Mechanisms for O2 dissociation over the BaO (100) surface.

We have investigated the atomic and molecular oxygen adsorptions on the various sites of the BaO (100) surface with both cluster models and the periodic slab models. We found that the atomic oxygen prefers to adsorb on the surface O2- to form the closed-shell peroxides with the binding energies of 83-88 kcal/mol. Such a high exothermicity provides a large driving force for the dissociation of molecular O2 on the BaO surfaces. As molecular oxygen approaches the BaO surfaces, the triplet ground state O2 molecule first binds electrostatically on top of the surface Ba2+ site. It further quenches to the singlet potential energy surface to form a covalently bonded O3(2-) species. We proposed a plausible pathway in which the O3(2-) species acts as the key precursor for further dissociation, leading eventually to the formation of surface peroxides O2(2-). This mechanism is helpful for the understanding of a series of related catalytic processes such as the oxidative coupling of methane, the NOx storage reduction, etc.