Principles of electrocatalysis and inhibition by electrosorbates and protective layers

Electrocatalysis or inhibition of electrode processes of a reactant R at the metal electrode M is due to the presence of surface layers S. These can be electrostatically adsorbed ions, covalently bound electrosorbates, neutral molecules or layers of oxides and salts, respectively. The influence of layers is caused by electrostatic or chemical interaction between S and R, by a geometric separation of M and R or, in thicker layers, by electronic effects. The influence depends strongly on the type of reaction which can be an outer-sphere electron transfer reaction (ETR), an ion transfer reaction (ITR) or a reaction with formation or breakage of chemical bonds (CBR). Typical examples are discussed for all types of layers and reactions with special emphasis on the type of interaction. Maximum effects are about two orders of magnitude for the electrostatic effect and geometric blocking and about three orders of magnitude for chemical and electronic effects. The theoretical interpretation by changing the pre-exponential factors, activation energies, activity coefficients of the activated complex and transfer coefficients is discussed, but evidence is rare and more detailed experiments are suggested.