Mass transport regime

For all reactions, the mass transport regime is controlled by the diffusion of the reacting ligand only, as the mercury electrode serves as an inexhaustible source for mercury ions. Hence, with respect to the mathematical modeling, reactions (2.205) and (2.206) are identical. This also holds true for reactions (2.210) and (2.211). Furthermore, it is assumed that the electrode surface is covered by a sub-monomolecular film without interactions between the deposited particles. For reactions (2.207) and (2.209) the ligand adsorption obeys a linear adsorption isotherm. Assuming semi-infinite diffusion at a planar electrode, the general mathematical model is defined as follows ... 

Mass-transport regime. A high mass-transport coefficient that leads to a greater uniformity of pollutant concentration in the reaction layer near the electrode surface and to generally a higher efficiency... 

Consider a Newtonian incompressible fluid containing a component A in high dilution (<0.05M) and moving under creeping flow conditions within a relatively high porosity porous medium. The solid surface adsorbs instantaneously the eomponent A. The mass transport regime (convection and/or diffusion) is expressed by the value of the Peclet number, defined... 

It follows that in dynamic electrochemistry the applied potential represents the driving force for charge transfer (usually electron transfer) and the current that flows is a measure of the rate of the reaction. Electrochemical experiments of this type are classified as voltammetry or amperometry, and several of the most important techniques are considered in subsequent chapters of this volume. These techniques differ in the form of the potential signal applied to the working electrode, the type of mass transport regime employed and the current response measured. For example, cycKc voltammetry, discussed in Chapter 2.1, utilizes a triangular potential waveform with... 

The space time yield is a measure of the rate of production per unit volume of reactor and is normally quoted in units such as mol dm h . The space time yield is proportional to the effective current through the cell per unit volume of reactor and hence on the current density (overpotential, concentration of electroactive species and the mass transport regime), current efficiency and the active surface area of electrode per unit volume. 

Although flow cells have great advantages over tank cells in terms of both current density and control of the mass transport regime, they commonly suffer from the problem that the conversion per pass is low. For this reason it is common to run... 

In such systems the rate of corrosion will depend on the mass transport regime in the solution. Increasing the rate of transport of oxygen to the surface must increase the rate of corrosion. 

T steady-state mass transport regime the concentration profile in the diffusion layer corresponds to that of the steady state and the limiting current density depends exclusively on convection conditions. 

T 1, non-steady-state mass transport regime the concentration profile inside the diffusion layer changes with time and the effect of convection on I =j E,X) is negligible. 

Until the previous paragraph, the electron transfer and mass transport steps in the sequence (1.17)-(1.19) for the simple electrode reaction have been considered in isolation, although it was recognised that the rate of the former increases with overpotential, Equations (1.31)-(1.34), while the maximum rate of the mass transport step depends only on the bulk concentration of the electroactive species and the mass transport regime. Equation (1.44). 

Under complete mass transport control, the rate of reactant supply or product removal determines a limiting current, /l, and this is the maximum possible current for a given reaction. The limiting current, /l, is determined by the mass transport regime close to the electrode surface, characterized by the mass transfer coefficient, k, and related to the relative velocity, v, between the electrode and the electrolyte ... 

Improving the mass transport regime In parallel plate cells this is achieved either by increasing the electrolyte flow rate or introducing turbulence promoters into the interelectrode gap. 

Furthermore, the ultrasounds contribute very significantly to the enhancement of the mass transport regime by convection, i.e., the transport of Fenton s reactants and products towards/from the cathode. 

Nevertheless, the overall performance of electrochemical processes is established by the complex interaction of different parameters that may be optimized to obtain an effective and ecmiom-ical mineralization of pollutants. The principal parameters that determine an electrolysis performance are (i) electrode potential and current density, (ii) current distribution, (iii) mass transport regime, (iv) cell design, (v) electrolysis medium, and (vi) electrode materials. Even if we still remain far from meeting all the requirements needed for an ideal anode, significant steps have been made toward the production of better electrode materials [3]. 

Eigure 6.1.4.2 shows the relationship that exists between the range of useable scan rates and electrode radius subject to the condition that ohmic drop is negligible and that the dominant mass transport regime is linear diffusion (9). 

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