posted on 2013-07-18, 00:00authored byVasilii
I. Avdeev, Alexander F. Bedilo
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-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.