EC’ process

An example of such a catalytic EC process is the oxidation of dopamine in the presence of ascorbic acid (4). The dopamine quinone formed in the redox step is reduced back to dopamine by the ascorbate ion. The peak ratio for such a catalytic reaction is always unity. 

In this example24 redox catalysis kinetics is governed partly by chemical reaction, i.e., the scission of C6H5S02CH3. For given concentrations of pyrene and sulphone at sweep rate v one can find values of klk/k2 from published graphs23 in the case of EC processes. 

Mullins, L.J. Leary, J.A. "Fused Salt Electrorefining of Molten Plutonium by the LAMEX Process", I and EC Process Design and Development, Vol. 4, Pg. 394, October 1965. 

Yoon, S. M., and Kunii, D., Gas Flow and Pressure Drop through Moving Beds, I EC Process Des. Develop., 9 559 (1970)... 

The simple EC process is less common, though the reverse of the reaction above would clearly be a case in point, and a better known example is ... 

The first cathodic wave was studied by cycling the potential across it at various scan rates and the peak potentials were found to increase as indicative of a reversible, diffusion-controlled system, with ° = — 1.43 V vs. SCE. However, at sweep rates 20mV/s the peak anodic current is much smaller than expected which was interpreted by the authors as indicating that the reduced species undergoes a subsequent chemical reaction, i.e. an EC process. 

Funk, J.E. and Reinstrom, R.M., Energy requirements in the production of hydrogen from water, I EC Process Design Dev., 5,336,1966. 

A similar role is played by irreversible follow-up reactions, but the possibility of a mixed kinetic control by the two steps of the EC process should then be taken into account. A simplifying assumption is that the follow-up reaction is so fast that the conditions of zone KP prevail. It corresponds to the maximal influence of the coupled chemical step. The dimensionless expression of the cathodic trace of the irreversible voltam-mogram is then given by (see Section 6.2.1)... 

FIGURE 2.31. Concentration profiles in steady-state (stirring or circulation electrolysis showing the various region of interest for a simple electron transfer reaction (top) and EC process with a fast follow-up reaction (bottom). 

Meissner, H.P. Kusik, C.L., "Aqueous Solutions of Two or More Strong Electrolytes-Vapor Pressures and Solubilities", I EC Process Des. Develop., 1973, 12, 205... 

Example Consider the dissociative EC process CF2CI2 + e —> + CFCI2 . 

Fig. 7.13. Potential energy diagram of dissociative EC process CE2CI2 + e E" + CFCb".

Alatiqi presented (I EC Process Design Dev. 1986, Vol. 25, p. 762) the transfer functions for a 4 X 4 multivariable complex distillation column with sidestream stripper for separating a ternary mixture into three products. There are four controlled variables purities of the three product streams (jCj, x, and Xjij) and a temperature difference AT to rninirnize energy consumptiou There are four manipulated variables reflux R, heat input to the reboiler, heat input to the stripper reboiler Qg, and flow rate of feed to the stripper Lj. The 4x4 matrix of openloop transfer functions relating controlled and manipulated variables is ... 

R.L. Laurence and G. Vasudevan. Behef function combination and conflict management. Ik.EC Process Design and Development, 7(3) 427-433, 1968. 

Thomas, D.G., and Mixon, W.R., "Effect of Axial Velocity and Initial Flux on Flux Decline of Cellulose Acetate Membrane in Hyperfiltration of Primary Sewage Effluents," I EC Process Design and Development 11, 339-343 (1972). 

Thomas, D.G. and Watson, J.S., "Reduction of Concentration Polarization of Dynamically Formed Hyperfiltration Membranes by Detached Turbulence Promoters", I EC Process Design and Develop., 1968, T, July, 397. 

When the EC process is forced to occur within two opposite insulating flat substrates the technique is termed CEC and leads to thin single crystals, their thickness ultimately defined by the separation between both substrates (Thakur et al., 1990). This technique can be universally applied, as the parent standard EC technique, to the synthesis of a variety of thin single crystals of conducting and insulating molecular materials. 

