A heterogeneity of surface sites for superoxide (O2-) stabilization on thermally reduced TiO2 (P25) were identified by cw EPR. Two main groups of adsorption sites exist; site I due to O2 - adsorbed at oxygen vacancies, [Vac ⋯ O2 -], and sites II-III due to O2- adsorbed at five co-ordinate Ti4+ centres. Electron transfer from the precursor Ti3+ centres to O2 (forming O2-) could be prevented at each site through selective site blocking with probe molecules (CO2 and Ar). CO2 preferentially adsorbs at the five co-ordinate Tinon-vacancy3+ centres, while Ar preferentially adsorbs at the Ti3+ centres associated with the oxygen vacancies (Tivacancy3+).

Interaction of Molecular Oxygen with Oxygen Vacancies on Reduced TiO2: Site Specific Blocking by Probe Molecules. Green, J., Carter, E., Murphy, D.M.: Chemical Physics Letters, 2009, 477[4-6], 340-4