Oxidation-reduction potentials and their relation to the catalytic activity of transition metal oxides

Calculations of binding energy of oxygen in 3d transition metal oxides were carried out on the basis of an ionic model taking into account the crystal field effects. Good agreement was obtained with experimental data concerning variations of binding energy of bulk oxygen in isovalent series. The oxygen binding energy in bulk and on the surface, the free energy of oxidation of lower to higher oxides, and the oxidation-reduction potentials in aqueous media vary with the atomic number of the metal ion according to the same patterns due to the fact that variations of ionization potentials dominate over the variations in crystal field stabilization energy. The extraordinary high oxidation-reduction potential of the Fe2+Fe3+ system explains its relatively low catalytic activity as due to strong oxygen binding. At very low oxidation-reduction potentials (e.g., in Cr3+Cr4+ system) the catalytic activity is also low due to depletion of surface oxygen. Maximum catalytic activity occurs at intermediate (5–20 kcal/mole) oxidation-reduction potentials.