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The adsorption of molecular oxygen on GaN (0 0 0 1) and ( 0 0 0 1 ¯ ) surfaces has been explored by employing density functional theory. Our calculations indicate that O2 prefers to be dissociated on these surfaces in parallel orientation, and the dissociated O atoms are combined in fcc site (on both surfaces) or in hcp site (only on GaN ( 0 0 0 1 ¯ ) surface). The lengths of the formed Ga O bonds at the two polar surfaces agree with that in ideal Ga2O3 bulk, and these reactions are greatly exothermic. From the potential curve, no obvious energy barrier is found during the process of O2 dissociated adsorption on GaN (0 0 0 1) surface.

The adsorption of CH 2 O at low-coordinated sites (edge and corner) of MgO (0 0 1) surface has been studied using cluster models in the framework of density functional theory. Two adsorption states of CH 2 O, i.e. heterolytically dissociative adsorption and molecular adsorption were found. In the former case, the CH 2 O adsorbed over Mg 3C –O 3C pair site was dissociated into two separate subspecies, “CHO − ” and “H + ” through the C–H bond cleavage with rather low activation energy, 4.58 kcal/mol. Due to the quite similarity in adsorption energy, it can be expected that both two adsorption states, namely molecular and dissociative adsorption, should simultaneously exist for CH 2 O over Mg 3C –O 3C pair site of MgO (0 0 1) surface. In analogy, for CH 4 , C 2 H 2 and CH 3 OH, the edge and corner sites of MgO (0 0 1) surface exhibit the catalytic reactivity toward the C–H bond cleavage, thus leading to the dissociative subspecies, which was reinforced by the quantum chemical calculations and the respective experiments.

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.