Metal oxides: O2- chemistry and dynamical effects on oxide reactivity

Abstract

Doping of CeO₂ with calcium introduces defects and oxygen vacancies and leads to a strong increase of the catalytic activity. Desulfurization of SO₂ with CO involves oxygen abstraction from the lattice and charge-transfer (CT) excitation; this reaction runs at a 70°C lower temperature on the doped substrate. The doping reduces the CT energy cost and the oxygen binding energy, but oxygen abstraction by CO is still not favorable for the lattice at 0 K. The charge state of the ion depends on the Madelung potential, which depends on the lattice structure. Introducing changes in temperature is found to generate vibrations of sufficiently large amplitudes that oxygen anions and cerium cations sometimes can be found at positions where they are sufficiently destabilized so as to be reactive. As the CT energies and oxygen binding energies depend on the instantaneous positions of the ions, active sites appear and disappear at the surface dynamically. The activity of the catalyst substrate is a dynamical quantity that depends on the amplitudes of thermal motion of the surface ions.