Surface reactivity of MgO oxygen vacancies: electrostatic mechanisms in the formation of O2- and CO- species

Ferrari, Anna Maria; Pacchioni, Gianfranco
August 1997
Journal of Chemical Physics;8/8/1997, Vol. 107 Issue 6, p2066
Academic Journal
The interaction of O2 and CO gas-phase molecules with oxygen vacancies on the MgO (100) surface has been studied by means of cluster models and ab initio wave functions. It is found that the surface oxygen vacancies, or Fs centers, exhibit a high reactivity toward O2 and CO at variance with the regular MgO surface. The reaction proceeds through the formation of radical anions, O2- and CO-, via the transfer of one electron trapped in the surface cavity to the empty levels of the adsorbed molecule. The resulting surface complexes, X-/Fs+ or X-/Fs2+ (X=O2 or CO), are bound by electrostatic forces. Although the mechanism of the interaction is the same for the two molecules, the details of the energetics are different. O2 spontaneously removes the electrons trapped in the MgO oxygen vacancies to form the stable O2- superoxide anion. On the contrary, CO- forms only at finite temperatures and is a metastable species. The different behavior can be rationalized in terms of electron affinities of the two molecules. The calculations are useful also for the spectroscopic characterization of the radical anions at the surface. The calculations of electron paramagnetic resonance (EPR) hyperfine coupling constants and, for CO, of the vibrational frequencies indicate that the experimental spectra are consistent with the existence of O2- and CO- surface species. The analysis of the vibrational shifts shows that the coordination mode of CO is C-down and not O-down. © 1997 American Institute of Physics.


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