Oxygen adsorption on reduced TiO2(110) was investigated using temperature programmed desorption and electron-stimulated desorption. At low temperatures, two O2 molecules could be chemisorbed in each O vacancy. These molecules did not desorb upon annealing to 700K. Instead, at 200 to 400K, the two O2 converted to another species, which had four O atoms, i.e., tetra-oxygen, that decomposed at higher temperatures. In contrast, when only one O2 was adsorbed in an O vacancy, the molecule dissociated upon annealing above ~150K to heal the vacancy in agreement with previous results.

Tetraoxygen on Reduced TiO2(110) - Oxygen Adsorption and Reactions with Bridging Oxygen Vacancies. Kimmel, G.A., Petrik, N.G.: Physical Review Letters, 2008, 100[19], 196102