DEFECT EFFECTS ON H(2) DISSOCIATIVE ADSORPTION ON THE NI(100) SURFACE

The dissociative adsorption of a hydrogen molecule on the nickel(100) surface with point defects is investigated using the embedded-atom met hod (EAM). The potential-energy surfaces (PES) for H-2 dissociation on both perfect and imperfect Ni(100) surfaces are presented, based on t otal-energy calculations. It is clearly shown that as the H-2 approach es the Ni(100) surface along the entrance channel, the H-H bond is pro gressively weakened while the H-metal bonds begin to form; finally the H-2 is adsorbed on the surface in the form of two independent H atoms . This dissociation process is affected by the vacancy and impurity at oms existing in the Ni substrate. The activation barriers (E(a)) for t he dissociation of H-2 through various pathways are calculated. The ba rriers for the dissociation of H-2 On the perfect Ni(100) surface are found to be low (about 0.08-0.09 eV, corresponding to different dissoc iation pathways). The existence of vacancies enhances the dissociation of H-2 by lowering the activation barrier height and providing more a dsorption sites. However, the impurity atoms (Cu, Pd) can impede the d issociation of H-2 on the Ni(100) surface by increasing the activation barrier height. The adsorption heat of H-2 chemisorption on the conta minated Ni(100) surface is also calculated. It is found that the effec ts of impurities on the dissociation of H-2 vary with the dissociation pathways and the impurity sites.