Relation of Electrocatalysis to Catalysis

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

Although catalysis in electrochemical reactions was probably first specifically recognized by Frumkin at a conference in Leningrad in 1939, a first and perceptive definition of electrocatalysis seems to have been by Busing and Kauzmann in 1952 (72) in terms of the ability of various electrode surfaces to promote the velocity of the rate-determining step of the reaction. In this respect, their definition preceded the common use of this term in North America in the 1960s by some years, when it was applied to the activities of fuel-cell electrodes by Liebhafsky (7i). 

It is a point peculiar to electrochemical reaction kinetics (77), however, that the rates of charge-transfer processes at electrodes measured, as they have to be, at some well-defined potential relative to that of a reference electrode, are independent of the work function of the electrocatalyst metal surface. This is due to cancellation of electron-transfer energies, O, at interfaces around the measuring circuit. In electrochemistry, this is a well-understood matter, and its detailed origin and a description of the effect may be found, among other places, in the monograph by Conway (77). 

When a chemical intermediate step in an overall electrochemical reaction sequence is rate determining, for example, an adsorbed radical recombination step or a first-order dissociation step involving an adsorbed intermediate [e.g., of RCOO in the Kolbe reaction (75)], then the general principles of heterogeneous catalysis do apply more or less in the usual way. However, even then, at an electrode, it must be noted that its surface is populated also and ubiquitously by oriented adsorbed solvent molecules (2, i) and by anions or cations of the electrolyte (7). The concentrations and orientational states of these species are normally dependent on electrode potential or interfacial field (7-i). 

Conditions for Electron Charge Transfer with Adsorption of an Intermediate 

---