Titanium Superoxide as a Carrier of a "Long-Lived" Superoxide Anion: An Ab Initio Investigation

The photoinduced generation of a superoxide anion on the surface of a semiconductor photocatalyst is usually attributed to the reduction of O₂ with conduction-band electrons. In the current work, the reaction of TiO₂ with O₂ giving rise to TiO₄ in superoxide and peroxide states has been investigated with ab initio (CAS, CCSD) and DFT (B3LYP) calculations. The ground triplet state and two substates (open-shell singlet (OSS) and closed-shell singlet (CSS)) of a doubly degenerate excited singlet state (a¹Δ) are considered as reactive states of oxygen, participating in spontaneous or photoinduced processes, respectively. The triplet and OSS singlet states of TiO₄ contain O₂^- as structural units and can be defined as titanium superoxides. Both states have energy less than the level of the initial pair TiO₂+O₂ by about 30 kcal/mol. The CSS state of TiO₄ has a diperoxide structure Ti⁴⁺(O₂^2-)₂ and also lies in energy below the initial pair TiO₂+³O₂. Titanium superoxide is considered to be the carrier of an "exceptionally stable" and "long-lived" superoxide anion, which was earlier synthesized or detected on the surface of TiO₂. The low-energy location of the conical intersections on the way from reagents to ³TiO₄ allows us to explain the literature data on the spontaneous generation of the "long-lived" superoxide anion on the TiO₂ surface.