Electronic Structure of Oxygen Radicals on the Surface of VOx/TiO2Catalysts and Their Role in Oxygen Isotopic Exchange

Abstract

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+(3d1) ion to the adsorbed oxygen atoms resulting in the formation of surface oxygen radicals: atomic O–, superoxide O2–, and ozonide O3–. These radical species exhibit higher reactivity compared to the surface oxygen species stabilized on fully oxidized diamagnetic V5+(3d0) 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 R1 oxygen exchange reactions on vanadia catalysts. The obtained data suggest that O– radicals formed as short-liv...