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

O 2 chemisorption on reduced, rutile TiO 2 (110) with various concentrations of oxygen vacancies (O v ) and bridging hydroxyls (OH b ) 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 OH b ’s by exposure to water (i.e., 1 O 2 per OH b ). Surfaces with few or no O v ’s or OH b ’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 ∼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 OH b concentration is enhanced—without changing the concentration of subsurface Ti interstitials. The results indicate that the reactivity of TiO 2 (110) is primarily controlled by the amount of electron-donating surface species such as O v ’s and/or OH b ’s, and not Ti 3+ interstitials.