Catalysis of Redox Reactions by Surfaces

In aqueous solution, electron transfer between two solute redox reactants A+ and B may occur either by an outer-spher (o.s.) or an inner-sphere (i.s.) redox mechanism (ET stands for electron transfer) 

In the o.s. reaction, the ion pair A+ - B is formed in a first step. The corresponding equilibrium constant can usually be obtained from simple electrostatic models. In this ideal case specific chemical interactions can be neglected and the rate constant of the E.T. step follows the theory of R.A. Marcus (see for example Marcus, 1975, or Cannon, 1980). In the i.s. reaction each of the three steps in reaction (9.2) may determine the reaction rates. The lability of the coordinated ligands at the 

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

A classical case of heterogeneous inner-spheric electron transfer has been demonstrated by Gordon and Taube (1962) on the oxidation of U(IV) by Pb02 by using 180 in Pb02, they could show that both O 11 ions in the product UO + are derived from the oxide lattice  

A typical redox reaction sequence between an (usually organic) reductant R and iron(III) (hydr)oxide can be schematically expressed as 

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