SURFACE REACTIVITY OF MGO OXYGEN VACANCIES - ELECTROSTATIC MECHANISMSIN THE FORMATION OF O-2(-) AND CO- SPECIES

The interaction of O-2 and CO gas-phase molecules with oxygen vacancie s 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 vac ancies, or F-s centers, exhibit a high reactivity toward O-2 and CO at variance with the regular MgO surface. The reaction proceeds through the formation of radical anions, O-2(-) and CO-, via the transfer of o ne electron trapped in the surface cavity to the empty levels of the b y electrostatic forces. Although the mechanism of the interaction is t he same for the two molecules, the details of the energetics are diffe rent. O-2 spontaneously removes the electrons trapped in the MgO oxyge n vacancies to form the stable O-2(-) superoxide anion. On the contrar y, CO- forms only at finite temperatures and is a metastable species. The different behavior can be rationalized in terms of electron affini ties of the two molecules. The calculations are useful also for the sp ectroscopic characterization of the radical anions at the surface. The calculations of electron paramagnetic resonance (EPR) hyperfine coupl ing constants and, for CO, of the vibrational frequencies indicate tha t the experimental spectra are consistent with the existence of O-2(-) and CO- surface species. The analysis of the vibrational shifts shows that the coordination mode of CO is C-down and not O-down. (C) 1997 A merican Institute of Physics.