Transfer of stored electrons between TiO2 polymorphs during photocatalytic H2 production in methanol-water medium

Mechanical mixtures of platinum-loaded and bare anatase, rutile and brookite TiO2 polymorphs were studied in the photocatalytic production of hydrogen (PHER) in methanol-water slurries. Mixtures of TiO2 and Pt/TiO2 materials show non-additivity of catalytic properties as a function of the nature of titania polymorphs involved. Addition of bare rutile or brookite to Pt/anatase leads to a drop of the PHER rate. By contrast, addition of bare anatase improves PHER activity of both Pt/rutile and Pt/brookite. These observations are coherently explained by directional transfer of long-live stored electrons between the particles in suspensions. Hydrogen release in the dark after addition of Pt/TiO2 to pre-illuminated bare TiO2 give evidence that such transfer is fast and have a pronounced directionality. The accumulated electrons are preferentially transferred from anatase to rutile or to brookite. A simple mechanistic framework and a phenomenological kinetic model are proposed that correctly describe the observed variations of PHER rates.