Heterogeneous catalysis in solution. Part 25.—Contrasting behaviour of reduced and oxidised platinum surfaces in the catalysis of the reaction between cerium(IV) and mercury(I)

Abstract

The kinetics of the reaction between Ce^IV and Hg²⁺₂ in 1 mol dm^–3 HClO₄ have been studied at 25 °C in the presence of a large rotating platinum disc catalyst. When the disc was cathodically preconditioned, the catalytic rate was first order in cerium(IV) and zero order in mercury(I). It was also directly proportional to the square root of the rotation speed and exhibited a low activation energy, which showed that the reaction was mass-transport controlled. There was excellent agreement between the catalytic rate and the limiting current density for Ce^IV reduction (converted to rate units by Faraday's law), as would be expected from an electrochemical model of the catalysis. Diffusion coefficients of Ce^IV and Hg²⁺₂ in 1 mol dm^–3 HClO₄ were determined from the limiting current measurements. Cyclic voltammetric sweeps after the catalytic runs revealed the presence of underpotential-deposited mercury layers on the platinum surface, and the number of mercury monolayers was evaluated.

In contrast, when the platinum disc was anodically preconditioned, the catalytic rate was smaller, of fractional order in both reactants, and independent of rotation speed. The catalysis was therefore surface-controlled. Kinetic and cyclic voltammetric experiments pointed to the presence of two types of adsorbed mercury species on the oxidised platinum surface which strongly influenced the catalysis. Exposure of the oxidised disc to a solution of Hg²⁺₂ ions before the addition of Ce^IV resulted in an increased catalytic rate because the mercury(I) ions had partly reduced the oxidised surface.