Oxygen Photochemistry on TiO2(110): Recyclable, Photoactive Oxygen Produced by Annealing Adsorbed O2

The photochemistry of oxygen adsorbed on TiO2(110) at 30 K and annealed up to 600 K is investigated. UV irradiation results in exchange of atoms between chemisorbed and physisorbed oxygen. Annealing chemisorbed oxygen to 350 K maximizes these exchange reactions, while such reactions are not observed for oxygen that is dissociatively adsorbed on TiO2(110) at 300 K For oxygen annealed to 350 K, the exchange products photodesorb in the plane perpendicular to the bridge-bonded oxygen rows at an angle of 45 degrees. In contrast, chemisorbed O-2 photodesorbs normal to the surface. Remarkably, the chemisorbed species is stable under multiple cycles of UV irradiation. Atoms in the chemisorbed species can be changed from O-18 to O-16 and then back to O-18 via the exchange reactions. The results show that annealing oxygen on TiO2(110) to similar to 350 K produces a stable chemical species with novel photochemical properties. Possible forms for the photoactive species include O-2 adsorbed in a bridging oxygen vacancy or tetraoxygen.