Is There a Nanosize for the Activity of TiO2 Compounds?

Reactivity of TiO2 compounds is investigated vs nanosize using atomic hydrogen adsorption as a probe. Small clusters with (TiO2)(n) stoichiometry (n = 1-24) of lowest energy are compared with infinite systems like polymers and Surfaces. The binding energy for the most reactive Clusters reaches similar to 3.5 eV while it is 2.8 eV for the smallest one (the TiO2 molecule) and 2.30 eV for the most reactive surface (rutile-110). No systematic relation with size emerges from this analysis which predicts a large variety of individual structures. High H binding energy is associated with the presence of poorly coordinated 0 atom at the surface and with a favorable reducibility of the substrate. The local topology of the adsorption site and its flexibility play a major role in reactivity, However, the reactivity of nanosized TiO2 clusters is correlated with small electronic gap and low lying LUMOs.