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CHAPTER 2
Electrochemical Electron Transfer:
From Marcus Theory to Electrocatalysis
E. SANTOS
Faculdad de Matem´tica, Astronom´ayF´sica, IFFaMAF-CONICET, Universidad Nacional de C´rdoba,
C´ rdoba, Argentina, and Department of Theoretical Chemistry, Ulm University D-89069 Ulm, Germany
W. SCHMICKLER
Department of Theoretical Chemistry, Ulm University, D-89069 Ulm, Germany
2.1 INTRODUCTION
Early attempts at a theory of electrochemical reactions were focused on the hydrogen
evolution reaction. In retrospect, these attempts were premature, because catalytic
reactions, of which hydrogen evolution is a prime example, cannot be understood
without detailed knowledge of the electronic structure and the interactions of the
electrode and the reactant, information that can only be obtained by quantum chemical
calculations requiring high speed computers. Therefore, these valiant efforts were
unsuccessful, even though some qualitative aspects, such as the role of tunneling
events investigated by Gurney [1931, 1936], or the role of the adsorption of inter-
mediates [Parsons, 1958; Gerischer, 1956; Trasatti, 1972], were elucidated. It has
been claimed, with some credibility, that this preoccupation with hydrogen evolution
delayed the development of theoretical electrochemistry by at least a decade [Bockris
and Khan, 1993].
The first successful theories of electrochemical reactions, proposed by Marcus
[1956] and Hush [1958], dealt with outer-sphere electron transfer, and focused on
the role of solvent reorganization, while electronic interactions and catalysis played
a minor, merely formal, role. This remained largely true for the quantum mechanical
version of that theory developed by Levich [1970], and even for the first theories
describing electron transfer involving the breaking of a bond [Sav´ant, 1993;
German and Dogonadze, 1974; German and Kuznetsov, 1994]. Generally, it was
assumed that the electronic interactions between the reactant and the electrode are
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