ABSTRACT
Three possible approaches to calculating potential dependent properties of redox and electrocatalytic reaction are discussed. First, it is described how the self–consistent calculational modeling of surface adsorption and reactions as already used in surface chemistry studies might be extended to include changes in electron chemical potential by adding charges to the surface model. Problems here center on the limitation on potential adjustment that relates to model size and electrolyte composition and structure. In the second approach, the potential variations are made by modifying electrode surface atom ionization potentials so that the Fermi level as given by semiempirical cluster or band calculations shifts. This approach focuses on adsorbed molecules and their interactions with the electrode surface, for which changes in potential affect the donor–acceptor bonding interactions. Within semiempirical atom superposition and electron derealization molecular orbital theory, predictions of potential dependent decomposition of adsorbed water molecules and adsorbed carbon monoxide oxidation reactions over platinum electrodes have been made in this way. The effect of electrolyte molecules in this approach is manifested through steric and covalent bonding interactions only. For the third approach, the focus is on calculating mechanisms for oxidation and reduction reactions of species either in solution or adsorbed. In this case self–consistent ab initio calculations are performed for the reacting system with a remote noninteracting electron donor or acceptor species present. As the reacting system traverses the reaction surface, a structure is reached where its electron affinity matches the donor’s ionization potential so that the reacting system and the donor are in equilibrium. The ionization potential of the donor determines the potential of the reduction reaction. The transition state structure may come before or after the electron transfers; in recent studies of outer sphere O2 reduction, these have been found to coincide. Oxidation reactions are treated in the same way with an electron acceptor present: At the electron transfer equilibrium, its electron affinity corresponds to the potential. Such studies can also be accomplished without donors or acceptors by the equivalent approach of analyzing the intersections of reactant and product Born–Oppenheimer potential energy surfaces. For reactions outside the Helmholtz plane, the electron potential is well defined, but, as for the semiempirical Fermi level shift approach, for studying reactions of adsorbed molecules away from equilibrium in the over–potential region, often the case for electrocatalysis, setting the correct electron potential requires knowledge of the change in electrochemical potential that occurs through the double layer.
