Kinetic study of the reduction of nickel oxide near its antiferromagnetic-paramagnetic transition. Influence of the magnetic structure of the reactant on the kinetics of nucleus formation on its surface

Abstract

The influence of the Néel transition on the rate of reduction of a well crystallized NiO powder sample (mean diameter 2500 Å) by hydrogen has been investigated. The kinetic study concerns the rate of nucleation and the rate of interface progress, in the domain 200–300°C. A novel experimental procedure, together with Mampel's theoretical model, were used for the determination of the energy of activation of nucleation.

The progression of the reaction interface is slightly accelerated near the Néel point. Except for this effect, the rate of interface progress obeys the same Arrhenius law (same pre-exponential factor and activation energy) below and above the Néel point (E_i= 28 kcal mol^–1). An important decrease (by a factor of 2.9) of the overall rate of reaction is observed just above the Néel temperature. This decrease is a consequence of a much slower nucleation rate, which brings about a decrease of the area of the reaction interface by the same factor. The activation energy of nucleation is 34 kcal mol^–1 below, and 45 kcal mol^–1 above, the Néel temperature.

The change in magnetic state and the accompanying small change in crystallographic structure of the bulk nickel oxide at the Néel temperature thus seems to have a major effect on the surface phenomenon, i.e. nucleation. Some possible causes are discussed.