If M is unstable then ipb/fpf will be less than unity. Its magnitude will depend upon the scan rate, the value of the first-order constant k, and the conditions of the experiment. At fast scan rates the ratio ipb/ ip, may approach one if the time gate for the decomposition of M is small compared with the half-life of M-, (In 2jk). As the temperature is lowered, the magnitude of k may be sufficiently decreased for full reversible behaviour to be observed. The decomposition of M- could involve the attack of a solution species upon it, e.g. an electrophile. In such cases, ipb/ipf, will of course be dependent upon the concentration of the particular substrate (under pseudo-first-order conditions, k is kapparent). Quantitative cyclic voltammetric and related techniques allow the evaluation of the rate constants for such electrochemical—chemical, EC, processes. At the limit, the electron-transfer process is completely irreversible if k is sufficiently large with respect to the rate of heterogeneous electron transfer the electrochemical and chemical steps are concerted on the time-scale of the cyclic voltammetric experiment.1-3... 

The simplest example of an EC process is one in which structural isomerization follows single electron transfer, e.g. reaction (15),44 and many examples of this type of reaction have been reported.28-45"46... 

The solid line of Figure 23.1 gives the calculated trace for Equation 23.1 with a moderate heterogeneous charge-transfer rate (a quasi-reversible E system). The circles give the calculated response on the basis of Equations 23.2-23.4, in which the initial product B reacts rapidly to give X, which is oxidized on the return sweep. The anodic wave therefore arises from the electroactive decomposition product (X) of B. The cathodic (forward) scan is therefore an EC process and the reverse scan an E process, so that the overall cyclic mechanism might be referred to as EC,E (Eq. 23.2-23.4). 

By comparing the problem given by Eqs. (3.186b)-(3.193b) for aCE mechanism with Eqs. (3.186c)-(3.193c) for an EC mechanism, it can be easily inferred that the current corresponding to the EC process can be deduced with a similar procedure to that followed for a CE one (see Appendix D). So, the solution is ... 

Indeed, from Eq. (3.205a) for a catalytic process (under kss conditions) and Eqs. (3.218b) and (3.199c) for CE and EC processes (under the more restrictive dkss conditions), it can be inferred that a true stationary (independent of time) current-potential response for a catalytic reaction (reaction scheme (3.IXa, 3.IXb)) can be reached even for large electrodes (planar diffusion), whereas for CE and EC... 

Finding rigorous analytical expressions for the single potential step voltammograms of these reaction mechanisms in a spherical diffusion field is not easy. However, they can be found in reference [63, 64, 71-73] for the complete current-potential curve of CE and EC mechanisms. The solutions of CE and EC processes under kinetic steady state can be found in references [63, 64] and the expression of the limiting current in reference [74], Both rigorous and kinetic steady state solutions are too complex to be treated within the scope of this book. Thus, the analysis of these processes in spherical diffusion will be restricted to the application of diffusive-kinetic steady-state treatment. 

For this purpose, we will consider a CE mechanism (reaction scheme (3.VII)), since the derivation of the expressions for an EC process can be carried out by following a similar procedure to that of a CE process. In this case, as previously, we will consider that the pseudo-species (= cB + cc) and species cD have a purely diffusive behavior. So, it is also supposed that dss/dt= 0 ( >ss = cB — Kcc), which fulfills, in a spherical diffusion field ... 

The voltammetric response for the reaction scheme (3. XI) depends on the difference between the formal potentials of both electrochemical steps, ACt°, and on the equilibrium and kinetic constants of the intermediate chemical reaction. If AE = Ef.2 — Ef j [Pg.217]

Of the wide range of this type of processes, this study focuses mainly on three first- or pseudo-first-order reaction mechanisms (see reaction scheme (7.VI)) catalytic, CE, and EC processes which are the most analyzed in the Electrochemistry literature. 

Pseudo-First-Order Electrocatalytic Processes EC Process... 

A. Schumpe, W.D. Deckwer, Gas holdups, specific interfacial areas, and mass transfer coefficients of aerated carboxymethyl cellulose solutions in a bubble column, I EC Process Des. Develop. 21 (1982) 706-711. 

Johnson, G.E., Kunka, L.M. and Field, J.H., "Use of Coal and Fly Ash as Absorbents for Removing Organic Contaminants from Secondary Municipal Effluents", I EC, Process Design and Development 4, 323-327, 1965. 

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