Electrochemical reactor secondary reactions

The largest exchange current density, j0, of the reaction has to be selected, if possible, since economic limitations are always prevalent in scaled-up engineering. However, with the development of nanodispersed substrates and carbon-supported metal catalysts, this limitation becomes a secondary consideration. At this point, it is important to say that most of the reported values of j usually refer to simple reactions on pure metal substrates using different shapes of electrode designs in a certain and single electrolyte. Thus, the measurement of the real j0 value at select industrial conditions of the electrochemical reactor has to be performed that is, experimental measurements cannot be avoided [4,5]. 

The major sources of dilute, metal ion liquors are identified within the metals production/processing and chemical industries. Problems associated with traditional methods of metal ion removal are highlighted and the developing role of electrochemical techniques is discussed. Electrode and cell reactions are illustrated via typical examples from laboratory and industrial practice. The need to select an appropriate cell design and to control the reaction conditions is emphasised via consideration of the problems caused by secondary reactions. Important design criteria for electrochemical reactors are summarised. Available reactors are classified according to the nature of the product which may be metal flake or powder, a metal deposited onto a disposable substrate, a metal ion concentrate or an insoluble metal compound. The applications for electrochemical techniques in environmental treatment are illustrated by examples which show features of reactor construction and their typical performance. Current trends are summarised and recommendations are made for further work in critical areas. 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primary cells. The discharging reaction is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondary cells), however, the reaction is reversible that is, application of an electrical potential across the electrodes in the opposite direction will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, electroplating, and electrowinning industries are based. 

The coupling of enzymatic and electrochemical reactions has provided efficient tools, not only for analytical but also for synthetic purposes. In the latter field, the possibilities of enzymatic electrocatalysis, for example, the coupling of glucose oxidation (catalyzed either by GOx or GDH) to the electrochemical regeneration of a co-substrate (benzoquinone or NAD+) have been demonstrated [362-364]. An electroenzymatic reactor has also been developed [363-364] to demonstrate the production of biochemicals on a laboratory scale. NAD(P)+ derivatives immobilized by covalent attachment to polymer matrices or protein backbones have been used in enzyme reactors [365, 366]. Another important coenzyme ubiquinone can be regenerated at an electrode [367, 371] and applied to drive secondary enzymatic reactions with the participation of membrane enzymes (e.g. fumarate reductase). 

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