We show that metal particles encapsulated by thin silica shells can catalyze redox reactions on their surface. The surface charge of both silica-coated and uncoated colloidal silver particles has been varied by chemical electron injection, using controlled amounts of borohydride ions. The charging and discharging processes were monitored by time-resolved UV-visible spectroscopy, making use of the influence of the surface charge on the position and intensity of the silver plasmon band. We show that the charging and discharging rates can be controlled by means of silica coating, since they are limited by the diffusion of borohydride ions through the silica pores.