  The dissociation of a hydrogen molecule on MgO(001) films deposited on
Mo(001) surface is investigated systematically using periodic
density-functional theory method. The unusual adsorption behavior of
heterolytic dissociative hydrogen molecule at neighboring surface oxygen and
surface magnesium, is clarified here. To my knowledge, this heterolytic
dissociative state has never been found before on bulk MgO(001) or metal
supported MgO(001) surfaces. The results confirm that, in all cases, the
heterolytic dissociation is much more favorable that homolytic dissociation
both energetically and kinetically. The energy difference between two
dissociative states are very large, in the range of 1.1 eV ~ 1.5 eV for Mo
supported 1 ML ~ 3 ML oxide films, which inhibits, to a great extent, the
homolytic dissociation in the respect of reaction thermodynamics. The energy
barrier of heterolytic dissociation are about 0.5 eV, much lower that the
barrier of homolytic dissociation. The transformation reaction on thick films
will be more endothermic. Passing through heterolytic dissociation state have
significantly lowered the reaction heat and the energy barrier for obtaining
homolytic dissociative structure, which make the homolytic splitting of H2
easier on 2 ML oxide films. The results provides a useful strategy for
enhancing the reactivity of the nonreducible metal oxide.
