Heterogeneous catalysis in solution. Part 19. The effect on oxidation–reduction reactions of certain sparingly soluble salts and other solids

Abstract

The addition of various semiconducting solids (silicon and metal sulphides) and of two ionically conducting solids [copper(I) and thallium(I) iodides] appeared either to catalyse or inhibit the rate of iodine formation by the reactions [Fe(CN)₆]^3–+ I^– and Fe³⁺+ I^– in aqueous solution. The curious effect of CuI on the latter reaction was investigated in more detail. Adsorption experiments demonstrated that [Fe(CN)₆]^3– and I₃^– chemically attacked these solids. Their chemical instability in the presence of the oxidants or the reaction mixtures was confirmed by thermodynamic calculations and the observed kinetic effects could then be explained. Only silica and silver(I) iodide did not affect the rates of the reactions; adsorption tests and thermodynamic calculations showed that these two solids were stable under the experimental conditions. The catalytic effect produced by a solid is therefore genuine only if kinetic and/or thermodynamic evidence has established its chemical stability in the reaction mixture.