Diffusion-controlled oxygen-reduction

In the work of Alodan and Smyrl," the pH profiles over the particle were simulated by assuming a diffusion-controlled oxygen reduction reaction at the cathode and by setting a limiting diffusion current density of 0.1 itiA cm, calculated for a diffusion layer thickness of 20 pm and O2 concentration of 0.26 mol m. ... 

TsujimuraS, Kamitaka Y, Kano K. Diffusion-controlled oxygen reduction on multi-copper oxidase-adsorbed carbon aerogel electrodes without mediator. Fuel Cells 2007 463-469. 

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

It has been emphasised that the oxygen reduction reaction is diffusion controlled, and it might be thought that the nature of the metal surface is unimportant compared with the effect of concentration, velocity and temperature that all affect /Y and hence. However, in near-neutral solutions the surface of most metals will be coated (partially or completely) with either... 

For diffusion controlled corrosion reactions e.g. dissolved oxygen reduction, and the effect of temperature which increases diffusion rates, then by substituting viscosity and the diffusion coefficients at appropriate temperatures into the Reynolds No. and Schmidt No., changes in corrosion rate can be calculated. 

Dissolved oxygen reduction process Corrosion processes governed by this cathode reaction might be expected to be wholly controlled by concentration polarisation because of the low solubility of oxygen, especially in concentrated salt solution. The effect of temperature increase is complex in that the diffusivity of oxygen molecules increases, but solubility decreases. Data are scarce for these effects but the net mass transport of oxygen should increase with temperature until a maximum is reached (estimated at about 80°C) when the concentration falls as the boiling point is approached. Thus the corrosion rate should attain a maximum at 80°C and then decrease with further increase in temperature. 

The results of this study demonstrated that the rate of oxygen transfer across a clean air-water interface was diffusion-controlled on the time scale of SECM measurements. The rate of this transfer process was, however, significantly reduced with increasing compression of a 1-octadecanol monolayer. Figure 28 illustrates this point, showing approach curves for O2 reduction recorded with the monolayer at different surface pressures. The transfer rate was found to depend on the accessible free area of the interface, as described by the following equation ... 

A complication that occurs on a low at.% Ru electrode is that, owing to the low Faradaic currents (low Ru content) and hence large Rt value, currents due to other trace redox reactions, e.g. oxygen reduction, become more detectable. This reveals itself in a phase-angle of 45° as co 0 as trace oxygen reduction would be diffusion-controlled. The impedance corresponding to this situation can be shown to be the same as that in Equation 5.3, with U(p) expressed by the relationship ... 

Generally, the reduction is achieved under deaerated conditions to avoid a competitive scavenging of Cjoiv and H atoms by oxygen. These atoms are as homogeneously distributed as the ions and the reducing species, and they are therefore produced at first as isolated entities. Similarly, multivalent ions are reduced by multistep reactions, including disproportionation of intermediate valencies. Such reduction reactions have been observed directly by pulse radiolysis for a variety of metal ions (Fig. 2), mostly in water [28], but also in other solvents where the ionic precursors are soluble. Most of their rate eonstants are known and the reactions are often diffusion controlled. 

The role of those layers on the course of corrosion is essential since the main cathodic process involved is the reduction of dissolved oxygen which is at least partly diffusion controlled at usual corrosion potentials. Thus, the existence of an additional solid phase at the electrode generally decreases the diffusion rate of oxygen and therefore modifies the corrosion potential and the corrosion rate as well. 

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

Corrosion current density — Anodic metal dissolution is compensated electronically by a cathodic process, like cathodic hydrogen evolution or oxygen reduction. These processes follow the exponential current density-potential relationship of the - Butler-Volmer equation in case of their charge transfer control or they may be transport controlled (- diffusion or - migration). At the -> rest potential Er both - current densities have the same value with opposite sign and compensate each other with a zero current density in the outer electronic circuit. In this case the rest potential is a -> mixed potential. This metal dissolution is related to the corro-... 

The reduction of oxygen by copper(I) is faster than that of the iron(II) complexes 5x 104M 1 s-1 for CuI(phen)2 [52] and 4xl04M 1 s 1 for Cu1 (histidine [76]. It is this relatively fast autoxidation that limits the usefulness of copper complexes as mimics of superoxide dismutase under conditions of high superoxide concentrations [77]. Copper(II) catalyses the dismutation of superoxide at near diffusion-controlled rates kcat = 8 x 109 M 1 s-1 [78,79],... 

Activation control of an overall dissolution rate can, of course, reside in the reduction process, in the oxidation process, in a mixture of both, or in a mixture including some transport control. The reduction process is usually more influential in determining the overall rate. Thus, in the absence of transport control, the kinetics of the electrode process for reduction of hydrated protons, or water molecules, or dissolved molecular oxygen plays the major role in metal dissolution kinetics. Indeed the literature confirms the conclusion that many of the systems seen in experiment or in practice are diffusion controlled that most of the rest are under mixed diffusion and activation control and that those with some activation control... 

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