Abstract: With an improved EHMOmethod, three modes (flat-lying, vertical, and in-clined insertion) for adsorption and activation of dioxygen on (100), (110) and(111) surfaces of Na₂Oand of K,Ohave been exaniinecl, and the interaction ofthese dioxygen adspecies with CH₄ and with CH₃·(radical) from gas pliase liasbeen investigated.The results indicate that both oxides tend to forni less cliargedadspecies of dioxygen, with the flat-lying adsorption on (110) surface most favor-able energetically.All these cliemisorbed dioxygen species are capable of interactingeffectively with CH₄ and Abstract ing one hydrogen atom from the CH₄ molecule andtheir tendencies to reassociate with CH₃ ·, which would easily lead to deep oxida-tion of the fragments of hydrocarbons, are enhanced with inereasing negativecliarges on them.In comparison with Na₂O, K₂Ohas a little stronger tendency tostabilize less charged dioxygen adspecies; this has a close relation with the knownexperimental fact that K⁺ showed better promoting effect than Na⁺ in improving C₂-selectivity in methane oxidative coupling (MOC ).

Key words: Chemisorbed dioxygen species, Methane oxidative coupling, EHMO approach