Mass transfer active electrode

The solution velocity in this case is one of the major factors that control the corrosion potential and the corrosion rate in the active state of the alloy. The effect of mass transfer on electrode kinetics is discussed in detail in Chapter 3. Figure 4.11 correlates the... 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

The dimensionless limiting current density N represents the ratio of ohmic potential drop to the concentration overpotential at the electrode. A large value of N implies that the ohmic resistance tends to be the controlling factor for the current distribution. For small values of N, the concentration overpotential is large and the mass transfer tends to be the rate-limiting step of the overall process. The dimensionless exchange current density J represents the ratio of the ohmic potential drop to the activation overpotential. When both N and J approach infinity, one obtains the geometrically dependent primary current distribution. 

Very refined measurements at various positions of the packed bed were made by Jolls and Hanratty (J6), who used an active sphere (electrode) in a packed bed consisting of 1-inch inert spheres. The overall mass-transfer data for the turbulent flow regime suggest a dependence of... 

DSA-02 oxide-coated Ti anode (DSA = Dimensionally Stable Anode) is placed, thus creating a large cathode volume. The effluent solution flows perpendicularly through the electrodes with a typical flow rate of 0.5 dm3 s-1. The flowthrough metal electrodes have an active area approximately 15 times their geometric area. The cell allows air sparging to increase the mass-transfer. The current efficiency is about 40% when the inlet concentration of the metal ions is 150 to 1500 ppm and the concentration at the out-let is about 50 ppm. The cell is currently used for the treatment of recirculated wash-waters from acid copper, copper cyanide, zinc cyanide, zinc chloride, cadmium sulphate, cadmium cyanide and precious metal plating and washwaters from electroless copper deposition. Since the foam metal electrodes are relatively expensive the electrodes... 

In the search for cheaper materials, an problem often encountered is that the intrinsic activity is so low that it leads to much thicker electrodes, which in turn is counterproductive as it leads to high mass transfer losses. 

There are a lot of further innovative cell constructions in the literature that may also be suitable for electroorganic syntheses, from laboratory up to industrial scale. Examples are rotating electrodes, application of ultrasound, and packed or fluidized particle bed three-dimensional electrodes for increasing the active electrode area and enhancing the mass transfer. A short overview is given, for example, in [1,2]. 

Zone III the E vs ln(l — j/ji ) logarithmic curve corresponds to concentration polarization, which results from the limiting value ji of the mass transfer limiting current density for the reactive species and reaction products to and/or from the electrode active sites an increase inji from 1.4 to 2.2 Acm leads to a further... 

The catalyst support impacts the rate of a catalyzed reaction, the reaction pathway (quantities and species of intermediates and products) and the resistance of the catalyst to deactivation. In DBCP reactions, powders had higher rate constants than beads, presumably due to reduced mass transfer limitations alumina yielded a faster rate than C, which had a faster rate than PEI/silica. Sorptive capabilities of the supports may also play an important role Kovenklioglu found that supports which sorbed 1,1,2-TCA more strongly had higher reaction rates, and Farrell concluded that TCE sorption to Fe cause higher reaction rates on Pd/Fe electrodes than on pure Pd electrodes. It is also clear that supports influence reaction products, but the correlation between a given support and pathways/products it promotes is not yet understood. The choice of support can also affect its resistance to deactivation this implies that catalyst supports may be tailored to maximize activity over the long term. 

Highest sensitivity is reached when high enzyme activity within a thin layer is used and effective external mass transfer is provided. Under these conditions, substrate measurement can be managed down to the range of 1 micromolar with imprecision below 2 %. Therefore, owing to their limited sensitivity, "normal" enzyme electrodes are applicable only to metabolites present in the micro and millimolar concentration range. 

Under working conditions, with a current density j, the cell voltage E(J) decreases greatly as the result of three limiting factors the overvoltages r a and r c at both electrodes due to a rather low reaction rate of the electrochemical processes (activation polarization), the ohmic drop RJ in the electrolyte and interface resistance Re, and mass transfer limitations for reactants and products (concentration polarization). 

Let us consider the electrode kinetics associated with charge transfer from an n-type semiconductor particle to an electrode. As indicated by Albery et al. [164], the crucial difference between the electrochemistry of a colloidal particle and an ordinary electrochemically active solution phase species is the number of electrons transferred from the particle to the electrode may be large and will depend upon the potential of the electrode. Fig. 9.5 shows the model for an encounter of a particle with an electrode used by Albery and co-workers. kD is the mass-transfer coefficient for the transport of the particles to the electrode surface. In the simplest case, wherein it is assumed that the lifetime of the transferable electrons (majority carriers of thermal or photonic origin) is greater than the time taken by a particle to traverse the ORDE diffusion layer, this is given by... 

To develop any electrochemical process, a voltage should be applied between anodes and cathodes of the cell. This voltage is the addition of several contributions, such as the reversible cell voltage, the overvoltages, and the ohmic drops, that are related to the current in different ways. One of these contributions, the overvoltage, controls the rate of the transfer of electrons to the electrochemically active species through the electrode-electrolyte interface when there is no limitation in the availability of these active species on the interface (no mass-transfer control and no control by a preceding reaction). In this case, the relationship between the current that flows between the anodes and the cathodes of a cell and the overpotential is... 

The second item that needs to be fixed is the number of species and the reactions, including the stoichiometric coefficients and also the kinetics of the processes. In this context, in electrochemical oxidation processes it is important to discern between two types of anodes those that behaves only as electrons sinks (named nonactive) and those that suffer changes during the electrochemical oxidation which influence on the treatment (named active electrodes). In both cases, the main processes related to removal of the pollutant that involves irreversible oxidative routes. Consequently, the reductive processes are less important and it can be presumed that in the cathodic zone only hydrogen evolution occurs. Nevertheless, if some organic compound can be reduced at the cathode, the mass-transfer and the reduction processes must be included in the model scheme. 

When a typical active material is employed as the anode, a number of additional species generated on the electrode surface must also be considered. They can influence the process performance, causing additional chemical reactions on the electrode surface if the redox couple remains at the surface (i.e., Pt/PtO), or in the bulk solution if the electrogenerated species are dissolved (i.e., A1/A13+). A scheme outlining the processes that need to be considered in the anodic electrochemical zone is shown in Fig. 4.3. The first process to be taken into account is the formation of oxidized species on the electrode surface. These species can either remain on the surface or move toward the bulk zone. In the latter case, mass transfer to the bulk zone and possible chemical reactions in this zone must be considered. 

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