own lifetime can reach the electrode surface by diffusion. This is possible only for particles produced close to the surface, within a thin layer of electrolyte called the reaction layer. Let this layer have a thickness 5,.. As a result of the electrochemical reaction, the concentration of substance Red in the reaction layer will vary from a value Cg at the outer boundary to the value Cg right next to the electrode within the layer a concentration gradient and a diffusion flux toward the surface are set up. 

It can be seen here that the larger the valne of k, the thinner will be the reaction layer and the more readily will the particles avoid getting involved in the electrochemical reaction and instead, participate in the reverse chemical reaction. However, becanse of the increase in concentration gradient, the flnx to the surface and the current density will still increase. 

The application of a potential also modifies all the acid-base equilibria in the vicinity of the interface, which, in turn, establishes a reaction layer on both sides of the interface. However, the bulk composition of each phase remains unchanged on the short experimental time scale, and the dissociation equilibria are maintained during a potential sweep. 

Sapundzhiev, H., Grozev, G., and Elenkov, D., Influence of geometric and thermophysical properties of reaction layer on sulphur dioxide oxidation in transient conditions. Chem. Eng. Technol. 13, 131-135 (1990). 

Figure 5.37 depicts the stationary distribution of the electroactive substance (the reaction layer) for kc—> oo. The thickness of the reaction layer is defined in an analogous way as the effective diffusion layer thickness (Fig. 2.12). It equals the distance [i of the intersection of the tangent drawn to the concentration curve in the point x = 0 with the line c = cA/K,...

Fig. 5.37 Steady-state concentration distribution (reaction layer) in the case of a chemical volume reaction preceding an electrode reaction (Eq. (5.6.12)) K = 103, kc >oo, A 1 = 0.04s1, D = 10 5cm s i is the effective reaction layer thickness...

Orui et al. [169] studied the stability of La(Ni, Fe)03 with 10 mol% Sc203-l mol% Al203-stabilized Zr02 (SASZ) and found that LNF was more reactive with SASZ electrolyte than LSM. For the cell sintered at 1100 and 1200°C, a reaction layer was clearly visible at the LNF/SASZ interface. The thickness of the reaction layer increased with the sintering temperature and the layer was identihed as the oxide-containing La and Zr by the TEM/EDS analysis. 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

The relative importance of reaction with respect to diffusion can be described in terms of the nondimensional (second) Damkohler number [30-36] (also called Thiele modulus), in terms of the reaction layer thickness [37,38] or in terms of lability criteria [39,40]. 

A quantitative discrimination between labile and nonlabile complexes is made by comparing the diffusion timescale with those of the association/dissociation reactions (or alternatively, the reaction layer, /i (equation (58)) and the diffusion layer, <5, thicknesses (e.g. equations (15), (18) and (19)). 

The concentration at the electrode surface is much smaller than in the absence of a reaction the more so the faster the reaction. The concentration profile is squeezed within a reaction layer whose thickness, /i, is small compared to the diffusion layer the smaller, the faster the reaction ... 

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