The characterization and stability of super-oxide radicals (O2-) over polycrystalline TiO2 (Degussa P25) was investigated using electron paramagnetic resonance (EPR) spectroscopy. The adsorbed oxygen molecules act as efficient electron scavengers and were therefore used to indirectly probe the sites of electron transfer at the surface of the anatase component of the mixed phase P25 material. A distribution of various stabilization sites on the surface was identified by analysis of the g values and further confirmed by identification of several well-resolved 17O hyperfine patterns. For the first time, on a polycrystalline TiO2 surface evidence for stabilization of superoxide radicals specifically at anion vacancy sites was presented by EPR. These radicals, labelled [Vac...O2 -], were characterized by the spin Hamiltonian parameters of gxx = 2.005, gxx = 2.011, gzz = 2.019, and17OAxx = 7.64 mT (Ayy = A zz > 1mT). The [Vac...O2 -] radicals exhibited pronounced reactivity under the influence of thermal, photochemical and chemical treatment as compared to the remaining surface O2 - anions bound at non-vacancy sites. The extent of site occupancy was found to be sensitive to the oxygen adsorption temperature and the extent of O2 -radical migration on the surface. Thus, the stability and lifetime of the surface O2 -anions were directly correlated to the structure of the adsorption site itself at the anatase surface of P25.

Evidence for O2- Radical Stabilization at Surface Oxygen Vacancies on Polycrystalline TiO2. Carter, E., Carley, A.F., Murphy, D.M.: Journal of Physical Chemistry C, 2007, 111[28], 10630-8