Chemical Reactivity of Reduced TiO2(110): The Dominant Role of Surface Defects in Oxygen Chemisorption

O-2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (O-v) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature-programmed desorption, and electron-stimulated desorption. On the annealed surface, two oxygen molecules can be chemisorbed per O-v. The same amount of O-2 chemisorbs on surfaces where each O-v is converted to two OHb's by exposure to water (i.e., 1 O-2 per OHb). Surfaces with few or no O-v's or OHb's can be created by exposing the hydroxylated surface to O-2 at room temperature, and the amount of O-2 that chemisorbs on these surfaces at low temperatures is only similar to 20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O-2 increases by more than a factor of 2 when the OHb concentration is enhanced-without changing the concentration of subsurface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species such as O-v's and/or OHb's, and not Ti3+ interstitials.