Recombination of atoms on surfaces. Part 13.—Oxygen atoms on Zn_xFe_3–xO₄ and Ni_xFe_3–xO₄

The catalytic behaviour of the spinels Zn_xFe_3–xO₄ Ni_xFe_3–xO₄ towards the recombination of oxygen atoms has been studied in the temperature range 20–50°C. The variation of the energy of activation for the reaction with x shows a maximum at approximately x= 0.6 in both the systems. This is believed to be due to the fact that the rate-determining step of the reaction involves the acceptance of an electron by the catalyst, and so is influenced by the Fermi level in the materials. There are two factors which determine the electronic nature of the catalysts : the delocalization of the electrons and the number of charge carriers. They produce a maximum in the Fermi level in each system for an intermediate value of x between 0 and 1. Consequently, there is a maximum in the activation energy pattern.