Colloidal platinum catalysts for reversible photoredox processes

Abstract

The quantum yield for formation of redox ions from the zinc(II) tetra(N-methyl-4-pyridyl)porphine-photosensitised reduction of methyl viologen is ca. 0.7, but the products undergo diffusion-controlled reverse electron transfer. Using ultrafine colloidal Pt particles, supported with polymers, waxes or surfactants, it is possible to intercept reverse electron transfer at modest concentrations of Pt, but the catalysts are not selective. Some degree of selectivity can be obtained if a positively charged Pt colloid is used in conjunction with a less hydrophilic viologen, but under such conditions steady-state irradiation does not produce H₂.

When a surfactant viologen (SV²⁺) is used as the support for Pt colloids, the reducing equivalent can be transferred rapidly to the Pt core. The lifetime of the reduction product (SV⁺) depends upon the number of electrons resident on the particle and increases with increased charge. This is due, at least in part, to dimerisation of the viologen radical. Again, steady-state irradiation in the presence of a neutral zinc porphyrin gives no detectable yield of H₂.