Reactivity of Hydrous Ferric Oxide Containing Metallic Cations

Abstract

Hydrous ferric oxide prepared by precipitation contains the bound water in the form of OH groups. Its dehydration occurs as an endothermic reaction between 100 and 200°C. The amount of bound water increases with increasing amount of amorphous material within the solid oxide. The crystallization of the amorphous solid which proceeds on ageing in water is retarded by addition of metallic cations to the oxide; especially Cu-ions show a very strong retardation effect.
The reactivity of the oxide containing different kinds of metallic cations and different amounts of the bound water, has been investigated in a number of reactions. The amount of hydrazine decomposed on the oxide increases linearly with the amount of bound water. The rate of dissolution of the oxide in acid solutions obeys the equation, d[Fe³⁺]/dt = k (H⁺)^0.5 (A⁻)^m, where (H⁺) and (A⁻) denote the concentrations of hydrogen ions and anions and m is a constant. If it is assumed that the bound water forming the Fe–(OH)–Fe bridge provides the active sites this equation may be explained. The rate of the adsorption of a gaseous CO–O₂ mixtures follows the Elovich equation and the initial rate increases with the amount of bound water. Finally, the temperature of the exothermic transformation of magnetite into α-Fe₂O₃ is examined. Cr, Li and Cu-ions showed an accelerating effect on the reaction, and Ti and B-ions a retarding effect.