Electron transfer and dimerization of viologen radicals on colloidal TiO2

Abstract

The reaction of several viologen radical cations with colloidal TiO₂ particles of 70 Å radius has been studied in detail. The electron-transfer reaction from methyl viologen radicals (MV⁺) to the TiO₂ particles is controlled by the rate of the heterogeneous electron-transfer step. Protonation of the reduced particle follows the electron-transfer reaction in a temporally well separated reaction. Analysis of the subsequent equilibrium stage, but before protonation occurs, allows an estimate of the charge carrier density in the colloid. The effect of added Pt, either as a separate colloid or by photodeposition on the TiO₂ particle, has also been studied. Kinetic analysis indicates that a mixture of Pt and TiO₂ colloids results in adsorption of the Pt particle on the TiO₂ colloid. However, the adsorbed Pt colloid reacts independently of the TiO₂ particle.

Heptyl viologen radicals (HV⁺) catalytically dimerize in the presence of TiO₂ particles in a heterogeneous reaction between an absorbed HV⁺ and a dissolved radical. The competition between dimerization and hydrogen evolution can be directed towards the latter reaction either by reducing the pH or by loading the TiO₂ particles with Pt. The unsymmetric C₁₄MV⁺ viologen radicals aggregate even in the absence of any colloid. At low pH values and high Pt loadings they can, however, be directed towards hydrogen evolution.