Electrolysis processes, comparison

Table 9.2 Comparison of main advantages and limitations of the three main brine electrolysis processes...

The use of aqueous chloride electrolysis in comparison with molten salt systems has the disadvantage of producing a powdered lead cathode deposit in comparison with molten lead. The production of chlorine can be common to both systems but can be avoided in the aqueous system if iron leach solutions are used as the electrolyte, or proton permeable membranes are used to allow for a separate anolyte solution composition. No clear preference has emerged to date from the many process options examined. 

Table 6.19.6 Comparison of the three chlor-alkali electrolysis processes [estimated by data of Bergner (1994) Bergner, Hartmann, and Staab (1987) and European Commission 001)].

At times it can be useful to carry out a sort of reversal electrolysis, i.e. reoxidize the reduced species (or reduce that oxidized) immediately after generating it to re-obtain the initial species. As shown in Table 3, the comparison between the amount of electricity (in C) consumed in the forward process, Qf, and that consumed in the return process, Qr, can be useful in the identification of the electrode mechanism involved in the process under study (even if in general it is less simple than appears). 

The electrode processes on the voltammetric and the preparative electrolysis time scales may be quite different. The oxidation of enaminone 1 with the hydroxy group in the ortho position under the controlled potential electrolysis gave bichromone 2 in 68% yield (Scheme 4.) with the consumption of 2.4 F/mol [21], The RDE voltammogram of the solution of 1 in CH3CN-O.I mol/1 Et4C104 showed one wave whose current function, ii/co C, was constant with rotation rates in the range from 1(X) to 2700 rpm and showed one-electron behavior by comparison to the values of the current function with that obtained for ferrocene. The LSV analysis was undertaken in order to explain the mechanism of the reaction which involves several steps (e-c-dimerization-p-deamina-tion). The variation of Ep/2 with log v was 30.1 1.8 mV and variation of Ep/2 with logC was zero. Thus, our kinetic data obtained from LSV compare favorably with the theoretical value, 29.6 mV at 298 K, for a first order rate low [15]. This observation ruled out the dimerization of radical cation, for... 

Lithium metal is produced commercially by electrolysis of a fused eutectic mixture of hthium chloride-potassium chloride (45% LiCl) at 400 to 450°C. The eutectic mixture melts at 352°C in comparison to the pure LiCl melting at 606°C. Also, the eutectic melt is a superior electrolyte to LiCl melt. (Landolt, P.E. and C. A. Hampel. 1968. Lithium. In Encyclopedia of Chemical Elements.C. A. Hampel, Ed. Reinhold Book Corp. New York.) Electrolysis is carried out using graphite anodes and steel cathodes. Any sodium impurity in hthium chloride may be removed by vaporizing sodium under vacuum at elevated temperatures. All commercial processes nowadays are based on electrolytic recovery of the metal. Chemical reduction processes do not yield high purity-grade metal. Lithium can be stored indefinitely under airtight conditions. It usually is stored under mineral oil in metal drums. 

These aspects do not form an unmixed blessing, however. If the metal must be reduced by electrolysis, the process may become energy intensive. Thus attractive solutions to this problem are reduction of more valuable gold by less expensive zinc and of more valuable copper by scrap iron. Finally, in view of the large amounts of waste water formed as by-product, one may be trading an air pollution problem for a water pollution problem. A comparison of the two types of processes is given in Table 10.6. 

It follows from Equation 6.12 that the current depends on the surface concentrations of O and R, i.e. on the potential of the working electrode, but the current is, for obvious reasons, also dependent on the transport of O and R to and from the electrode surface. It is intuitively understood that the transport of a substrate to the electrode surface, and of intermediates and products away from the electrode surface, has to be effective in order to achieve a high rate of conversion. In this sense, an electrochemical reaction is similar to any other chemical surface process. In a typical laboratory electrolysis cell, the necessary transport is accomplished by magnetic stirring. How exactly the fluid flow achieved by stirring and the diffusion in and out of the stationary layer close to the electrode surface may be described in mathematical terms is usually of no concern the mass transport just has to be effective. The situation is quite different when an electrochemical method is to be used for kinetics and mechanism studies. Kinetics and mechanism studies are, as a rule, based on the comparison of experimental results with theoretical predictions based on a given set of rate laws and, for this reason, it is of the utmost importance that the mass transport is well defined and calculable. Since the intention here is simply to introduce the different contributions to mass transport in electrochemistry, rather than to present a full mathematical account of the transport phenomena met in various electrochemical methods, we shall consider transport in only one dimension, the x-coordinate, normal to a planar electrode surface (see also Chapter 5). 

A comparison of the E°s would lead us to predict that the reduction (it) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving 02 are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na+ (iii), so the net result of the electrolysis of brine is the production of Cl2 and NaOH ( caustic ), both of which are of immense industrial importance ... 

Kolbe electrolysis also allows some comparisons with analogous homogeneous reactions with regard to dimerization, substitution, or addition reactions of the generated radicals. Photolytic or thermal decarboxylation of diacylperoxides is a source of alkyl radicals similar to those afforded by the Kolbe electrolysis. The anodic oxidation of propionate has been compared with the thermal decomposition of dipropionyl peroxide [28]. Examination of the yields shows that reaction between radicals is favored in the electrochemical process, whereas in peroxide decomposition hydrogen atom abstraction from the solvent or the substrate occurs to a higher extent. This illustrates the effect of the higher radical concentration at the electrode. 

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