Heterogeneous catalysis in solution. Part VIII. Catalysis of the aquation of the bromopenta-amminecobalt(III) ion by metallic platinum

Abstract

The catalysis by platinum of the aquation of [Co(NH₃)₅Br]²⁺ in 0·01 M-HClO₄ solution at 25°C was investigated with a large rotating disc of the metal. The rate of reaction was found to be surface-controlled and dependent on the electrochemical pre-treatment of the catalyst. The potential of the platinum was a major factor in determining the catalytic rate; the lower the potential, the faster the reaction. Addition of Br^– and [Co(NH₃)₅H₂O]³⁺ ions lowered the platinum potential and so increased the catalytic rate, in marked contrast to catalysis by HgS and AgBr¹. In the absence of any background electrolyte other than HClO₄ the results conformed well to a model which postulated the establishment of adsorption equilibrium, according to a Langmuir isotherm, prior to reaction.