O ₂ interaction and reactivity on a model hydroxylated rutile(110) surface

Recently several theoretical studies have examined oxygen adsorption on the clean, reduced TiO ₂(110) surface. However the photocatalytic behavior of TiO ₂ and the scavenging ability of oxygen are known to be influenced by the presence of surface hydroxyls. In this paper the chemistry of O ₂ on the hydroxylated TiO ₂ surface is investigated by means of first-principles total energy calculations and molecular dynamics (MD) simulations. The MD trajectories show a direct, spontaneous reaction between O ₂ and the surface hydroxyls, thus supporting the experimental hypothesis that the reaction does not necessarily pass through a chemisorbed O ₂ state. Following this reaction, the most stable chemisorbed intermediates are found to be peroxide species HO ₂ and H ₂O ₂. Although these intermediates are very stable on the short time scale of MD simulations the energetics suggests that their further transformation is connected to a new 300 K feature observed in the experimental water temperature programmed desorption (TPD) spectrum. The participation of two less stable intermediate states, involving terminal hydroxyls and/or chemisorbed water plus oxygen adatoms, to the desorption process, is not supported by the total energy calculations. Analysis of the projected density of states, however, suggests the possibility that these intermediates have a role in completing the surface oxidation immediately before desorption.