Photochemistry of chemisorbed and physisorbed O$_{2}$ on reduced rutile TiO$_{2}$(110)

The ultraviolet (UV) photon-stimulated reactions of oxygen on TiO$_{2}$(110) are studied. For chemisorbed O$_{2}$, the photochemistry depends on the O$_{2}$ coverage. For small coverages, only $\sim $14{\%} desorbs while the rest either dissociates during UV irradiation, or remains molecularly adsorbed on the surface. For the maximum coverage of chemisorbed oxygen, the fraction of O$_{2}$ that photodesorbs is $\sim $40{\%}. However when physisorbed O$_{2}$ is also present, $\sim $70{\%} of the initially chemisorbed O$_{2}$ photodesorbs. Experiments using O$_{2}$ isotopologues show that UV irradiation results in exchange of atoms between the chemisorbed and physisorbed oxygen. Annealing chemisorbed oxygen to 350 K maximizes these exchange reactions. The exchange products photodesorb in the plane perpendicular to the bridge-bonded oxygen rows at an angle of 45\r{ }. Remarkably, the chemisorbed species is stable under multiple cycles of UV irradiation with physisorbed O$_{2}$, and the atoms in the chemisorbed species can be changed from $^{18}$O to $^{16}$O and then back to $^{18}$O via the exchange reactions. The results show that annealing oxygen adsorbed on TiO$_{2}$(110) to $\sim $350 K produces a stable chemical species with interesting photochemical properties. Possible forms for the photoactive species include O$_{2}$ adsorbed in a bridging oxygen vacancy or tetraoxygen.