Redox system reduction hydrogen evolution

Fig. 13. (a) Schematic representation of the formation of mixed potential, M, at an inert electrode with two simultaneous redox processes (I) and (II) with formal equilibrium potentials E j and E2. Observed current density—potential curve is shown by the broken line, (b) Representation of the formation of corrosion potential, Econ, by simultaneous occurrence of metal dissolution (I), hydrogen evolution, and oxygen reduction. Dissolution of metal M takes place at far too noble potentials and hence does not contribute to EC0Ir and the oxygen evolution reaction. The broken line shows the observed current density—potential curve for the system. 

From an electrochemical point of view this type of catalytic reduction can be conceived as two coupled electrode processes. In the case of this hydrogenation reaction, the catalytic and electrochemical reduction differ from each other only in the means for the achievement of the reduction, e.g. molecular hydrogen, redox systems, or pure electrochemical means. Similar to heterogeneous catalytic processes, electrochemical reactions tend to occur as a sequence of very simple steps. For example, hydrogen evolution occurs as two steps, with two alternatives for the second step, corresponding to two reaction routes ... 

Corrosion is an electrochemical process that consists of at least two reactions that compensate each other electronically in open circuit conditions. The anodic metal dissolution is counterbalanced by the cathodic reduction of a redox system within the electrolyte. Various processes may serve as cathodic counterreactions. The most common are oxygen reduction and hydrogen evolution in acidic electrolytes. 

Sometimes the value of the redox potential attains even at low current densities such values that another simultaneous process is possible. For instance the cathodic reversible potential of the Ti++++/Ti+++ system with the same concentration of both kinds of ions is about tc° = 0.04 V if platinized platinum in a solution of sulphuric acid is used as a cathode the evolution of hydrogen commences at a potential also near zero and the ourrent efficiency with respect to the reduction of Ti++++ ions will be comparatively low. Much better results can be achieved by replacing the platinum by another material, suoh as lead or graphite whioh have an appreciable hydrogen overvoltage, whereby the deposition potential of hydrogen becomes more negative as oompared with the potential of the Ti++++/Ti+++ system. 

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