Electronic Structure of Oxygen Radicals on the Surface of VOx/TiO2 Catalysts and Their Role in Oxygen Isotopic Exchange

The electronic structure of oxygen radicals formed by adsorption of gas-phase oxygen on partially reduced sites of supported vanadium oxide catalyst V4+Ox/TiO2 has been studied by periodic DFT. The unpaired electron density in the radicals is transferred from the paramagnetic V4+(3d(1)) ion to the adsorbed oxygen atoms resulting in the formation of surface oxygen radicals: atomic O-, superoxide O-2(-), and ozonide O-3(-). These radical species exhibit higher reactivity compared to the surface oxygen species stabilized on fully oxidized diamagnetic V5+(3d(0)) ions. Oxygen isotopic exchange over O- radicals has been investigated by the climbing image nudged elastic band (CI-NEB) method. We show that molecular oxygen can exchange with the lattice oxygen of the surface paramagnetic radicals V5+O- with low activation energy of about 14 kcal/mol, close to the value experimentally observed for some heterolytic RI oxygen exchange reactions on vanadia catalysts. The obtained data suggest that O- radicals formed as short-lived intermediates at elevated temperatures are likely to be the active sites of the oxygen exchange following the R1 mechanism. The properties of oxygen radicals and their possible role in catalytic oxidation processes taking place over bulk and supported metal oxide catalysts are discussed. It is suggested that oxygen radicals can be the active species in catalytic oxidation reactions.